Building a NiMh pack from 176 cells?

[BatteryCharger]

I happen to have a cheap source for a LOT of quality 2500mah AA NiMh batteries. I want to build a 13.2 volt, 40AH battery pack, which would require 176 cells. :faint:

Is it a bad idea to try and make a pack with that many cells? I know I can do it with fewer, but not for less money. I want to make it in several individual peices so I can replace them if necessary.

If I go ahead with it, would it be better to wire them as (16) individual 13.2v strings (2.5AH x 16 = 40AH) or, (11) individual 40AH, 1.2v packs?

Also, aside from accurately measuring the capacity of each battery, how should I match the cells?

 

[zipplet]

Have you thought about how you are going to keep a pack that large nicely balanced? Usually one could use a slow charge (0.1C) to help balance a pack but I don't think that's going to work well with 176 cells with many many in parallel.

Will you charge them by disconnecting parts of the pack and charging individually? Also, please please use fuses or you might wind up with a fire on your hands...

Edit: You can use a nice smart charger like the LaCrosse BC-900 or the Maha 9000 (forget the exact model) to not only cycle the batteries for maximum capacity initially but also to figure out the capacity of each and help you match them up - but with 176 cells this will take you a few weeks

 

[SilverFox]

Hello BatteryCharger,

Interesting project...

I would suggest that you build 16 strings. This would allow you to use a hobby charger to charge 2 strings at a time in series. This still means that you will have to do 8 runs for a complete charge, but you could use multiple chargers.

You first match cells on capacity, then on voltage under load. Be sure to run a couple of standard charge/discharge cycles before you move on to matching. The closer you match, the better the pack will perform, but with that many cells close matching may not be possible.

If I were working on something like that, I would order 250 cells, and pick the best 176 from that.

Tom

 

[BatteryCharger]

Thanks for the info guys...a little more though...

Basically I'm building a "12 volt" general useage battery, to replace a deep cycle lead acid 40AH AGM battery I used to use that died. It weighed 40lbs, the NiMh should weigh more like 12. :D

My charger is an automatic 9.6-18v, .9A or 1.8A unit. So if I charged everything at once it would be about a C/22 charging rate. Would that work or should I seperate them out to charge them? I won't be in much of a hurry.

As for matching them, I really don't have time to measure the capacity of each cell. Is there a sort of "quick and dirty" way to do it? For example, charge them, let them rest overnight, apply a 5A load, and measure voltage?

 

[lctorana]

...a LOT of quality 2500mah AA NiMh batteries...

I hope for your sake these are not Energizers...

 

[Mr Happy]

Thanks for the info guys...a little more though...

Basically I'm building a "12 volt" general useage battery, to replace a deep cycle lead acid 40AH AGM battery I used to use that died. It weighed 40lbs, the NiMh should weigh more like 12. :D

My charger is an automatic 9.6-18v, .9A or 1.8A unit. So if I charged everything at once it would be about a C/22 charging rate. Would that work or should I seperate them out to charge them? I won't be in much of a hurry.

As for matching them, I really don't have time to measure the capacity of each cell. Is there a sort of "quick and dirty" way to do it? For example, charge them, let them rest overnight, apply a 5A load, and measure voltage?

OK, I'd go for the sixteen 13.2 V strings, and then wire all the strings in parallel. The reason is that this helps to keep the current evenly balanced between the strings (putting 12 cells in series averages out any differences between individual cells).

If you are going to charge at a C/22 rate, then I think you can just charge the whole caboodle as a single pack. It is normally advised not to charge NiMH in parallel, but when you have such a low charging rate this advice is not so applicable. All the strings will see the same voltage at each end, so each string will eventually get charged. Just be aware that it might take 30 hours or so before you have a full and even charge on the whole pack.

What you ideally will want to do is have some way of checking the temperature of the cells, and stop charging if any string starts to get too hot. But my feeling is that this is not going to be essential.

 

[travelinman]

Just make sure you've got a smoke alarm nearby! :candle:

 

[Burgess]

Test it OUTDOORS !


:devil:

_

 

[BatteryCharger]

Well, this thing should be safer than my old battery. It was capable of 2,400 AMPS short circuit - as I carefully tested with a wrench across the terminals. :shakehead It literally melted through the box end of a steel wrench. I'll probably put a 150 Amp breaker as a limit for the output of the entire thing, maybe a few individual high amp fuses between strings.

I'm not supposed to name the cells because "they don't represent the quality standards expected by the customers" but it rhymes with smuracell 2650.;) That's why I'm getting them cheap, they are cosmetic rejects.

A 42.4 Amp Hour NiMh pack should be fine surging up to 150 amps for short periods, shouldn't it?

 

[lctorana]

Maybe, if there's an electronics surplus store, you can find some of those massive high-current fuses meant for railway use etc.

 

[mdocod]

Few things to consider:

with 11 cells in series, you'll potentially have a peak fresh off the charger voltage in excess of 16V, make sure the devices you are intending to power are going to be okay with this, or revert to a 10 cell series design.

