I have an older electric bike, it's a 2011 Trek FX+ series with a small Bionx brand battery on the back, and I'm looking to potentially solder in a larger battery on it that I got from a broken electric skateboard, I just I don't want to fry anything from overpowering it. Here are the specs of the batteries;

Bionx Battery
Lithium Ion

Electric Skateboard Battery
Lithium Iron Phosphate

I really am uncertain of what the main concerns are with a project like this...Is it the voltage difference or the amperage difference that matters most? Obviously what I want to avoid is overloading the battery or components and making it burst into flames... As cool as that may look going full speed down a hill...

  • \$\begingroup\$ Welcome to EE.SE! This appears to be a reverse engineering, modification, or repair question. Please be aware that such questions must involve specific troubleshooting steps and demonstrate a good understanding of the underlying design of the device being discussed, so that you can ask specific, focused questions that can be answered concisely. More information can be found here: Is asking how to fix a faulty circuit on topic?. \$\endgroup\$ – Dave Tweed Mar 1 '18 at 11:46

If the bike is designed to use a 40 volt battery, I would expect it to operate poorly, if at all, with a 20 volt battery.

  • \$\begingroup\$ Does the amperage make any difference? Since it's over 3 times more amps? \$\endgroup\$ – Zach Prater Mar 1 '18 at 5:09
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    \$\begingroup\$ @ZachPrater 20Ah is not amperage! It is battery capacity. \$\endgroup\$ – Chupacabras Mar 1 '18 at 5:42
  • \$\begingroup\$ Would it be fair to say that the battery can of course run for one hour at 20 amps compared to 6.4amps, but because the voltage is half, the run time would be cut in half? Or would it be that the runtime is longer, but the power that the motor could get would be less? \$\endgroup\$ – Zach Prater Mar 1 '18 at 5:52
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    \$\begingroup\$ If the motor is designed to run on 40 volts, it will run much slower, and with lower power, on 20 volts. The ampere-hour capacity is irrelevant if the motor can't move the bike at a useful speed with the new battery. \$\endgroup\$ – Peter Bennett Mar 1 '18 at 5:56
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    \$\begingroup\$ @ZachPrater, again, not "20 amps", but "20 Ah", 20 Amper-hours. You just need two skateboard batteries, and you will ride about 3 times longer. \$\endgroup\$ – Ale..chenski Mar 1 '18 at 6:07

Your ebike will not function on 20 volts if it expects 40 V input.

However, if you had a second of these batteries then pit them in series, so 20V + 20V will give you 40 volts. This would also give you 800Wh which will almost double your range.

Its unlikely your new battery pack will fit in where the old one does, so its going to need some thought around a strong and safe mountpoint.

Also, your present charger won't be able to charge the new battery. Ideally you would keep the skateboard chargers and undo the battery pack, then recharge each 20V battery separately.

If your bike has regenerative braking, that may be an issue because that circuit won't match the charger characteristics.

Finally, the display will probably have erroneous readings for "remaining battery" so expect to be riding blind, like a car with no fuel gauge.


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