TysonScott said:
I am going to have a 28s8p battery pack on my ebike. It will be on each side of the frame, so i was planning to make both sides their own separate 14s 8p battery pack (with bms) then use a dongle to connect them in series before going into the controller. Is this safe to do?
That depends on the BMS each pack has. If you look at the many other threads about seriesing multiple packs, you'll find discussions about this, but if the BMS FETs are not able to handle (not just be rated for!) the full voltage of the entire seriesed pack, then as soon as a BMS shuts off for whatever reason, it will blow up the FETs in that BMS.
That failure may be "silent", no outward sign. It may fail stuck-on, too, so that it continues to let the pack drain despite whatever was causing it to shut off, so if it had a cell already empty, you'll keep draining it, possibly until it actually reverses and perhaps even starts a fire, either at the time, or later during charging, from the reversal-damage (since the BMS may no longer be able to stop charging either, depending on the specific BMS design).
The "cheap" BMSs will probably not have FETs that can do what you want. You may have to replace any that come on it with ones that can. You may also have to determine if the BMS's power supply design can also handle the full dual-pack voltage--if it can't, it may fail and cause the BMS to not work at all (whether this leaves the pack stuck "on" or "off" depends on the BMS design and the failure mode).
If you get a BMS that uses a contactor instead of FETs, then as long as the contactor is rated for the full voltage of both packs, it'll be safer than the FETs would be.
will it function correctly? (draw equally from each pack).
Anything in series will always see exactly the same current from everything that is in the series circuit.
Im just wanting conformation before i order 2 14s bms's.
The best solution BMS wires is to get a single 28s BMS, and make an interconnect cable from one side to the other that you can leave hooked up once installed that connects the most positive half of the pack to the BMS that is in the most negative half of the pack, and connects the two together. See the BMS's specific instructions on required wiring / connection order so that you don't blow anything up on it when connecting that most positive half of the pack. There have been threads about doing this kind of wiring.
Also for charging would it be best to charge each individually to the same voltage or could i wire the charge cables from the bms's in someway to charge them both at the same time? would i use a 28s or 14s charger for this? (i would guess 14s but the current would be split between the battery packs)
If they're in series, and then you also parallel them (to charge both with one 14s charger), you just shorted across one entire pack, and this may start a fire, or just blow up the BMS in one, etc.
You can't charge as 28s either, because each BMS has a separate charge port, and can only protect against overcharge if you use those ports separately. (the same is true of using regen--the BMS cannot protect against overcharge or overcurrent, etc.).
The only safe way to charge simultaneously with two separate BMS in series is to use two electrically-isolated*** 14s chargers. One on the first pack, and one on the second pack, and no electrical connection of any kind between the two chargers' outputs.
***isolated means that none of the input wires electrically connect to any of the output wires, and vice-versa. For instance, the Meanwell HLG-series LED PSUs or the Grin Satiator that I use as chargers are like this.
A cheap plastic-cased charger I have here somewhere that I don't use is NOT like this, and the AC neutral side of it's two-wire cord is electrically connected to the negative output.
A metal-cased charger on my brother's trike has the case grounded to the ground prong on the AC side, *and* the negative output.
So neither of those types can be used for your purposes.
If any charger you have or get doesn't specifically say it's Isolated, and a measurement of continuity from every input wire to every output wire doesn't fail, you can't use them for this.
Otherwise, you will have to use one 14s charger to first charge and balance one half of the pack, and then the other, taking at least twice as long as with two, or with a 28s charger and a single 28s BMS.
In any of the 14s BMS cases, I'd recommend dual-port BMSes, so that charge and discharge are separate ports. It may simplify charging wiring and procedures with less risk of problems.
The single 28s BMS can be either common or dual port.
It may be simpler to use two 28s BMS, and split the pack into two parallel halves of 28s 4p each. Then everything can be simply paralleled and have one charger, etc. For this you need common-port BMSs, and should not use ones with separate charge/discharge ports.
