Saturday, May 31, 2008

Considering Different Batteries

The original CivicWithACord design used 18 8-volt golf cart batteries. These are commonly used in the EV community since they are widely available and have been used for several years. After some debate on the Google Group and dealing with clutch-link interference issues, I'm tending towards 12-volt group-31 AGM batteries for the initial design. I think the AGM batteries will be safer due to a solid electrolyte and the entire design might be cheaper because we won't need acid-proof battery boxes.

I've done an initial analysis of several battery types using the spreadsheet here.

At first glance, I was really tending towards using the Deka 9A31 batteries. They were cheaper than the Optima group 31 batteries, put out some good current and had higher amp-hours. Only after talking with a few folks and looking at specifications did I think the Optima group-31 batteries were the better choice.

Here is a quote from Mark Freidberg from the OEVA:

Hi Tim,
I've been disappointed with the Dekas and won't be purchasing anymore. Their capacity has fallen off after little more then 100 cycles even with careful charging. One thing that is nice about the Soneils is that one battery at a time can be replaced instead of the whole pack all at once. And the Paktrakr has allowed for careful monitoring on discharge to see which batteries fall off first. Four of the Dekas have been replaced by Odyssey PC2150s. Its too early to tell how much better the Odysseys will prove to be.
The other Group 31 size AGM to consider is the Discover EV31A-A. So far rumor has been that they have been holding up well but its still early in the game as they are still pretty new on the market.
All the AGMs and Gels I've tried though are stinky on the discharge, if not as much as golf cart floodies. So thats disappointing. As stinky is another way of saying not healthy to breathe. My battery box has an exhaust fan, but still its an imperfect setup. IMO, its good to have batteries separate from the passenger compartment like a truck or sedan, and/or to have non/low-toxic battery chemistries like li-ion.

Mark Freidberg
After comparing specifications on the Deka 9A31 and Optima 31 batteries, it seems like the Dekas only have a cycle life of 150 full discharges while the Optimas have a cycle life of 350 full discharges. The Optimas have a slightly shorter amp-hour range but could last twice as long and put out more current. Since both batteries are group-31, I'll try to make the battery racks fit both and have people choose their own group-31 battery.

Using 12-volt group-31 batteries also enables the use of the Belktronix controller system and doesn't interfere with the clutch slave cylinder like the 8V golf-cart batteries do.

Rob Conelly provided the following picture of how the Optima 31s are placed in his system. I assume there's still six in the rear trunk for balance.

Rob's system has air conditioning and the original power-steering pump tied to the rear shaft of the Warp9 motor. For the initial open-source design, I'm planning on using manual steering and not having A/C to save range and complexity. I'll see if Rob is willing to share his battery rack design.

Next up: Controller choices

No comments: