# Thinking of upgrading to NiMh, need help



## adric22 (Jan 17, 2008)

I have a 24V Currie scooter and the battery pack is due for replacement. I was thinking of using off-the-shelf AA batteries of the NiMh type instead of the AGM batteries it has now. I think I'll need 60 cells to get me the same voltage and amp-hours.

The trouble is, I'm not sure what to use for a charger. Won't it need to be charged differently from lead-acid? Any suggestions on this front?


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## Wiredsim (Jul 4, 2008)

I guess the big question is how to charge, you basically have three choices: 


Charge all the cells as a single pack(or two 12v packs)
Charge each cell individually manually by taking them out of the pack and putting them in a smart charger
Find a way to wire the batteries to allow you the charge them individually and then draw current as a pack in series
Are you looking at Energizer type 2500ah AA's? What is your current pack capacity? I am assuming it is around 6-8ah? If you get F-type cells(14ah) or full capacity D cell(9-10ah) you can cut back on your total cells and run a single pack in series instead of three packs of twenty. This would make it easier to charge. You could always buy a few 4 or 8 cell chargers and modify them to fit a D/F cell, something like this.

For example:

http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=325

Or

http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=835

If you do a cost comparison I think you will find it will be roughly 1Ah/$1 with any NiMh cylindrical cells. For example you can buy Energizer NiMh AA's, 4 for $10, that will give you exactly $1 per Ah. I think if you get a decent price on any larger cells your looking at roughly the same price range. For example : http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=3606 

That is 10 D cells at 10Ah capacity each, for a total of 100Ah for $80, that's 1.25Ah/$1. Even better! Not to mention the total pack resistance should be lower as you have less cells, you can get higher current D/F cells that run 3C or even 5C warrantied. 

Does anyone know if it be possible to wire individual charging wires to each cell and use a simple diode/relay circuit to isolate each cell while charging? 

Otherwise I think it would be possible to create a manual switch plate that would be operate as a bus plate for every battery. Then you could shift the plate physically to alter the circuit or replace it with a different plate to charge.

The big question would all the extra effort and time be worth it in battery life and capacity vs charging them is a series pack?

Or you could always go Lithium(not liFePO4):

http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=4057


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## Wiredsim (Jul 4, 2008)

Check it out:

"Since NiMH batteries are not as prone to the overcharge combustion problem as Li-Ion, their voltage measurement requirements are not as stringent. In addition, since voltage is not a useful indicator of the SOC of this battery [7], the voltage measurements do not require the same accuracy as Li-Ion, and individual cell voltage measurements are not necessary. Instead, the measurements can be reduced to a few segments of series connected cells, e.g., 6 segments of 8 cells each in a 48 cell pack."

That is from a BMS design document from the NREL! I am willing to go out on a limb here and say it is probably accurate.. 

http://www.nrel.gov/vehiclesandfuels/energystorage/pdfs/3a_2002_01_1918.pdf


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## Wiredsim (Jul 4, 2008)

This is an interesting thread:

http://endless-sphere.com/forums/viewtopic.php?f=14&t=5316&st=0&sk=t&sd=a

He basically is just stringing NiMh C cells together and using a thermistor to do a Temp cutoff. I have read that with NiMh a temperature based charging system is more accurate then voltage based.

Its an interesting read, and a dirt cheap method also..

A lot of people are using Ping batteries for their scooters and e-bikes though:

http://stores.ebay.com/PingBattery

Mind you if you want a 20Ah 24V pack your looking at $500~. This includes the BMS and charger however.

So for Ping's its roughly $25 per 24V Ah with BMS and charger included, I got this number from completed ebay auctions of $480-520 for the 24v 20Ah Ping battery.

If you bought 40 13Ah F type cells for $12.55 each, total of $502. Put them in four packs of 10 each, throw in a Klixon @ $5 x 4 for a total of $522. Total of 26Ah @ 24Volts. According to Knuckles in that thread you could use a 24v SLA charger with the Klixons for Thermal protection.. Otherwise use a smart Temp based charger, those usually run around $30-40. So potentially $540 to $560 total, total of $21 per Ah.

The advantage with LiFePO4 is smaller size, lower weight(11Lbs vs 21Lbs), and potentially longer life(1500-2000 cycles vs 1000).

However the NiMh according to specs offer a better max power draw of 80Amp continous and 120Amp peak draw.


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