Does a higher Ah battery mean more power?
The Ah (ampere-hour) rating of a battery is an indication of how much energy it stores and can deliver.
It’s often said that a 50Ah battery can deliver 5 amp for 10 hours or 50 amps for 1 hour. This is very simplistic and is not how it works.
Let’s focus on lead-acid batteries (still the most commonly used) and they come in two broad categories: auto batteries and deep-cycle, sometimes called ‘leisure’ batteries.
Both can be rated in Ah but they work in different ways.
As a general rule, a higher Ah battery is better for delivering more current, which implies more power in watts. A higher Ah rated car battery will generally deliver more cold cranking amps (CCA), while a deep-cycle battery can deliver medium currents for a longer period of time.
Video – What are battery Ah? How AH relates to battery power
Does a higher Ah battery give more power?
Power is measured in watts and watts equals current in amps multipled by battery voltage.
This means that higher amp-hours will inevitably mean more power.
12 V 50Ah battery power = 50 x 12 = 600 watt-hours
12V 75Ah battery power = 75 x 12 = 900 watt-hours
Deep cycle vs auto batteries
What does Ah mean on a battery?
A more useful way of rating a car or truck battery is using CCA or Cold Cranking Amps.
Auto batteries are also rated in Ah but CCA is a great way to determine the cranking power available for turning over and starting a cold car or truck engine.
Auto batteries range from 40Ah to 75Ah for larger engines and deliver hundreds of amps in a short time, usually in under 30 seconds.
This kind of battery has a high C rating, which is an indication of it’s ability to deliver high currents in relation to its rate capacity.
If a 50Ah battery can supply 500 amps for 30 seconds or so, then it’s C rating is C10 i.e. it can deliver 10 times it’s Ah rating in amps to crank a car engine.
There’s a bit more to it than this, but these are the basics.
How deep can you discharge a car battery?
Auto batteries are great for supplying high current in a short time but aren’t suitable for delivering sustained amps over time.
Once a car engine fires the car’s alternator immediately begins charging it very fast, so it’s never in a deep discharged state.
In fact, auto batteries shouldn’t be discharged regularly more than around 15% of their capacity.
Each time an auto battery is deeply discharged its plates are damaged and it never quite charges back up to full capacity. It loses more and more capacity each time it is deeply discharged.
How are deep cycle batteries rated?
Deep-cycle batteries don’t have CCA (Cold Cranking Amps) because they are not designed to deliver high currents for any length of time.
A typical lowest size for leisure batteries is 100Ah and they can deliver medium current for hours on end without damage.
A lead-acid deep-cycle battery can be discharged up to 80% of it’s rated capacity, but 50% is often recommended, as this increases the overall life.
This is very important when sizing batteries for applications such as solar energy storage. You need to double designed battery life to account for the fact that 50% of the Ah is just not available!
How to measure amp hours in a battery
The easiest way to determine how many Ah are left in your battery is to measure the terminal voltage with a multimeter.
The voltage level can be anything between just over 11 volts to almost 13 volts and gives a rough indication of the state of battery charge.
Important: The battery should sit unused, neither charging or discharging, for several hours before the test. This makes sure there is no chemical activity on the internal plates.
The table below shows the percentage discharge compared to voltage:
Table – Lead-acid Ah discharge level vs terminal volts
Depth Of Charge % (12 V Pb)
Is a higher Ah car battery better?
Although car batteries are rated in Ah as a measure of their capacity, a much more useful measure is given by CA and CCA. Both of these are higher with higher Ah.
CCA (Cold Cranking Amps) is a standard introduced by the Battery Council International and states the cranking amps a battery can deliver at 0 degrees F (-18C) for 30 seconds.
This is very cold and not much use for people living in the southern states, for example.
CA (Cranking Amps) is perhaps a more useful standard, which states the maximum cranking amps that a battery can deliver at 32 degrees F for 30 seconds.
CA amps will always be higher for the same amp-hour rating and is probably more useful as a buying guide. It’s sometimes known as Marine Cranking Amps (MCA).
Will a higher amp hour battery last longer?
All other things being equal, then yes, a higher Ah battery will last longer.
It all depends on the load characteristics, but for the same load at the same temperature, the higher Ah battery will last longer.
It’s just a matter of dividing the Ah (amp-hours) by the current in amps to give hours runnng time.
Run time for 50Ah battery with a 100 watt load = 50Ah/8.33 amps = 6 hours
Run time for 75Ah battery with a 100 watt load = 75Ah/8.33 amps = 9 hours