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which battery voltage is bestThe most common question I get as a solar consultant is…

What voltage should I use for my battery bank? And Why?

Almost everyone who is new to solar, wind or micro-hydro assumes that 12 volts is the way to go.

It seems obvious. You can buy 12 volt lighting, 12 volt coffee makers, 12 volt fans, and many other 12 volt appliances. You can even buy batteries that are 12 volt. So it would seem that 12 volts is the magic voltage!



In the early days the choice was easy, you could only buy 12 volt inverters and 12 volt charge controllers, however 12 volts is likely not the best choice for a renewable energy system today.

The most common battery voltage is now 24 volts and soon 48 volt systems will be the most popular. What are the best choices for your first battery bank?

Homesteaders building renewable energy systems should avoid 12 volt battery banks with the following exceptions:

1. RVs/motor homes
2. Travel trailers
3. Very small systems as for a small cabin or tiny home

rv solarRVs and motor homes already have 12 volt starting batteries and house batteries as well as 12 volt lighting, hot water heater controls, air heating controls and refrigerators. It makes no sense to retrofit systems that are already working fine. The same goes for travel trailers.

ms2012Just limit your inverter to 2000 watts such as Magnum Energy’s MS2012 and you will have a rugged and reliable inverter/battery charger. The MS2012 is a low frequency (big, rugged and heavy) pure sine wave inverter with a massive power factor corrected (PFC) battery charger that works well with fuel powered generators. How to charge your batteries with a generator. The MS2012 has a peak output of 2800 watts for 5 minutes. Divide 2800 watts by 12 volts and the surge amperage is 233.

Use a DC 250 amp circuit breaker for your disconnect and a 300 amp class T fuse with holder for your catastrophe fuse

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. Add a set of 4/0 cables and you have a setup that will run a microwave, television, radios, toaster, hot plate or even a small air conditioner. Magnum does make an MS2812 that will also work nicely but unless you watch your loads carefully you will need a larger breaker, fuse and cables. Sometimes that extra 800 watts will start even a good sized air conditioner and the A/C unit only needs a lot of current when it starts up. Once it is running, it is not that big of a load (maybe 200-400 watts).

A cabin of this size might get by with a 12 volt battery system.

There are also times when I will recommend 12 volts for a tiny renewable energy system for a cabin, cottage or tiny home. If my fellow off gridder only needs a 2000 watt or smaller inverter, than 12 volts might be the way to go.

The problem with designing a 12 volt system is the inability to grow in the future. But still there are the weekend warriors that only want to operate a small fridge and a few lights or they have a propane fridge and propane lighting. 12 volts might work fine for them.

It is rare that I recommend 12 volts but it does happen or sometimes the homesteader has already bought their equipment by the time we get to talk.

As solar (wind and small hydro) electric systems get larger, the currents we have to deal with are getting huge!

(Remember that VOLTS X AMPS = WATTS) or (WATTS/VOLTS = AMPS)

For example:

4000 watt 12 volt inverterLet’s pretend we have just purchased a new 4000 watt pure sine wave 12 volt inverter.

If it is a decent quality inverter, it will likely be able to surge (for 5 to 30 minutes) at about 6000 watts. When we divide 6000 watts by 12 volts we get a maximum surge current of 500 amps. This is a huge amount of current.

Imagine how large your inverter cable would need to be to handle 500 amps?

Where are you going to find a DC circuit breaker or DC fuse that can handle up to 500 amps?

When you go shopping for those products you will soon realize they don’t exist or are very rare.

A large DC breaker for the solar industry is 250 amps. I have seen two 250 amp DC breakers paralleled together to make a 500 amp breaker. But even if you are lucky enough to find one (and pay up to $800 for it), you still need to get the current from the battery bank to the inverter using copper battery cables.

In this example you would need at least two 4/0 cables in parallel for the positive and two 4/0 cables in parallel for the negative. The cables could add up to another $800 in unplanned expenses.

It just isn’t practical or affordable.


Magnum Energy’s high quality MS4024 inverter/battery charger.

If you were to build that same system using 24 volts, the maximum (surge) current would be cut in half or 250 amps. DC 250 amp breakers ($90-$200) are readily available as well as 300 amp DC class T fuses ($70-$120).

