


Discussion
Battery calculations
When field charging lipos, how do I figure out how many charges I can get from a particular SLA battery? For example, if I have an 18AH SLA battery, how many times could I charge a 2200mah lipo at 2C?




Cape Town, South Africa
Joined May 2008
176 Posts

I suppose a rough calculation can be done by looking at your lipo battery, getting thrown arround by the decimal place is the first thing, your 2200mAh battery is 2.2Ah. Your source is 18Ah divide by 2.2 = 8.1 times that you should be able to charge the battery. Although I have found that my charger starts beeping "low source" before my field battery getts to 12V.
So the likelyhood that you can recharge your 2200 mAh Lipo probably 6 (maybe 7) times. That's what I think there might be other answers. T 



The other thing that factors in the the calculation is how many cells are in the LiPO. It takes twice as much energy to charge a 4cell 2200 as a 2cell 2200. Terry's answer is correct if the Lipo is 2 cells (charges at 8.4 volts) or maybe even 3 cells. But if it is a 4cell Lipo or more, the charger must draw more current than 2.2A in order to step up the voltage to charge the 4cell pack (needs 16.8 Volts).
And all chargers have some losses in the conversion. Some energy is lost in the form of heat. 



Alos it should be pointed out that a deep cycle Pb should never be discharged greater than 80% and 50% is a much better stopping point if you expect it to last very long.
A simple means to monitor Ah removed per charge and for the day as far as that goes is to place an inline watts/ mAh (Ah),volts,amps. between the source battery and the charger. I have used my Inspector watts meter for this often. http://www.westernrobotics.com/pdf/...tor_Manual.pdf Charles 


Melbourne, Australia
Joined May 2006
6,407 Posts

Assuming you are not going to discharge the SLA more than 50% then for a 12V/18Ah SLA you have approximately:
12 x 18/2 = 12 x 9 = 108Wh energy available. Recharging a 2.2Ah lipo takes about: no_cells x 4 x 2.2 = ~9 x no_cells Wh So you can get about (12 x 9) / (9 x no_cells) = 12/no_cells recharge cycles out of your SLA Hence if they are 3S packs you can get 4 recharge cycles without overdischarging the SLA. 
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