NiMH to Li-Ion Battery Conversion

[Astudillo]

Hey guy's,
My girlfriend got an old E-bike from her stepmom with an 7Ah NiMH battery pack that only does 5/6Km(we life in the flattest county on earth).
I saw this battery conversion as a fun new project. So I checked what voltage the pack should be, the charger said 24v(20cells x 1.2v). I also checked a few online sources that does the conversion and they sell 25.2V so I went for a 6S6P config.

Everything went fine, the house didn't went up in flames and installing the BMS was quite easy. So today we tested the new battery pack on the bike, and all worked well, except the gauge on the bike that has 3 leds for the battery indication was blinking on one led as if the battery was low on voltage. Witch was a little bit weird being fully charged at 25.2v. So I started reading a bit more about NiMH cells that 1.2v is the nominal voltage and the full charge is 1.5 volts witch makes the pack 30v instead of 24v.
Is this true? or are there NiMH cells that only do 1.2v?

I'm a little confused now if I should/can add a extra cell pack to make it 7S and 29.4v to get the full capacity of the pack.
The gauge say's the battery is almost empty on 25.2v but the cutoff voltage on the controller is 20v. so that should be midway between 20 and 30v.
How can I find out if the controller and motor on the bike can handle the voltage?
The bike is a Batavus Pharos.

Hope you guy's can help.

Cheers,

Raymond

 

[flippy]

you can go 7S no problem. you are right in the range of 7S lipo with the numbers given.

but i would recommend charging only to 4.1V (and use a BMS) so the pack lives as long as it can.

fun fact: they used the exact same controller for the lipo versions they sold later and they sucked really bad. the quality of the lipo cells was atrocious so you dont even see them secondhand.

 

[Astudillo]

Hi Flippy,

Thanks for the quick reply! I will make the pack 7S and of course use a BMS.

Cheers,

Raymond

 

[amberwolf]

BTW, the charger it came with is likely designed to shut off when the voltage *drops* (something that happens to NiMH when it reaches full charge), and/or if it has a third wire to the battery, when the pack reaches a certain temperature (or changes a certain amount of degrees in a certain amount of time). It's called Delta-V / Delta-T.


As long as you have a BMS that will shut off charging current when any cell reaches full, you could still use that charger for the Lithium pack. But if the BMS fails to shut off charging current for whatever reason, the NiMH charger may continue to supply current and overcharge the Lithium pack.


So you may want to find a charger that's designed for Lithium charging at the number of series cells you end up with.

 

[flippy]

As long as you have a BMS that will shut off charging current when any cell reaches full, you could still use that charger for the Lithium pack. But if the BMS fails to shut off charging current for whatever reason, the NiMH charger may continue to supply current and overcharge the Lithium pack.

trusting the bms to cutoff at overvoltage seems like a good way to burn your house down.

my guess is that raymond lives in the same flat country i do. i can recommend or sell the proper charger as i have a account at the mean well importer/dealer so i can get them fairly cheap.

 

[Astudillo]

Hey guy's,

Thanks for all the info! Originally I wanted to use the plug of the original charger, but the wires where so thin that I wasn't comfortable running 6 amps (spec sheet said 1.020mA for best lifecycle x 6P) through it. So on the battery side I drilled out the contact pins and glued a nice XT60 plug with some nice 16AWG wire, so no hot wires there. As a charger I use my iCharger 4010 Duo I use for my multicopter battery packs.

For use at home I think im fine with my current charger, but maybe its also smart to have a more portable chargers in the future. @Flippy any thoughts/advice on that? And as I think you are also living in the Netherlands do you know a good vendor of BMS's that ship fast to NL?

Cheers,

Raymond

 

[flippy]

dont run the max current. charge only as fast as you NEED to. not as fast as it can handle.

send me a message, i might be able to help.