Now we can use one 4/0 cable ($80-$200) for the positive and one 4/0 battery cable ($80-$200) for the negative which is a very common size and also readily available.

Now let’s use 48 volts with the same 4000 watt inverter that can surge up to 6000 watts. The maximum surge current is now only a mere 125 amps or 6000 watts divided by 48 volts.

As a general rule you would use a 175 amp DC breaker ($90-$200) as a DC disconnect for this inverter with a 200 amp class T fuse ($60-$90) and 2/0 inverter cables ($40-$100 each) which contain 1/4 the copper that 4/0 cables are made of.


Outback’s FM60

Just the above mentioned reasons should be enough to make you carefully consider using 24 or 48 volts for your next renewable energy battery bank but there is more.

Let’s look at the specs of one the most popular charge controllers on the planet, Outback Power’s FM60.

It is a 60 amp MPPT (maximum power point tracking) charge controller and the 60 amp rating refers to the battery voltage.

The higer the voltage of your battery bank, the more power output the FM60 can handle.

At 12 volts the Flexmax 60 (FM60 | $400-$600) can only handle up to 750 watts, 24 volts – 1500 watts and at 48 volts -3000 watts of solar modules. You may have to buy multiple charge controllers at 12 volts or even 24 depending on the size of your solar array.

All charge controllers are the same, the amperage rating is based on the battery voltage; the higher the voltage the more power the controller can handle.

You may have noticed in Outback’s specs they even have a wattage limit (3750 watts) for folks with a 60 volt battery bank. Although I have not seen a 60 volt battery bank, it is likely to become common in the future for the same reasons 48 volts is so popular now.

 There is a good reason to choose 24 volts instead of 48 (or 12 volts instead of 24) in certain situations.

If you only have one string of batteries it might make sense to lower the battery voltage so you can have two strings.

In the early planning stages, you make the decision to build your battery bank from eight 6 volt batteries such as Trojan’s L16s. If you wire them all in series you now have a 48 volt battery bank.

Sounds great but what if you have one battery fail? How to find the defective battery in your bank. Since your inverter and charge controller need 48 volts, your system is now down until you get a new battery.

1 string

A better scenario would be to wire your 8 batteries into two groups of 4 and parallel the strings to make a 24 volt system. Now if you have a battery failure you can operate your system on 4 batteries while you wait for a replacement.

Having trouble with a battery or two? Check these guys out…


2 strings

We always recommend two strings no matter what voltage you decide for your battery bank. Then you will always have one string to “limp” with until you can get a replacement. Learn how to make your batteries last as long as possible.

Deciding whether to design and build  a 12, 24 or 48 volt battery bank is one of the bigger (and first) decisions you will make when deciding to live off the grid.

If you have any questions or something to add on this topic please comment below or contact us.


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Why Are Batteries Rated in Amp Hours (AH) and How Can One Battery Have Different AH Ratings?

Big Red 2 Volt Battery by SurrettePart of designing an independent power system is calculating the amount of amp hours (AHs) your battery bank will need to keep your system operating between charges.

All manufacturers provide amp hour ratings for their deep cycle batteries to try and make battery selection easy.

However the problems begin when you see the same battery will have multiple AH ratings. And they can be very different???

How can one battery have multiple AH ratings and which one should I use?

The first thing to remember when looking at amp hour ratings is that C20 is the most often quoted and most used by renewable energy experts.

If you don’t care why there are different ratings, use the C20 (20 hour rate) and always use the C20 when comparing batteries. If you are curious keep reading…


A “C” rating is simply a battery’s capacity (or AH/amp hour rating) when discharged over a specific period of time.

This rating is acquired by adding a specific size load to a battery to make it completely dead in a 3, 5, 8, 10, 20 or 100 hour period.

For each test the battery is discharged until the battery reaches a voltage of 1.75 volts per cell. Discharging a battery to 1.75 volts per cell is considered to be fully discharged. For example: a 6 volt battery is discharged until the voltage reaches 5.25 volts.

Battery Voltage (Nominal) Ending Battery Voltage
12 VOLT 10.50 VOLTS
24 VOLT 21.00 VOLTS

Amp Hour Rating of Deep Cycle Batteries

If the specific load discharges the battery in 5 hours, the manufacturer adds up the AHs the battery produced (in that 5 hour period) and calls it a C5 rating.

