Suggestions for an electric bike pack

[Elektrobot]

I'm considering taking on a project of installing an electric conversion kit on a bicycle, and would like to make my own battery pack.

The brushless motor I plan on using is rated 36V 500 Watts

The motor controller is rated 36V 22A (+/- 10% or so is fine, it can take it)

I'd like something with at least 10Ah total- but more than 15Ah is not needed.

Rank of important factors are:

1. Price
2. Weight
3. ....have I mentioned price?


If I could keep the weight of the entire pack under 25 pounds I'd be thrilled- but its looking like around 30 is a more realistic goal at the price point I'm willing to spend. I plan on packing them in a plastic project case and zipping it up in a duffle that attaches to a rack mounted above the real wheel.

Do any manufactures pack their SLA's with helium? :thinking:

Any tips on finding lowest cost/lowest weight solution would be appreciated.

 

[HarryN]

Hi - neat project. 36 volts takes you into expensive territory fast for charging if you plan to actualy charge at 36 volts. You can save a lot of money if you can break the pack voltage in 1/2 or 1/3. That puts it into Triton II territory.

22 amps from a 10 AH pack so about 2 C - that is not terrible, but but still a pretty heavy drain for all but very decent cells.

If you want cells that can take abuse, then nothing beats SLA but I don't think it will do it for weight.

In NiMH, which is going to be the next "value" level, the r/c technology is mostly developed around the sub-C size, with a max around 5 AH, so this would require at least a D size cell to reach 10 AH. I am not sure about pulling 22 amps from a D size NiMH - you might have to go for a NiCd cell to do that. Certainly, 2 x sub C cells in NiMH can do this current and capacity.

I have 2 each R/C packs from cheapbatterypacks.com which can be used to see if that is in your budget or not. There are many good pack builders, they are just my example here for budget concepts.

Using their configurator for a nominal 18 volts, sub C, NiMH, 5 AH cells. http://www.cheapbatterypacks.com/tstuff/v7/buildcustompack.asp?sid=1511506

Then 18 volts nominal is 16 cells in series. The 5 AH cells I used were out of stock, but the 4 AH cells are $ 1 / each less so I chose them for an example. It is about $ 100 per pack x 4 (2S2P) = approx $ 400 for 10 AH at 36 volts.

Getting this in D cells for the 10 AH rating would likely save 10 - 20% on price but probably not much in weight. (2S1P vs 2S2P). (Thse guys do not offer D size packs, but someone can make them up for you.)

The higher end sub Cs weigh in at almost 3 oz / each. Assuming a 2S2P setup at 3 oz each that is 4 x 16 cells x 3 oz = 12 lbs. (approx)

Li Ion and LiPo packs are lighter but more money.

A decent charger and power supply will be pushing $ 200. You will want a decent charger to take care of the $ 400 worth of packs.

I hope you have a strong bike to mount all of this to. BTW - I would strongly consider mounting the pack up front - it is far easier and cheaper to fix the front end than the rear.

 

[HarryN]

Actually - here you go.

http://www.onlybatterypacks.com/showitem.asp?ItemID=11920.37

Just add heavier leads in checkout - maybe 13 guage wire. I have no idea about those particular cells but it is an example you can find with a search on "NiMH d size pack"

I know it is fun to solder / weld up your own pack, but I would not bother trying to weld these up myself. There are so many pack commercial and small pack builders who have already done the suffering for you. I am not sure, butIIRC, cpf'r LuxLuther makes packs ?

 

[Elektrobot]

Thanks for the advice and links!

As for 22 Amps being 2C for a 10 Ah pack, the motor running at 36V 500 Watts should be pulling about 14 Amps. I plan on pedaling the bike from a stop until I get up to speed in a few seconds- then use the electric motor so there won't be sudden huge current demands from the motor to get a mass moving. It will pretty much be used to keep an already moving object moving.

I will look into getting the system voltage down, but I'm not sure how the controller and brushless motor would respond.

I'm excited about this project- its something I should have tried a long time ago. I live in an area where I have a butcher shop, grocery store, drug store, hardware store, several restaurants and my small business all located within 4-6 blocks. I'm tired of jumping into the car for these short trips several times a day, but I don't really want to go on a "bike ride" every time I need to run an errand. I figure adding motor to my bike will let me use it without breaking to much of a sweat or making it a chore to use.

How much easier can transportation be than jumping on a bicycle and flipping a switch after pedaling a few times?

I hope it works out to be as good as it sounds!

 

[HarryN]

Hi - you don't need to drop the 36 volts to the motor and controller, just for charging. Example - use 3 series 12 volt packs (in series) during the "use", but when charging, disconnect them so they charge up individually.

The challenge is that only a very few chargers can charge 36 volt ( 32 cells in series) packs in one shot - and they tend to be much more expensive than ones that can charge up to even 24 volts.

Have you considered an electric scooter instead ?

 

[chew socks]

You could look into RC Car Lipo's....some of them can discharge at 20C or more. Hobbycity.com has some very nice prices. If multiple packs are hooked up in series and parallel you could get the right voltage and many amp hours.

 

[Norm]

I will look into getting the system voltage down, but I'm not sure how the controller and brushless motor would respond.

Most controllers have a low voltage cut-out my 48 Volt controller cuts out at 42 Volts to protect the batteries. Just be careful about motor ratings, my 350 Watt motor draws 17 Amps at 48 Volts = 816 Watts ?? Not too sure how they rate it as a 350 Watt motor? :huh2:
Or I could be getting false readings because the current is drawn at some pulsed frequency. I am no expert when it comes to understanding how these things work, although my controller does say current limited to 17 Amps and that is exactly what my flat out current is.

There are some very bright people around here and no doubt someone will be along shortly with the correct answer.

Runs on 4 X 12 V 7 Ah SLA's, lighter batteries would be a bonus, If you can keep your centre of gravity low SLA's may be OK, I use them because of the cost of lighter alternative.

500 Watts may be overkill depending on your weight, I'm only 60 kg and my top speed unassisted on the flat is around 40 KPH.

Just remember the lower your voltage the higher you current will be for the same output. I use a 24 volt charger to charge 2S 2p but run all batteries in series when the, bike is running i use parallel Anderson power pole connectors when charging and have a link inserted when I ride.

Norm