I don't want to make a battery pack but...

[englishvalencia]

I have a few of these left over from a solar project...

http://www.ebay.co.uk/itm/Super-Rechargeable-Protable-Li-ion-Battery-DC-12V-6800mAh-Lithium-ion-EU-Plug-/351353498255?pt=LH_DefaultDomain_3&hash=item51ce4cea8f

Is there any reason why I shouldnt buy a few more and put them in serial and parrallel (2S4P, for example) to give me a battery for my bike?

It seems that these 12v units have a maximum current of 1-2A. That's my main concern. Presumably, if I have enough of these, they will produce enough current to power my 250W motor at 24V.

I would charge each unit separately.

Should I have any concerns about doing this? Given that I already have a few of these batteries, it would save me a few quid to make up a battery this way. I"m not experienced enough to make a 24V Lithium battery myself from scratch and would like to avoid the huge cost of a dedicated ebike battery.

 

[Fordulike]

I have a few of these left over from a solar project...

http://www.ebay.co.uk/itm/Super-Rechargeable-Protable-Li-ion-Battery-DC-12V-6800mAh-Lithium-ion-EU-Plug-/351353498255?pt=LH_DefaultDomain_3&hash=item51ce4cea8f

Is there any reason why I shouldnt buy a few more and put them in serial and parrallel (2S4P, for example) to give me a battery for my bike?

It seems that these 12v units have a maximum current of 1-2A. That's my main concern. Presumably, if I have enough of these, they will produce enough current to power my 250W motor at 24V.

I would charge each unit separately.

Should I have any concerns about doing this? Given that I already have a few of these batteries, it would save me a few quid to make up a battery this way. I"m not experienced enough to make a 24V Lithium battery myself from scratch and would like to avoid the huge cost of a dedicated ebike battery.

I think you're right to be concerned about the low current output of these batteries.
You would need to use at least ten batteries in series parallel to provide adequate power and current for a low powered controller.

BTW, what is the current rating of your controller?

 

[jerrysimon]

At 24v it will draw higher current than 36v. I would guess these would lag (voltage would drop too much) under the required current drawn.

With 4P you will get 4-8A max which will almost certainly not be enough current for a 250W motor running at 24v.

Just using the formula P = VI works out at 10A and in practice your 250W motor is likely to draw a lot more under heavy load.


Regards

Jerry

 

[Fordulike]

I forgot to mention what Jerry kindly stated, the voltage sag would be horrendous

 

[englishvalencia]

I am grateful fir the advice.

The problem is that I know nothing about my bike, other than it has a 250W motor at 24V.

I've already used 2 of the avove units in series to give me 24V, just to test my bike (I bought it as not working on Ebay last week). Everything worked and I was so happy. The lights came on and the motor spinned in accordance with the throttle pressure. Given the maximum current of these battery units, I can't get the motor to keep spinning when weight is applied.

I'm curious - would putting 2 drill batteries like these...

http://www.ebay.co.uk/itm/191558541387?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

in series work for giving me enough current? I'm assuming that these types of batteries allow for much more current, given that they have to power drill motors. The advantage of these batteries is that I can charge them with a dedicated charger. I know they have small capacity but I could also buy more as I went along. Any thoughts?

 

[Deleted member 4366]

Those first batteries are totally inappropriate for an electric bicycle. They have three low quality 18650 cells in them. For a 24v ebike battery, you need 28 good quality 18650 cells or 35 ordinary ones plus an appropriate BMS. It would be much better to buy a proper ready-made 24v battery.

The drill batteries are not much better. You' d need about 8 of then to get enough power to get your bike rolling.

 

[jerrysimon]

In theory yes, if the batteries can supply the higher current draw. In practice the packs probably have their own cutout so once the current gets to a certain point they will cut off.

Many people, myself included, have used cells from these type of drill packs (though I don't know about those, seem too cheap) which can be sourced separately or extracted from the drill packs to build your own battery.

I did this because I wanted smaller, lighter, battery packs 3-5Ah or less because the distance I commute does not warrant a large heavy 10Ah + battery.

However if you need 10Ah or more its just so much easier to buy an off the shelf battery with built in BMS etc.

Edit ..Dave was reading my mind.

Regards

Jerry

 

[Fordulike]

I'm curious - would putting 2 drill batteries like these...

http://www.ebay.co.uk/itm/191558541387?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

in series work for giving me enough current? I'm assuming that these types of batteries allow for much more current, given that they have to power drill motors. The advantage of these batteries is that I can charge them with a dedicated charger. I know they have small capacity but I could also buy more as I went along. Any thoughts?

Still wouldn't be adequate for your bike. The bike will run, but you will get the same problem of it cutting out under load, and even by some miracle it did work, the usable range would be pitiful.

Sorry to be the bringer of bad news, but you should really try to look into purchasing an electric bike specific battery

 

[Fordulike]

If funds are tight, would you consider going the SLA battery route?

 

[englishvalencia]

Yes, the batteries I removed (dead) were SLAs so that's the original battery chemistry for my bike. I may just replace them like-for-like. They weigh so much, though. I was just looking for a lighter solution.

 

[Fordulike]

Yes, the batteries I removed (dead) were SLAs so that's the original battery chemistry for my bike. I may just replace them like-for-like. They weigh so much, though. I was just looking for a lighter solution.

Depending on when your batteries were manufactured, you may be able to purchase some more modern, slightly better performing SLA's.

I don't think they have got any lighter though, but somebody here might be able to give you an insight on any advances in SLA technology