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.
Power is measured in watts and watts equals current in amps multipled by solar 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
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.
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.
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.
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).
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