Poll

How can my wounded NiMH AA cells be used?

Build them into a clever BMS, see comments below
1 (50%)
Use them carefully in non-critical gadgets
0 (0%)
Forget it, recycle them!
1 (50%)

Total Members Voted: 2

Voting closes: May 18, 2017, 09:35:04 AM

Author Topic: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?  (Read 1470 times)

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DamonHD

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I've let > 100 NiMH hybrid/LSD AA cells [GP ReCyko], nominal 2000mAh each, get run down to << 1V due to a bad software bug in my valve code.

If they could be brought back to some sort of life, say 1Ah each ~1Wh, I'd quite like to harness them to make a 100Wh addition to my 12V-nominal solar off-grid system.

To do it sensibly seems hard, because I imagine the balancing issues will be horrible.  I am envisaging a microcontroller looking after each string, say, and making sure that cells don't get too unbalanced (though maybe it could just tell me to take bad ones out manually).

Is there a sane way of doing anything more useful with the cells than recycling them?

Rgds

Damon

PS. At this moment I'm trying to weed out any really unhappy cells by attempting to charge them in a smart/fast charger...
« Last Edit: March 20, 2017, 02:12:49 AM by DamonHD »

mab

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #1 on: March 19, 2017, 02:19:38 PM »
As I understand it Nimh cells charge like NiCd - best way to equalise a string is charge until they start heating up (fast charging), otherwise 16hrs at 0.1C.

If your cells are a bit of a mix, I'd full charge them individually, then do a rough capacity measurement on each one (removing and duds), and sort them into matched sets of 10 (12v) as best you can.

I'm not sure how well they'll work if you're going to use them as parallel strings when charging off solar though - to fully charge and balance them you may have to do the strings separately, as Nimh like NiCd don't have a rise in voltage when they're fully charged, so one string will take more charge and heat up whilst the others sit there part charged; maybe a microcontroller to monitor the temps of the individual strings and disconnect each string when it starts to heat up?

Warpspeed

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #2 on: March 19, 2017, 04:32:15 PM »
If you can buy a batch of these brand new and all matched, you may stand a faint chance.  But more likely you will be mixing a bunch of cells from different manufacturers with different ages and past history of abuse.
To get anything worthwhile is going to take a lot of cells, and that just means a lot of potential trouble.

There are people breaking up Lithium batteries taken out of Chevy Volt and Nissan Leaf cars, and you can buy these second hand Lithium cells of around 65Ah capacity.
Just one of those cells, and 65 Ah will reach your power storage goal very easily.

DamonHD

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #3 on: March 20, 2017, 01:42:10 AM »
These are all cells from one manufacturer [GP ReCyko]; they will have had different amounts of abuse at the hands of my software error!

Rgds

Damon
« Last Edit: March 20, 2017, 02:12:33 AM by DamonHD »

george65

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #4 on: March 20, 2017, 04:37:48 AM »

Sounds to me  I'm sorry like an exercise in futility and wasted resources and effort.

I'll play devils advocate and lets say you get 1.5Ah out of each cell. That will give you 15Ah out of the pack.
I have 2 SLA batteries sitting on my desk in front of me I have been using for playing with arduino projects and other things that are 9 ah each.
I just saw a seller on ebay selling 2 yo cells the same size 2 years old taken from a UPS they want $5 ea for. you could wire these 2 batteries together in 2 min and have more power than what you are talking about.  Doing it with AA's just seems like too much effort and resources to make it worth while.
I was also looking at several sellers adverting good ( tested) 100ah 12V deep cycle batteries for $100.

Another thing, is your current battery setup NIcad, lead acid or something else? If it is different, how are these cells going to fit in with regards to voltages, Discharge curves, charging and everything else?

How much time is going to go into the pack you want to make? How much resources in the form of wire, solder, power for your iron, holding/mounting the packs, wasted power in testing and then you don't know how long the get up will last before it all becomes a waste.
Yeah, I get you want to utilise a resource but like a lot of recycling, sometimes that is more energy intensive and uses more resources than it's worth.