For the sake of convenience, I think charging it slowly as an entire pack as Mr. happy said would be acceptable with the slow charge rate, HOWEVER; build the pack in such a way that each series string can be isolated and removed if need be. Once a year or once every 6 months or so, it would be best to run each of those individual series packs on a few exercise cycles followed by it's own independent 0.1C for 16 hour balance charge. A charger that could give you some information about the amount of charge taken, or energy removed during a discharge cycle would be ideal to weed out under-performing strings. Have a spare string or 3 separate of the whole battery that you exercise once and awhile to keep on hand for a swap-in-partial-pack-repair-situation.

Considering all the hassle involved with building the pack, testing cells, wiring things up and somehow shoe-horning it into some sort of fairly rugged enclosure, followed by the enormous amount of work that's going to be required to keep the thing properly exercised, balanced and working, even if I were getting the cells for FREE I wouldn't bother with this project. Maybe if it were D or F cells in smaller quantity I might consider it...

FYI: you can buy a brand new 12V ~50AH deep cycle marine gel cell battery for ~$150, probably less if you look around...

Assuming you were getting 200 smuracells for free, after considering all the materials you will need to build this pack up in a "nice" build, (shrink-wrapping individual strings into packs, installing plugs on all individual series packs, building a "bus-bar" unit with female plugs for all the strings to plug into to that wired them up in parallel, some large "lug" connections, plastic enclosure, various fasteners, threaded rods, nuts etc....)... I'd be willing to bet you are going to have at least $75 bare minimum in materials not associated with the cells just to build this. That's assuming some really serious wheelin and deelin and maybe a barter or 2 swung up in there....

Is it going to be worth it?

Spend the time and money on a LiFeP04 pack if you want a serious upgrade. Building this AA pack will need to be looked at from the perspective of "because I can" and not necessarily "because I should." It'll be a fun project.

-Eric

 

[travelinman]

I run a string of 6, 2 volt 270 A/hr Lead Calcium alloy cells hooked in series to produce vast quantities of power for my ham radio emergency station.

These batteries are considered very stable and safe as they are Lead - Calcium alloy. However as with any cells, the more out of sync with each other, the more danger there is in drawing down the weaker cell which results in disaster.

I do an equalizing charge every 3 months to keep the danger down, and by equalizing I mean each of the cells MUST be kept to within 0.050 Volts of the average of the bank of cells. This is per CD technologies recommendations.

Be very very very careful of any bank of cells that does NOT consist of cells very closely matched.

This is not just data relevant to Lead Calcium cells, it relates to all banks of any type of cell. The more amperage you have, the more danger you are playing with.

Do a search for the "off grid" solar types, they have lots of great info on hooking different types of cells together.

 

[BatteryCharger]

Considering all the hassle involved with building the pack, testing cells, wiring things up and somehow shoe-horning it into some sort of fairly rugged enclosure, followed by the enormous amount of work that's going to be required to keep the thing properly exercised, balanced and working, even if I were getting the cells for FREE I wouldn't bother with this project. Maybe if it were D or F cells in smaller quantity I might consider it...

FYI: you can buy a brand new 12V ~50AH deep cycle marine gel cell battery for ~$150, probably less if you look around...

Well, the problem with lead acid batteries is that they're made out of lead. :cool: 50 pound batteries just aren't very portable. Plus, you really shouldn't drain them all the way, so a 50AH battery is more like 20-30AH realistically. If I built a 50AH NiMh pack, I can use the whole 50AH without feeling guilty.

What I think I'm going to do is line up 11 batteries side by side. Then, I'll make a small wood frame around them from 5/8" square hardwood. Then, I'll put a peice of 1/8" hardboard on the top and bottom. To secure each battery, I'll use a set screw through the wood frame, pushing on the negative contact like a spring. Then I can make as many of these packs as I want and parallel them. This way I can easily replace any individual cell or pack in a few seconds. I'll probably put the whole thing in a plastic toolbox or something similar and easily available...I'm not even sure what size it will be yet. :green:

I'll try and make one pack tomarrow and post a pic.

 

[Mr Happy]

What I think I'm going to do is line up 11 batteries side by side. Then, I'll make a small wood frame around them from 5/8" square hardwood. Then, I'll put a peice of 1/8" hardboard on the top and bottom. To secure each battery, I'll use a set screw through the wood frame, pushing on the negative contact like a spring.

That's a good option except for heat dissipation. Wood is an insulator and the cells will get warm during charging. They might get too warm when all boxed in like that.

 

[travelinman]

I don't think you will want to drain a NimH cell all the way down to zero. It won't give you very many cycles that way. I could be wrong about them, but I know the big lead calcium cells I use are rated NOT when drawn down to zero volts, but down to 10.8 volts.

I wasn't suggesting you use 6, 90 lb cells unless you are King Kong, I was attempting to draw a comparison between the dangers of using cells that have different capacities. With that many cells, you cannot possibly know when one of them will crap out on you and create an imbalance, then a nice bonfire with all that wood around it.

 

[BatteryCharger]

I don't think you will want to drain a NimH cell all the way down to zero. It won't give you very many cycles that way.

I'm aiming for about 1v per cell, or 11 volts "dead". That way I should be able to use the full 2000-2650mah raiting on the battery, where as a lead acid you should only use half the mah rating or less.

I don't think heat is going to be much of a problem with my cells, since I don't plan on charging or discharging at a very high rate. I'll probably drill some ventalation holes in my cell packs though. I'm also going to break down and buy a BC900 charger so I can test the mah...just trying to charge all these batteries 4 at a time in my Maha charger is going to take years!