What's the basic definition of an amp-hour? For example:
A 12V battery is rated at 100Ah.
Does this mean that the battery will be discharged in 1 hour if the load is 100A@12V?
Thanks.
Electronic – the basic definition of an amp-hour
ampere-hour
Related Solutions
It is bad practice to connect batteries in series when they don't have the same capacity. The battery with the smaller capacity will be empty before the larger one, resulting in a lower voltage for the smaller battery. At that point things will start to get interesting as the larger battery will start to charge the smaller one through the connected circuit and with reversed voltage. The cell is not designed for being reversed and charged and bad things may happen a.o.: leaking acid and exploding. Neither of these situations are desirable. This is also the reason why most manuals of battery operated devices urge to replace all batteries at the same time.
Your hunch that batteries have a current limitation is correct. In general, it's hard to tell the current rating [A] from capacity [A·h]. You have to look it up in the datasheet. A lot depends on the design of the battery.
For example: coin cells with 500mAh capacity may have only 3mA max current.
Another (opposite) example: automotive starter battery with 40Ah capacity may have 500A max current.
Lead-acid batteries are interesting in this respect, because there are two distinct types.
- Starter lead-acid batteries are designed specifically to deliver high peak current for a short period of time. Deep discharge, however, dramatically shortens the life of a starter battery. So, it's not suited for routine operation at high depths of discharge. Your typical starter battery in the automobile works at very shallow depth of discharge.
- Deep cycle lead-acid batteries are designed (as name suggests) to discharge further. But they can not provide as much instantaneous current.
Here's an example datasheet for a deep cycle battery. Have a look at the nominal capacity on p.1. Notice that capacity depends on discharge current (i.e. the rate of discharge).
-
Depth of Discharge Starter Battery Deep-cycle Battery
100% 12–15 cycles 150–200 cycles
50% 100–120 cycles 400–500 cycles
30% 130–150 cycles 1,000 and more cycles
(Source.)
p.s. If you want to read-up, here's an excellent web site on batteries - Battery University.
Best Answer
1) the battery may not remain at 12 V during the whole discharge (and may start from over 12 V when fully charged). However (depending on the selection of the cut-off voltage (lowest voltage it discharges to), 12 V may be a reasonable value.
2) when discharged very quickly (and 1 hour is quick), most batteries tend to deliver less total charge. A more usual discharge rate (especially for claiming capacity) is C/10 -- a 10 hour discharge. This your battery may be expected to deliver 10 A for 10 h before falling to less than 12 V.