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

Part 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 |
---|---|

2 VOLT | 1.75 VOLTS |

4 VOLT | 3.50 VOLTS |

6 VOLT | 5.25 VOLTS |

8 VOLT | 7.00 VOLTS |

12 VOLT | 10.50 VOLTS |

24 VOLT | 21.00 VOLTS |

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.**

Hopefully 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.

### THE SLOWER YOU DISCHARGE YOUR BATTERIES, THE MORE STORAGE CAPACITY THEY WILL HAVE!

### 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. 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:

- The under sized battery bank will not make it through the cloudy or calm periods resulting in more generator run time.
- The undersized battery bank will need replaced prematurely as it will be deep cycling too much and too often.
- The undersized battery bank will likely be chronically undercharged.
- 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 |

Using 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.

### CAN A BATTERY BANK BE TOO BIG?

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:

**VOLTS X AMP HOURS = WATT HOURS**

**6 V X 225 AH = 1350 WATT HOURS**

We have two batteries.

**1350 WATT HOURS X 2 = 2700 WATT HOURS**

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.

{ 35 comments… read them below or add one }

I am now not certain the place you’re getting your info, however good topic. I needs to spend some time learning more or figuring out more. Thank you for magnificent info I was on the lookout for this information for my mission.

Hi John,

Thanks for the positive feedback. All the info on this website is from personal experience instead of regurgitated info from other websites. We make mistakes and try to help others learn from them…Jody

Dear Sir,

IF we test the battery at C5 and found that it is fully discharged at 3 hours not 5 hours

Assuming that 100% is discharged through the whole 5 hours then if we say 85% that means 4.25hrs

Is that means that battery is poor produced or what?

is that can be a cause for refuse the battery as quality as in standard IEC 60254 for 2005

Dear Jody,

Please could share with me your email I want to share with some battery information. I need your assistance in determining how good is the battery. I am based in Zimbabwe and will be hosting PV Installer’s training so would also need assistance in sizing the battery bank for Solar home system

Hi Jeffree,

My email address is jody@solarhomestead.com. I would be happy to help you in any way I can…Jody

Hello, If you put batteries in series doesn’t the amperqage stay the same and voltage double?

In your article; 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

(twice the 1.24 kWh C10 rate because there are two batteries)

Jerry M

Hi Jerry,

Thanks for your comment. You are correct. When you put two batteries in series the voltage will double but the amperage (amp hours or AHs) stay the same. The capacity of the bank doubles. If one battery can store for 1.24 kWh then 2 batteries can store 2.48 kWh. The amperage has not changed, the voltage has. Amps X Volts = Watts. Double the voltage and you will double the watt hours or kilowatt hours of the battery bank.

In the example of the Trojan T105 each battery can store 207 amp hours at 6 volts or 1.24 kWh (207 AH X 6 V = 1240 watt hours or 1.24 kWhs). Add a second battery in series and now the equations is 207 amp hours at 12 volts (207 AH X 12 V = 2.48 kWhs) The amperage has not changed but because the voltage is double so it the capacity. Hope that makes sense. Take care…Jody

I just added a 150 watt solar panel to my vessel.

My guess is that the sun will do its job in Florida for about 8 hrs per day.

I want to run a Engel 40 quart DC freezer for 24hrs per day. The specs say it draws up to 2.5 amps

My guess is that my under the counter frig will draw 5 amps per hr max.

My anchor light is LED and uses negligible power

All other uses lets say they total 1 amp per hour if figured on a 24 hr day

My solar should put out 8.3 amps for 6-8 hrs per day

I have a D-8 battery as a house battery. It’s a ranking type so I want to take your advice and get as much battery as possible. I think I can fit 3 group 24 12 volt gel batteries in the box. If they are rated at 74 AH will I get 3 times that by using three batteries? If there rated at 74 amp hrs, is that usually a 20 hr rating or 100 hr rating?

PS when aboard I can also run my Gen on occasion and my Truecharge 40 amp charger that has setting via toggle switches for Flooded Gel or AGM batteries.

