# DIY NiMh Battery Pack



## FunOhmer (Jan 31, 2014)

I have a question, I'm struggling with the calculations though.

I was wondering whether building your own NiMh battery packs would be a feasible endeavour? I was planning on purchasing SUB-C size NiMh cells. However, my issue is I need to work out how many cells would be needed to make a reasonable battery pack, and how many of those batteries would be need to power the car.

6000mAh capacity
Voltage 1.2v

These can be purchased cheaply online complete with tabs for easy soldering, or I could use a battery welding device.


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## racunniff (Jan 14, 2009)

FunOhmer said:


> I have a question, I'm struggling with the calculations though.
> 
> I was wondering whether building your own NiMh battery packs would be a feasible endeavour? I was planning on purchasing SUB-C size NiMh cells. However, my issue is I need to work out how many cells would be needed to make a reasonable battery pack, and how many of those batteries would be need to power the car.
> 
> ...


"Reasonable" really depends on your design goals. How heavy and aerodynamic is your car (i.e. - what is your typical Wh/mi power consumption?), what range are you going for, what kind of motor will you use (that is - what voltage levels are required?), etc.

With those details, a better assessment could be made. For example, assuming a "reasonable" 300 Wh/mi average power consumption and a maximum depth-of-discharge of 80%, you would need a nominal 11.2 kWh pack to get a nominal 30 mile range (300*30/0.8). With your cells being a nominal 6*1.2 = 7.2 Wh capacity, you would need nearly 1600 cells to get to that level.

The series/parallel configuration would depend on the voltage levels you need, but 96v is about the lowest I've seen for an effective street automobile. You would make parallel groups of 20 cells and string 80 of those together in series to get to your voltage level.

I'm not very familiar with NiMh charging characteristics. Lead and LiFePO4 charge just fine in parallel, but NiMh may not (NiCd for example cannot really be charged in parallel because of the weird charge curve). Hopefully you do not need 1600 separate micro-chargers...


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## racunniff (Jan 14, 2009)

racunniff said:


> I'm not very familiar with NiMh charging characteristics. Lead and LiFePO4 charge just fine in parallel, but NiMh may not (NiCd for example cannot really be charged in parallel because of the weird charge curve). Hopefully you do not need 1600 separate micro-chargers...


This article suggests that charging NiMh in parallel is tricky just like NiCd: http://www.powerstream.com/NiMH.htm


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## FunOhmer (Jan 31, 2014)

racunniff said:


> This article suggests that charging NiMh in parallel is tricky just like NiCd: http://www.powerstream.com/NiMH.htm


Thanks for the info Racunniff, so would you say it's unfeasible? I'm getting quotes from China for that amount so if the price is good I'd still be happy investigating further.

The proposed car would be a Land Rover Freelander 1, with some aero adjustments such as a an underbelly cover and front dam to help improve the efficiency. Really, I want a range of 75 miles minimum, so I guess more cells will be needed


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## racunniff (Jan 14, 2009)

FunOhmer said:


> Thanks for the info Racunniff, so would you say it's unfeasible? I'm getting quotes from China for that amount so if the price is good I'd still be happy investigating further.
> 
> The proposed car would be a Land Rover Freelander 1, with some aero adjustments such as a an underbelly cover and front dam to help improve the efficiency. Really, I want a range of 75 miles minimum, so I guess more cells will be needed


Well, that's a pretty hefty requirement. I'd use 450 Wh/mi as an estimation point to be a little conservative (for reference the ElectroJeep is about 400). Doing the math - pack size is ( 450 Wh/mi * 75 mi range / 0.8 DOD margin ) = 42 kWh pack (the ElectroJeep has a 31 kWh pack and about 55 miles range). This is roughly 6000 cells. You probably want an AC system with that heavy vehicle, which frequently means higher voltage - I'll use 300V. 300/1.2 = 250 groups in series, each group with 24 cells in parallel.

I would personally say this is not feasible. Even if someone dropped the 6000 cells in your lap for free, your personal labor stringing them together is huge. Assuming (big assumption!) that you could do a single group in, say, an evening of 4 hours, that would take you a full year of every single week night just to get your pack built. And this does not take into account the BMS which you would need to design and implement. Additionally, with that high number of cells, some amount of cell failure is guaranteed, so you would want to design your system to be robust in the face of multiple single cell failures, plus make it easy to swap out cells as needed.

I would instead recommend you go with prismatic LiFePO4 like so many of us have. The ElectroJeep is 96 cells of 100 Ah each - much easier to assemble and maintain. You could go with 96 180 Ah cells and probably get close to 100 mile range. This would not be cheap - but it would be feasible.

Do note that, even with LiFePO4, I'd recommend a margin for some amount of cell death. I allow 10% (that is, I buy 10% more cells than my design requires). On the ElectroJeep, I've suffered about 2% cell death to date, so I'm feeling pretty happy. But it is much easier to swap out one out of 96 dead cells than a few out of 6000.


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## dougingraham (Jul 26, 2011)

Lets compare to what I have in my car since I know it better. Your requirements with the heavier vehicle and longer range will be about 3 times mine. I have a 51 cell pack of 100 AH LiFe cells. This is 16.3 kwh. To get that size pack of the 6AH 1.2V NiMH cells would require 2267 cells. To best utilize these I would place as many in series as possible. The voltage limit is 340 volts of the motor controller. You can almost do this if you parallel by 8 but the voltage during charge will exceed the rating of the motor controller so lets parallel by 9. This gives a 252S9P pack with a nominal voltage of 302 and a rating of 16.3 kwh. I dont know the continuous discharge rating of these particular cells but to match my 100AH cells they would need to be able to do 8C (45 amps). This is a reasonable assumption. So it can be made to work at least on a small car. I paid $6885 for the pack I have in the car so you would need to pay less than $3.04 for each cell just to match the cost.

You will have problems with charging these cells in parallel. You cant use the traditional technique of watching for the voltage to start back down. But you can do a CC/CV charge similar to what we use for LiFe with a conservative CV point. Or you could do a Delta Temp cutoff although 252 temp sensors seems excessive.

But why would you want to do this? LiFe have higher energy density both wh/kg and wh/li, last longer, cost less, have a higher charge/discharge efficiency and charge faster. This does not consider the additional complexity of the 2268 interconnects as compared to the 51 I have in my car.

I would say that you can do this but unless they are giving you the cells for $1.50 each or less I wouldn't consider it cost effective or practical.


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## Karter2 (Nov 17, 2011)

Before i bought any of those cheap 6 Ahr cells, i would want to see some independent test results for capacity and discharge performance. 
"D" cells have been shown to have up to 10Ahr capacity, ..but they are more than 3x the volume of a sub "C"


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## samwichse (Jan 28, 2012)

http://www.batteryspace.com/nimhrechargeablecellm-size12v26ah52arate.aspx

Maybe if you shopped around, you could find a cheap price on some larger capacity M sized NiMH cells?


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## Tesseract (Sep 27, 2008)

samwichse said:


> http://www.batteryspace.com/nimhrechargeablecellm-size12v26ah52arate.aspx
> 
> Maybe if you shopped around, you could find a cheap price on some larger capacity M sized NiMH cells?


Erf... that M cell doesn't compare favorably to LFP at all. For example, here is a 100Ah CALB cell for $145. Note that the CALB cell delivers ~10x the Wh for just 3.6x more price; or to put it another way, it is less than 1/3rd the cost per Wh of the NiMH cell.


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## samwichse (Jan 28, 2012)

Right. But the OP is looking to make a NiMH pack for some reason. The larger format cells seemed more realistic than 6k small ones.


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