Battery Vehicle Society

I'm looking for a better pack for this project:

http://www.jetpower.co.uk/gtcar_home.htm

The turbine has a 208v 3 phase 400Hz generator which defines the buss voltage at around 300V. To get the project up and running, two 24 x 4.5Ah strings of UPS batteries were used, but now their power and to some extent energy are quite limiting.

So I’m considering NiMH or Lithium. I have developed the motor and battery controls which fully protect the pack. Float voltage, minimum discharge voltage and both charge/discharge currents are all individually regulated.

Two strings of 240, 10AH NiMH appear to be an option and don’t require individual cell battery management, for example:

http://www.batteryspace.com/nimhrecharg ... te1pc.aspx

http://www.ebay.co.uk/itm/D-Rechargeabl ... 8915790344

I’m familiar with lithium polymer and have converted a small motor bike to electric using 200, 10Ah Li poly cells. I did my own analogue battery management for this with 10 series groups i.e. 30 – 42V system. For the MG, power looks to be more important than energy, so LiFePO4 types are perhaps the best option. Particularly if they can get away without battery management or a very simple system. For the MG we are looking at 100 cells in series so the amount of work to repeat the analogue system I did for my bike would be prohibitive. For example:

http://www.batteryspace.com/LiFePO4-401 ... screw.aspx

http://www.ev-power.eu/index.php?p=prod ... 20ah-promo

Anyone got any suggestions, for example a LiFeP04 system with management scale able to 100 cells for reasonable cost. Budget 2-3K.

Nice project Is your budget for the bms only or for the pack and bms Sorry couldn't help laughing a little bit.

What is the max current it charges at?
What is the max current it discharges at?
What are the max and minimum system voltages?

Small size lifepo4 cyclindrical/prismatic do not stand large charge currents well in my experience unless they are A123 20ah pouch cells which are very good but expensive. If you use lithium you will need a bms and a good one especially if you use Lipo instead of Lifepo4!

If you used a Maximite or Picaxe master you could use four of my own BMS stacked 25 cell slave boards for a 100 cell lifepo4 system. A couple of days work to assemble the slave boards. Cost say £500 or £5 per channel might be a bit cheaper with bulk parts.

For the easy cheap solution I have a lot of panasonic nimh cells in 6 cell high current sticks which can take 50A charge & 100A discharge. http://www.ebay.co.uk/itm/320788907644? ... 1555.l2649
I would rather see them in use than on my shelf so if you want to do a deal off e-bay we might be able to come to an arrangement. They are a doddle to assemble with 6mm bolts or threaded rod.

Very interesting project..

Ive got some LiPo cells here capable of very high power (20C continuous), and with good energy density. I also have a small and cheap high/low voltage protection system with extremely low power draw. All in stock in the UK..

One thing you didn't mention was how important was cycle life? The cells I have are good for about 250-300 cycles before 80% capacity is reached.

If this sounds any good for you, drop me a line. steve@jozztek.com

Thanks for the replies, food for thought there.

We are looking for a 25 - 30KW contribution from the battery. So a suggested 100A limit for a pack with a system voltage of around 300V. The 24 x 12V lead acid system was float charged at 336V and draw restricted to limit depression to 240V. There is no regenerative braking, the expectation being you would have the turbine running so this does all the charging. Charge current can be regulated to a value much lower than the discharge if need be. When cruising there may only be 10 - 20A spare from the generator anyway.

Calendar life is more important than cycle life. The turbine is very inefficient so it is not really a practical vehicle to cover any distance in.

What sort of voltage do the NiMH and Lifepo4 drop to at 100A? I bought some supposedly 11Ah D NiMH off eBay but they were only 4.5Ah and got pretty hot at even 10A. Were supposed to be able to do 30A. Seller refunded me including the return postage which I was pleasantly surprised at.

The battery lives in the spare wheel well and the lead acid solution weighed approx 90Kg. So within budget I guess we would look to fill this space, with several strings to help power if possible.

Assuming you parallel at least two strings of 6.5ah nimh, 40 sticks in each string, you are looking at ~80kg for 300-350V approx at 13ah max capacity.

Drawing at 100A will give a 50A draw per string and probably a max of 8ah useful capacity.

Add another parallel string for 120kg and you will add at least another 5ah making a useful 13-15ah pack.
The peukert effect will be less as load is shared so you will get more out of each cell.

I can supply 120 sticks at £6.50 each collection only. No bms reqd.
The cells sticks also have temp sensing strips so you can detect an individual overheating cell with a very simple circuit.

http://www.youtube.com/watch?v=WX-_azD2 ... re=related

You might wonna look at the above video before it dissapears like their website.

It's a 400+ HP electric car using Lifepo4 cells. First they used Headway cells, now i think it's got 300 (100S3P) 20Ah A123 pouch cells. Lipo is more expensive and won't last as long as Lifepo4 but will give more instantanious power then the above. It's basically all up to how much you want to spend on it.

My experiance + long term research on cell discharge;
Lipo cells can take up to 100C for 2-3 seconds. 70C for 5 seconds. 30C for 50% capacity
Lifepo4 cells can take only 30C tops for 2-3 seconds, 10C for 5 seconds. Probably keep it at 2C for 50% capacity

Charging wise, i don't exceed 5C on Lipo but am confident they can do more. Lifepo4 is supposed to charge even faster and 10C is being done in RC applications.

With regards to cell balance. I think that even Nimh packs deserve a balance function and certainly it's well worth the effort to do this with Life or Lipo to be safe and enjoy a long battery life with peak performance.

How much power can the generator deliver? I think you would only need about 15-20KW to be able to maintain 70mph long distance. If you want to start and stop a lot without regen then obviously you could want more. The battery pack is just there as a giant capacitor.

Joe

EDIT; new website for the BMW; http://www.rimac-automobili.com/evaluat ... vehicle-11