I think in the grand scheme of things you might be better off to concentrate on something with a more substantial and likely more reliable return before worrying about something that is going to have as little as what you are proposing.

DamonHD

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #5 on: March 20, 2017, 04:43:37 AM »
I think it is probably futile to wire up the AAs as you suggest, in terms of time and other resources, but I want to try to work it out rather than assume.

Currently my system is based on a (rather ill) 12V gel lead-acid bank, but it has had ~200Wh of LiFePO4 in it in the past, so I nominally have various methods of accommodating other chemistries and taking some cycling load off the gels.

http://www.earth.org.uk/expanding-off-grid-PV-system.html

Rgds

Damon

Bruce S

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #6 on: March 20, 2017, 07:55:00 AM »
DamonHD;
Sorry for the late post, but having run stuff with a mix of NiCd and NiMh , it really depends on how far down they went.
IF this is say 0.8V per cell. I'm betting you'll be okay.
I've brought back ones from as far down as 0.5 and they are still working. I usually de-rate them to 2/3 full cap.
My cells range anywhere from the 325mA up to 2000mA. The higher densities have a harder time bouncing back, but rarely are they completely toast.
In my regiment; I charge them aback up to full cap over 15hrs , then put them into a solar light for testing over the next few days.
This actually works the batteries backup , the bad ones will not last like the others. A full days charge has my 2000mA units lasting a minimum of 6 hrs.
Hope this helps
Bruce
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DamonHD

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #7 on: March 20, 2017, 08:38:13 AM »
Picking a few at random out of the box:

0.88V

1.28V

1.30V

The charger has not rejected any as "bad" yet, which it sometimes does to really ill cells.

Rgds

Damon

Bruce S

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #8 on: March 20, 2017, 09:54:36 AM »
They're all probably good. I would charge the 0.88V one by itself and give it a good workout. Pretty sure it'll be just fine.

Curious, how many in the back do you have?
And more importantly , what was the "bug" in the program?

Bruce S
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Simen

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #9 on: March 20, 2017, 01:43:31 PM »
If the lowest are no lower than 0.8V, i would guess close to all of them should get up to rated capacity. It's 0.0V that kills capacity in NiMn cells

I have 2 NiMh AA cells rated 2500mA that i forgot in a flashlight for 18 months, and they measured 0.00V ;) I trickled them carefully with some mA's until the reached around 1.0V, and then put them in the charger. My charger can charge/discharge cells in cycles, and stop when max capacity are reached, and these cells ended up at ca. 1700mA.
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Mary B

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #10 on: March 20, 2017, 02:34:08 PM »
Should be able to use an Arduino or other small micro to build a smart charger that could monitor x battery banks and charge them separate. Use all the I/O lines, one to turn the charge on to that bank, another to monitor the temperature. Software would be relatively straight forward poll each bank once  a minute and it it is starting to heat turn the FET off that is passing charge to it.

Warpspeed

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #11 on: March 20, 2017, 03:00:18 PM »
That would work well for only a few cells.
The original poster is suggesting >100 cells.
I just cannot imagine monitoring and individually charging over one hundred individual cells.

DamonHD

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #12 on: March 20, 2017, 03:22:18 PM »
They're all probably good. I would charge the 0.88V one by itself and give it a good workout. Pretty sure it'll be just fine.

Curious, how many in the back do you have?
And more importantly , what was the "bug" in the program?

Bruce S

My charges does do the cells individually.  And hasn't refused one yet, about 60 or so in at a guess.

I have another ~40 sets of valves to collect, so maybe 240 cells in them, most of which have been run down.

The bug was an error in start-up.  We don't like GPIO pins to float on our board because that ends up eating power, so all unused pins get pulled to one rail or the other at boot.  And experiment showed that GND is a bit better than Vcc.  UNLESS a couple of your pins now have pull up resistors on them for input buttons.  Whoops: drained the batteries in 6 weeks instead of the planned 1Y or so.

