Wiring of a separate port BMS as common port

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Sep 19, 2020
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TL;DR: the BMS has a port for charge and one for power. It can charge through the power port. Will it also balance through it?

I have a generic BMS I salvaged out of a dead e-scooter battery. I want to use it on an external battery that'll go on the stem of another e-scooter as a range-booster, in parallel with its original one.

Problem: the BMS is separate-port, and I have no need for a separate charge lead since the battery is to be charged from the main connection through the parallel arrangement.

The BMS does take a charge through the power port; logically it must for e-braking purposes, but I've tested it anyway and confirmed that the charger's light does go red if connected there. It follows that I could just ignore the charge port altogether, and charge the battery from the main connection.

I have one doubt, however: will the BMS balance the cells if it's charging through the power port, or does it specifically need an input on the charge port for balancing to activate?

Here is how I intend to wire it (right is B-).
 
If you charge thru the discharge port, the BMS cannot stop the charger from overcharging the battery in the event that either the charger voltage is higher than full pack voltage, or that the cells are mismatched and need balancing during charge, such that some of the cells charge to higher voltages than others and may exceed their safe max voltage.

The only way the BMS can do it's job is if charging is done thru the charge port, and discharging is done thru the discharge port.

If you tie the ports together, then there is no discharge *or* charge protection, because the body diodes in the FETs of each port bypass the other port, so the BMS can't do it's job of protecting the cells.

A common port BMS probably wires the two sets in series, back to back, rather than in parallel, for that reason. In that design, turning off either set of FETs should prevent current flow thru the BMS in the appropriate direction to let it protect the cells.
 
amberwolf said:
that the cells are mismatched and need balancing during charge, such that some of the cells charge to higher voltages than others and may exceed their safe max voltage.

All right, so it won't balance when charging through the discharge port.

Is there a problem if I parallel both the outputs leads *and* the charge leads of both batteries, but separately?

I mean paralleling main output 1 to main output 2, and separate from that, charge lead 1 to charge lead 2? This way I wouldn't tie the ports together, but the same charging plug would still charge them both. I'm wondering if the outputs being paralleled would mess with the process.

If that doesn't work, can I disconnect the main outputs from each other, but charge both batteries in parallel from the same charging lead?
 
Fallingwater said:
All right, so it won't balance when charging through the discharge port.
It also won't cut off charging if any cell goes too high.

Is there a problem if I parallel both the outputs leads *and* the charge leads of both batteries, but separately?

I mean paralleling main output 1 to main output 2, and separate from that, charge lead 1 to charge lead 2? This way I wouldn't tie the ports together, but the same charging plug would still charge them both. I'm wondering if the outputs being paralleled would mess with the process.
Yes, the outputs being paralelled during charging can be a problem under the specific circumstances of:

If
The BMS does not disable the discharge port during charging
and
One pack has to stop charging for any reason
then
The charge current can still flow out the discharge port of the BMS that hasn't shut off the charge port back thru the parallel connection thru the body diodes of the discharge port of the BMS that did shut off charging, and keep charging the cells of the pack that is trying to stop charging.


The same thing can happen during discharge:
If
The BMS does not disable the charge port during discharging
and
One pack has to stop discharging for any reason
then
The discharge current can still flow out the charge port of the BMS that hasn't shut off the discharge port back thru the parallel connection thru the body diodes of the charge port of the BMS that did shut off discharging, and keep discharging the cells of the pack that is trying to stop discharging.

If that doesn't work, can I disconnect the main outputs from each other, but charge both batteries in parallel from the same charging lead?

Yes, that will allow parallel charging without any issues I can think of.

Just remember to also disconnect the charge ports during discharge.

Kind of a PITA....but the only way I know to be sure of preventing a problem and still parallel things.

Well, not the *only* way: Diodes on the input and output of each pack, paralleled only on the side of the diodes *away* from the pack, would also do it, but then you have wasted power and voltage drop across the diodes in both charge and discharge, and hte cost of the diodes and the trouble of setting them up and making sure they won't overheat depending on the load they see, etc. There are already pack-paralleling diode units available commercially, but I have no idea how good or bad any of them are. There's also threads about how to do this DIY style, what diodes to use, etc., somewhere around here. :)
 
amberwolf said:
Yes, the outputs being paralelled during charging can be a problem under the specific circumstances of:
Very clear, thank you very much. Really should have gotten there on my own, but I was writing at 4AM and my brain couldn't brain. :D

Kind of a PITA
Eh, I'll survive, at least until I get my hands on a common-port BMS and rewire this thing properly. I don't have switches beefy enough to handle main discharge, so I figure I'll just use an XT60 on the output and an XT30 on the charge line, and alternate them as needed.

I'm going to be the sole user of this scooter, so it doesn't bother me all that much that there's a specific procedure to follow that anyone else is bound to forget.

Glad I found a way, this way I can have this thing ready to ride immediately. Thank you!
 
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