Is it safe to use a spare battery as a range extender battery (via charging port mod)?

[William Knox]

I have a battery that lost half its charge, it holds the same power but simply stops working at the 50% charge. I suspect half of the cells are destroyed somehow due to going up a really steep hill after a long ride, I still don't really know why. Nonetheless, I was sent a new battery from my scooter company as it was covered under the warranty, and they didn't ask for the old one back.

Long story short, with the new battery replaced, I retrofitted the old "1/2 capacity battery" into a charging pack by attaching a appropriately rated cable and plug to it.

With this pack plugged into the scooters charging port, I end up getting the extra range that I expected, which seems like a success.

Should I be worried about damaging my batteries, my BMS, controller, or other components? I make sure to never have the pack plugged in if the charge between the two batteries are far off, so I make sure that they are both at full capacity.

Again, the battery specs are identical, they are for the new E-twow Booster GT, they are both rated at 48v with 10.5Ah. FYI: The provided wall charger outputs 48V at 3.0A.

Any feedback would be greatly appreciated.

 

[amberwolf]

there's a few threads about this specific situation with more info if you poke around, but the basics are:

the only safe way to do that is if both batteries are at the same state of charge voltage level at the time of connection.

if the voltages are different, then since there is no current limiting on the input to the onboard pack, and it's hardware (connectors/wiring) ccould be damaged, or the bms could be damaged, or the cells of whichever pack is lower voltage could be damaged, depending on how high the voltage of one pack is compared to the other.

additionally, if the discharge rate of the system is enough higher than the charge port is rated for, the current flow from the extra pack into the system may exceed the charge port's ability to handle it, and be damaged.

it could be the actual hardware (connectors, wiring) or it could be the bms's charge fets that could fail.

the former will probably be obvious heat damage, melting plastics, etc., but the latter wont' be obvious until you either find the onboard pack no longer charges at all (and the addon pack doens't help range anymore), or worse that it can't stop a charger from overcharging the cells, as those fets are there specifically to let the bms turn off charging input when cells are already full, especially if there is a large imbalance between the cells, and to let it turn off charging input if the charger voltage causes the cell voltages to go too high (over hvc) for any reason (charger failure, etc).

 

[William Knox]

Very useful feedback, thank you Amberwolf.

It is concerning knowing that the 10.5 amps that comes out of the extender battery is 3.5 times the amps that comes out of the wall charger (3 amps).

Perhaps that's the reason I have never seen extender batteries for this scooter, that attach this way, perhaps the potential for damage is apparent. Even still, I seems to have no apparent issues so far, so I'm less inclined to not use this setup.

I'm wondering if I could wire a plug that connects to the internal wires that connect the on-board battery to the controller, kind of like a parallel set up, but I'm almost sure that would fry some other part of the hardware.

The best way to use this extra battery is to just swap it out, but the scooter doesn't make it at all easy to do that. I could make a physical switch in the on-board wiring, that allows me to cut in-between the two batteries, as though they are both on-board. So when one dies out, then the next one gets switched into the circuit, pretty much the same results.