Originally Posted by GoingWalkabout
On another thread dealing with electric
motors, someone made the following statement. I haven't seen any reference material to the this claim but would sure like to understand if it is correct.
"One large bank will provide more usable power than two smaller ones. Battery output capacity drops as a function of how quickly you pull power out see Pukerts laws."
It's known as "C Rating". It is generally written as C5, C10, C20, C100 etc.
where the number is the number of hours to fully discharge the battery.
If you have a nominal 200Ah battery bank and you discharge it at 10A, it will last for about 20
hours or to put it another way, you are discharging at 1/20
th of the Amps of its rated Ah capacity. That is C20
If you discharge it at 20A, i.e. at 1/10
of it's rate Ah that would be C10
If you discharge it at 40A, i.e. at 1/5
of it's rate Ah that would be C5
Battery capacities are usually stated at a C20 rate. If you discharge them faster than that, you get less Ah out of them. Discharge them slower and you get more Ah out of them.
Take for example a fairly common deep cycle 6V L16 battery which is nominally 370 Ah. Reliant L16-AGM | Trojan Battery Company
That page gives the actual Ah capacity of the nominal "370Ah battery" at various discharge rates.
If you fully discharge it at 18.5A (C20), it will provide power for 20 hours and put our 370Ah.
But if you discharge it at 74A (C5), it will only put out 290Ah and last less than 4 hours, not 5.
Conversely, it you discharge it at 3.7A ( C100), it will put out 392Ah and last 106 hours.
So to go back to the original question.
Assume you have two batteries each rated at 100Ah and you have a 10Amp load.
Connect the batteries into one 200AH bank and your 10Amp load is discharging at the C20 rate and will give you 200Ah.
Run your 10Amps through one battery and you will be discharging at the C10 rate and will probably get about less than 90Ah out of it. So the two batteries only give you a total of 180Ah.
Assume you are cycling your batteries down to 50%, in the first case, you have 100Ah usable. In the second case, only 90Ah usable.