View Full Version : battery mah draw question

03-21-2009, 04:47 AM
i brought some new batteries to lighten up my mono i went higher c rating smaller mah and iam running 2x2s in series instead of 4s 2p my question is is there a way to work out how much capacity i used from the battery. e.g 2 minute run and it took 58 minutes to charge battery back up at 1c. batteries are 30c 2500mah
thanks scott

03-21-2009, 05:03 AM
58 mins is as good as an hour.
1C for 2500mAh is 2.5A
2.5A for 1 hour is 2500 mAh
Therefore you used 2500mAh in 2 minutes run time.
This is 75A draw rate.
This equates to 30C on a 2500 pack.

2S+2S is the same a 4S, you knew that bit.
Either way 75A draw will take a 2500mAh 30C setup 2 mins or less.

Your batts and your call.
If you like the performance as is then all is good.

03-21-2009, 05:27 AM
the thing is tho as the battery charges the amps going in to the batteries drops plus the batteries were still at 3.8volts, batterie temps were 40deg c (104f) and i know from the data logger that my max amp draw is 45 amps, so 2.5amps (batt cap) divided by 45amps (amp draw) multiple by 60= 3.3 minutes run time, and this would be at continues max amp draw not real running were amp draw falls away as boat gets up on plane and top speed

03-23-2009, 04:23 AM
2 minute run and it took 58 minutes to charge battery back up at 1c this means you pull 2500mA out of your pack (100%), this is too much you should not pull 70 or 80% out of a Lipo pack, I'm afraid, if you did this few times, the batts are toasted!
As Simon said, 2500mA in a 2min run and batt is 100% empty after this time means you are pulling 75A average!!! you probably pull a lot more burst???

03-23-2009, 04:43 AM
but i have data logged this setup 4-5 times and max amp draw is 45 amps. when you charge a battery it doesnt take a full 1c charge the whole time. i propped up on sunday and ran for 3 minutes batteries were 45 deg c (113f) so there is no way its pulling 75 amps

03-23-2009, 04:53 AM
Your charger does not show the "mah" when charging the battery?
When a lipo is fully charged each cell has 4.2 volts, and when charging as it is near completion the amps fall. The charger starts at the amps you set but later on (at half charge capacity?) the amps fall automatically and towards the end for 10 minutes or more it is 0.2 amps charging. This is what I see on my chargers.

03-23-2009, 04:57 AM
what logger do you have? Eagle Tree? if yes it tells you how many A or mA you draw during the recoring time

03-23-2009, 04:59 AM
no the charger doesnt show mah. thought there may have been another way of working it out. i dont run my boats for anymore than 2-3 minutes at a time any way, i dont like to run the batteries down to were the low voltage cut off drops in

03-23-2009, 05:02 AM
yes have the use of the eagle tree logger, my mate has all my runs on his computer will have to go threw them and have a look or try the logger this weekend,

03-23-2009, 05:11 AM

At 45 Amps your max. runtime is 3.33 mins { (2500*0.06)/45 = 3.33) }

When run time is 2 mins. { mah = (amps*time)/0.06 hence mah = (45*2)/0.06 = 1500 mah }

Assumptions: 45 amps average load, battery fully charged 2500 mah.
There is nothing else that I can think of.

03-23-2009, 05:25 AM
thanks disar, i had worked out 3.3 minutes just thought there may have been some other way of working it out. but i didnt even think of the data logger so will give that a try, was trying to work it out if i go racing (5 lap oval) how many races i would get out of a charge
thanks scott

03-23-2009, 05:28 AM
Back to the original question...

is is there a way to work out how much capacity i used from the battery
then you have it... Eagle Tree!
Once downloaded onto you PC, it'll give you the max amp draw and the cumulative mAh, this one is the one you are interrested in, again it should not be higher than 70-80% of the batt. capacity 2000mAh max for a 2500mAh pack.
The max amp draw sould be less than the C rating 75A in your case.

i dont like to run the batteries down to were the low voltage cut off drops in
Remember that when the cut-off activates, (let's say 3.2V witch is the lowest you can admit) this is under load, if you look at the graph (on the Eagle Tree software) you will see that right after you stop pulling 45A peak, the voltage raise a little bit because the cut-off is measured under load!

03-23-2009, 05:37 AM
The eagle tree yes, shows everything, but I would recommend that you buy a charger (I use chinese ones) that shows mah, does balancing and shows individual cell voltage. As time passes the lipos voltage and capacity starts falling especially if not treated right. And most important (because I damaged two 2200 mah lipos) when you see the speed going down bring the boat out.

03-24-2009, 07:45 AM
my charger shows individual cell voltage but not mah, i will data logg it on the weekend and see what comes up,
thanks for your help