If another smaller load discharges the battery in 20 hours, the manufacturer adds up the AHs the battery produced (in that 20 hour period) and calls it a C20 rating.

  • A C3 rating means the battery has been completely discharged over a period of 3 hours. SUPER FAST DISCHARGE
  • A C5 rating means the battery has been completely discharged over a period of 5 hours. VERY FAST DISCHARGE
  • A C8 rating means the battery has been completely discharged over a period of 8 hours. FAST DISCHARGE
  • A C10 rating means the battery has been completely discharged over a period of 10 hours. FAST DISCHARGE
  • A C20 rating means the battery has been completely discharged over a period of 20 hours. MEDIUM DISCHARGE
  • A C100 rating means the battery has been completely discharged over a period of 100 hours. SLOWER DISCHARGE

Let’s look at the AH capacities of a common solar battery, the Trojan L16 6 Volt battery. According to Trojan, the ratings are as follows:

Battery "C" Rating Battery Capacity Load Available Energy
C5 HOURS 303 AH 60.6 AMPS 1.82 KWH
C10 HOURS 340 AH 34.0 AMPS 2.04 KWH
C20 HOURS 370 AH 18.5 AMPS 2.22 KWH
C100 HOURS 411 AH 4.11 AMPS 2.47 KWH

If you were to bring home your new Trojan L16 battery and add a 60.6 amp load to it, the battery would last about 5 hours (before reaching 5.25 volts) and give you 1.82 KWH of electricity. However, if you took the same full battery and added a 4.11 amp load, you would get about 100 hours with a total energy capacity of 2.47 KWH. That is a 36% gain in storage capacity.

Big Red 2 Volt Battery by SurretteHopefully you are starting to see there is no such thing as 370 AH battery (as the L16 is often assumed) because the total capacity changes depending on the load applied to the battery.


Why does a battery produce more power over 100 hours than over 5 hours?

The main reason is heat. The faster you discharge a battery, the more heat will be produced due to resistance in the battery itself. Think of it like a battery cable. If you were to force 20 amps through a 2/0 battery cable, most of the electricity would pass through as there would be almost no resistance

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. Now force 1000 amps through your 2/0 battery cable. The electricity would pass through, but the cable would become very hot causing waste heat. A battery operates the same way. The more amps you remove, the more waste heat created.

The 20 hour rate or C20 rate is the most common rating used in the solar industry but you need to be aware there are shady battery manufacturers and installers that will inflate their AH ratings by using the C100 or 100 hour rate. This mistake is also commonly made by inexperienced solar installers.

It is best to use the C20 rate when designing your renewable energy system even though it is more likely your batteries will be discharged over 100 or more hours.

The 20 hour rate will be about 10% less (than C100), adding some margin to your battery bank. The other reason is that home energy systems generally have highs and lows when it comes to power consumption. Even if the battery bank is discharged over 100 hours it was not likely consistent. There might have been 5 hours of medium loads like fridge, freezer and small electronics, 1 hour of hairdryer use, 20 hours of computer operation and 70 hours of nothing. This is not the same as 100 hours of a small load.

The last thing to consider when discussing AH ratings is the fact that under-sizing a battery bank will always result in poor performance for four reasons:

  1. The under sized battery bank will not make it through the cloudy or calm periods resulting in more generator run time.
  2. The undersized battery bank will need replaced prematurely as it will be deep cycling too much and too often.
  3. The undersized battery bank will likely be chronically undercharged.
  4. The undersized battery bank will not even operate to its rated potential as it will be discharging at a fast rate (maybe C5 instead of C20).

Over-sizing your battery bank will result in much better performance. ALWAYS!

If you double your battery bank, you will always get more than double the storage capacity.

REMEMBER: The slower you discharge your battery bank, the more capacity you will get from the sane bank.

Let’s prove this using Trojan T105 6 volt golf cart style batteries. Specifications as per Trojan:

Battery "C" Rating Battery Capacity Load Available Energy
C5 HOURS 185 AH 37.0 AMPS 1.11 KWH
C10 HOURS 207 AH 20.7 AMPS 1.24 KWH
C20 HOURS 225 AH 11.3 AMPS 1.35 KWH
C100 HOURS 250AH 2.50 AMPS 1.50 KWH

Trojan T105 225 AH at 20 hoursUsing two T105s we make a 12 volt battery bank by wiring the two batteries in series.

