Can I charge an ebike from another ebike battery?

Question

I have a fiido d2s, and I love it. I'm not sure if I could upgrade to a bigger bike, because it fits so well in the back of my car and public transport. Anyway, it doesn't have a detachable/replaceable battery.

I've used this: https://www.amazon.co.uk/gp/product/B07TG8WB27/ref=ppx_yo_dt_b_asin_title_o09_s00 and the charger that came with it while out and about to extend my range. It basically takes DC power, converts to AC, and then converts back to DC. I believe the bike's battery is about 280 Wh and this gives me another 80 after some loss, give or take.

But I'd really love to double or 2.5x my range. I've looked up tutorials like this one:

, which end up with a battery that can charge a scooter (or ebike) using the DC input that comes with the bike for charging.

But getting enough batteries alone (aside from other parts and me probably screwing things up) costs almost as much as one of these: https://www.ebay.co.uk/itm/Bicycle-Ebike-lithium-li-ion-Battery-24V-36V-48V-10Ah-20Ah-1500W-350W-500W-1000W/174033645919 (not almost as much, but also not insignificant).

Some of the ebike batteries have 36v, 2a discharges which is the same as the charger that came with my bike. Is there any way to take those and charge my bike's internal battery?

Edit: I appreciate all the suggestions. For the record, I know that charging and riding at the same time will work. I do it with my 100 Wh battery pack already. And I know I use more electricity riding the bike than can be pumped into it. But I think if I could charge at the maximum (36v, 2amp) while riding the bike, I could extend the continuous riding time from 3 to 5 hours and maybe the total riding time to 7 or 8. I’m sure I could do this if I just bought a 300+ Watt-hour battery pack. Something like this: https://www.amazon.co.uk/dp/B07DVGYDL5/ref=cm_sw_r_cp_api_i_vTlEEbJ1GRQ9R

But I can get a lot more watt-hours for a lower price and in a friendlier shape with the ebike batteries, vs that thing.

I’m wondering more if there’s a simple process to connect the discharge port of the ebike battery to the dc input of the bike. Or does it need some sort of conversion?

Answer 1

Yes in theory but the gain is not practical. You’re better off swapping batteries than trying to jury rig something.

Let’s do the calculations.

You note that the AC charger puts out 36VDC at 2 amps. That’s 72 watts per hour.

Even a modest eBike has an 8Ah battery, for a total of 36x8 or 288 watt-hours.

At 72 watt-hours per hour charging it takes four hours to charge. So you’re trying to fill your “gas tank” with a very tiny spigot, so to speak.

Your internal batteries BMS charging may stop taking a charge well below the output of your external pack. My 36v bike for example wants to see 42v on its input. So you’d need a boost circuit.

Add to this charging inefficiencies which are usually on the order of 85-90% with each step and it’s not worth it.

So just get another spare pack and exchange when your empty. Or rebuild your current pack so it’s larger but scale the BMS accordingly.

Answer 2

Here is a video I found which is very detailed and shows how to add a second battery to your bike:

Video: How To Add 2nd Battery to the Fiido D2

Here's the link to the battery which was recommended a ways down in the comments section:

36V 10Ah Battery Supplier

In the video he explains that you are adding the second battery in parallel. Thus both batteries will charge and discharge at the same time.

Note the warning to FULLY CHARGE BOTH batteries BEFORE connecting them!

You're also on your own finding the connectors, shrink tube, wire, etc. Good luck!

Answer 3

Hmmmm...

IMO if you get a schematic drawing for the bike you should be able to purchase a 2nd battery and find a method to connect that battery in parallel to the original.

Having a second battery in parallel would mean both would charge & discharge at the same time. This should increase the life of both as heat from charging/discharging is a major cause of LiIon battery wear.

Regarding the idea of plugging the 2nd battery into the charge port, without seeing the schematic I don't agree this is inefficient but I am also concerned this may not be an option. The bike may have a direct connection from the charge port to the battery (read: no energy loss). If this is the case then you could have a 2nd battery pack with simply plugs in and your range is doubled. OR there could be electronics which would disable the battery while charging (say to prevent the batteries from over heating while charging). If this is the case, then plugging in the 2nd battery would literally disable the bike while it is charging.

If you can locate the schematic for the electrical system I'd be glad to walk through this.

Answer 4

My apologies if I have misunderstood the question, but connecting the output of a secondary battery to the charge input of the primary would not work "in the field" most, if not all such batteries include a BMS (battery management system) this limits the charge current, often not more than 2A (to avoid damage and fire etc). The primary battery output (when running the motor) should be in the region of 10A (36V 10A will provide 360W). In short, you'll be filling a bucket with a thimble. If you're adamant that you will run a secondary battery, you may find it beneficial to find the primary input on the controller, attach the second battery in parallel, et voilà, more range. If you need further support with this, I'll be happy to help, not to blow my name own trumpet but.. I have some silly letters after my name, mainly BEng.

I neglected to mention, a 36v ebike battery is 36v nominal, the charger is 42v continuous, so if you do charge using a second battery, you'll get charge when the battery is 42v to around 41v after that, the second battery will be sat doing nothing as the primary battery BMS won't even realise it's being charged.