Captain Jim

Hi Jody,

What about the charging of T105 RE – 6V battery? Can you please explain it..?

I find your website very informative. I have a few questions:

1) Can the battery system be installed in the basement which is also used as a storage/play room?

2) What is the C20 rating or capacity of 8 Trojan L16’s connected in in a 12 volt configuration? How would that C20 rating or capacity change if connected in a 48 volt configuration?

3) Is there an ideal voltage configuration ( 12, 24, 36 48) when incorporating all of the system components or does it totally depend on the size?

4) When a 7500 watt hour load is required per day (this includes a refrigerator, a freezer and a gas fired boiler { 3 amps max for the burner and 2 fractional circulators} and some misc. small items) will a battery package of 8 Trojan L16’s be adequate?

how many batteries do i need for 62 kw steady load if attached to solar panels 300 watts each x 300 wil this suffice for 24 hours load

I need C20 type Lithium ion batteies in india

Best regards

Mehernosh Shroff

Seaworthy Shipping Services +++++

Solar Power Conversion of Tugs Rigs Vessels Ports , + solar power propulsion

Towage Salvage Tug owners+ India’s No 1 OSR Tier II Response & clean up Experts +Storage tank Clean up + Tracking and safety of manpower

CRANE BARGES

21 western India house , Sir P M Road Fort Mumbai

mobile +91 9322403761 , 9820744303

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Email :seaworthy@vsnl.com ,growmormeher@live.in ,seaworthyshippingservices@gmail.com

http://www.indiamart.com/seaworthyshipping/

Hi,

Thank you so much for your inquiry. Unfortunately operating such a large electrical load with solar would be cost prohibitive as well as you would not have enough space to mount all the solar modules, batteries and other equipment. It is just not possible to supply that much of an electrical load with solar. Sorry and good luck in your adventures in India and across the world…Jody

Dear Sir/mam,

If i m having 12v, 100 AH battery. then how can i find out the charging profile.

I have prepare a 12 A charger.it max out voltage is 15 V. but by using it i m get 1220 gravity. Pls suggest me how can i increase the gravity.

Hi,

Thank you for your comment. Your battery should never be charged at more than 10-13 amps making your 12 amp charger perfect. You should be able to get the specific gravity high enough with that charger. Make sure your battery is fully charged, then fill with distilled water and perform an equalization charge by bringing the voltage up to 15 volts for a few hours. This will boil the electrolyte and hopefully remove sulfate from your battery’s plates and bring it back to high specific gravity. If this doesn’t work your battery is starting to wear out and will need to be replaced eventually. If you tell me the brand and model number of the battery, I will try to find out more for you. Hope this helps. Jody

Hi,

I cannot totally agree with you on your calculations, unless I’m assuming something very different.

E.g:

The C10 rating on a 207Ah battery = 20.7A constant current per hour for 10 hours;

i.e. on a 6V Cell, it is P=VI, or 6V x 20.7A = 124.2W per hour?

So I guess my question/comment is:

Why do take into account the C Factor by multiplying the 124.2W by 10 to get 1.24kW. Surely this is not the ACTUAL kWh delivery, but rather over a period of 10 hours?

Thank you,

Hi,

Thank you for your comment. You actually do seem to agree with my calculations and your calculations are correct.

The misunderstanding is the difference between a kilowatt and a kilowatt hour.

You stated:

“The C10 rating on a 207Ah battery = 20.7A constant current per hour for 10 hours;

i.e. on a 6V Cell, it is P=VI, or 6V x 20.7A = 124.2W per hour?

So I guess my question/comment is:

Why do take into account the C Factor by multiplying the 124.2W by 10 to get 1.24kW. Surely this is not the ACTUAL kWh delivery, but rather over a period of 10 hours?”

The correct way to state this is:

“The C10 rating on a 207Ah battery = 20.7A (Amps) constant current per hour for 10H (hours);i.e. on a 6V Cell, it is P=VI, or 6V x 20.7A = 124.2 VoltAmps or Watts

So I guess my question/comment is:Why do take into account the C Factor by multiplying the 124.2 Watts X 10 Hours to get 1242 watt hours or 1.242 kilowatt hours (kWh). This battery WOULD produce 1.242 kWhs over a period of 10 hours”

A kilowatt (kW) is a measure of

instantaneousoutput which could be thought of as kWh/h.The two hours cancel themselves (kW1/1) leaving us the kilowatt.

