• Howdy! we're looking for donations to finish custom knowledgebase software for this forum. Please see our Funding drive thread

Parallel Charging into three different batteries with separate BMS's

Beison8000

1 µW
Joined
May 31, 2022
Messages
3
Hello Friends,
Apologies if this has already been discussed but I searched and didn't see the specific answer.

Scenario:
  • I have one bike with 3 batteries that are in obscure locations (front loading cargo bike).
  • The batteries are all 48V, two are the same pack make/model with but different amount of wear, and one is from a different company.
  • They all are about 15ah. They all have roughly similar voltage sag performance under ~1000w load (2-3V sag when isolated and monitored)
  • Each pack has a charge/discharge port. Each pack can be charged from the XLR port. Each pack calls for the same 54.6V 2.5A charge current
  • I currently have a 3-way parallel connector attached to the discharge port of each pack and the bike functions well with this setup.
  • When I recharge the bike, I disconnect each pack and individually charge, then test voltage, then reconnect. This is not ideal because it is a hassle to remove the batteries from their different location and charge individually
  • I would like to wire a 3-way parallel connector with a pig-tail that I run to one mounted location where I can attach a charger, connect the bike overnight, and come back with all cells fully balanced and charged.
Question:
  • Will this setup work the way I'm thinking? If one pack finishes charging first, will the BMS restrict flow to that pack, while letting the other packs finish charging and balancing?
  • Will there be any undesirable backflow of current between the 3 packs with this setup?
  • Am I missing anything else here? Is there a better way to do things?
 
I would but a Schottky diode on each branch of the 3-way connector to prevent back flow.
This will reduce final charge voltage by about 0.25V, so you won't get quite 100% full.
Not necessarily a bad thing IMHO.
Shouldn't interfere with balancing, but you could attach the charger directly to each 3-4 times a year to make sure.
 
One of my bikes uses 3 separate battery modules with 3 separate BMSs. I charge/ discharge them all together in parallel with no issues. But I built them all from the same cells and used identical common port BMSs.
 
I would but a Schottky diode on each branch of the 3-way connector to prevent back flow.
This will reduce final charge voltage by about 0.25V, so you won't get quite 100% full.
Not necessarily a bad thing IMHO.
Shouldn't interfere with balancing, but you could attach the charger directly to each 3-4 times a year to make sure.
Not a bad idea.

These BMS's are always a bit of a mystery to me... I come from the RC world and if you ever are curious about the health of your pack you just voltage test the charge on each individual cell. very easy because the balance connector is right there. In this case, if I want to have the same level of fidelity I need to open the packs and start poking around with my multimeter. ...not the worst thing to do but then I need to re-glue and waterproof the housing. More of a pain than I'd like to do.

Out of curiosity, if I went this route could I also connect one of the programmable DC-DC power supplies like a Raiden RD6018 or something similar? I'd set the voltage to 54.6V and current to 7.5A (3 batteries * 2.5a each).
 
Last edited:
One of my bikes uses 3 separate battery modules with 3 separate BMSs. I charge/ discharge them all together in parallel with no issues. But I built them all from the same cells and used identical common port BMSs.
Yeah... my packs are second-hand bargin finds haha. They appear to be working really well but are all a bit different so I want to be a bit more cautious.
 
If the BMS have separate charge (C-) and discharge (P-) pads, then you have to disconnect the discharge ports from each other when charging, whether or not you parallel the charge ports.

If you don't, then the battery being charged will feed current back into the discharge ports of all other batteries that are paralleled with it, *even if their BMSes have turned those off*, because the FETs are just diodes, not switches, when fed backwards.

This is also true when riding. If a pack has an internal problem that drops it's voltage so the BMS tries to shutdown,, and is paralleled with another pack whose voltage remains higher, then that pack will feed current back into the pack that has a problem and is trying to prevent potential cell damage (unsuccessfully).

Also, even if the packs are full, they can't prevent charging from regen (not a common circumstance, but it happens).


If the BMSes use the same charge/discharge pad, probably marked as P-, then the BMS can safely disconnect the cells from charge current under any of these conditions, and you can parallel them for discharge and/or charge without diodes, etc. (assuming the BMS are functioning correctly and remain so).


Otherwise, the "safest" way to parallel the packs is with the diodes on the discharge ports while riding,

You will also need them on the charge ports, if you leave those connected while riding, because the same issue exists with those.


The diodes in either case must be able to block the full voltage of the pack, minimum, preferably much higher, and they also must be able to handle the worst-case current that will ever flow thru them in any situation. They make them in forms that can bolt to heatsinks to shed the power they will waste as heat (you can calculate how many watts that will be by the votlage drop across them at the current you will draw thru them).


I do NOT recommend any of the battery combiner / battery blender devices unless they actually show you that all they are is such diodes; if they are "smart" then you can't know exactly how they will operate (none of them appear to have detailed info that explain this, many have conflicting statements or impossible operating modes advertised) , and there are failure modes of them (because they use FETs, not diodes) that can leave your packs unprotected.
 
… where I can attach a charger, connect the bike overnight, and come back with all cells fully balanced and charged.
No, I’d stick with your old routine. Charging unattended is already risky. Charging three parallel packs, of varying quality, unattended is not smart.

My pack is a conglomeration of parallel and series packs and two chemistries, but each pack has matched cells within them, so they’ve never gone more than 0.01v out of balance. And I only charge to 90%. I do shorter rides now (less than 30 miles), so I bulk charge my pack at 8A for around an hour before riding, and ride. I’m within sight during charging though. With my routine I charge before each ride, so if I want to go in a long ride, I can, and 8A is a conservative charge rate for how they are set up, so not hard on the cells. Even with this Frankenstein configuration, I feel safer than charging an normal pack unattended.
 
Lithium batteries catch fire at an explosive rate burn hot them in water then put a little salt in there and watch that thing grow some weird ass colors I don't know if this should be done because I don't know what's in the water now but a chemical reaction is fun.
Not recommended or endorsed be safe.
 
Back
Top