NiMH battery experience?

R Potter · 06-14-2008, 04:23 PM

Hello all, I'm new to this forum and this is my first post. I've recently ordered a new van from SMB West.

What I've been thinking about is batteries. In particular I've been thinking about NiMH batteries. They have a lot of virtues. They have considerably less weight and volume than AGM battreries. They also can be truly deep cycled -- up to ~85% of their AH rating (see
http://site.greenbatteries.com/document ... s-nimh.jpg

to see an example) and with the right charger can undergo more than 1000 charge-discharge cycles. I've used them for hobby applications and the ones I've used have been reliable.

The downside of NiMH batteries is the cost. They have been ~3 times the cost for an equivalent power level. However the price has been dropping. For instance at this site:

http://www.batteryspace.com/index.asp?P ... rodID=2617

you can find a premade NiMH battery pack with a 52AH capacity (true capacity 0.85 x 52 = 44 AH effective capacity) for $500. That works out to be ~$11.30 per effective AH (@ 12v). If you are willing to construct your own battery pack you can buy batteries with soldering tabs here (for example):

http://www.batteryjunction.com/1dni10rebawi.html

for $7 per AH (~$8.25 per eff AH). I can't vouch for these sites because I've never used them, but I hope the general trends are reliable.

For comparison I went to this site to find the price of AGM batteries:

http://www.bdbatteries.com/marinebatter ... agodF2SGWQ

There you can get a 4D battery with a 205 AH rating for $490. Which works out to ~$2.40 per AH. However because the effective rating of AGM batteries is about half their rated value, that means the effective price of AGM batteries is ~$4.80 per AH.

So the price ratio of NIMH batteries is now less tha twice as high as AGM ($8.25/AH vs $4.80/AH).

Has anyone thought about this, or even better experimented with NiMH batteries?

Thank-you.

**BroncoHauler** · 06-14-2008, 04:31 PM

Nothing to contribute, but a question.

How are the vibration and heat tolerance of NiMH batteries?

Herb

R Potter · 06-14-2008, 05:04 PM

Quote:

Originally Posted by Bronco_hauler

Nothing to contribute, but a question.

How are the vibration and heat tolerance of NiMH batteries?

Herb

NiMH batteries have a reputation for good thermal properties.tolerance. See here for example:

http://www.txobattery.com/pdf/presentat ... t_tech.pdf

As to vibration, I haven't seen any mention of that, but the links I posted in the original post were for regular D cell batteries put together in series and parallel to make 12v packs. My experince with radio controlled cars is that they survive crashes without any apparent issues. I have a 2001 Prius that has NiMH batteries and under regular driving conditions they have lasted very well the last 7 years.

Greg In Austin · 06-15-2008, 08:21 PM

I am not sure I agree with some of the things suuggested so far about NiMH batteries...

There is no question that NiMH battery technology is some of the best battery technology out there, but some of this depends on application.

NiMH is the ONLY proven traction battery technology; they were used in the Toyota Rav4EV, Gm EV1, Chevy S-10E among others. The Rav4EVs are still out there running 100 miles on a charge. All of these used large format NiMH batteries

The current generations of Hybrids are all using NiMH batteries, BUT they are NOT large format NiMH batteries. There are NO large format NiMH batteries available today.

THis battery technology was developed in the 1990s by ECD/Ovonics (Energy Conversion Devices) with help from money from the US ABC (Advanced Battery Consortium). This technologu was purchased by GM, but when they won their law suit with CA and stopped the EV1 program, they sold the technology to Texaco. Two weeks later Texaco merged with Chevron. Chevron then spun off Cobasys to own this technology. Chevorn is squatting on the patents and they do not expire until 2014. Recent rumor has it that GM is trying to re-aquire the tech.

Although you might be able to use a large amount of NiMH capacity, they require a BMS (battery management system) to control the charge and avoid thermal issues for larger format batteries.

