Probably because certain battery chemistrys change their capacity based on load. For example, AA alkaline cells have a very high Mah rating (Can't remember an exact number), provided they are discharged slowly... Which is partially why they will run for so long in a clock or remote control. However, put them in something like a particularly demanding digital camera, and they won't deliver near the same number of Mah. Which is why chemistrys suited for high drain, such as NIMH, can be so attractive as an alternative to alkaline cells... they actually retain much the same Mah rating on comparitively heavy discharges.
Kewl, then since that amp draw of a batter charger while charging a 4S1P batter is pretty low a 32Ah @ 20Hr battery might be just the ticket for use at the field to charge my Lipo packs. Right? Makes sense? (I think)
I hope I'm not double posting this. I had logged in, written my question, and then when I tried to submit...poof it was gone and it said I was no longer logged in.
Long story made short I'm looking for a battery to carry to the feild with me for those days where the fun could outlast my supply of ready charged Lipo packs.
I ran across a wheel chair battery rated for deep cycle that said it was rated for 32aH @ 20Hr.
I thought the 32Ah told me everything. I thought it was stating that the batter was good for 32 amps at rated voltage for one hour. So then I read the @ 20Hr and get confused.
Can anyone unconfuse me?
Those wheelchair batteries will do the job. But, they are expensive for their capacity.
That 32 Ampere Hour capacity is based on discharging that battery over a period of 24 hours. Discharging that same 32 Ampere hour battery at 32 amperes should last one hour. But it won't, due to the chemistry of those lead acid batteries. You could find that battery will last much less, maybe even 15 or 12 hours, depending on how good that battery is.
Check out those deep cycle marine batteries, available at many locations. Don't buy those marine "engine starting" batteries, they often are nothing more than a standard automotive battery with handles. I did buy one of them, and it only lasted one year.
My batteries for charging consists of a pair of 120 Ampere Hour Deep Cycle Marine batteries, connected in parallel. Yeah, they stay in my truck for the entire flying season.