Q about adding second battery to eBike. (Not sure which best forum).

Chickereeeee

A friend of mine has the correct vendor-supplied add-on battery for his ebike. It can only be connected to the original battery, and not replace it, as there is all sorts monitoring circuitry between bike and battery 1 to get the correct current flows. There is a different 'conversation' between battery 1 and battery 2 (the add-on) to get THAT connection working properly and safely.

Previously, on another bike, he just tried connecting two batteries in parallel, and buggered both. Luckily, they did not burst into flamers.

TL:DR - NO!

knightstyle

I think the problem is that different batteries have different characteristics so if you get two identical batteries you could just connect them in parallel and double the capacity. After all that is what is inside the battery, lots of cells some in series to give the voltage and some in parallel to give more current and capacity.

But just connecting any battery, probably a mismatch, is asking for trouble.

hivosco

If you are going for an off-the-shelf e-bike it's easier to have one that is supported so you can continue to repair it and use shared knowledge. There are often third party parts for things, and it's not rocket science, but it's easier to find for anything that is supported or quite popular.

Cheap batteries are low quality, and especially regular lithium are exponentially more likely to catch fire if they are low quality.

Although fires and heating are overall rarer for Lithium Iron Phosphate (LFP), instead of li-poly or li-ion, it's still a safer bet for quality, reliability, to go with a brand name that have a good reputation behind them.

I wouldn't personally trust any old seller, you want a trusted name behind the battery pack itself, and the cells they use.

If you have an off the shelf e-bike it's usually easier to get a second battery and swap it.

forgotmyname

https://forums.moneysavingexpert.com/discussion/comment/81995240#Comment_81995240

When you say they have management is that just for charging?

When you add a 2nd battery what is going to balance the current draw from the aftermarket battery?

WIAWSNB

https://forums.moneysavingexpert.com/discussion/comment/81996372#Comment_81996372

They balance eachother, like any two batteries in parallel.

You wouldn't connect a discharged battery to a charged one, as one would feed the other. You connect them in similarly charged states - they'd deplete pretty evenly.

That's the idea, leastwise.

Or, I'll fit a switched socket, so it disengages the internal battery when I plug in the new.

forgotmyname

They balance each other unstably, you can draw a lot of current from the pack with the least resistance and then when you
stop pulling all that power the pack with the higher voltage RAPID charges the other pack. With no protection in place the
only limit on the charge rate is the wire diameter.

What happens when it tries to charge the other pack at 300+ amps? Because it will not be using the charging protection it
will be using the fat cables your using to power the motor.

WIAWSNB

Why would one battery 'rapdily' charge the other?

forgotmyname

Because the bike will not draw current equally from 2 mismatched batteries so when one battery goes lower than the other
there is nothing to prevent a rapid charge of the weaker battery except the thick cables that supplies power to the motor.

You could get hundreds of amps charge rate unless something in the line prevents that but the battery connection to the
speed controller/motor etc is almost a direct link unless you put some management into it.

When they build laptop batteries etc. they pick the cells with the closest match for capacity and resistance etc. Even then
you still get one cell that can fail long before the others.

If that one cell in one pack goes lame the stronger battery can charge it with almost no limit to the charge rate.

WIAWSNB

In parallel, surely the draw will match the battery capacities, but the voltage will remain the same in both?

Ie, a larger battery will provide more amps than t'other, as their respective voltages fall at the same rate.

forgotmyname

Depends on the batteries resistance. You need to match them not just stick two random packs together.

If it was a low powered output then it's probably going to be fine but an Ebike is going to draw a fair bit
of power so the packs need to be closely matched.

When building packs or charging before use they state that the batteries need to be 0.1v to 0.2v variation MAX
but most builders are more cautious and keep the variations below 0.1v before using the packs or connecting them
together. Putting 2 packs together the high voltage output cables will be the only limit to the charge rate for the
weaker battery.