A kilowatt hour is a

totalunit of energy that equals a kilowatt (kW) being produced (or consumed) for one hour.This could be expressed as kilowatt x 1 hour = 1 kWh

Think of a hose:

You have a hose that is producing 1 gallon per hour (1 gallon/hour).

instantaneousOperate the hose for one hour and your hose has produced 1 gallon.

totalHope this helps…Jody

Hi,

I live in a country where electricity is a major problem. I use a power inverter with 4 T105 batteries that last me 8 hours every day before power is back.

Now I need to replace my T105 batteries by some brand cheaper that could equal the same amount of time. Any suggestions ?

Hi Jean,

The T105 is generally available anywhere in the developing and developed world. They are used so much because they are so easily obtained. However Trojan has used this to their advantage and charge a premium (more money) for this convenience. There are dozens of other brands that are equivalent to the T105.

If you can find a cheaper 6 volt lead acid battery that has close to a 220 amp hour (AH) rating than use that.If you have found a particular brand you are unsure about contact us.To sum it up you are looking for a 6 volt 220 AH (amp hour) golf cart battery. One last consideration is it MUST be a lead acid unsealed battery. Do not buy maintenance free or marine batteries. They are not the same as what you need.

Good luck….Jody

Excellent article, thanks for answering questions related to this.

I would be interested to know how you would determine what a C100 battery rated at 1300aH is equiv to at a C20 rating. This question relates to having to enter battery amp hours that are assumed to be C20 where the user has a C100 1300 ah rated battery.

Hi Mark,

Great question. There is no way to convert from C100 rating to C20 that I know of unfortunately as all batteries perform differently. All we would know it the rating will be lower as more heat will be created when discharging the batteries 5 times as fast. Personally I would consider it a 1100 AH battery at C20 but that is only experience, not real theory. The manufacturer will definitely know if you can get in touch with them. Another thing to consider is ” If you design the system to operate for at least four days (98 hours) without recharge, then you could use the C100 rating”. Thanks…Jody

I would appreciate if you could suggest which battery should I use for a 2Kw solar setup. C10 or C20 type battery.I will be using 12V- 150Ah lead acid battery.

Hi,

Thank you for commenting. C10 or C20 simply relates to how much power a battery will produce if discharged over a period of 10 hours (C10 rating) or 20 hours (C20 rating). All deep cycle batteries have a C10, C20, C100 etc rating. The “C” rating does not refer to a specific type or brand of battery. Go to our page how to choose your first battery bank to help you with battery selection. I would highly recommend you use 6 volt true deep cycle batteries if they are available to you or you have not already purchased the 12 volt units. Six volt batteries have much thicker plates and are made for the abuse a solar system typical has to deal with. If you already have the 12 volt 150 AH batteries don’t worry about it. The money you saved by buying them likely makes up for the difference in lifespan. Thanks and have a great day…Jody

In your T105 example, how are you able to increase your capacity while wiring in series?

I’ve always thought that wiring in series will increase voltage but capacity stays the same; while wiring in parallel will increase capacity but voltage remains the same.

Hi Nick,

Thank you for your good question. Capacity of your battery bank is measured in watt hours or kilowatt hours. For example if you have one T105 220 AH battery at 6 volts you have a capacity of roughly 1320 watt hours or 1.320 kWhs. If you have two of them you have doubled your capacity to 2640 watt hours or 2.640 kilowatt hours whether they are wired in parallel or series.

You are just confusing capacity with amp hours.When you wire batteries in series the voltage increases but the amp hours stay the same.

When you wire batteries in parallel the voltage stays the same but the amp hours increase.If you have one T105, you have 220 amp hours at 6 volts or 1.320 kWh (6V X 220 AH).

If you have two T105s in parallel you have 440 amp hours at 6 volts or 2.640 kWh (6V X 440 AH).

