3 scooter batteries in series: too good: 54 V!!!?

For 2 months now I have an Aldi (low end) e-bike. The Li-battery allows 25-35 km travel (depending on terrain and wind conditions) on max power assist. According to specs, the battery has a voltage of ca. 40 V when fully charged, going down to a cutoff point of ca. 35 V when empty.

For a relatively cheap 2nd battery solution  (to increase travel distance for another 20- 30 km) I put 3 brand new motorcycle batteries in series. Charged individually with a good (LIDL) charger, the batteries give a whopping 17.3 V, with a halogen light load15 V!!

I don't dare to hook this up to my bike! Of course I can bleed the 3 batteries to ca. 13.5 V to make it safe, but so much capacity is lost!

There has to be a common solution/ circuit: 55-41 V, ca.10 A, to a stable 40-36 V.

Any suggestions???

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Are you sure your voltage measurements are correct? I suspect you are using a digital multimeter with a flat battery, which will give incorrect (high) readings.
BobS (author)  liquidhandwash4 years ago
You are right!!!

A 'Elco' (similar to radioshack) store clerk told me the same thing. He had quite some experience modding ebikes: he told me that they usually can handle even double voltage, but under no circumstances reverse polarity. He also uses the scooter batteries, since he blew up his Lion battery by overcharging...He also told me that circumventing the 23 km/h speed limiter is real simple!

Since my 2 month old bike is still under warranty I'll have to wait doing this mod. Later I'll do this mod under his guidance, and if OK, I'll post the results
cie4 years ago
I picked up a 36V XLR scooter battery charger for Schwinn S600 S750 ST1000,GT GT750 Razor MX500 MX650 (Electronics) from Amazon for only $9.99 US. Works great on my electric quad's 36v battery. They offer a wide selection there if that helps. Can't beat the price. :)
frollard4 years ago
Do you take your lithium charger with you?

Will it take a DC input?
Will it take a variable DC input?

in so, use dc input from the motorcycle battery(ies) to charge the lithiums while they are being used...
It wouldn't be too hard to make a switcher for that kind of load. 50 odd V is commonly used in telecoms circuits, so there are loads of that kind of circuit out there.
PS you'd need to settle for 36 V output.