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Swapping out a lead acid electric bike battery for a Lithium Ion one. Answered

Hi folks.

I have a fairly old but still quite serviceable Thompson Euro Classic electric bike that runs on a 36v lead acid electric battery pack with a brushless motor.

I've just been offered a 36v lithium ion bike battery that I would love to use but I'm not too sure if it would be the right choice, my main reason for the change is simply weight consideration, the current battery pack comprises of three 12v batteries and weighs in at a hefty 28lb the replacement weighs a little under 4lb, this would very obviously make quite a difference in the bikes 87lb overall weight.

However weight isn't the only consideration, I would like to extend the range of my bike and with a little work and some suitable wiring and a couple of switches I could quite easily use the battery as a second power source for longer rides, I fitted a large plastic  tool box onto a rear carrier when I first bought the bike which would easily accommodate the battery or I could possibly even add an external carrier for it behind the seat, the current battery is good for around fifteen to twenty miles depending on how many hills I hit & how much I feel like pedalling, I hoping that an additional battery would double the range meaning I could rely on it to visit nearby towns without having to worry about finding somewhere I can give the battery a booster charge.

My main worry is about damage to the motor, I know very little about electric bike motors and having read so many stories about these batteries suffering from overheating problems and even catching fire I am more than a little concerned I could damage to my only transport, I usually ride motorcycles but thanks to a couple of health issues I'm currently grounded so apart from walking which can be pretty limiting this bike is my only way to get around without spending a fortune on public transport

I'm looking for advice from people who know about electric bikes and their motors who could give me some simple advice in layman's terms on how to proceed, please don't blind me with all sorts of technical jargon as it really wouldn't help, I'm not stupid it just isn't my field of expertise.

Thanks in advance for your help.


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4 years ago

I'm not good at it but I'll try to keep it simple -- Make sure that your Lithium pack has a Battery Management System -- it's a board that comes with some packs and it makes sure they don't catch fire.

If you don't know if it has one, or know that it doesn't have one, I'd advise not to buy it. Check out what happens when they're upset :

I'm working in a similar conversion (link at the bottom) and I learnt with it that as long as you get it with a voltage similar to your old Lead Acid pack, you should be fine.

The controller input is a range, and if you're within this range you should be fine. My bike was rated 60v but it was really 66v charged and 55 discharged. Now my Lithium pack is 67.2v charged and 59 discharged. Same range. I'm fine.

If you're not fixed on that deal I'd recommend finding a battery pack with BMS such as this :


This is a bit higher voltage but a good looking pack :


My Ible with more info on my conversion :


Reply 6 months ago

Thanks for the great info. I have a 48v 1000w fat tire mongoose ebike that I love but the lead-acid batteries are going fast. For many reasons, the most being weight, I want to replace w a li-ion type.
Since you are an expert, can I ask u some questions? If so:
Does the one above you recommended from ebay have a battery mgt system? If so, where does it say that?
With my 48v 1000w lead battery what would you recommend a lithium battery...48v 1000w??
What is AH and do I need to care about that?
Mic from Michigan


4 years ago

I have done a few mobility scooter "transformations" to get rid of the old style batteries.
In terms of range a change to a Li-Ion battery of the same voltage is always a plus -even if it just for weight you saved.
The problems however are not just limited to changing to a suitable (and usually costly) charger.
Li-Ion batteries don't like to be discharged below a certain level, which can happen if you have someone at the wheels who can't walk more than a few meters....
As you still have two legs to work the bike I strongly suggest to put a battery monitor in the thing.
Best to use one that automatically cuts the battery off once a certain voltage is reached.
Li-Ion batteries are not even close to what proper deep cycle batteries allow and it will be costly to replace them.
After two premature fails I installed battery cut off's that I set to about 0.5V over the rated limit of the installed battery.
Of course I got a few rescue calls after that and for some people I additionally instally a LED battery gauge that was calibrated to show yellow at about 60% charge level and red with a little to spare to anyone home.


4 years ago

Ah, I wouldn't worry about frying the motor. If the controller and the motor remains the same, as long as you put the same voltage in the input, the system should behave as before.

If you have a 36v pack that means 40v charged, as Lead Acids top at 13.4v. The Lipo pack I recommended is 45v charged it seems, so you'd be gambling with that pack.

You could probably find lower voltages though, or verify in your controller what the input range is. Bad news is that most chinese controllers don't specify...


4 years ago

Couple of things to look at on the batteries:

Amperage. I suspect that the motor will be the limiting factor on current, but I could be wrong. Google the terms Volts, amps, and resistance* for a fairly simple understanding. V/R=A .

True voltage- with different chemistry, there are slight differences in V.

Assuming that the resistance of the motor determines current, (amps) And that the true voltage is pretty close, then the light battery should be fine.

*somebody will probably note that a motor also has impedance, etc, but I think we can think in terms of resistance for now. Close enough.

If it'll help, just post all the numbers from data plates on batteries and motor. We can help from those #s.