Can I build an EV bike battery using Prius HV modules?

May 22, 2022
Hello I just bought an electric bike conversion kit (without the battery) and I want to know if I can save $250 by just building my own battery instead. I have 28 spare Gen 3 modules because I did a repair and replaced them all.

The kit that I bought as shown in the file attachment, is for 36v at 450w. If each module is 8v fully charged and I connect four then I have ~32v so it seems fitting. But aside from that I don't know if I'd be able to connect it to my kit somehow using a BMS (Battery Management System) and any other wiring I would need. Most tutorials online are of Lithium batteries and I haven't seen one of NiHM like what the prius modules are. I saw reviews on the page where people were powering the kit up with 12v motorcycle batteries so maybe there is some versatility.

Have any of you ever done something like this?


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You'll need 5 modules, which if I'm not mistaken will be pretty big and heavy.

That kit is nasty (not in a good way). Brushed motors are bunk; rag joint sprockets are atrocities. If your time and your results per effort are worth anything, that kit will end up much more expensive than a simple plug and play hub motor kit like Ebikeling.
I'm on a budget so $150 seemed OK to me for this kit. If I do decide to go through with it could you help me with educating me on how to wire it up? The alternative is refusing it from Amazon when it gets to me tomorrow and getting my refund and then buying a gas engine for the same price. I'm in the rural desert and that was my first option but my bike frame doesn't fit one so I went with this. But coincidentally I was gifted a frame that would fit it, yesterday
greenteaoolong said:
The kit that I bought as shown in the file attachment, is for 36v at 450w. If each module is 8v fully charged and I connect four then I have ~32v so it seems fitting. But aside from that I don't know if I'd be able to connect it to my kit somehow using a BMS (Battery Management System) and any other wiring I would need. Most tutorials online are of Lithium batteries and I haven't seen one of NiHM like what the prius modules are.
You can use any chemistry you like, as long as it can supply the current required by the controller (for 450w at 36v, that/s 450/36=12.5A), *and* it has the correct voltage range required by the controller (for a 36v controller, that's about 42v full charge, and about 28-30v empty; the controller may state it's actual range on it's label; not all do).

You'll need five sticks in series, I think, to come closest to meeting the voltage range. I don't know what current yours are capable of.

You must also use the correct charger for the chemistry you're using--NiMH uses a specific kind of charging method. Since you won't be using the Prius BMS with it, the typical NiMH charger balances them by overcharging them a little, and the cells then *drop* in voltage and create heat with that wasted energy. The charger stops charging by detecting the voltage drop, and/or the rise in temperature. There are other charging methods for NiMH, but that's the common "small pack" method to self-balance it.

I have not seen a small-pack NiMH BMS; they may exist but you'd have to poke around. A BMS for other chemistries will not work.

If the cells don't get out of balance during use, you don't need the balancing charge method, and can use a charger of the correct current limit for your cells (I don't know what that is, but there are old threads about the Prius NiMH sticks around here that probably have it), that is of the correct voltage for the number of cells you have in series x 1.2v - 1.4v (usually the 1.4v is the "overcharge - self balance" version).

I recommend against paralleling any of the sticks (for more range or more current), because of the way the chemistry works.

Charging them in parallel doesn't work because of the voltage-drop-when-full behavior of this chemistry--the first one to fill up will drop in voltage and start not only taking all the charge current, but begin draining from the other sticks in parallel, and will rapidly heat up, possibly causing a fire.

Discharge in parallel ddoesnt' have that specific risk, but if you forget and leave them in parallel when you charge...see above. There's some other risk for discharge I forget (been too long since I used these) but there is at least one thread about NiMH cells that covers it, possibly by Fechter.
Regarding the kit, it's probably better than the typical gasoline engine kits; I see at least one rider with one of those on the side of the road in my area trying to fix something about the drive every week or two. I don't know what percentage of riders that is, but I only hear them a couple times a month.

Any of the sprockets that clamps to your spokes is likely to be hard on the wheel; you may want to keep a spare one around. A couple of the roadside-repair bikes Iv'e seen were trying to fix their wheel (though I couldn't see exactly what they were doing, it wasn't tire-related and appeared to be something about the sprocket or spokes).

(I hardly ever have time to stop and help anyone roadside in the very few places I would ever trust being able to do so; most places they're stopped I would never willingly stop there; too much risk of bad people doing bad things).
Now, before you decide which thing to build:

What kind of range do you need?

At what speed?

Under what riding conditions?

