Upgrading an electric bike battery

Thread Starter

zprater1

Joined Feb 28, 2018
3
Hey ya'll, long time lerker, first time poster, and I'm just looking for advice on a project I'm working on...

So 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 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
260Wh
6.4Ah
40v

Electric Skateboard Battery
Lithium Iron Phosphate
400w
20Ah
20v

So I'm trying to decide on how to go about this? Or if the voltage is just the biggest concern in a situation like this and I can put something like a step up/down voltage transformer... I'm fairly savvy, this is just a bit over my head. Any tips or advice would be appreciated!
 

dendad

Joined Feb 20, 2016
3,259
Welcome :) This site has given me a lot of enjoyment.
Your battery will need to be 40V like the old one. A larger current and Amp Hour capacity is ok, as long as you have the correct charger. Remember, Lithium based batteries make pretty good bombs if charged incorrectly.
Any gain you get from a bigger capacity 20V battery may well be lost in a boost converter, although it may be worth a try. Probbly it will be cheaper just to get the correct battery. Do not use the bike 40V charger on the skateboard 20V battery!
 

Thread Starter

zprater1

Joined Feb 28, 2018
3
Welcome :) This site has given me a lot of enjoyment.
Your battery will need to be 40V like the old one. A larger current and Amp Hour capacity is ok, as long as you have the correct charger. Remember, Lithium based batteries make pretty good bombs if charged incorrectly.
Any gain you get from a bigger capacity 20V battery may well be lost in a boost converter, although it may be worth a try. Probbly it will be cheaper just to get the correct battery. Do not use the bike 40V charger on the skateboard 20V battery!

Ah, very good! That explains it perfectly, thank you so much for this! I suppose then I'll be looking at a 40-48 volt battery with a higher amp hour so I can get longer run time. This is great!!
 

Thread Starter

zprater1

Joined Feb 28, 2018
3
The new battery needs to be the exact same voltage as the original.
Are you thinking that an 8 volt difference could harm the system?

A 40 volt battery seems really hard to find for some reason, everything I find seems to be 36v, 48v, or 50+ volts.
 

dendad

Joined Feb 20, 2016
3,259
The battery chemistry and number of cells needs to be the same if you want to use the existing charger. The Amp Hour rating can be bigger, it will take longer to charge.
You cannot for instance, use a LiOn charger on a lead acid battery or the other way around.
A LiOn battery charger needs to have a connection to each cell for charge control, and charge in the correct way to prevent a big bang.
 
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