When you look at a battery, there will be three specifications. These are:
- Volts (V)
- Ampére hour (Ah)
- Watt-hour (Wh)
Now, you are interested to know what the second one is.
Ampére hour or Ah stands for the capacity of the battery. It’s a measurement of how much current (amps) you can draw for a specified time.
A Word About C-rate
Let’s say you have a 12V battery with 100Ah. This means that you can draw 100amps for one hour, 50 amps for two hours, or 20 amps for 5 hours. But this doesn’t mean you should. Let me explain why.
Depending on the battery’s chemistry, there are different discharge and charge rates, called C-rate.
For lead-acid, this is C/5 or 0.2C. You can charge/discharge a 100Ah lead-acid battery at a rate of 20Amps (100Ah*0.2C).
For lithium, this can be 1C or higher. You can charge/discharge a 100Ah lithium battery at a rate of 100Amps (100Ah*1C).
You need to respect the charge and discharge C-rate the manufacturer gives you because that will give you the most capacity and the longest lifespan. The reason for this is internal heat generation. When you charge or discharge the battery quickly, it generates heat which is lost energy.
You have to take this into account when you select an inverter. Read my article about selecting the right amount of batteries here.
Ah doesn’t tell everything about a Battery
Ah alone doesn’t tell you the stored energy inside a battery.
If you have a small 2Volt battery at 3Ah and a second battery at 12Volt and 3Ah, which one holds the most energy?
The second one. Because 2V*3Ah= 6Watt hours and the second one has a total of 12V*3Ah=36Watt hours.
That’s why we express battery stored energy in Watt-hours (Wh), and not Ampére hours (Ah).
Is a higher Ah battery better?
Generally yes. But it goes together with voltage. Let me explain.
If you have 4 12V, 100Ah lithium batteries and configure them in a 12V 400Ah battery with a 3,000W inverter, you will draw a current of 250amps!
Now if you take the same battery and configure it to 24V 200Ah, then you will have 125Amps, which is much better.
Your wires will be less thick which will save you a significant amount of money.
How long will a 100Ah battery last?
It depends on the chemistry of the battery. For lead acid this will be 2.5 hours. Because the c-rate is 0.2C and you can only discharge a lead-acid battery to 50% to not damage the battery. The formula: 100Ah*0.2C= 20Amps for 5 hours. Because it’s only half, it’s 20Amps for 2.5hours, or 10 amps for 5 hours, and so on.
What is mAh?
1 Ah = 1,000mAh
mAh or milli ampére hour is just 1,000 times smaller than Ah. On small batteries like AA batteries you will see 3,000mAh, this is the same as 3Ah.
Is Ah always the same?
No, if the battery gets discharged quickly (more than the recommended C-rate, heat will generate, and the capacity of the battery will reduce.
Does a higher ah battery give more power?
No, a higher Ah value doesn’t give you more power.
For example, a 12V 400Ah battery holds 4,800Wh. A 48V 100Ah battery holds 4,800Wh too.
These two batteries are completely different but have the same power contents.