Extending 36/48V from eBike charger to eBike on external bike rack

Probably.

Start with the easy stuff... what's the current and voltage of the mains wall wart?

And also do we know how much of the charging system is in the charger and how much in the bike/battery pack?

The worry is that the charger might try to sense the battery voltage, unlikely, but if did then increasing the resistance of the connection would upset it.

(My 36V bike charger is rated 36V 2A but its open-circuit output is 41.5V.)

nib

Chinese scooter and ebike power brick chargers are usually Constant Current/Constant Voltage supplies. ie yours supplies 41.5V (=4.15V per cell in a 10S pack, slightly under the 100% voltage of 4.2V for extra lifetime) if you take less than 2A, but if you try to take more than 2A the voltage drops off. This follows common lithium ion charging practice, where you charge at rated current until the battery starts to reach the termination voltage and then tail off the current.

If you have a substantial resistance, this causes undercharging. Let's say there's a 0.5 ohm resistance in the cable. The charger reaches 41V at 2A, but there's a 1V drop in the cable so the cells are only seeing 40V. The charger drops the current to maybe 0.5A, so now there's only a 0.25V drop in the cable and the cells see 40.75V. Effectively what'll happen is the cells spend a long time trickle charging at the top of the curve, with the current gradually dropping. This will make charge termination take much longer. It probably won't fry anything, although it's possible any management controller gets confused.

So I wouldn't do it. Either extend the mains side of things, or work out some other way to provide a CC/CV supply close to the battery (eg you might be able to find a CC/CV boost converter and run it from a 12V lead acid).

I'm assuming it *is* a Chinese ebike, and not some fancy Bosch thing with a proprietary charger?

Theo

The basic charging process is here. CC-CV-Off

LFP chemistry is a bit more forgiving, and is used in fairy lamps. But it also has about half the energy storage. Still, they are using it in some cars (China and to a small extent, USA). LFP can take more "abuse". Whereas the kind of Lithium cells in a laptop, they're easy to piss off. That's why we have precision chargers, with precise voltage thresholds to limit damage to the batteries. And damage to you.

The Lithium ion have a "recommended charging temperature" and an "acceptable charging temperature range". If you've just come back from a throttle-wide-open ride, the batteries hidden inside the bicycle can be quite warm. You would allow an hour for them to cool off, before plugging in the charger. (It helps if the bicycle LCD display, lists a pack temperature parameter.)

Lithium does not like operation below 0C, so at the end of the cycling season you would bring the bicycle in the house.

They also don't like elevated temperatures, like when not being used. Even sitting a BEV in the sun in a tropical climate, is thought to "wear" on the batteries a bit. (It may cause the tape used inside, to dissolve in the electrolyte.)

As an eBike operator, it pays to learn the "lore" of Lithium Ion batteries, so you do not intentionally make them angry and spout fire.

I would only buy an eBike with a key-locked removable pack. No "battery-in-frame" designs for me. You want better access to the battery pack, so you can "evaluate" it easier.

One advantage of the key-locked packs, is you can buy spares. But this also encourages price gouging. One bicycle, the spare pack, the price was set to double what the normal price per kWh should be. While a key-locked pack, you could stock a spare, it's not the best usage of money from a raw power storage perspective.

Paul

It is a Specialized bike with their "own" motor, battery connector, and charger. So not a fancy Bosch thing but probably similar.

Cheers

Dave R

Neither 12V options seem appealing - just adding more cost and complexity than required - plus extra conversions will waste more power.

You could run 240V to the destination - terminate inside a large waterproof enclosure, that can house the charger as well.

Pick a large enough CSA cable[1] and there ought not be much to worry about. If you used some 4mm^2 flex, then that would add 9.2 mOhms/m round trip resistance. So say you added 3m that gives you , and the charger is running at 5A, that would drop 5 * 0.0276 = ~0.14V, which I can't see having much impact on a 36V or 48V supply.

You can get butyl flex in 4 and 6mm^2, e.g:

Charge times might not be that long - a 15Ah battery is toward the larger end for a ebike - so it may change in 2 to 3 hours anyway.