The Prius is a very good example. The Prius MFD (multi function display) has a SOC (state of charge) meter, but when the meter is "full" the battery is at 80% SOC, and when the meter shows empty the battery is at ~40% SOC. The Prius does not allow the battery to go outside these limits and thus takes very good care of the battery. When the SOC of NiMH goes much above 82% you hit thermal issues and heat + Batteries = shorter life.

Most large format NiMH batteries are actually made up of a number of smaller prismatic cells that are then physically bound together within a metal case to keep them from expanding and breeching their case under thermal pressure.

There has been much talk about Li batteries over the past few years, but NiMH is the ONLY proven technology for large format traction type batteries.

You are not specifically talking about a traction application here, but the depth of charge descibed could be a problem. The lack of a BMS could also be a problem. You cannot overcharge these batteries.

NiMH is just less forgiving.

AGM and Pba batteries are more forgiving and can take more abuse when there is no BMS.

.

Greg In Austin · 06-15-2008, 08:34 PM

Things I forgot to include in the previous post....

The Prius battery pack is made up of 20+ smaller NiMh batteries.

It also has a cooling systemthat includes a fan to pull air from the passenger cabine to keep them cool if they are charging to close to 80% SOC on a high ambient temp day.

The EV1 and S-10E both actually used the air conditioner and sunted air conditioning from the passenger cabin to keep the batteries cool ...a priority.

Replacement NiMH batteries for our S-10E will run around $8k ...ouch

Oh.... and there is also a lot of speculation about what will happen when the NiMH patents expire. You can expect Li to again take a back seat for larger format applications.

You can expect to hear a lot less about Hydrogen fuel cells and FCVs.

Did anyone see the recent press release that Toyota is breaking ground for a new battery manufacturing plant? It was Toyota/Panasonic that improved on the technology from ECD/Ovonics. THose are the batteries in the Rav4EV. ...and it was Chevron that shut down Toyota with threats of a lawsuit.... that was settled... and now Toyota has to use a less efficient smaller format...

.

R Potter · 06-16-2008, 05:53 PM

Quote:

Originally Posted by Greg In Austin

I am not sure I agree with some of the things suuggested so far about NiMH batteries...

There is no question that NiMH battery technology is some of the best battery technology out there, but some of this depends on application.

NiMH is the ONLY proven traction battery technology; they were used in the Toyota Rav4EV, Gm EV1, Chevy S-10E among others. The Rav4EVs are still out there running 100 miles on a charge. All of these used large format NiMH batteries

The current generations of Hybrids are all using NiMH batteries, BUT they are NOT large format NiMH batteries. There are NO large format NiMH batteries available today.

THis battery technology was developed in the 1990s by ECD/Ovonics (Energy Conversion Devices) with help from money from the US ABC (Advanced Battery Consortium). This technologu was purchased by GM, but when they won their law suit with CA and stopped the EV1 program, they sold the technology to Texaco. Two weeks later Texaco merged with Chevron. Chevron then spun off Cobasys to own this technology. Chevorn is squatting on the patents and they do not expire until 2014. Recent rumor has it that GM is trying to re-aquire the tech.

Although you might be able to use a large amount of NiMH capacity, they require a BMS (battery management system) to control the charge and avoid thermal issues for larger format batteries.

... [snip]

You are not specifically talking about a traction application here, but the depth of charge descibed could be a problem. The lack of a BMS could also be a problem. You cannot overcharge these batteries.

NiMH is just less forgiving.

AGM and Pba batteries are more forgiving and can take more abuse when there is no BMS.

.

Greg, I'm not sure either, that's why I asked. It's clear that you have practical experience with NiMH for traction batteries. Do you think that they are just a bad idea for house batteries. In those applications the current draw requirements would seem to be less (?).

I agree with your point about large format batteries not being available. That's why I mentioned packs of D cells. I've had some experience with packs of smaller cells for RC applications and they seem to be very reliable and temperature is not an issue, though the currents involved are small. When using them for house batteries, do you think air cooling is good enough?

