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.