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Thread: EV conversion to a 1950's French lightweight motorcycle: advice please

              
   
   
  1. #1
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    EV conversion to a 1950's French lightweight motorcycle: advice please

    Hello,
    Name is Graham and I'm from England, a petrol head and just retired!
    I have restored a 1950 French Terrot MT1, a 2 stroke bike of 100 cc, 3 speed gearbox. Having huge issues with a non running engine, so thinking electric.

    Thoughts are a 1800 Watt motor in the position of the existing engine using the chain drive to the original rear sprocket, twist grip control with 48V Lipo batteries.
    I have found such a kit, less batteries on ebay, 106 which seems good to me, but it is the battery spec that foxes me, hence this thread.

    What battery spec should I be looking for please?

    I would like 30 miles range of country lane riding.
    Hope you can help me.
    Graham.
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  2. #2
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    Howdy Graham,

    That's a stunning like machine right there!
    No doubt the anoraks will have a few words to say about defiling it in such a fashion.

    1.8kW is only about 2.5hp equivalent, are you sure that will suffice?
    If it's the kit I'm thinking of then it's the 4500rpm brushed DC motor setup; I've not seen one used on here and know very little about them but you'll want to consider your gearing carefully.
    You don't want to be straining the motor at low speed so you'll have to gear for a compromise between your desired top speed and the capability of the motor in question.

    Funds willing, I'd be more inclined to take a look at a hub motor and dispense with the chain altogether.
    This will leave you with less maintenance, less noise, and a lot more space within the frame for your pack, controller and charger.

    The tougher challenge will be building a pack to replace the 'showpiece' of the bike which won't look... well, a bit shit really.
    For 50km you'll want about 5kWh as a ballpark, which you should expect to run you about 1500 or so.
    At 48V, 3No of the 40kWh Leaf modules would do the trick most readily if you can get your hands on them - they'd actually fit quite nicely in that space and could be dressed up relatively easily as well.
    That would give you a pack with a nominal voltage of 44.8V, 4.98kWh-rated, and happy to supply ~200A which is way more than you'll ever need with that motor.
    Naturally there are countless other battery options outside of the above, but for someone on their first forey into such things, it's hard to beat the Leaf packs for convenience.

    You'll be well advised to pick up a small BMS to look after them for you - there are plenty of options around for that voltage level from set-and-forget arrangements at the top end, right down to manual monitoring at the bottom, depending on your tastes and capabilities.
    You'll need a charger as well - the Meanwell units are the most popular at that voltage level AFAIK and are fairly readily available.

  3. #3
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    Thank your for your detailed reply! There are just 3 of these bikes in the UK, quite unknown here, and hated by the French!

    Bought nothing yet and still mulling over what to get.
    The original engine was 3 bhp with 3 speed box.

    You have made me think...the kit is a brused DC motor with a very small primary gear to it which makes me wonder if it is right.
    If I use a hub motor with custom lacing to the 19" x 2" rim I can swap it out for the restored wheel BUT keep te original engine ( and obv no chain) which would keep the looks.

    There is room on the rear pannier for the controller etc and I hope the batteries.

    It is the battery pack that bothers me the most in all this. Do your really mean 1500 Euros?

    Time to hit ebay again!
    Thank you for this help!
    Graham.

  4. #4
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    Ah yeah, appropriate gearing assumed, a 2.5HP equivalent electric motor will easily surpass a 3HP engine on performance terms alright, so no concern there.

    The market price for 1kWh of traction capable batteries at the moment is ~300.
    This isn't to say you can't do better, merely a guide as to where you should draw the line between reasonable and unreasonable pricing.
    For example, I just picked up a complete 40kWh Nissan Leaf traction pack for 4500 on Monday, which puts the price/kWh in that instance at a touch over 100/kwh, so a complete bargain.
    I'm not breaking that particular pack however as I'm refitting it to my car (so you're outta luck on that one I'm afraid), buy buying big, or complete, will generally get you better pricing.
    Bottom line, the batteries are the most expensive component of any EV - this is the principal reason why the Hyundai Kona, a 22k car in base spec, is an ~40k electric car!

    As to getting away with the conversion without removing the motor - that would require some substantial paniers!
    Lithium batteries are state-of-the-art at ~280Wh/L meaning that for 5kWh's you should expect to require at least 20L of storage space for the battery alone (so think 5 gallon drum volume at a minimum)
    The charger, motor controller, BMS (recommended) and sundry associated electronics will easily take up another bit on top of that.


    NOW - all of the above being said, I get the impression that this bike won't be expected to travel all that fast, so my baseline assumption of 10km/kWh may be somewhat punitive as that's a guideline for an average speed of ~100km/h; so lets look at it another way.

    1.5kW peak, and 48V supply, means that it'll use about 40A at max draw allowing a bit for various inefficiencies. So discounting any speed association, if we assume max power as the norm (worst case scenario), then a 40Ah pack will last 1hour of constant use. A 40Ah pack at 48V is ~2kWh.

    Hence the question now becomes, in riding the bike regularly as it is, how much of your time do you spend at >half throttle? (ie: how often are you using a significant amount of the available power)
    If you spend most of your time at fairly heavy throttle, then the above will give you a good indication of the endurance you should expect from any given battery size; if you tend to do a lot more coasting or light throttle riding, then you'll do better.


    There are lots and lots of different ways to make these sorts of things work, and I could easily harp on about the bits and piece that are worth including, but if you're on a minimal budget, the priority will be get the thing to move. With that in mind, I'd be inclined to think long and carefully about your motor and controller, as that's the bit that will stay with you - batteries will come and go so don't worry so much about that. Personally I'd take whatever time I needed to get the funds for a decent hub motor kit (motor and controller) and have that integrated, as you propose up top, to a refurbished wheel. It'll be the most discrete and space effective option, and at the speeds you'll be doing, the handling issues which can come with that sort of unsprung mass won't be of any concern. It will also remove chain noise from the experience, which is well worth doing in any EV conversion IMO. Furthermore, many of those kits include a small 48V battery pack - a little bigger than a 1L tetra pack carton. They aren't cheap per-say, but they are swappable, scalable, and have an off-board charger which means one less thing to worry about in your build (something like this for example, although make sure you don't have to be able to pedal to get it to start as can be the case with some of these kits as they're ostensibly for "assist" purposes).

    For very limited duty, these batteries would fit in some reasonably sized panniers - Although I don't know what sort of range you'd get. Based on the numbers above, that 17Ah battery would be good for about 20mins at full throttle, but you could always buy/build a bigger one later.

    In any case, just some food for thought - but a good example of a well integrated solution off the shelf and something that might help you preserve the aesthetic to some degree at least. I'd still be inclined to dump the engine mind, even if just to remove the additional weight load on the motor which will have a fair bearing on the current required for starting and acceleration. If you look at the styling of a lot of the more hipster EV offerings on the market at present, they're broadly vintage styled with a faired in engine bay so I wouldn't discount getting creative with some fiberglass and making tasteful covers for a much bigger powerplant either.

    But as I said, the first priority is to make it move!

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