Battery calculations - RC Groups
Thread Tools
Sep 05, 2011, 03:01 PM
Don't Midair Me, Bro!
split_s's Avatar

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?
Sign up now
to remove ads between posts
Sep 05, 2011, 03:19 PM
Registered User
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.
Sep 05, 2011, 03:26 PM
Registered User
dmcquinn's Avatar
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 4-cell 2200 as a 2-cell 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 4-cell Lipo or more, the charger must draw more current than 2.2A in order to step up the voltage to charge the 4-cell pack (needs 16.8 Volts).

And all chargers have some losses in the conversion. Some energy is lost in the form of heat.
Sep 05, 2011, 03:37 PM
Southern Pride
everydayflyer's Avatar
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 in-line watts/ mAh (Ah),volts,amps. between the source battery and the charger.

I have used my Inspector watts meter for this often.

Sep 06, 2011, 12:33 AM
Registered User
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 over-discharging the SLA.

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Airship Thrust Calculations Narwhal Modeling Science 27 Jun 18, 2011 03:26 AM
Discussion what are the basic calculations to make a small RC glider?? sri Beginner Training Area (Aircraft-Electric) 1 Mar 23, 2011 02:55 PM
Question What Rm Value is Correct for Calculations? FlyingW Power Systems 11 Mar 18, 2011 10:28 PM
Prop Thrust Calculations Slow Flyer Utah Parkflyers 2 Aug 06, 2001 06:11 PM
Calculations mt_100 Electric Plane Talk 7 May 24, 2001 10:45 AM