Cell balancing in 5S25P

Just registered into this amazing forum.. My question is mostly related to BMS.

I am planning to build 5S25P configured 18.5V 65Ah pack using Samsung INR18650-30Q.

Do you have any suggestion for cell balancer chip with pack protection ?

How the cell balancing will be done in the individual parallel branch containing 25 cells ?

Welcome to ES!

The cell balancing chip is not just a chip. It is a component called a BMS or sometimes a PCM. The do the same thing, which is to protect your cells from over charging, over discharging and balancing them when fully charged.

To balance all those 25p cells you will need to connect one of the balance wires to your string of parallel cells. Balancing that many cells could take quite some time so learn about keeping you battery in good balance so you will never have to go through a two or three day wait for you pack to balance.

As for manufactures, I have no recommendations. Perhaps somebody can help you with that.

So what are you going to be powering with that big battery?

it depends also on how much amps you plan on using? For a 5s4p pack I build to power a ryobi 18 volt hot glue gun I use a cheap 15 amp 5s bms that cost me 8 dollars. It doesnt have any balancing function, but they sell 5s bms with a balance function.

As long as you plan to slow charge your battery pack, it might stay in balance. I always use a balance charger to charge my 5s battery.

If you plan to fast charge your battery, you might seriously consider getting active balancers. For a 25p pack they might be worth the cost. I use one on my 4s20p lifepo4 110ah pack and operation is excellent it can balance at up to 10 amps per cell, it will keep up with fast charging. electricarpartscompany sells them and they have ones for 5s, I paid 92 dollars for my 4s balancers. Balancing doesnt get any better then that, it doesnt bleed the high cells, it transfers the high cells to the low cells, not wasting any power and balance during charge and discharge. As you can see in picture I have them connected to a regular 4s balance connector. I just use a regular 4s bms to control the actual over/under voltage, the 4s bms also balances at 60ma but won't keep up when fast charging.

You don't need to balance the parallel cells... They all self even out to the same voltage. You just need to balance the series groups. But with that many in parallel you have to watch out for one cell going bad and overheating as the others discharge into it... That's where cell level fusing can get important.

I am working on an AUV and the pack will be used there.

I am designing my own BMS !

eventually I ended up finding a controller chip which is ISL94203

I liked the idea about cell fusing. I think it will solve the protection headache that I was worried about for cell balancing in parallel.

Or a simpler parallel level fusing. Fusing between cells in your parallel cells is a lot of fusing and once popped will cause your bms to cut off (if your bms works) or if your fuse between parallels pops, then your bms will cut off. 4 or 5 fuses between parallel groups vs a whole lot more if you put them between each cell.

Here is a thread you might want to follow...

https://endless-sphere.com/forums/viewtopic.php?f=2&t=93560

Impressive thread !

Any off the shelf fuse is available rather than using fuse wire for cell fusing ?

Please dont bother with cell fusing on diy packs. Especially these small ones. The cells you use already have protection built in of they go thermal or overcurrent beyond spec. The cell will simply appear dead. No reason to make such a unreliable diy solution.

The only situation where cell level fusing should be considerd if you have a dozen different brands, types and ages of used cells. If you use quality cells from a reputable brand you have nothing to worry about and just use nickel strips. But dont forget you make the series strips first and then you make 1 or 2 parralel strips depending on design because those dont carry anny current.

I am going to use only Samsung INR18650-30Q and all are brand new.

First I will solder each branch i.e. 25 in parallel and make 5 sets. then I will connect them in series. Is that what you mean Flippy ?

No. I mean you need to construct the whole pack. FIRST you make the series connections and THEN you can put the parralel connections on top of those.

The series connections carry the current so those have to be directly on the cells. The parralel strips do nothing in current so those go on top.


See this topic. Read it all and look at the pretty pictures i uploaded.

https://endless-sphere.com/forums/viewtopic.php?f=14&t=93112&start=25