Splicing in an identical battery with only power wires (+ve & -ve), no BMS connection. Thoughts?

[mrkelkel]

Was sick of driving the car for short trips (<5km), and it just so happened that Ninebot had slashed prices on one of their China-road-legal scooters.
1890 yuan (265 USD) later, this arrived:


Rides great (even with no front suspension) and does 35 km/h (22 mph) on flat ground, but range is abysmal
The 48V12Ah pack is only good for 30km, and worst of all it's using a proprietary BMS, proprietary signalling and connectors, so a drop-in replacement is out of the question.
I can, however, buy an identical 12Ah pack from the same battery OEM which has a built-in BMS, but it doesn't "talk" with the bike controller and the pack only has power wire outputs. (+ve and -ve).
So here's what I'm thinking: charge both packs to 100%, and splice the second pack into the first pack's +-ve & -ve wires. The bike only talks with the first pack, but both packs would be in similar condition. Thoughts on this approach?

 

[E-HP]

A couple of questions. Per your diagram, it looks like in stock form, the factory battery is charged via the discharge connections. Can you confirm?
It's not clear how the new battery is normally charged. Can you confirm that it is also charged via the discharge connections (not a separate port on the battery)?
If both answers are yes, and both batteries are charges to the same voltage, then you can safely parallel them per the diagram and discharge and charge in that configuration, and each battery should be protected by their respective BMSs during charging/discharging.

 

[mrkelkel]

A couple of questions. Per your diagram, it looks like in stock form, the factory battery is charged via the discharge connections. Can you confirm?

Yep, the battery does charge & discharge on the same set of terminals, its only connector also has only one pair of +ve/-ve wires.

It's not clear how the new battery is normally charged. Can you confirm that it is also charged via the discharge connections (not a separate port on the battery)?

Yep to this too, the charging port has a direct connection to the controller too, albeit through two fuses. I'll be charging at 2A so they won't be an issue. Both batteries are also 48V 13s lithium packs.
Thanks!

 

[Superb_Raccoon]

It's gonna be LIT!


At least use a balancer.

Example, not a recommendation or endorsement:

Dual Battery Module 20-72v / 40A

High Amp dual battery module. This module hooks up two batteries of the same nominal voltage in parallel and combines their amps - Electrify Bike

www.electrifybike.com

 

[mrkelkel]

It's gonna be LIT!
View attachment 345907

At least use a balancer.

Example, not a recommendation or endorsement:

Dual Battery Module 20-72v / 40A

High Amp dual battery module. This module hooks up two batteries of the same nominal voltage in parallel and combines their amps - Electrify Bike

www.electrifybike.com

Hehheheh

Never thought something like this would be commercially available, gonna see if something similar is available in my neck of the woods. Thx

 

[docw009]

It seems the bike is capable of regen braking, which puts a high voltage on the battery. Usually. they use a battery with a one port BMS to protect from overcharge during braking, I'm not familiar with regen, never wanting to have it, but I have read some threads about DIY ebikes with regen causing battery issues.

I think the second battery probably should be the same way, with a one port BMS. Now a splitter would remove the headache of making sure both batteries are always at the same voltage when connected together, but I think would prevent regen braking from working.

All stuff to think about. I used to run packs in parallel for hundreds of miles and never thought much about all the pitfalls.

 

[mrkelkel]

It seems the bike is capable of regen braking, which puts a high voltage on the battery. Usually. they use a battery with a one port BMS to protect from overcharge during braking, I'm not familiar with regen, never wanting to have it, but I have read some threads about DIY ebikes with regen causing battery issues.

I think the second battery probably should be the same way, with a one port BMS. Now a splitter would remove the headache of making sure both batteries are always at the same voltage when connected together, but I think would prevent regen braking from working.

All stuff to think about. I used to run packs in parallel for hundreds of miles and never thought much about all the pitfalls.

Yep there's regen, but it maxes out stock at 2A and around 1kW. Doesn't sound like much (my e-moped does around 3kW) but was pleasantly surprised when I tried out the one-pedal (one-twist?) mode, didn't need to touch the cable drum brakes at all if I eyed the regen braking distance correctly.

I used to run different chemistries (Li-ion & LiFePo) in parallel on an older, dumber bike, always had regen on and that didn't seem to cause much trouble, so I'm hoping all the smart stuff on the new scooter would be chill with this too. If pack 2's BMS calls it quits the stock pack should still handle the 2A of regen, but only one way to find out I guess.

 

[E-HP]

Hehheheh

Never thought something like this would be commercially available, gonna see if something similar is available in my neck of the woods. Thx

You won't be able to charge the paralleled packs. Each battery has to be charged separately.

 

[mrkelkel]

Done! The new parallel pack is operating nicely. Decided to splice into the charging harness with an extra set of connectors so everything is reversible if required. Current to and from the parallel pack and the charging harness shouldn't exceed 10A, so the stock 18AWG wires should be sufficient.


Replaced the 5A fuse with a 25A fuse, identical to the fuse on the main harness:


Also added a 32A Wago splicer for potential third & fourth packs (or fast charging) down the road


And it's all done Under full throttle (20A discharge) the scooter app only reports 11.8A from the stock pack, so it's pulling around 8-9A from the parallel pack, and the stock BMS doesn't seem to mind as well.