LIPO question - but not for RC

[davew]

Hi, Hope this is OK on here but I think you RC guys here will be able to help better and talk in a language i understand better than the Bike forum I use. Anyway,

I have an electric bike, had it for about 8 years and is well used but good. It is powered by 3 off, heavy SLA batteries. These are connected in series to give the required 36v, 12A. The batteries are shot and I am exploring the idea of using LIPOS to both save weight and for experimentation purposes before I build a more radical kit based bike.

Lipo batteries are used in many modern e-bikes but are generally viewed as potentially dangerous and other technologies are often prefered.

I am proposing to purchase 4 off, 5s, 5000mah packs from HK. By wiring each pair in series to give 10s and then those pairs in parallel I would have 36v (nom) and 10ah capacity. With 15c rated cells the peak power requirement of approx 18amps draw should be easily achieved and life in terms of recharging cycles should be good.

For a start are the above statements true?

With RC models, battery management is generally taken care of by the ESC but on e-bikes a battery management system (BMS) is fitted within the battery compartment. This is where I am starting to struggle.

The BMS really does two things - it shuts down the motor when the voltage gets too low and on better systems also manages balancing of cells during the charging process. Manufacturers of cheaper e-bikes claim that "Lipos don't go out of balance" and leave out that facility!

I don't want to spend well over a hundred quid on batteries and mess it up so am asking for your advice!

Without opening up the battery case every time I charge the cells, pulling them out individually and balance charging on my RC charger what is the alternative? I could charge with a readily available "intelligent" 36v Lipo charger that are sold for these type of applications but I wouldn't be balancing. Should I plan for a mixture of the two methods even though it would be a lot of messing about?

What low voltage warning devices are available? Something simple such as a warning buzzer?

Any suggestions with wiring schematics etc would be gratefully received!

Thanks, Dave

[Steve]

PM xygax - he is God when it comes to all this electrickery stuff.

[Swashbuckler]

I would never charge lipos in situ no matter how much hassle it is to remove them; if something goes wrong with a pack or cell the damage will not be limited to the packs, unless of course you could make a fireproof battery container on the vehicle. I also would always use a quality lipo charger that balances, because even though the packs rarely go far out of balance having a charger that monitors individual cells will prevent a single cell from being overcharged which is where damage is done.
As far as a warning device I would use the Spartan lipo guard which is programmable and works up to 12s 44.4v nominal. It has a high intensity warning led that pulses normally and lights up solid during high current draw or when the voltage dips below the programmed level. 10S will give 37v nominal as you said. A data logger might be a good investment for you too. A great tool when experimenting with power setups. Good luck.

[GravityKills]

>> Lipos don't go out of balance

That doesn't hold for RC LiPos. Please search the web for "LiPo fire" to get an idea what you're dealing with. They burn like fireworks.

Putting 5 Ah packs in parallel can be dangerous:
- one cell fails
- the pack is 4 V short
- the other pack reverse-discharges into the failed pack

That aside, I think the bike will run.
But you must prevent deep discharge of LiPos. It causes internal damage that is irreversible and increases the risk of having a fire later.

PS: Just to give you some idea: Those packs are about the size you're considering (probably 2x 6s 5Ah, but in the video one pack survives).
The smoke is pretty corrosive: (Lithium fluorophosphate on Wikipedia: "It is very destructive to mucous membranes")
If this happens in a place where people live / sleep, good night...
[ame]http://www.youtube.com/watch?v=vaWVovBHcTc[/ame]

[xygax]

Hi, Hope this is OK on here but I think you RC guys here will be able to help better and talk in a language i understand better than the Bike forum I use. Anyway,

I have an electric bike, had it for about 8 years and is well used but good. It is powered by 3 off, heavy SLA batteries. These are connected in series to give the required 36v, 12A. The batteries are shot and I am exploring the idea of using LIPOS to both save weight and for experimentation purposes before I build a more radical kit based bike.

Lipo batteries are used in many modern e-bikes but are generally viewed as potentially dangerous and other technologies are often prefered.

I am proposing to purchase 4 off, 5s, 5000mah packs from HK. By wiring each pair in series to give 10s and then those pairs in parallel I would have 36v (nom) and 10ah capacity. With 15c rated cells the peak power requirement of approx 18amps draw should be easily achieved and life in terms of recharging cycles should be good.

For a start are the above statements true? YES

With RC models, battery management is generally taken care of by the ESC but on e-bikes a battery management system (BMS) is fitted within the battery compartment. This is where I am starting to struggle.

The BMS really does two things - it shuts down the motor when the voltage gets too low and on better systems also manages balancing of cells during the charging process. Manufacturers of cheaper e-bikes claim that "Lipos don't go out of balance" and leave out that facility! Rubbish they do and you will need a balancer
I don't want to spend well over a hundred quid on batteries and mess it up so am asking for your advice!

Without opening up the battery case every time I charge the cells, pulling them out individually and balance charging on my RC charger what is the alternative? If you do it properly then you will get the most battery cycles from you packs
I could charge with a readily available "intelligent" 36v Lipo charger that are sold for these type of applications but I wouldn't be balancing. Make sure that the cells never exceed 4.2vpc on charge and to do that you need a balancer get one fit it into the battery compartment. It needs to be the type that dosent consume any power when there is no charge current.
Should I plan for a mixture of the two methods even though it would be a lot of messing about?

What low voltage warning devices are available? Something simple such as a warning buzzer? Ebay they work ok (lipo fuel guage)

Any suggestions with wiring schematics etc would be gratefully received!

Thanks, Dave

comments in gory colour...
Basicly you will need to use a balancer to maximise teh life if you dont you almost certainly will get cells that go out of tolerance and that not only is a life issue its a fire hazzard... there are little voltmeters available for 5S / 6S and thay may help you with this project ....

Steve

[Grant_beltcp]

If I was building an electric bike, unless it was for insane performance I would look more towards A123 cells.
Have a search on the net for Killercylce, worlds fastest Electric bike and uses A123 cells.

They can be charged using standard balance chargers, or even better you can just put a power supply directly onto the packs and charge them in 10-15 mins.

You dont need to worry as much about discharging them too low, or balancing every charge or taking them out of the bike to charge. It may cost a little more for the cells though.

You will probably find that you dont need as high capacity packs going from SLA's to Lipos/A123's as their discharge cycles are far more effecient.

[davew]

Many thanks for the replies. I have a reasonable knowledge of e-bikes - probably more than helis! - I've owned one and been interested in the technology for the last 8 years.

Lipo batteries are used in parallel in some commercially available bikes so the perceived problems are overcomable.

I am aware of the dangers of using lipo batteries, have seen the videos and in fact a pal burnt out his Flasher 500 last week by hammering it to death by running a totally shot 3S in series with a brand new 3S.

I am familiar with A123 cells but they are more expensive and there's loads of soldering of tabs etc to carry out! If I decide to upgrade my diy power pack to 48v, 20ah how many cells will I need (answer nearly 300 with the commonly available 3.3v, 1100mah variety). Same energy achievable with 12 lipos.

[lex7919]

You can get A123 cells in much larger capacities than 1100mah, And IMHO Grant has give a very credible alternative there by Using A123's you have all of the benifits of lipo but without the negatives and for a road going machine i feel its the best solution to the proposed problem.