Greeting guys! I have two batteries, both 35C rated, but the first is 2300mAh, and the Second 4200mAh! Both are 3S lipos... Is it possible to connect them in parallel? Or are there going to be any problems?
Thanks in advance!
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A very accurate model of a battery is an ideal battery in series with a resistor equal to the batteries internal resistance. When under an external load the voltage at the battery's terminals is the no load voltage minus the voltage drop across the internal resistance.
For batteries in parallel and under a load, it is the terminal voltages that must be equal. This will differ from the no load voltage ( ideal voltage). For two batteries in parallel with the same zero load terminal voltage, the ratio of the current supplied when a load is switched on by each battery would be the inverse ratio of the internal resistance.
The instance a load is switched on, the battery with the lowest internal resistance would supply more current and this has nothing to do with the batteries capacity. Depending on the capacity of each battery, it's contribution to the total current, and the discharge characteristics, the no load voltages difference (Bat1 - Bat2) could be positive, negative or zero. As long as each Battery can handle the entire load without damage none of this imbalance in discharge rate really matters much until one of two things happens.
1) The load is suddenly switched off
2) One of the batteries reaches it's full discharge voltage before the other.
In case 1, each battery's voltage will instantly increase. I measured this increase on 4 parallel 18650's under a 1 amp load to be about .2 volts. Each battery in a parallel pack will increase in voltage by the product of their contribution to the previous total current times their internal resistance at whatever SOC, they are at. For different battery types there will no doubt be a voltage imbalance and current will surge from one battery to another. The higher the previous load current the higher this voltage difference will be. If it were only .3 volts and the combined internal resistance of the batteries was 50mOhms the instantaneous surge current would be 6 amps, which may exceed one or both max current rating. Of course this current would discharge one and charge the other and rapidly balance the batteries.
But there is a slight potential to cause one or both to suffer a capacity loss
Case 2 can have devastating impact on a battery. Say one battery safely can discharge down to 2.8 volts and it's parallel partner can drop to 2.5. If there is no protection circuit the 2.5 battery will happily continue to drain current into the load after the 2.8 is already in dangerous territory. The best solution is individual protection on each. The parallel pack could share a single protection circuit that shuts them down above the 2.8 volts. There would be a loss in usable capacity of the 2.5 volt battery, but at those voltages it wouldn't amount to much.
A good rule of thumb is to try to use batteries of near equal capacity and similar internal resistance. If you do use different capacity batteries try to make sure the smaller one has a higher internal resistance.
Example: 1/2 the capacity with double the resistance. This will make sure each battery follows the same discharge curve and the voltage surges from sudden load change shouldn't be a concern
IMHO paralleling unmatched LiPos is not worth the risk. It's perhaps OK in theory, and it may work fine in a particular case, especially if draw rates are low and the batteries aren't run down very far, but I think the possibility of different internal resistances etc. among the unmatched cells is asking for trouble. I fly parallel batteries routinely on large (600 class) helis and on my hex, but only of the same brand and specs. Even then on balanced charging after flight there are differences to be seen in the total mAh returned into the batteries. Seems to me that after a period of high load during flight one battery might end up temporarily at a lower voltage than the other (regardless of their "C" ratings being nominally equal). So then next you have one charging the other, at a completely uncontrolled and unbalanced rate. Maybe it won't cause a fire (experiencing a LiPo fire will cause a radical attitude adjustment regarding casual experiments, believe me) but it sure won't help battery life either.
As long as both batteries are charged to the same voltage (fully charged) before connecting them in parallel you will be fine. The parallel connection will even out the current draw between them, since they are forced to stay at the same voltage at all times. So you effectively get a 4200+2300 = 6500mah battery from parallel connecting them. Serial connecting different capacity batteries on the other hard, would be bad.. Very bad..
I flew ArduPlanes with 2 and 3 batteries in parallel for a year now, worked fine.