Archives

Alright, so everyone says NiMH batteries are too expensive, so I guess most people use a bunch of large, heavy lead-acid batteries. Now my question is this, why can't you just string together a whole bunch of AA NiMH batteries? I guess I just don't know enough to understand why you couldn't do that.

I mean, I suppose you just can't get enough energy out of them in an instant to run a car? But it seems like you could just suck energy out of multiple batteries at the same time and trick the car into thinking it's one battery couldn't you?

Most of it has to do with the terminals. Hybrid and EV packs that use that sort of cell have welded or screw terminals in order to make a strong, low-resistance connection. Otherwise you run into problems given the very high currents that run through those things. There are probably quality differnces between consumer-grade batteries and the commercial cells. Even if it's just consistency between batteries, that's a very big deal when you build a huge series pack like that.

Which is not to say you can't. Just that the results would be less than spectacular.

The first Prius battery pack was a bunch of D-cell batteries. (link (http://john1701a.com/prius/prius-history.htm))

The current Insight pack is STILL a string of D cells...

That’s basically an Insight battery pack too...

Didn't see Shifty's post before mine.. Check this out too:

http://www.milwaukeetool.com/us/en/news.nsf/vwPressReleases/05B7EA8847B096A48625710A006F210D?Open

BTW - 100th post :woot:

Not a dumb question.

Most converted EV's use heavy, flooded lead acid batteries because they're pretty tolerant of abuse & provide acceptable cost/mile over their lifespan.

Also, the large format cell size makes it easier to keep the pack in balance.

A big issue in linking multiple cells is that cell voltages tend to drift apart in regular use. Unless there's some means of monitoring and bringing them back into balance when this happens, you'll soon start killing individual cells through overcharging and over discharging - which can lead to a cascading failure of even more cells, quickly ruining the pack.

The average EV hobbyist just doesn't have the means for creating such a monitoring system, and it adds significant cost to the already higher initial cost of the "advanced" cell chemistry.

That said, Tesla Motors has the means:

The Roadster's pack is comprised of 6,831 individual lithium ion cells - with attendant monitoring system.

http://www.teslamotors.com/blog4/?p=59

AC Propulsion is doing the same thing with their eBox pack.

There are other issues with using smaller cells, having to do with putting the smaller batteries in parallel.
If one string of the cells develops a shorted cell, it will be at lower voltage than the ones it is in parallel with, and current can flow between the parallel strings of cells. Balancing becomes more difficult as well. Tesla spent 75% of their development money on solving those problems, which amounted in computer battery management for each series string of cells. Very complex system, beyond what most home built EV builders can handle.
Some photos of the Prius and Insight batteries.
http://www.99mpg.com/resources/articlesandblogs/batterypacksexpose/

What MetroMPG and Mike Dabrowski said. You'd have to have several parallel strings in series, each with its own separate charging and monitoring system and probably other problems I haven't even thought about. To really make an EV work you need large format cells. Which Chevron/Cobasys will no longer let us have.

. You'd have to have several parallel strings in series
.

Technically you do the opposite, several cell series strings in parallel.
The original cell voltage differences are already somewhat averaged at the series total voltage. for a series string.
You also need to think about charge balancing substrings, i.e. 7.2V or 12V as the ends/middle can become unbalanced. The only other comment to be added it is best to start with "matched" cells, i.e same Ah (off the shelf Ah are variable) which you may need to order directly.

Doable for NiMH but for Li ion the BMS with overcharge protection for each cell can be too expensive for the EVer.

Good Luck
Andrew

Alright, so everyone says NiMH batteries are too expensive, so I guess most people use a bunch of large, heavy lead-acid batteries. Now my question is this, why can't you just string together a whole bunch of AA NiMH batteries? I guess I just don't know enough to understand why you couldn't do that.

I mean, I suppose you just can't get enough energy out of them in an instant to run a car? But it seems like you could just suck energy out of multiple batteries at the same time and trick the car into thinking it's one battery couldn't you?

Actually, both NiMH and Lithium (Ion and Poly) are cheaper in larger form factors. It'd be more expensive by ~ 10x, charging and replacement would be a pain in the butt.

Its just than even "cheap" batteries are expensive for the power/energy draws you'd need.

Oops, I didn't read what I was typing and got that reversed. Yes, several series strings in parallel.