If you have two T105s in series you have 220 amp hours at 12 volts or 2.640 kWh (12V X 220 AH).

Hope that clears things up…Jody

If I add a 2 banks of 48 volt each separated from each other , one with L 16 and another with T 105 can i have one of them as a back up, being charged by the same array of solar panels?

Understanding that one switch will send the power to the inverter at anytime but never together. So I will keep the T 105 only for emergencies.

Thanks for the feed back

Hi Miquel,

Thanks for commenting. You can absolutely do what you would like to do. There are a few ways to go about it. Are the L16s and the T105s made by Trojan or what brand or brands are they? I can them look at the specs like bulk charge voltage and make recommendations from there. Please let me know…Jody

Please look for a new page called Installing and Maintaining Two Battery Banks in the coming weeks.

I would appreciate if somebody can suggest me a way to revive deep discharged lead acid type batteries.

Hi,

Thank you for you comment and question. If they are indeed lead acid deep cycle (you can take the caps off the top and add distilled water) and they are not too far gone, you should be able to help them a lot.

First buy lots of distilled water from a pharmacy or wherever you can get it. Open all the battery caps and fill each cell until the lead plates are covered about 1/4″ or 6mm roughly. Next you will need to give the batteries a good equalization charge. Y

our battery manufacturer will tell you the correct voltage for the equalization charge.Basically when you equalize batteries you are purposely overcharging them in an attempt to remove the sulfate that has built up on the plates and you are boiling/mixing the electrolyte (acid). You will also need to purchase a battery hydrometer. It measures the specific gravity of the electrolyte.You will get the recommended specific gravity readings from your battery manufactureras well.I would apply all the current you can (up to 13% of the battery bank’s C20 amp hour rating) to your batteries until you reach the equalization voltage and hold it at that voltage for a specific amount of time (tell me what your battery bank is made up of and I will tell you how long).

Keep an eye on the electrolyte and make sure it NEVER gets below the plates.Let the batteries rest for a few hours. Fill the batteries with distilled water to the recommended height. Give the batteries another equalization charge until all of the cells have the same specific gravity (S.G.) and the S.G. recommended by your battery manufacturer. This should bring your batteries back to as new as possible.. They will overflow while being charged. Only fill them once you know the batteries are full (after the first equalization charge).DO NOT FILL YOUR DEAD BATTERIES WITH DISTILLED WATER BEFORE YOUR FIRST EQUALIZATION

Read http://solarhomestead.com/deep-cycle-battery-maintenance/ for info on keeping your batteries working well.

Good luck and let me know if you need more help or clarification…Jody

Thanks a lot Jody. I will try this out.

But, tell me if it is a deep discharged dry cell

Battery, e.g., 12V 7Ah, is there a way to revive it.

I will look forward to your solution.

Thanks

Hi Again,

Unfortunately you cannot do anything to help your dry cell battery. Time to buy a new one.

Thanks…Jody

Hi all!

My name is Tony Long, I review deep cycle batteries. I had a terrible problem understanding amp hour ratings for Deep Cycle Batteries, thank you for clearing this up in your article.

You are most welcome. If there is anything I can do for you please let me know…Jody

Why the C10 rating is recommended for Solar application? What is the major difference in terms of battery construction, especially plates, for C10 and C20 operation.

NICE EXPLANATION.

I HAVE 4 PEACES OF EXIDE 12V 32AH (C3) VALVE REGULATED LEAD ACID BATTERY, JOINTS IN A SERIES AT MY E-BIKE. BUT WHILE CHARGING WITH 48V 6AMP CURRENT, THOSE BATTERIES BECOME OVER HEATED JUST WITH IN 3 HOURS.

PLEASE SUGGEST ME WHAT WILL BE APPROPRIATE CHARGING AMPERE FOR THOSE BATTERIES? HOW LONG THEY SHOULD BE CHARGED?

If a battery is fully discharged down to 1.75 volts per cell (full cell voltage about 2.25 volts) than a 6 Volts battery shall have to be discharged to 5.25 volts.