Those are going to primarily determine whether any of these things are practical for your needs.
Woah this is all very informative. So the kind of riding conditions I'll be taking on is rural desert for miles and miles and many miles out here in the Mojave. There are many offroad vehicles and dirt roads. There are dirt bike tracks which I won't be going on. There's a lot of cross country open country bumpy road. There are some nearby mountains and hills as well. I'm starting to think more and more I should go with gas engine.

I need as much range as possible. A noisy engine isn't a problem. Amazon delivers my package tomorrow and I can still refuse it and get my refund. I'm thinking if ordering an engine tonight before I go to bed. I have an option from Walmart website for $130 or an online shop in Los Angeles for $170 but it comes with 6 month warranty. What do you think I should do.

I was also told on PriusChat by some experts that these batteries will discharge very fast and give poor performance. I would have to buy a $250 battery from amazon.
Discharge rate of the batteries you can't really know until you know their capacity (assume they are half their original capacity, and just grin at any extra you get), *and* the actual power usage of your system in your actual conditions (which we can guesstimate, but it requires numbers from you to do this).

If you use small batteries at a very low rate, they discharge slowly and can last a long time. If you use them at a high rate, they discharge quickly and last much less time.

I doubt that for your broadly-stated range goals and conditions that a $250 battery would get you anywhere near what you want.

To help you figure out how much battery (or whatever) you'd need, you would need to tell us the actual range you need, at the actual speed you need that range at.

Wherever there is anything other than perfectly flat roads, you also need to know the slopes, so you can figure out how much power it really takes to go somewhere at a certain speed. Note also that rough roads take more power, as do sandy conditions or other conditions where tires sink in instead of staying on the surface. Anything where the tire doesn't just push you along the surface, but instead has to move some of the surface out of the way, will take more power. How much more...depends on the surface and the vehicle/rider weight and the speed and the tire type/size/etc.

If there are winds, then they add to your speed when riding into them, and subtract when riding with them, and crosswinds will probably add to your speed in one way or another. A 5mph headwind means that riding at 10mph is actually taking the same power as riding at 15mph. A 20mph headwind means riding at 20mph is actually like riding at 40--it will take so much more power you probably won't be able to maintain your speed.

So there are quite a few things that affect power usage, and that all affects range. It also affects whether a given system can even do the job you may ask of it--it might take a lot more power than that little motor system, or the little gas engines, can deliver.

With numbers, a generic wh/mile figure can be guesstimated, which then lets you figure out how much battery it will take to go a certain number of miles at those speeds under those conditions.

Without numbers for speed or desired range or slopes/etc, I could only guess that for "miles and miles and many miles" and "as much range as possible" you would need a huge trailer full of battery or a trailer full of generator to keep the small one constantly charging (or directly powering the controller via the generator). I'd guess it will

This doesn't include going up hills or mountains; I doubt the motor could handle much of that, with the trailer full of battery or generator.

I don't know how well those little "chinagirl" style gas engines can do that either; or how reliable they'll be in the dusty and dirty conditions you'll have there, especially if it's not on smooth roads (they seem bad enough here in the city in Phoenix from my limited observations).

I guess the good news is that it's still a bicycle so even if the motor fails or runs out of battery (or gas) you can still pedal it. :)

If you want to see how the range vs speed vs conditions / etc thing works, you can play with the simulator at , and the trip simulator linked from there.

The same thing applies to the gas engine; it's just not simulated there, and you'd need to use a conversion page to see how much gas is equivalent to a wh of battery.

You can guesstimate that a 1hp gas engine will be about equal to a 750w electric motor system (but they aren't necessarily equal because it depends on how each is driving the wheel and the way it's being used, and under what conditions).
I have a number of Nisan Leaf modules and looked long and hard at putting them onto my ebike, the only way I could see to do it was by putting the battery on a trailer.
On my freighter trike, I put 5 of them in the cargo area and they work fine, weight is not a problem for the three wheeler. Have no idea what the range is, the human/saddle interface is the limit for range. Amberwolf suggested that I borrow his idea and mount a couch/bed. <grin>
PS. As others have said here, you will need to use 5 modules. I run my battery from 39.5 volts to 37 volts, and the motors seem quite happy.
I had also assumed the OP was talking about Leaf modules, but they're Prius modules which means NiMH chemistry. That's lower power density and higher self-discharge rate than the lithium cells we are used to.
Seems like you’re heading in the wrong direction. I would take chalos advice and get rid of that kit that you bought from Amazon. So you ended up with one of the worst options you could get for an E bike conversion and now you want to order a gasoline engine before you go to sleep? I think you should consider slowing down and contemplate your options before buying the wrong thing again. If you want to spend a lot of time working on your bike then get a gasoline engine. I would suggest getting a hub motor set up that’s virtually maintenance free.