Multiple (mounted) Batteries - No Parallel or Parallel?


Active Member
Hello Friends,

I get to start my first thread since joining and looking for experience/feedback from those who have installed batteries in parallel on their ebikes.

Currently I have Bionx powered bikes and multiple batteries (errr, 7 of them). When I go for long rides I simply carry the extra batteries with me, swap them on the downtube mount and pedal on. I have no choice of course with these propriety motors and batteries, but I made it work.

A summer project I'm considering would be to build out a bike with 2WD and 4 batteries. The motors/controllers would be from (9C's and Phaserunners) and the batteries would be 4 identical 52V units, still finalizing that choice.

To start, while I know it's possible to charge in parallel (been doing lots of research on this) that is not my plan. I will charge each battery with it's own charger. I do understand with parallel configurations you want the batteries to be matching voltages (within 0.1V, no more than 0.5V is what I have read) when you parallel the discharge connections connected to the controllers/motors.

I would also be installing a breaker to each controller/motor for safety and the ability to disconnect from the battery/batteries when needed.

I see 3 possible configurations and here are my initial newbie thoughts on each:

1 - No Parallel
Because I would be building out a 2WD setup I would start with 2 of the 4 batteries connected. Half way through the ride I would manually switch to the second battery for each controller/motor. I realize the individual motors may draw more (rear versus front most likely) and may have to change the battery for the rear over ahead of the doing the same for the front - any thoughts would be appreciated.

-Simplicity. Plug and Play.
-Never work about State of Charge. If the batteries are mis-match they will work just fine with this configuration.
-Charge cycles - still learning about this but in theory if I only use the first battery for each motor on a ride, the second battery requires no recharge. I could 'rotate' between the batteries as needed and on the longest rides, simply use up all 4 batteries worth of capacity.

-Miss out on the better balance of having multiple batteries working together drawing down from their full charge.
-Having to swap connections mid ride. If there is an issue with a battery, will not discover until a long way from home possibly. (Yes the 2WD configuration gives some piece of mind for this however)
-Balance draw down between the 2 motors, if one motor draws more/needs more Ah's over the distance of the ride.

*Small related note, in configuration 1 here I do realize I could possibly bring the wiring together in a central spot and if I do I could then swap batteries to motors around as needed, even a 3 batteries for the rear motor and 1 battery for the front motor typical use option, more will be known when I test.

When I build this bike up I will start of course in this configuration and see how it works. This thread is to gain feedback from others with experience to help me plan for this project.

2 - Full Parallel
All 4 batteries in parallel connected to the 2 controllers/motors.

-One big system, fully automated while riding.
-Battery draw down balance no longer an issue - both motors sharing all 4 batteries worth of capacity for the ride.
-Most accurate reading on the Cycle Analyst as it monitors the voltage of the 4 batteries operating together as one.

-Parallel connecting 4 batteries after recharging.
-Charge cycle for all 4 batteries after every use.
-Most complicated to troubleshoot if there is issues.

3 - 2 x 2 Parallel
While riding today I realized I could actually consider separate parallel batteries, one per motor controller.

-Still achieve all 4 batteries in parallel configurations (no connection swapping mid-ride)
-Better draw down balance than no parallel but not as good as the Full Parallel configuration.
-Easier to troubleshoot than Full Parallel.

-As mentioned the balance of draw down is not as good as Full Parallel (depending on the load the motors each draw over the ride).

At first when I thought of the 2x2 it gave a sense of less complicated but now writing it out feels like a 'half way' solution stuck in the middle. I wanted to include it though for discussion and feedback regardless as I appreciate those who post and will help me decide what to do.

Those are my thoughts. As you can see there is really no wrong configuration persay, rather I'm just seeking the guidance of those who have done this before me to help determine which configuration would be best to plan for long term.

Thanks again for reading and look forward to the replies.

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