30A BMS on a 48V 750W motor, what cells do I need to use?

[pishta]

according to the math, this will only draw about 21A but am not sure what these pull on initial start up. Im attemping to build a pack for this hub motor combo using some harvested 18650 batteries. I got 5 different 18650 battery makes ranging from 1.5A to 10A discharge ratings. Im not sure how the discharge rating is factored into a pack setup. If the BMS can allow 20A+ to the motor, what should a single battery in a 13s6P be rated for? Does the 20 get divided across the 13 series cells for <1.5A each or is it some parallel formula? The 10A batteries are E-bike pack: light blue LG LGDBMH11865 with discharge capacity

Discharging:

2.50V Cutoff 620mA Standard 10000mA Maximum

and I have 5 and 6.5A batteries too. Most of the batteries came from physically damaged E-bike batteries besides the 240 1.5A cells that came from 30 small 4s2p UPS systems. I built a 10s2P pack for a small 36v scooter out of the 1.5A batteries before I knew the A ratings and it sucks. It never charges to full green according to the 3 light scooter meter and it has no power and yellow drops to red battery as soon as I goose it. All the 1.5A/2400mah batteries I used tested >2400 mah on my lipo battery charger/capacity tester.

 

[Chalo]

Have you made a battery before, from cells that weren't fished out of the garbage? There might be safe ways to do what you're suggesting, but a n00b isn't going to nail it on the first try.

If you must try this kind of salvage, I suggest starting with a solar charged led driveway light or something else that can burn vigorously without causing a serious problem.

 

[James340]

I am going to point out the obvious. You did this before and it didn't work.This could fail catastrophically.

 

[amberwolf]

. I got 5 different 18650 battery makes ranging from 1.5A to 10A discharge ratings. Im not sure how the discharge rating is factored into a pack setup.

The lowest capability cell in series with other cells limits the entire pack's capabilities to that.

If you use a bunch of different kinds of cells to build a pack from, you should assume that the worst cell is the limit of the pack for whatever it's worst at (current, capacity, voltage sag, etc).

You'd also want to load test every cell at the current rate that you expect it to have to handle, and if it can't do it, don't use it. (to know what it can take you have to look at the cell's spec sheet from the manufacturer, and see what it should do at that current, and then see if your tests done under the same conditions as the spec sheet says also show that).

If the BMS can allow 20A+ to the motor,

A BMS doens't allow or disallow a certain amount of current.

It is a protection device, so if it ahs a 20A current limit, it will turn off all power at that limit. It's a last-ditch line of defense to keep you or your system from damaging cells in a way that can lead to a fire. It's not intended to be relied upon for everyday limiting of either current or capacity.

You need to use a controller that has a current limit lower than what your battery is capable of, or that is programmable so you can lower it to below what your battery can do.

Or you need to use a battery that can easily supply more than your controller will ever be able to ask for.

what should a single battery in a 13s6P be rated for? Does the 20 get divided across the 13 series cells for <1.5A each or is it some parallel formula?

If you only need 20A maximum then you divide that by the number of parallel cells, and that's what each cell must be able to provide. Any cell that can't, shouldn't be used.

I would recommend making sure that you use cells capable of at least twice or three times the load you'll place on them, becaue they'll perform better, waste less power as heat inside the cells, and be less likely to be damaged when an interconnect between them fails and takes some part of the pack out of the circuit (or some of the cells themselves degrade beyond the abilty to handle the load).

Most of the batteries came from physically damaged E-bike batteries

That can put them at risk of being damaged themselves, even if you can't see it, and such damage can lead to a fire. Since you can't see the damage, you can't know when that might happen, or what conditions might cause it. If it happens, it will just suddenly happen, and you won't be able to do much about it but get away from it and call the fire department, hoping it doesn't ignite your home or whatever else is nearby.


I would strongly recommend not using these cells, and instead go to somewhere like cleantecauto or batteryhookup, and buying a used-EV car battery module, and using that as the basis for your battery. It will be much much less likely to have a problem, and probably last for many years, and perform far better than a pack built from recycled garbage cells of at best unknown properties or worse, known-bad properties, with a bunch of interconnects between them all that must be perfectly reliable and not fail or else force cells to take more load than they can, and be damaged....