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Connection of two LiPos

Hi all,

I want to use two LiPos on my copter to increase flight time and ensure some backup. According to forum posts many simply connect two batteries in parallel. However, this does not seem to be a good idea since you always have to ensure that both batteries are almost equally charged and none of them can fail. I was thinking about some approach using diodes but then recuperation currents can not be fed back to the batteries.
Any thoughts on this? In your opinion, what is the best approach for the connection of two batteries?

Thanks in advance.

Charge them in pairs with balancing, and connect them in parallel. Use identical batteries. Other solutions are complicated and just add a point of failure to the system.

Definitely overthinking it.

As long as both batteries are the same voltage class (as in both are 4s, etc) and as long as both are relatively close to the same state of charge (similar resting voltage), you’re perfectly fine. They don’t even need to be the same total capacity (c rating).

When you connect them together, their voltage will equalize rather quickly. In operation, they will discharge together like any other battery. If they are different capacities, then still work fine together. The voltage of both packs will remains equalized as they discharge. A smaller capacity pack is not going to reach empty faster than the larger pack. It’s just going to discharge slower, matching the voltage of the larger pack.

You should definitely NOT be putting massive diodes on it.

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I do this on my large Octo. I have 2 6s 10000 packs

I made a parallel cable and works great.

Like mentioned above I do charge them in sync mode so they are at the same charge level

But what if one battery fails? Then the other one will drive a high current and the faulty battery may explode, as mentioned in another post. @smartdave did you look at the currents flowing between the batteries?

@Pedals2Paddles, I thought diodes are a good way but why do you think this is a bad idea?

I also found a battery decoupler, which seems to solve my problem somehow. See here.
What do you think of this?

This is what I use and it has served me well

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The thread you linked to is a battery that “exploded” in flight. If that happens, it’s not going to matter if you’ve isolated the batteries. The copter is going down in flames.

Diodes create a voltage drop and would be a significant point of critical failure. Also, diodes that can flow typical LiPo voltages at 50-100 amps would be enormous and impractical. I just spent a few minutes trying even find something like that and I found nothing. This is just not something that is done.

Regarding the reverse current decoupler, that would be remarkably safer and more reasonable than a stack of giant diodes. IMO, it is still a waste of 20 grams and adds a critical point of failure. This seems like a solution in search of a problem. There aren’t a bunch of copters falling out of the sky in a ball of flames due to catastrophic short circuit parallel LiPo failures. But if you really want that piece of mind, that’s probably the way to do it.

I feel that if you buy good quality batteries, and don’t run them down to zero you really shouldn’t have any problems

I think the take away here is the more hardware in the power system the more points of failure as Matt said. Components, connections, solder joints, etc. I think the advantage goes to zero… The one time I had a battery fail and catch fire was after it crashed.

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I use two similar batts in parallel in all my quads and no problem yet.

battery decoupler it is just a stack of giant diodes :smiley: and 50A for a large copter is way to low. It will definitely burn out and cause crash (as I said, it is just a point of failure)
On the other hand, I never found a failed lipo which went short circuit, the most common failure is open circuit.

yeah you’re overthinking it a bit. Just make sure theyre both the same i.e: same cell count(1s,2s,3s, etc) or mah capacity.

There is other tech out there as well

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Well, for most users it’s enough to just connect both batteries in parallel.
Using diodes is not the best idea - a lot of power will be converted to heat.
We developed special instant switching circuit for our tethered drone and we’re planning to release it but it’s going to take a few months and due to production process won’t be cheap.

Thank you all for the replies. I will connect them now in parallel.
@JulianZ I’m looking forward to your solution :slight_smile:

I will write a post when it’s ready :slight_smile:

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Hi @xfly, how about this thread? I’m thinking to put my batteries in parallel and I have same question.

Hi @giliardi, I put them in parallel and it of course works. However, still not my favorite solution. So I’m open for new ideas on that :slight_smile:

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