We are now going to add a load that will drain the two batteries over a 10 hour period to make our calculations easy. Using the C10 rate above, our battery bank will produce 2.48 kWh (twice the 1.24 kWh C10 rate because there are two batteries) if we add a 20.7 amp load at 12 volts. For our purposes we now have a 2.48 kWh battery bank.

If we double our battery bank, we should get a 4.96 kWh battery bank as it is twice as big.

However when we double the battery bank to four T105s and change our load to 22.6 amps at 12 volts (the C20 AH rating) we would now have 5.40 kWh bank because we can use the C20 rating instead of the C10 (10 hour rating). There are double the batteries meaning double the time it takes to bring the batteries to 10.50 volts or 1.75 volts per cell.

Our new battery bank is not 4.96 kWh, it is 5.40 kWh, an increase of 9%.

Obviously a real battery bank would not be discharged in exactly 10 or 20 hours but you can now see that adding to your battery bank will only make it better.


Not really. However your charging source must be large enough to bring the battery voltage to the manufacturer’s recommend bulk voltage.

In our experience you need a charging source that is at least 3% (in watts) of the watt hours of your battery bank’s storage capacity.

For example we will make a battery bank from two Trojan T105s and use the C20 rate as C20 is the industry standard.

From our chart above we see the Trojan T105 has 1.35 kWhs or 1350 watt hours of storage. This was found with the following formula:

6 V X 225 AH = 1350 WATT HOURS

We have two batteries.


In our experience we would need a charging source (solar array etc.) that is 3% of the total watt hours.

2700 WATT HOURS X 3% (0.03) = 81 WATTS

For a battery bank made of two Trojan T105s you should have at least 80 watts of solar to bring the battery bank up to its recommended bulk voltage.

Remember this is just from experience and is not written in stone. As long as your charging source will bring the battery bank’s voltage up to the manufacturer’s recommend bulk voltage, you have not oversized your battery bank.

In a properly designed power system having a huge battery bank will not cause any problems.

The only major difference will be that the batteries will likely perform better than their C100 rating (or 100 hour rate) and never get discharged or cycled very much.

When a solar system has been properly designed there will always be excess energy available to keep the batteries topped off. Deep cycle batteries used in off grid power systems do not need to be cycled hard or have deep discharges. They are fine being full 99% of the time. In fact solar batteries will last the longest if they are not discharged deeply.

Each time a bank is cycled, that is one less cycle it can provide.

Something like a golf cart battery may only handle 500 or so of these cycles while a high end Rolls/Surrette can handle 2500 or more.


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Routine equalization charges are vital to the performance and life of a flooded lead acid battery…

particularly in a solar, wind and less so in a micro-hydro power system. During battery discharge, sulfuric acid is consumed and soft lead sulfate crystals form on the plates. Learn more about making your battery bank last as long as possible.

The importance of regular equalization of your battery bank.If the battery remains in a partially discharged condition, the soft crystals will turn into hard crystals over time

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. This process, called “lead sulfation”, causes the crystals to become harder over time and more difficult to convert back to soft active materials.

Sulfation from chronic undercharging of the battery is the leading cause of battery failures in solar systems.

In addition to reducing the battery capacity, sulfate build-up (flooded lead acid and sealed batteries) is the most common cause of buckling plates and cracked grids.

Deep cycle  batteries are particularly susceptible to lead sulfation. Normal bulk charging of the battery can convert the sulfate back to the soft active material if the battery is fully recharged.

However, a solar battery is seldom completely and fully recharged, so the soft lead sulfate crystals will harden over a period of time.

Having troubles with a battery in your battery bank?


The golf cart battery should be equalized every three months or so depending on use and abuse.SLOW OR PREVENT SULFATE FROM BUILDING UP ON THE LEAD PLATES – Only a long controlled overcharge, or equalization, at a higher voltage can reverse the hardening sulfate crystals.

Normal charging will reduce sulfation, but to remove it, the battery must be equalized.

