Recently I did an experiment, which is I shorted a fully charged 3S(~11V) 2200mAh Li-Po battery with a heat plate (measured 2.8 Ohm) for about 10 minute. The heat plate get hot (which is what i intended), and i monitored the battery voltage. The voltage drop quite fast to ~8V within 10 minute. I'm afraid that it will damage the battery because the battery getting warmer. But then, i measured the voltage and get ~10V after removing the heat plate.
So, my question is:
(1) People said the safe voltage point for Li-Po battery is 3V for each cell. So, 3S battery safe point should be 9V. How should i determine the 9V point? With the heat plate(load) attached and measure the battery voltage? Or detach everything from battery and measure the battery voltage?
(2) How do i know my battery is fully drained? 0V? Or can I load it with the heat plate and left it until empty current? *Not dare to try because maybe it will explode ? I don't know. I just tried 10mins at most.
(3) Based on calculation, my 3S battery is 8C max current supplied. 2.2 X 8 = 17.6A max! My heat plate draw about 4A. Hence, it is under 17.6A and should be safe right? And, 2200mAh battery, divide by 4A, which gives me about 33mins runtime. What about if I left it there for an hour? the battery will over-discharge and damage/explode?? Lol
my 3S battery is 8C max current supplied. 2.2 X 8 = 17.6A max!
The 2.2Ah rating is a battery-life rating (or an energy rating). The maximum recommended current could be higher or lower than 2.2A.
Of course most batteries are intended to last more than an hour, but I don't know if there's any harm in draining it faster than that... You'll have to check the datasheet, or wait for someone who knows more about Li-Po batteries than me.
And, wiring cells in series DOES NOT MULTIPLY THE CURRENT CAPABILITY OR THE AMP-HOUR RATING. It ONLY multiplies the VOLTAGE.
The voltage immediately after disconnecting the load is the true cell voltage - the further drop when
the load is attached is mostly internal resistance losses, and isn't to do with the chemistry of the cell.
However the drop of 2V at a modest 3ish amps when the battery is nominally rated for 17A means
your battery is not very healthy (or you measured the voltage too far from the battery terminals).
The internal heating is consistent with 6W = (10V-8V)*3A dissipated in the internal resistance.
You didn't short anything BTW, a short is a massive overload, not within the intended operating
limits of the battery.
MarkT:
The voltage immediately after disconnecting the load is the true cell voltage - the further drop when
the load is attached is mostly internal resistance losses, and isn't to do with the chemistry of the cell.
However the drop of 2V at a modest 3ish amps when the battery is nominally rated for 17A means
your battery is not very healthy (or you measured the voltage too far from the battery terminals).
The internal heating is consistent with 6W = (10V-8V)*3A dissipated in the internal resistance.
You didn't short anything BTW, a short is a massive overload, not within the intended operating
limits of the battery.
So, the heat plate (2.8 Ohm) connected directly to the battery is OK? In any way, high current draw will introduce internal heating? Correct me if im wrong.
over discharging a lipo won't put it into flames, but it will kill it very fast !
if the cells go under 2.5V each, a decent LiPo charger won't allow you to charge it again - Sure you can still use tricks to "rescue" it, but it won't be stable as it should be, and none of its characteristics will be true (capacity, discharge current, max current discharge etc... will be far lower than they were) , it will heat more at the same current etc...
in the RC community, a security rule says that you shouldn't discharge it under 80% of its capacity
then, with a constant discharge current of 4A , you shouldn't use a fully charged healthy 2200mAh pack for more than 24mn -
@MarkT you're quite right, but the internal resistance increases with age , and when the battery is not used as it should be (over charged, over-discharged ? , ...... )
If you want to be safe with a series set of cells, put a
voltage monitor between each cell.
As the battery ages, the cells will not all age at the
same rate.
One will fail first.
If you need to let the monitor circuit sit on the cells
for some time, you can use a FET with the proper
biasing, like a 2N7000, as an isolator when not
sampling.
The amount you see under heavy load is not that
unusual. Don't expect to get 100% of the A-Hr rating
under heavy loads.
Look at some manufacture discharge curves.
Dwight
In rc use it is not uncommon for the terminal voltage to drop below the rated mins for short periods.
Terminal voltage is not therefore a good indicator except off load.
For this reason timers are used to ensure the battery is not damaged, RC radios are fitted with them as a mattr of course these days.
What type of Lipo battery is it?
For high current loads , the best type of batteries are those used in hi power electric models which are specifically designed for high discharge rates.
Heres a typical example.
This battery has a 70C discharge rate which means it can deliver over 100 amps for a short time.
There are a large range of capacities and C ratings of these types of batteries.