What do people think about Lipos? (racing packs) in parralell for EVs

Let me know how you make out with the LiPo's. We also own www.bigrcmodels.com and can get anyone that's interested great pricing on bulk orders of LiPo products. Here's an easy to use chart to match up volts to mah and see what inventory we have that matches up. http://www.bigrcmodels.com/lipo_batteries_launch.asp

you might want to check your website. I checked the price on the 10AH 22.2v 6cell and it came up $9,999.00 a little steep, I payed $300 each for mine. Must be a misprint.

Colin, Yes we have used these to raise the voltage on an xm3500li to 93v.(4 10AH 22.2v wired parallel to create a 40AH 22.2v added battery) The drawback as you said is you need a different charger. These batteries have much more power to weight and space than the LIFEPO4 batteries but are usually more expensive. I figured I could fit 80AH@72v nominal(88v full charge) of lipo batteries in my 3500li that would give me about 50mph top speed and about an 80 mile range at 35mph. Does anyone know if you can charge these type of batteries with the TSL series chargers from thundersky?

you are correct, parralleling all the cells when under charge will balance out their SOC (absolutely wont work when the cells are not under charge, and will cause all kinds of balance issues).

however, before the cells are parralleled, they must first be taken out of the series circuit (if you dont you short *all* the cells which results in smoke fire and other bad things)
this requires a switching mechanism on *each* cell thats rated to full discharge current (but not necessarily voltage).

if going the solid state route (mosfets and such) you need devices rated to *full* pack voltage and current.
the on resistance easily double the voltage drop of the pack under load.

if going the contactor route, the contacts need to be sized big enough not to cause to much voltage drop.

both approaches increase weight size and cost of the pack while reducing performance.

Matt

Colin, did you check the votage on the batteries when you got them? $15 is a great price for those batteries. I keep hoping the price will come down on these. Then we'll be talkin some range and speed with these electric motorcycles. They can be very volitile though as you found out. Don't feel like the lone ranger on that one. At least you didn't pay to much for them. Anyway the charger you bought, was it a balancing charger? From the sound of it your batteries were 4 cells. hyperion makes good chargers, check out thier website. Sorry for your loss...don't give up.
Bill-

I forgot to say, sometimes if the batteries have been drawn down to low it will say no connection or if the balance lead isn't hooked up. Can you still get those for that price?
PS Don't have anymore brainwaves man.

when one of the cells is at an extreme end of the charge spectrum (ie almost fully discharged or charged), then it will sort of work. but the two cells will only balance voltage, not state of charge.

It is only when the voltage of both cells are held at an extreme end, that both are at the same state of charge.
normally this is done while on charge, rather than discharge.

voltage at no load is a *very* deceptive way of determinig state of charge, yet parrallel balancing relies on voltage balancing.

so two cells at 3.65v each, can be a good 20% SOC apart from each other.
this is actually worse with good quality cells, as the charge and discharge curves are so flat at the middle.

parrallel balancing is also *very* slow when the voltage isnt far above the chemistry reaction voltage.
so the current flowing between a cell at 3.65v and a cell at 3.6v will be minute.

the amount of energy (and AH) a cell at 4.2v can give before it falls back to 3.7v is insignificant relative to its capacity. its just surface charge, caused by the capacitive nature of the two electrodes near each other absorbing the charge due to the reaction side of things going open circuit due to running out of reactants (ie fully charged).

Matt

hi colin,

not a chemist, but an electrical engineer in the making (im 2/3s the way there :p) who spends alot of time talking to the pros in the EV field.

not sure how the RC chargers are made, but yes, i think you should connect the big red and black leads as well as the 4 lead connector.

the 4 leads allow the charger to vary the current applied to each cell to keep the voltages balanced (which when the cells are near full charged will allow the charger to make sure the cells are at the same SOC, 100%).

the two big leads provide the charging current.

i must admit i havent played around with RC balancing chargers before, since all my lithium batteries are either single cells or come with a BMS that does the balancing.

if that doesnt work there is another solution.

you could put a 4.3v zener diode across each cell, such as this one:
http://www.jaycar.com.au/productView.asp?ID=ZR1401&CATID=33&form=CAT&SUBCATID=359

and use a lab powersupply to charge the pack.
just limit current to 0.25A, and wait for the voltage of each cell to reach 4.3v

Matt

yeah colin matts right, and I'm still sitting here with my mouth hanging open try to get all of what he said. I've used these chargers alot and thats right you have to plug both in. There are some that charge through the balance lead but they are all very low amp chargers. You'll be amazed at the performance of lipo. Yeah I learned the hard way to only have one lead exposed at a time myself. I wish these batteries came with standard connectors already soldered. Not everyone is a solder wiz. You should check out the balance pro cards at commonsense rc. Each card can monitor and balance up to 6cells, they are $39 each.

Thanks Matt, that explains something. I noticed that at full charge my 3500li had some cells up in the low to mid 4.2os with most around 3.38v average. After a quick run up and down the road they become very balanced and all line up. I guess that burns off that surface charge. I tried riding a little(half mile at full speed) to bring the cells in good balance and then charging again and it seemed to help my range. Is it possible that helps bring the low cells up? Unfortunatly I realize that isn't very scientific. Now I can monitor as I ride with the balance pro cards and realize the value of seeing whats going on under load. I guess now if I see any dropping way below the rest, I know they are going. I think ultimately a computer is needed to record the data and control charging. I guess the pak trakr keeps tack of that data but doesn't balance, it would be nice to have both.