If I take two identical lipo cells, say 2 x 10ah lipos both new, for instance one is at 3.4v, the other at 3.9v, now if I connect them in parallel to get a 20ah cell, what will happen to the cells?, will they balance out to the same voltage? will this balancing take time? hours, mins, days?, or will it happen rapidly? or is this dangerous? do the cells need to be accurately at the same voltage first before they are connected together?

So many questions, ive searched high and low for details on this but never found an answer yet never seems to be covered in any faq's.

Hello Stew,

With Li-Po chemistry and larger capacity cells, there is no problem at all.

Let's assume open circuit resting volts, and let's pick a cell internal resistance of 0.03 ohms. If we look at the worst case we would have a fully charged cell (4.2 volts) paralleled with a fully discharged cell (let's go with 3.4 volts, although that is slightly over discharged), the difference in voltage between the two cells is 0.8 volts. Using Ohms law we find that the initial current is going to start at a little over 26 amps and drop down from there.

For a 10 Ah cell, 26 amps is 2.6C, and 2.6C charging, while a little high, is not controversial.

However, let's look at what happens with a 1.2 Ah cell. The same math applies, so we would have an initial inrush current of a little over 26 amps. Now were are into an area where there is some controversy. I don't think anyone recommends charging at around 21.6C. On the other hand, this is only a pulse load, and Li-Po cells can handle much higher pulse loads than steady loads. This initial current only occurs when you hook the two cells in parallel, then the current rapidly drops off as the voltages equalize.

I have run a lot of tests on both Li-Po and Li-Ion cells doing this, and, if I remember correctly, the most I was able to measure was around 7 amps. Within 2 seconds the current dropped to around 3 amps, and after 10 seconds it was just a little over 2 amps and still dropping. I most likely missed the initial spike of current because my clamp meter was unable to pick it up, however the rapid decay in current indicates an exponential decay.

Now the question arrises - Can your 1.2 Ah cell handle a burst of 26 amps, followed by a second at 7 amps, a couple of seconds at 3 amps, and a few more seconds at 2 amps before dropping to a charging rate under 2C?

I have not tested every brand of cell, so I can't give you an answer. While I have suffered no ill effects, I don't have a good basis for understanding all the problems that could arise from doing this on a regular basis. When faced with uncertainty, you usually add "safety factors." In this case, the general consensus is to only parallel cells that are less than 0.5 volts of each other. This brings the initial surge down to around 16 amps and it will rapidly, within seconds, drop off to below 2C for most cells.

I find that, depending on the degree of imbalance, cells hooked up in parallel will usually be balanced in 10 - 30 minutes.

Tom

Thank you Tom,

hats off for a great reply [sm=thumbs_up.gif]

That has opened up the picture for me, so if my cells are below 0.1v in difference then i'll be ok. My electric bike project will now cometh forward!

I'm interested in knowing how long it may take to achieve equal voltage (or 0 current) in the above example of 4.2 hooked to a 3.4 volt.

Oops, just finished reading the answer, Thanks.

I have observed something that I can't quite figure out and it occurs with paralleling packs and also after chargeing the paralled packs.

That is after leaving them connected to each other (on charger or off) for hours. Upon disconnecting them the voltages are not equal.
Any idea why this may occur? I've noticed it on various packs 2p, 3p equal and unequal capacity packs.

Hello Guver,

As Li-Po cells age, their internal resistance increases. If you parallel cells of different internal resistance, they will equalize at the same voltage and charge properly, but as soon as they are removed from the circuit their voltage will drop a little. You can verify this by charging the cells individually and observing if there is still a voltage drop at the end of the charge.

Tom

I have left them hooked up to each other for hours , would that still apply?