Monday, September 15, 2008

Conundrums over Batteries

Ever since I heard the not-so-great cycle lifetime of the Deka 9A31 batteries, I've been scouring the internet to try and find a replacement group-31 AGM battery.

I've added several candidates to the spreadsheet above (click to enlarge) and, at this point, I'm leaning towards the Concorde batteries. They're made for aircraft, have a good Peukert exponent and have a reasonable cycle-life. They're quite a bit more expensive than the other choices, but that cost will be made up for in the lifetime of the battery. The spreadsheet above is also available in the Files section of the Civic-EV Google Group as the file BatteryAnalysis.xls.

It's funny. Every battery dealer I chat with is glad that I'm converting an electric vehicle, but they all tend to say that you have to choose between bad batteries and worse batteries. I've heard that the Odyssey batteries are the best, but they also come with the greatest price, and I don't know if I'm willing to drop $4000 on a pack yet...


Ross Cunniff said...

For what it's worth - I went with the Concorde batteries for my ElectroJeep project. I went through a similar analysis and ended up picking the Concorde Chairman AGM 1280T (designed for electric wheelchair applications, I believe).

I'm still waiting on a battery management solution... hopefully I'll have it in place before I start driving.

Ross Cunniff said...

Also, the other option I briefly considered was the Firefly Energy Oasis battery. But I could not figure out how to fit 26 of them into the Jeep (they are slightly larger than typical Group 31). Additionally, their cost and availability is still unclear.

TimK said...

Hi Ross!

Good to see all the progress on the ElectroJeep. I thought about adding the FireFly to the list, but I don't think it will be available for a few years. Thanks for the input on the Concorde batteries.

Marc C. said...

Tim, I read through your blog for the electric 914 and followed you over here. I am really interested in this project as I love Honda cars and currently drive an 04 Civic ICE. In regard to your battery situation, it seems to me that many people have used Trojan batteries in EV's and I was wondering if you considered the 31-AGM they have? I have not yet built an EV and have no personal experience with them. Just curious, the battery has a 20HR rate of 110 A/Hr.

TimK said...

Hi Marc,

Good point. I just looked at the specs for both the Trojan 31-AGM and the group 31 AGM battery (slightly oversized) from US-Battery. Both had a good number of amp-hours at the 20 hour rate; however, both had really bad Peukert factors of 1.25 which really shrinks your usable amp-hours in an electric vehicle. I should probably add them to the spreadsheet for completeness, but I'm not impressed with either.
Thanks for the heads up.

Marc C. said...

Tim, I did not realize that there was that large of a difference in Peukert factors among different batteries amoung the manufacturers. I did not see the 1.25 Peukert's on Trojan's site, can you educate me on how you arrived at this number?

TimK said...

Hi Marc, both sites list the 20hr amp-hr rate and the 5hr amp-hr rate. If you do the math and plug this info into the EV battery calculator at:

It will give you the Peukert exponent back. For the Trojan, you get 110 ahr or 5.5A per hour for 20 hours (1200 minutes) You get 83 ahr or 10.6A per hour for 5 hours (300 minutes). Plugging into the calculator, this gives a Peukert rate of about 1.255

An EV would probably draw this down in 1 hour, probably giving about 50 amphours of real storage.

Cheers, Tim

Marc C. said...

Thanks for letting me gleam a little bit of your knowledge. It helps me out in my planning.