As to the BMS, I was thinking of using one of the "smart charging" systems that come with these batteries for battery management. Is that likely to fail?

Thanks for the info.

Greg In Austin · 06-16-2008, 06:32 PM

RP,

If it were me, I am not sure that a pile of D Cells would be my choice. Our Honda Insight's battery is just that a whole pile of D Cells in series and parallel.

In that kind of a setup you will have to worry about temp and balance. Getting all the cells to continue to perform as originally designed or spec'd. What happens when that one cell starts acting up. Do you want to determine which one it is and dig it out for replacement? not me.

A pile of D Cell HiMH would not have the temp and expansion issues that larger format and large capacity would have.

We have been using NiMH batteries for a number of years around the house, and of the standard home size batteries I think the D Cells are the most disappointing. THey just don't seem to have the life that we think they should. AA seem to be fine and comparable to other non-rechargeable batts.

The D cells used in large packs like for the Insight are not your standard D Cells. The are that size, but not the same chemistry, and they do have the temp and expansion problem. These batteries are monitored for temp, and are cased to eliminate expansion.

If I were determined to use NiMH I think I would go with a larger format than D Cells.

...but that does come with more expense. It would take some sort of BMS in my opinion.

If you want to extend the life of NiMH they need to be used and charged gently. I don't think that our typical smart charging systems will work well for NiMH. The charging algorithms for NiMH seem to be much more complex and typically require computer control and sensor input.

I don't know, I just think NiMH for the type of use you are looking at would end up being more expensive and less satisfying. You still need a lot of amp-hours out of your SMB House battery, and that is not unlike a traction battery. The difference is how fast it comes out of the battery.

Don't get me wrong, I am a big NiMH fan. I think that after 2014 we will see a big change in what is available for batteries.

Pba is a proven and widely understood technology.

If you went with standard garden variety NiMH, then I suppose you could try and use the standard charging center technology, but you would also have to expec tto get similar life expectancy and performance.

The RC packs are usually pretty small. If you start putting packs together that are 10 or 20 or 50 times that size, will the charging technology scale and perform? I think that is where things start to break down.

The Tesla Roadster is using a giant pack of Li laptop batteries and it has a very complicated charging algorithm. Li has a thermal runaway problem which is arguably worse than the NiMH expansion issue.

Since smaller format NiMH are available, I think that if this were really viable that we might already be seeing it in application.

Bottom line, if you put your mind to it, I bet you could make it happen. So the real question is do you mind paying more? Do you mind putting the extra sweat equity into it? and are you willing to live with the performance IF it is not what you expect?

R Potter · 06-17-2008, 08:09 PM

Quote:

Originally Posted by Greg In Austin

RP,

If it were me, I am not sure that a pile of D Cells would be my choice.

Don't get me wrong, I am a big NiMH fan. I think that after 2014 we will see a big change in what is available for batteries.

Since smaller format NiMH are available, I think that if this were really viable that we might already be seeing it in application.

Bottom line, if you put your mind to it, I bet you could make it happen. So the real question is do you mind paying more? Do you mind putting the extra sweat equity into it? and are you willing to live with the performance IF it is not what you expect?

Greg,

Thank-you for your expertise and advice. I'm interested in this as a hobby sort of thing, but don't want to go whole hog with this.

After considering your advice, I'm thinking of putting together 10 D cells to make a 10 Ah pack and then experiment running some things around the house with it and monitoring the batteries. If I learn anything, I'll report back here.

Thanks again.

Randy

Greg In Austin · 06-17-2008, 09:03 PM

Randy,

That sounds like a good plan. 10 or even 20 should be manageable and somewhat inexpensive.

You might also look for a used or salvaged Honda Insight battery pack to play with; they are made of of D Size cells.

Good Luck and keep us informed along the way.