Can I connect an aux. battery's output discharge port (48V21A) to the input charge port of 48V13A main battery?

[ck1224]

I want to buy an aux. battery such that I can connect its output discharge port to the input charge port of the main battery so that I can use both
batteries as one main larger battery on a ride without having to stop and swap batteries out?

will this work, or not?

I know that some people have a toggle switch config where it toggles from one battery to the other battery with a wiring kit from Juiced Bikes.

 

[Sierratim]

I want to buy an aux. battery such that I can connect its output discharge port to the input charge port of the main battery so that I can use both
batteries as one main larger battery on a ride without having to stop and swap batteries out?

will this work, or not?

I know that some people have a toggle switch config where it toggles from one battery to the other battery with a wiring kit from Juiced Bikes.

The aux battery discharge port is intended to provide high currents with the bike's controller governing the actual current delivered the motor. The battery's BMS will monitor the cell voltages as the battery discharges, but not limit the current until a cell goes undervoltage. As it's unlikely that both batteries will have identical voltages when connected the aux battery will either provide or draw a large current from the main battery until the voltages become equal. As a minimum this will eventually shorten the batterys' lives. Worse case, it could cause overheating and fire. I wouldn't recommend this direct connection without a circuit to regulate current, as a minimum.

 

[ck1224]

The aux battery discharge port is intended to provide high currents with the bike's controller governing the actual current delivered the motor. The battery's BMS will monitor the cell voltages as the battery discharges, but not limit the current until a cell goes undervoltage. As it's unlikely that both batteries will have identical voltages when connected the aux battery will either provide or draw a large current from the main battery until the voltages become equal. As a minimum this will eventually shorten the batterys' lives. Worse case, it could cause overheating and fire. I wouldn't recommend this direct connection without a circuit to regulate current, as a minimum.

Thank you for the response. If I charge both batteries to full (around 53 volts or so) then it sounds like there would be little to no current draw from aux battery to main battery or vic-versa as they both would drain slowly together maintaining approximate voltages, correct? I wouldn't put a fully charged aux battery with a drained main battery or vice-versa. Someone on Rattan's FB group says they do this.

 

[Sierratim]

This setup is trying to use the aux battery as a high capacity charger for the main battery. There are no protective circuits on the main output terminals of the aux battery. The protective circuitry on the inputs to the main battery just monitors cell voltage, not current. This feels like a receipe for a problem to me. I haven't tried this, but I have several engineering degrees and have built and serviced a number of ebikes. I've also gone through a lot of the material at Battery University to try and be sure I don't cause any real problems in my messing around with these high energy batteries. IMHO, this isn't a good approach. YRMV.

If you want to proceed with this, please try it outside and don't leave it hooked up if you're not standing by.

 

[harryS]

Micah Toll, an ebike journalist, described this method on his website.

I was dubious. There's no path for the current in the aux pack to get to the motor. All it can do is recharge the main pack when the motor isn't drawing power. So I tried it out briefly , measuring the current out of the aux pack and the current out of the main pack.

This is what I remember. About what I expected. My controller could pull 21 amps max out of the main battery, The aux pack pushed a few amps max. Since they're close in voltage, the charging current out of the aux pack isn't going to be high. It works.

I didn't take it further, but I think that in an absolute mileage test, I'm likely to get more range running either pack separately, which Micah also said. I also think I'd get more range putting the packs in parallel so they share the load. Just conjecture, but believe running a pack at a lower current relative to its maximum improves the capacity/life. If that's true, you're better off running the bike on the 21AH pack, and using the 13AH pack as the charger.

Just my opinion. If you already have two batteries capable of running the bike, the fool proof method is to remove the first one when it's used up, and connect the spare. Then you don't have to worry about equalizing the voltages or worry about the charging process.

 

[BrianK]

Bafang sells a parallel cable for wiring in a second battery:

Parallel Battery Connector Cable– Ebike Essentials

Parallel Battery Connector Cable allows you to connect two of the same voltage batteries to increase your total Amp hours and range. (Example: If you have two 48V 17.5Ah batteries, this connector will allow you to install and use both batteries on your bike with your motor kit for a total of...

www.bafangusadirect.com

 

[hughresearch]

I want to buy an aux. battery such that I can connect its output discharge port to the input charge port of the main battery so that I can use both
batteries as one main larger battery on a ride without having to stop and swap batteries out?

will this work, or not?

I know that some people have a toggle switch config where it toggles from one battery to the other battery with a wiring kit from Juiced Bikes.

The long answer is no.