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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?

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    Normally a replacement swappable battery would be the right answer. Charging a battery from a battery is losing some power in conversion, and charging isn't 100% efficient either. So you're right either increase the size of the battery, or hack in some way to swap the battery pack. Do you want to keep your warranty ?
    – Criggie ♦
    Commented Mar 22, 2020 at 12:39
  • Is it feasible for you to use the battery less on your ride - to ration it out a bit more severely? After all, you are the main motive source.
    – Criggie ♦
    Commented Mar 23, 2020 at 18:45
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    If you have to ask -- no.
    – Daniel R Hicks
    Commented Jun 16, 2020 at 12:53

4 Answers 4

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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.

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    It isn't feasible to swap the battery as it is built into the bicycle. Check out the video link below.
    – NoCo Rider
    Commented Mar 22, 2020 at 23:37
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    @NoCoRider: "It isn't feasible to swap the battery" -- be that as it may, this is the only answer posted that directly addresses the question you asked, which was "Is there any way to take those and charge my bike's internal battery?". As this answer explains, even if you could theoretically recharge your bike batteries with external, they won't be able to charge them fast enough to be practical (unless you don't mind sitting around for hours not being able to ride while you wait).
    – Peter Duniho
    Commented Mar 23, 2020 at 6:18
  • That is correct, I run the battery out faster than it can recharge, and I’ve seen the consequences first hand when the battery dies and I charge with my battery pack while riding. But with my normal usage, the battery dies in about 3 hours. It takes about 5 hours to charge. I reckon if I could keep it plugged in all the time with a battery pack that’s big enough, I could drain the original battery slowly enough as to give me 5 hours of continual use. The problem is that my battery pack is too small, and I can’t realistically keep it plugged in all the time, because it’s not dedicated
    – Christian
    Commented Mar 23, 2020 at 13:13
  • I meant 5 hours continuous, which might give me 7 or 8 hours total (dependent on breaks).
    – Christian
    Commented Mar 23, 2020 at 13:28
  • USB-C, which is the high power output of the OP's battery pack, has a maximum output of 20V and 5A (100W). In order to charge the battery he MAY be able to use a large 36V battery d is plugged into the charge port of the ebike. To determine if this is feasible would require a schematic. Because these batteries have so much power one cannot just connect two of them together as this could create a very unsafe condition as described in the video.
    – NoCo Rider
    Commented Mar 26, 2020 at 6:04
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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!

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  • I have seen that video, but my knowledge of electronics is limited at best. I’d be quite afraid of ruining the bike...maybe if I can’t find another solution. Thank you for the answer!
    – Christian
    Commented Mar 23, 2020 at 13:17
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    @Christian If you're uncomfortable with wiring, it may work out best to pass your current bike onto someone else, and replace it with something that has the features you want. A replaceable battery.
    – Criggie ♦
    Commented Mar 23, 2020 at 18:44
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    @Christian, you might find an ebike shop which is willing to make the modification for you. The modifications are very straight forward for a person familiar with electronics. The key safety precaution is in the initial connection. If the two batteries are at a different voltage there can be a very high current which could overheat the wires starting a fire or even cause the a battery to explode. Once the batteries are connected then they will charge and discharge in parallel (at the same time).
    – NoCo Rider
    Commented Mar 30, 2020 at 17:07
  • You could also use the batteries separately (i.e. not connect them together), and switch batteries by swapping an external plug. In your linked video, I don't think the bag is robust enough to handle the weight of the battery, it could "rotate" to a side of the frame; I would at least add some straps to secure it. Finally, if you are still not confident to make such "dual battery" modification, you most suitable option would be to get a similar ebike but with removable battery.
    – Gras Double
    Commented Jul 29, 2021 at 3:18
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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.

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    "and find a method to connect that battery in parallel to the original." -- I think it unlikely there's a practical way to do that. Lithium rechargeable batteries used in bikes aren't just simple batteries with a positive and negative terminal. They include other contacts for battery management, and the motor/charge controller has to be programmed specifically to deal with whatever battery you've attached to it. I don't see any useful value in the suggestion here. :(
    – Peter Duniho
    Commented Mar 23, 2020 at 6:16
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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.

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