BALANCE THE INDIVIDUAL CELL VOLTAGES – Over time, individual cell voltages can drift apart due to slight differences in the cells. For example…in a 3 cell (6VDC) battery, if one cell is less efficient in bulk charging to a final battery voltage of 28.8 volts (2.4 volts per cell), over time, that cell might only reach 2.30 volts, while the other 2 cells charge to 2.45 volts per cell.

Solar Batteries must be equalized every 30-60 daysThe overall bulk battery voltage is 7.2V, but the individual cells are higher or lower due to cell drift. Equalization cycles help make all the cells the same voltage or equalize.

MIX THE ELECTROLYTE – In flooded lead acid batteries, especially taller cells (like the Trojan 6VDC L16 or Surrette 6VDC L16), the heavier acid will fall to the bottom of the cell over time compared to the standard 6VDC golf cart battery .

This stratification (dividing between the heavier distilled water and lighter battery acid) of the electrolyte causes loss of capacity and corrosion of the lower portion of the plates.

Boiling of the electrolyte from a controlled overcharging (equalization charge) will stir and remix the distilled water and acid into the battery electrolyte.


The ideal frequency of equalization charges depends on the battery type (lead-calcium, lead-antimony, etc.), the depth of discharging, battery age, temperature, and other factors. One very broad guide is to equalize flooded lead acid batteries every 1 to 3 months or every 5 to 10 deep discharges. Some batteries, such as the large L-16 type, will need more frequent equalization charges compared to a golf cart battery. The L16 tends to sulfate more quickly..

The difference in specific gravity between the highest cell and lowest cell in a battery can also indicate the need for an equalization.

Either the specific gravity or the cell voltage can be measured with the specific gravity of the electrolyte being the most accurate when deciding whether or not your battery/battery bank needs an equalization charge.

The battery manufacturer can recommend the specific gravity or voltage values for your particular battery.


All sealed or no maintenace batteries can be equalized



Sealed batteries do not tolerate equalization, could explode (in very rare cases), and cannot have any distilled water replaced (as there are no removable caps/covers) after an equalization charge.


Follow the manufacturer’s bulk/absorption charge recommendations strictly.

They are not supposed to gas (vent) and more importantly if they do, you will NOT be able to refill them with distilled water and they WILL be damaged if the voltage is allowed to go higher than the manufacturers recommendations.


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fm80 mppt controllerA charge controller is a piece of equipment designed to prevent the batteries in your system from overcharging.

It is similar to the voltage regulator in a car. However, unlike a voltage regulator in an automobile, charge controllers have advanced in ways never even imaginable to the hippies in Northern California, USA in the 1970s…

A typical 12 volt solar panel is capable of producing up to 17 to 22 volts. If the panel was left connected to a 12 volt battery, with no charge controller, the battery voltage would continue to rise until the battery/batteries become permanently damaged.

A charge controller can maintain healthy battery voltage by either:

  • short circuiting the solar panel
  • disconnecting the solar panel from the battery or by
  • diverting the excess electricity to a load such as an air heater, water heater or other electrical load.

Solar electricity can be either disconnected or diverted.

Wind and water turbines must remain connected to the batteries at all times so their electricity must be diverted or dumped in order to protect the batteries.


Shunt type charge controllers- these are the first to be developed and most crude form of charge controller on the market today. They are only used as solar charge controllers (not for water or wind power). They shunt (or short circuit) the energy from a solar panel when the battery is full. These controllers have a set on and off voltage with a hysteresis (difference) sometimes adjustable by the user and sometimes factory preset. When the batteries hit the high voltage setting the entire electricity source is shunted (short circuited) until the voltage hits the low voltage setting. This keeps the batteries regulated but the voltage can vary between the on and off settings. They are the least accurate type of controller.

Relay type charge controller – these were the second type of charge controller to be developed. They are similar in function to the shunt type except instead of short circuiting the panel output they open circuit between the solar module and the batteries. These controllers have a set on and off voltage with a hysteresis (difference) usually adjustable by the user. When the batteries hit the high voltage setting the entire electricity source is disconnected until the voltage hits the low voltage setting. This keeps the batteries regulated but the voltage can vary between the on and off settings and is not that consistent. These also can only be used as a solar charge controller (not for use with water power or wind power).

xantrex c40 pwn charge controllerPWM (Pulse Width Modulated) charge controller – This style of charge controller keeps the batteries regulated by disconnecting and reconnecting portions (or part) of the electricity available from the solar module several times per second keeping the battery voltage more constant.

