Paralleling batteries, separate charging ports & BMS

ynot

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Mar 5, 2022
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So we have an elderly bike with a 36 volt brushed rear hub motor, and a brushless Jump bike hub motor on front wheel.
Got two identical batteries for m' b'day, so put one on the jump hub and one on the rear hub. It is obvious now we have done a few trips that the rear hub is doing 80/90% of the work, so think it would be smart to parallel the batts.
Plan on fully charging both batts, to balance them, then connect them.

The questions I have,
Understand that if I charge one of the batts, through its port, the other will get charged through the new connection, bypassing one BMS. Should I connect both batteries to one charge port thus doubling my charge time?. Of course I could parallel connect the chargers. Or is it better to leave both ports as is and always charge using two chargers?
 
If both batteries are connected at the discharge port, only the one getting the charger will be protected during charge.

To charge safely, you need to disconnect the packs from each other and charge separately. This will be sort of a pain and if you forget something and connect them when the voltages are different, something could burn up.

I have a similar setup on one of my bikes with two parallel packs. What I did was install a single port BMS on one pack and I charge through the dual port pack. If the single port pack has a fault, it will disconnect during both charge or discharge.

You could also just take your chances and charge them together. One pack will be fully protected but not the other one. Charge in a location where a pack fire won't set your house on fire (always a good idea).
 
Similar situation and I use two chargers (same brand and model) to individually charge each pack and cut the charge time. They are always within a tenth of a Volt or two after settling for an hour so no problem re-connecting them back in Parallel (key switch on each pack).
 
if both packs have a bms, you can just connect them in parallel and charge/discharge at the same time. It would work as long as all the connectors/wires can handle the amps, and all wires are the same lenght. Just make sure both packs are close to the same voltage when you connect them together.
In my situation I have 12 li-ion packs each 25ah with their own bms, and I got them all connected in parallel. I been charging/discharging them for almost 2 years with no problems.
You would need something like a Y-connector to join the packs together, this will make one giant ah battery once they are paralled.
a 2 way connector.jpg
 
jonyjoe303 said:
if both packs have a bms, you can just connect them in parallel and charge/discharge at the same time.

This depends on the BMS type.

If it has common charge/discharge port, that works fine; it has only one current path out to the charger and load (and other paralleled batteries).

If it has separate ports that are always enabled (except for LVC/HVC conditions applicable to each port), current can flow out of the discharge port of one and into the other, during charge. This is a problem if one of the packs' BMS has tried to shut off charging because it has a high cell and is trying to bring it down.

For that case, the discharge port of all but one of the batteries would need to be disconnected from the parallel harness, during charge.



Similarly, for the same type of BMS, if the charge ports are left paralleled during discharge, one pack's BMS may shutoff it's discharge port to protect it against overdischarge, etc., but current will still flow out of it's charge port into the charge port of the other BMS anytime that other pack drops in voltage enough to draw this current. (which could happen at a fairly high state of charge if one pack just has a bad cell that has brought it's group down to LVC really early). Now that pack that tried to shut off discharge is still discharging, potentially seriously damaging the already-LVC'd cell(group), and if it goes on long enough, possibly even reversing it's voltage (not generally considered a happy condition ;) ).

That's an admittedly rare scenario...but it could happen under the right (wrong) conditions with that specific design of BMS.

For that case, the charge ports of all the batteries would need to be disconnected from the parallel harness, during discharge.


If the packs are healthy neither of these is likely to be an issue...but then you wouldn't really even need the BMS --it's there to prevent problems...bypassing it's ability to do so defeats the purpose of having it there in the first place. ;)
 
The information here is invaluable thanks for the responses.

Thinking that paralleling the discharge ports, and paralleling the charge ports (discarding one of course), and putting a 5 amp diode on each positive charge wire may be a good solution.

That of course will lower the charge level to the cells by the diode voltage drop, which will help the cells last, but may negate top balancing.
Thoughts about this would be welcome.

Range in this case is not a consideration as the bike will go further than my butt will allow.
Although an armchair mounted on bike might extend my range <grin>.
 
ynot said:
Thinking that paralleling the discharge ports, and paralleling the charge ports (discarding one of course), and putting a 5 amp diode on each positive charge wire may be a good solution.

That of course will lower the charge level to the cells by the diode voltage drop, which will help the cells last, but may negate top balancing.
Thoughts about this would be welcome.
Should work fine; if your charger is adjustable-voltage (some are, some aren't), you can compensate for the diode drop. Note that the diode-voltage drop itself will lower as current decreases near end of charge, making it less of a problem.
Range in this case is not a consideration as the bike will go further than my butt will allow.
Although an armchair mounted on bike might extend my range <grin>.

how about a couch? ;)
https://endless-sphere.com/forums/viewtopic.php?f=12&t=72225
vids here https://fadsihave.wordpress.com/category/couch/

If you have trouble with saddles, and don't need the control of the bike that they offer via the saddle nose, you could modify it to use a semi-recumbent suspended-mesh seat, dependign on the bike design (might require a whole different bike). Would probably be pretty easy to do for your trike, though.

See my SB Cruiser and CrazyBike2 threads for some seats I used; the best one is the StadiumChair used on the SB Cruiser for an upright seating. The StadiumChair Mk2 is a slightly different frame (but same nice seat) and offers clamp-on arms and other accessories (including, I think, drink-holders and possibly a tray).
https://endless-sphere.com/forums/viewtopic.php?f=2&t=67833
Early in the thread shows how I used a regular seatpost welded to the bottom of an ikea officechair to mount the first "chair" to the bike frame I started with; you can use that method to do the same without modifying the bike or trike, if space allows.


The more laid-back and more-back-supportive seat I built for CrazyBike2 is more comfortable overall but is only suitable for a more recumbent-built bike or trike. (the StadiumChair could be used on a more upright-seated bike or trike).
https://endless-sphere.com/forums/viewtopic.php?f=2&t=12500


My brother's trike (in the Raine Trike thread) uses a heavy padded powerchair seat which can have fold-up arms (but doesnt' on his as they were in the way).
https://endless-sphere.com/forums/viewtopic.php?f=2&t=80951
 
The couch would have been very handy in the '60's, would have saved a lot of time. Fantastic ride tho' & well done.
Had a Q'icke built in Mexico (think Rhodes car), went to wallmart & bought two steel office chairs & cut off legs then bolted them to the frame.
Sure wish I still had the 4 wheel bike when the jump wheels came about, 4 of them would have been great fun.
 
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