NiMH 48V 35 amp draw, charging? Balancing? Forget it?

Hello All,
Besides playing around with flashlights, I also screw around with electric scooters/electrified bicycles. Smoked my 36V 14AH AGM SLA pack from it going off balance so it is time to either upgrade to a 48V 20AH AGM SLA with four chargers or go NiMH.
Went to ebikes.ca and they have a 36V 12AH NiMH pack that handles 5C (60 amp discharge) that would fit my 35 amp requirement. Since they don't make 48V 12AH packs (they do make 48V 5AH) it makes me wonder if I am heading down the wrong direction. Since balancing will be an issue, I would either go with four 12V 12AH packs/chargers or two 24V 12AH packs with two chargers.
Am I creating a disaster by putting 40 NiMH F cells in series and loading them down at 3C (35 Amps?) peak? The 48V 12AH NiMH pack is 24 pounds VS 57 pounds for four B&B EB series AGM SLA batteries and I prefer NiMH depth of discharge ability VS SLA.
Figured with the RC guys, battery gurus and EV folks around here, maybe you could shed some light on loooong strings of NiMH cells running at high discharge rates. What kind of charger and how many would I need to get this mess of an idea running? Should I just go with the $450 48V 20AH SLA with four chargers or try out NiMH? This would be much simpler if A123 Systems sold 3.3V M cells with a 20AH capacity for $20 each but we have not hit 2010 yet.

Pardon me while I ramble.

I've been kicking around with the idea of an electric go-cart for a while now and I'm in the same boat as you. After a bit of reasearch at a number of battery retailer sites, I've concluded that NiMH simply is not cost-competitive with Lead-acid (PbA) when it comes to faceplate capacity ratings. NiMH has an undeniable performance and technical edge vs PbA, but it's far more expensive and no large-format cells/batteries are available.

If you ever get a line on some of the RAV4EV or EV1 NiMH batteries, let me know and keep it quiet from Darell since they would be excellent for your application and mine

Seriously - the RAV4EV batteries were 12Vx95AH and the EV1 batteries were 13.2Vx85AH, albeit perhaps a bit large for your application.

13AH x 48V will run you around $460 from Battery Space.

Another outfit (CleverChimp) is offering ruggedized 36V x 13AH packs for $425, but these have been designed for their Stokemonkey electric-assist device and have a 25A maximum discharge.

Perhaps you could score some decent tabbed D or F cells for cheap, but given the high discharge you're looking at, I would be hesitant to make my own pack without the equipment to spot-weld cells together.

I talk about price because that's an issue for me - I'm not about to sink several times the price of a go-cart on NiMH cells. Not sure what your price point is or performance goals are. In my case since this is a go-cart, I expect 10-30 minutes runtime, tops. With PbA, I can easily afford multiple battery packs and have them charged and ready for more than 1 round...

You are in the same boat as me, idle... what a rocky ride it is!

I've been doing research for awhile and have found some interesting solutions. First of all, those Chinese NiMH F cells really, really, really DON'T like any current draw over 15 amps...their series resistance kills the performance. The hub motor I plan on using will work fine with them as long as it uses four packs in series-parallel for a 48V 26AH 48 pound pack. ($1,036) The AGM SLA (Pb) batteries weigh in at 57 pounds so it is too close to be an option. Those four Pb batteries run $199.80 so you can see a huge difference in price.

I found some Sanyo 20AH NiMH M size cells and they are very proud of them... at over $67 per cell proud! Uhhhhh, I am not into a $2,600+ dollar pack at this point. Saft makes a 15AH F cell that would kick out the amps and is rated for up to 80A constant discharge for $21.75 each cell. $870 is more realistic but I fear the balancing problems and blowing the pack up because my wife and kids will use it.

From what I see, I am in a catch 22 situation at this point in time. The 35 Amp peak discharge rate will overload any less than $1,000 battery pack/charge combo out there so it looks like Pb is the only choice. As long as the pack lasts a year or two so Valance, A123 Systems or anyone else can build the C-LiFePO4 safe lithium-ion packs in M cell 20AH+ sizes for $25 each or less. The pack will probably have to last longer than a year or two then! Europositron is supposed to have their aluminum battery out by 2009 so maybe that will be an option in a few years.

Hello BentHeadTX,

The Schulze isl 6-330d will charge up to 30 cells. If you had two 20 cell packs, you could use a couple of Schulze chargers. With 20 cells it has a peak charge rate of 3.7 amps.

Keeping a large pack in balance can be a little bit more difficult. If you go strictly on voltage you end up not using very much of the batteries capacity. I believe the current trend is to monitor individual cell voltage and shut the pack down when a cell goes below 1.0 volts.

Tom