With the development of these charge controllers came anew and improved way of charging batteries using a bulk, absorption, float and equalization charge.

These are a great improvement over relay charge controllers as they are able to keep the battery voltage much more stable. They do not increase the output of the solar array but are more efficient than the previous controllers. Aside from being a charge controller, these units can have many features including digital displays, remote displays, load control, lighting control, and dump/diversion control.

tristar 60 amp 600 volt mppt charge controllerMPPT (Maximum Power Point Tracking) charge controller – This style of charge controller uses pulse width modulated technology to keep the batteries regulated but is able to also increase the output of the solar array by finding the maximum power point of the array (which is a higher voltage than the battery bank) and reducing the voltage to charge the batteries. This can result in up to a 30% increase in output of the solar array. It also allows for a higher voltage transmission from the solar array to the controller keeping wire losses to a minimum.

Outback Power’ s FM60 MPPT charge controller is our favorite as it is useful over a wide range of voltages and super efficient. Aside from being an MPPT charge controller, these units can have many features including lighting control, diversion control and an extra programmable relay. Some of these charge controllers can accept voltages up to 150 volts DC to make long distance transmission possible and efficient. There are MPPT controllers being designed now that are capable of input voltages up to 200 volts DC.

MPPT (maximum power point tracking) technology is constantly being improved and is very exciting for our industry. Learn more about MPPT technology.

Today’s charge controllers do more than just maintain correct battery voltage…


Prostar 15m PWM controller with digital displayDigital Displays – Some charge controllers have an optional or included (built in) digital display on the front of the unit.

This display shows the user how many volts, amps and/or watts the system is generating as well as keeps track of total power production and other pertinent info as available.

To the right is an photo of an installed front mount display for a Morningstar Prostar PS-15 PWM charge controller.

Remote Displays – Some charge controllers include, or have as an option, a remote meter that can be installed in another room or building. They usually display the same info as the installed digital displays but can be more conveniently located for easy access.

Load Controls – To prevent a battery from becoming deeply discharged by a load (such as a light or motor), a load controller is used. The load controller monitors battery voltage and disconnects the load from the battery at the disconnect voltage and does not reconnect the load until the battery reaches the reconnect voltage. These on and off settings are usually user adjustable. Common settings would be to disconnect at 10.5 volts and reconnect at 12.5 volts. Many charge controllers have a load control feature but can only be used as a charge controller or a load controller, not both. Two controllers would be required to perform both functions. Two exceptions to this are Morningstar’s SunSaver SS-6 and SunSaver Duo dual function charge and load controllers.

Lighting Controls – A lighting controller is a charge controller used to control lighting. They have several lighting functions such as dusk to dawn lighting or other on and off settings. They are used for security lighting, home lighting and billboard lighting. They are microprocessor controlled, fully automatic and also act as a load controller to protect the battery from deep discharge. Morningstar’s SunLight charge controller is an example of a good quality lighting controller.

Diversion (Dump) Controls – When charging a battery with a solar panel the energy source (solar panel) can simply be disconnected to regulate the voltage. When charging the battery with a water turbine or wind turbine the energy source cannot be simply disconnected. This would over-speed the water turbine or wind turbine and destroy it. Instead…the turbine is connected directly to the battery and the excess power is now dumped (or diverted) from the battery to a water or air heater using a diversion controller. This type of controller monitors the battery voltage and diverts all or a portion of the produced energy as heat to the dump load. The dump load can either be a useful load (like a hot water heater) or not useful and be dumped into an outside air heater. Many charge controllers have a diversion control feature but can only be used as a charge controller or a diversion controller, not both. Two controllers would be required to perform both functions. The Morningstar TS-45 is an example of a good quality diversion controller.

xantrex-battery-temperature-sensorExtra Programmable Relays – Outback Power System’s FM60, OutBack Power System’s FM80, Apollo Solar’s T80 and Xantrex’s MPPT60-150 charge controllers have a user programmable relay that can be adjusted to perform many functions. The relay can be used manually as a switch for pretty much anything you want to control remotely or the relay can be used to dump/divert electricity to a dump/diversion load, a lighting controller, a load controller (with an optional solid state relay). Another option is to use it to control an exhaust fan for your battery enclosure or an alarm to tell the user the batteries have not been charged for a set amount of time.

