Best way to connect multiple lipo's

[RobK]

I'm looking at a robot design that will need 8s for drive and 13s for Weapon. This is on a featherweight

What are the do's and don't around multiple batteries? I've had 2 batteries in parallel before and tech check has been happy, that just had a single fuse in the loom to protect both batteries, the burst current was such that either battery would have been able to supply the system. Series wiring seems very different?

I'm looking to series wire three batteries 4s 4s and a 5s. Them have a take off at 8s for the drive and the full 3 batteries for 13s for the weapon.

What is the right way? I can fuse each battery individually if needed? Thats the way i'm looking to go.

Or can i just fuse each feed off (fused at the burst current of lowest rated batteries it involves?) The 4s batteries will need extra capacity as they will be doing the drive and weapon.

Whats best, any advice appreciated. I had hoped weapon and drive would be 8s but i need to up the voltage on the weapon if possible

[Ocracoke]

I run my heavier class robots with their batteries in series to up the voltage. These are matching pairs of voltages and capacities so I end up adding in just the one fuse, treating them as a single battery in effect, like you mention for parallel batteries. Do you have a wiring diagram of how this would work? I wouldn't have thought takeoffs in the middle of the series chain would work as you describe but I don't claim to be particularly knowledgable with electrics. Perhaps it is my lack of understanding.

Whilst fusing them individually might make sense, I'd be more worried about exhausting the smaller capacity batteries first. I have that same concern with my batteries in series if I don't get enough time to charge them or change them around. Having a fuse at each stage of the series wouldn't have prevented that I wouldn't have thought?

[RobK]

Hi Lucy, thanks for the reply.

I'm Rob with the yellow Fruitcake robots, I've seen/ spoken to you at many events.

Going to try and upload a picture. I'm not sure if it would need some sort of protection on the return paths to prevent any reverse flow?

It is the individual cell protection I'm worried about. This robot will have an Arduino running some parts so I'm sure I could sort the code out to monitor the individual batteries and shut the systems down if a certain threshold is reached. But not sure how this all stacks up for tech check etc

IMG_20200928_211321_309.jpgIMG_20200928_211321_309.jpgIMG_20200928_211321_309.jpg

[Ocracoke]

Oh yes, hello!

I think it is worth making a prototype of this with some old SLA batteries or something with a load and measure the voltage at each take off to see what happens. You could put in a inline diode on the drive circuit to prevent a possible problem there but I'd possibly try wiring the battries up in series in one go and then having a take off for the drive with something to reduce the voltage inline to the drive like a potentiometer or something. That way, you'd only need the one fuse for all the batteries, which simplifies the wiring.

As far as cell protection is concerned, I am guessing the balance plugs connected to the Arduino to get a reading for each cell would work with a inline relay between the 4S batteries and the 5S battery to cut the circuit if it was needed but it feels like a lot of parts to go wrong.