I’m going to make up an external battery pack for an old cordless tool.
Thinking about different configs of cells.
At this point, I think I’ll be charging the batteries in an external charger - so I don’t have to worry about a BMS setup.
Is there any advantage to 2 in parallel with 4 of those in series vs. 4 in series with a second 4S in parallel?
Other than the 4S2P being easier to wire up in the external holders?
My poorly thought out logic makes me think the 2P 4S is better, but I thought I’d defer to the more knowledgeable here on the BLF.
If you don’t use a BMS, then YOU must be the bms. You and your voltmeter will need to do frequent monitoring.
In the tool packs i have seen 2P x 10S for the 40V ryobi. i assume you want a 16V pack so the easier and safer way is 2P x 4S, 2 in parallel with 4 of those pairs in series. The 2 cells in parallel are considered as one CELL by the BMS.
If you tried to parallel strings of 4 in series, then consider what happens if you have a bad cell (weak or shorted internally) in one of the 4S strings—current from the other string will flow into the defective string unless you have a switch or relay to isolate the 2 strings. It requires a more complicated monitoring and protection scheme, and i have never seen any device using parallel strings. It is too dangerous.
Hopefully you’re capacity matching these cells and also adding a balance cable at least —- I have a couple of 12v I made with Tesla 21700 and old NiCd cases —- The drill slows down way before the cells are too low —- I have a balance cable sticking out of the back and use a hobby charger to charge them — By being pretty closely matched, when charging they’re not far out of balance
Be careful with the type of charger you use, Make sure it keeps your cells in range —- 4S BMS on ebay are everywhere — then all you need is a 16v PS (16.8v max)
The problem with two separate packs —- when you connect the two together they need to be close to the same voltage —- I would do 2P4S
Thanks for the input.
I’ll be pulling the batteries to charge in an external charger.
Making up a semi-easy to replace battery back so I can swap individual batteries. So no need for balancing or BMS.
And I will be matching the cells. For right now, a set of 30Qs.
If I get inspired, I might try making up a pack with a BMS.
Then I would for sure be using cell balancing.
Could you point me to a BMS that would handle the load of an electric drill?
Preferably sold on US Amazon?
No need for cold temp sensor. High temp would be nice.
Thanks,
All the Best,
Jeff
What battery pack are you using —- Ebay tends to have BMS small enough for power tools (you really need to build your pack and use a DC clamp meter to see what amperage you need)
I would wire in some kind of fuse, with no BMS and Li-ion cells direct it’s best to have one
You’re a Flashoholic so you know how to monitor cells in multicell lights,this will be the same kind of thing —- For my drill I notice when it starts lagging, the cells are around 3.35v . Most of the energy is used up by then anyway.