# Choosing a small EV battery pack



## Duncan (Dec 8, 2008)

Hi

In your shoes I would use an 8 series by 2 parallel set of 16Ah Headways 

16 cells at 0.5Kg each = 8kg
16 cells at ~$20 each = $320

Half the weight and 320Amps continuous 480 amps burst

Or I would go for the Turnigy RC cells - lots of power - low weight - slightly higher risk

I am not fitting a BMS on my pack - but I will be acting as a BMS until I am sure it is stable


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## frodus (Apr 12, 2008)

Get some headway batteries (10Ah or 16, either are fine) and a balance charger for RC batteries.

24V is 8 in series, then 40Ah is 4 10Ah in parallel or you could do 32Ah and do 2 in parallel. Fairly easy to build especially if you get the building blocks. Manzanita Micro has headway batteries, great people to work with. 
http://www.manzanitamicro.com/products?page=shop.browse&category_id=29

For BMS and charging, get a FMA Powerlab V8. It'l charge and balance 8 batteries in series and you can do some measuring and discharging of cells. All you'd need is a 12V-32VDC supply, or a 12V-24V battery charger and a 12V-24V lead acid battery to supply power to the charger.
http://www.revolectrix.com/pl8_specs_tab.htm

That way, no BMS, simple solution.


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## lsuguy (Jan 26, 2012)

frodus said:


> Get some headway batteries (10Ah or 16, either are fine) and a balance charger for RC batteries.
> 
> 24V is 8 in series, then 40Ah is 4 10Ah in parallel or you could do 32Ah and do 2 in parallel. Fairly easy to build especially if you get the building blocks. Manzanita Micro has headway batteries, great people to work with.
> http://www.manzanitamicro.com/products?page=shop.browse&category_id=29
> ...


Thanks for the advice. It seems like the CALB cells that manzanita sells would be a better fit since I would not have to have any cells in parallel since they come in the 40AH variety. I remember reading in my research that it was recommended to avoid having cells in parallel if possible. I think it makes it easier to have a bms system as well since most I have seen cost more per cell it has to regulate. 

As for that charger I could not really see a price on it but it seemed like an ok option as long as it isnt too expensive. I guess as long as I get a charger that will balance the cells on every charge and not allow them to go over the rated voltage then that kind of eliminates the need for a bms. I guess I would still need a way to monitor the cell voltages during operation to make sure the battery is not completely drained. I want to make the operation of this pack as idiot proof as possible as I'm not sure who will operate this vehicle after we graduate.

Ive heard some people mention cell log 8 on these forums or somewhere else. Would that be a good way to make sure the pack is not overly discharged or would a more robust bms be advised?


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## frodus (Apr 12, 2008)

Sure, CALB are good, they do ~3C continuous (120A) or just over that.

It doesn't matter the number of cells in parallel, it's not per-cell for balancing. It depents on the number in series. Since the batteries are in parallel, only one balance channel is needed per group of parallel cells, so you'd need 8 channels total. Balancing is the same whether you chose headway or CALB. The only thing getting 40Ah cells does is make it easier to build a pack. If CALB's fit your requirements, then I'd chose that over Headway.

The charger should be around $250 IIRC, but consider it charges as well. There are other options as well, it was just one off the top of my head.

For monitoring, get a Cell-log 8 and modify it so it doesn't tap off the first 2 batteries, there are threads about the celllogs. They have an alarm output that can trigger a relay that trigger on LVC, HVC.


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