Need help with battery wiring

Hey forum!

So, I bought 2x 72v, 30A continous output batteries, 75A max.
I am planning on wiring it in parallel, to increase max amper. It has a BMS.
But, when I told the seller, he said this:

"This battery continuous current is 30A , it dertimes by BMS , it won't change to 60A even you use two batteries .
Our description has clearly clarified that , this battery can fit for 72V 750W-1500W ( Can't be more than 1600W ) ."

Is this true? Can I not wire up 2 in parallel to increase the max amperage for a 5kw ebike hub?

He may have thought you meant two in series. In parallel, you will be able to draw up to 60 amps. 30 from each. However, no battery likes continuous draw at its max output. You may find that you still get lots of voltage sag under the heaviest load, such as at start up, or full throttle up hills. The basic limiting factor, is if the battery gets a lot hotter than body temp, or air temp if its a very hot summer day, its suffering. If its too hot to hold in your hand very long, its getting killed. The good news though, is just 30 amps of 72v gets you going pretty good, so you should be able to cruise fast on the flat all you want with two batteries paralleled.

You might run into another catch 22 though, carrying two 72v batteries might affect the ride of the bike too much. Above 20 pounds of battery starts to make many bikes wobble at high speeds, and some, at fairly low speed.

You can parallel them following these two basic rules.

1 Charge separately, disconnect them to recharge. Don't let the parallel connection be used for one battery to charge the other.

2 Connect them only when the voltages are very close to exactly the same. So attach your Y cable to parallel them once they are full. Within a half a volt close enough.

If your controller is too strong, one bms may shut down before the other, leaving just one battery pulling the whole load. Two 30 amps batteries combined with a 40 amps controller would be a more reliable set up, vs a controller that maxes out both bms.


There is a way to hack the bms connections so one charger can charge both. I am not educated enough to tell you exactly how though.

Hey, thanks for the reply.

It's a MOSFET 120A controller, but I can set it to max amper usage, also will set it to ECO mode.

It can be set to show amper draw, if I keep an eye on it, any time except accelerating, I keep the amper draw below 60, would that be OK? It's 150A max that you can pull from those 2 batteries in parallel for a short amount of time, so what if I limit my controller to 100A for acceleration, but anytime other than that, keep it under 60?

I'll be getting a third battery later (May be a year or so), would that be too late to add to the old batteries? Or would their age not matter that much?

I'll probably charge them seperately, I got a voltage meter that I can set to 100 max, but it's not a digital one, so I have to be very careful with it.

Best way to know if your battery is getting used beyond what it can do and last, is to see how hot it gets. If your battery does not get above maximum possible ambient temp when you run it, its going to be happy. This is about 120f. twenty degrees or so above body temp. If you can't hold that thing in your hand, or put your hand on it after you discharge, you are killing it. it will get warm when you discharge, even when its way less than 60 amps. But if you are making it noticeably hot, you gotta lessen the amps.

You have to charge them separately, unless you do a modification to the bms wiring. You cannot charge one through the charge wire, then let it charge through the discharge wire.

If you have only one plug, that charges and discharges, you can charge them together. Faster though, on two chargers.


You can add more in paralell anytime. I do this, and have for years. I run lipo that works great for one year, ok for two, and can still be used at 3 or 4 years, but its weak. What I like to do is buy 10 ah of fresh cells each year. On a real long ride, i carry it all, paralelled, so the shitty old ones get very little of the amps.

Thanks for the help! Well I'm still waiting for shipping on the motor (Tomorrow is last day i can report item not arriving)

i have been waiting 4 weeks for shipping