Remote Temperature Sensors – An RTS or remote battery temperature sensor is used for temperature compensated battery charging. As a battery gets warmer gassing increases. As a battery gets colder it becomes more resistant to charging (requires a higher voltage to charge). The more the temperature changes, the more important a battery temperature sensor is. The image on the right is an Xantrex RTS used for the C40, and C60 charge controllers.


Bulk Charge – The bulk charge is the beginning of the three step process of most advanced charge controllers on the market today. During the bulk stage all current available from the solar panel is applied to the battery until the battery reaches the preset absorb voltage as specified by the charge controller or user

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. At this point the charge controller enters absorbing mode. For a 12 volt nominal lead acid battery this voltage might be anywhere from 14.0 – 15.0 volts.

Absorption Charge – During the absorb stage just enough current is applied to the battery to hold a preset absorb voltage for a set period of time. This stage is designed to prevent overheating and over-gassing of the battery. The current is tapered down to maintain battery voltage. The voltage for this mode in a 12 volt system is 14.0 – 15.0 volts. If the voltage is able to be maintained for the preset period of time the charge controller will enter float mode.

Float Charge – During float mode, a maintenance charge is applied to the batteries until there is no more excess energy available (the end of a sunny day) The voltage for this mode in a 12 volt nominal lead acid battery will be approximately 13.4 volts.

Equalization Charge – An equalization charge is a periodic boost charge applied to stir the electrolyte, level the cell voltages and complete the chemical reactions within the battery. It is usually done at a set period of time from every month to every three months depending on battery manufacturer recommendations. The higher quality charge controllers will perform this stage automatically. The voltage for this mode in a 12 volt nominal lead acid battery will be above 15.0 volts and up to 16.0 volts. For more information on battery equalization read our equalization page.


Most systems require a charge controller. However, very tiny systems do not.

As a general rule you need a charge controller if your solar modules are making more than 2 WATTS PER 50 AMP HOURS (AHs) of battery/batteries (at the same voltage).


For example: if you have a 12 volt, 120 amp hour battery (a typical large RV battery), any module 5 watts or less will not require a charge controller.


(120 amp hours divided by 50 amp hours) X (2 watts)=4.8 watts.

If your solar module is larger than 5 watts …you need a charge controller.


First you need to decide what type of controller is suitable for your application.

Things to consider include:

  • the distance from your solar array to the batteries. The more distance, the higher you might want the voltage to keep wire losses to a minimum. In this case consider an MPPT charge controller.
  • the open circuit voltage of your panels
  • is the voltage of your solar panel the same as the battery voltage? If not you are going to need an MPPT charge controller.
  • do you need to get every watt possible out of your array? If yes then you are going to need an MPPT charge controller.
  • is cost the most important factor to you? If yes, you might consider a PWM or relay type charge controller.
  • is this just a very small one panel system? If so the Morningstar SunKeeper SK-12 might be a good solution.
  • is your single panel system 40 watts or less? If so the Morningstar SunGuard SG-4 would be a good solution.

Battery Voltage – Charge controllers come in various sizes (current and voltage). To choose the correct controller you need to know your system’s battery voltage (12, 24, 36, 48 or 60) Most controllers are designed for either 12, 24, and 48 volt systems or a combination of two or three of these voltages. Some are designed for only one voltage. Morningstar’s Tristar TS-45 and Tristar TS-60 are user programmable using a serial cable, to any battery voltage between 12 and 48, making them suitable for even 32 and 36 volts systems.

Amperage – All controllers have a maximum current limit. They can be as small as 4.5 amps and as large as 100 amps. Please note this is the amperage between the charge controller and the battery and not the input amperage of the solar panel(s). Generally you should size your charge controller at least 25% larger than what is required. This allows the charge controller to operate cool and can also increase the lifetime of the unit.

Options – Many charge controllers contain different options as listed above. Be sure to check to check the specification sheet of your controller before making a purchase. It should give you a good idea of the options available for the specific charge controller and whether on not it will work for you.


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