So the batteries (2xAA) ran down to ~1.8V, just below where the brown-out detector won't let the MCU start-up...

All my fault.  Forced us to redeploy a huge number of units.  Rather embarrassing and expensive.  %-P

Rgds

Damon

Bruce S

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #13 on: March 21, 2017, 06:54:44 AM »

All my fault.  Forced us to redeploy a huge number of units.  Rather embarrassing and expensive.  %-P
Better to find it now than later.

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Bruce S

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #14 on: March 21, 2017, 06:55:49 AM »
I'm curious.
Are you using a timed charge? or using voltage cut-off? The NiMhs do not have the same nice dip that NiCds do. Close and means the same but not as detectable.
I gather that using a brown-out threshold you have sensors for low voltage shut down , what are you using for full-charge shut off?

Cheers
Bruce S
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DamonHD

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #15 on: March 21, 2017, 12:16:41 PM »
The devices destroying the AA cells are my doing.

The device that I'm using to recharge them is not made by me, and is one of these:

http://www.maplin.co.uk/p/maplin-nimh-and-nicd-aaa-and-aa-fast-battery-charger-with-lcd-display-n39cx

Rgds

Damon
« Last Edit: March 22, 2017, 01:14:54 AM by DamonHD »

Mary B

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #16 on: March 21, 2017, 02:07:23 PM »
No need to do individual cells in a string of cells. Monitor the string and match the batteries in the string as close as possible. These are not Lithium cells that explode easily.

That would work well for only a few cells.
The original poster is suggesting >100 cells.
I just cannot imagine monitoring and individually charging over one hundred individual cells.

Warpspeed

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #17 on: March 21, 2017, 03:58:24 PM »
Not so sure you can trust 100+ cells in series to behave themselves without individual attention for any length of time, especially if the battery is cycled hard and frequently.
If only one cell spits the dummy, the whole battery then has a problem.

mab

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #18 on: March 21, 2017, 05:55:50 PM »
OP was talking about using on a 12v system - 10 cell strings shouldn't be a problem.

DamonHD

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #19 on: March 22, 2017, 01:15:49 AM »
Thanks for all the input.

A number of 10-cell strings looks plausible, and if any one string seems to get out of kilter then I can probably rebalance it manually.

Rgds

Damon

Bruce S

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #20 on: March 22, 2017, 10:30:51 AM »
Damon;
How critical is the voltage level? what's in the in-rush current going to be?
IF the voltage has a brown-out level , you might want to think about going to a 11-cell setup. NiCds will voltage dip IF the startup current is high enough. OF course is bounces back up, but , if you have a brown-out setting then you'll need to look at a cold-start startup testing.
NOT completely sure where these are going to be in use at (inside or out) but ,,,
All the later banks I built 12V@20Ahr & 12V@16Ahr had the 11th battery, this was necessary to get past the voltage dip.

BTW: My 16Ahr bank is still working like a champ  ;)
I have my charge dump set to begin dumping at about 14.5Vdc this allows for the extra battery and allows for a good 70% recharge. I've done this it's entire life-span; owing to the fact that NiCds like being used.

Hope this helps

Bruce S
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DamonHD

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #21 on: March 22, 2017, 11:36:12 AM »
This would probably be a 'backup' supply to the RPi in the first instance (using <50Wh/day) and then the networking equipment (300Wh/d).  The first would be happy with voltages down to maybe 9V but the power is ~1W--2W.

I was thinking that the way to control inrush would be to have a slightly smart switching reg limiting current and voltage per NiMH string.  About time that I learned to play with inductors, 40+ years into messing with electronics...

Rgds

Damon

Bruce S

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #22 on: March 22, 2017, 12:36:53 PM »
Damon;
You might be thinking about this too hard , even 2 watts at 9V ain't gonna do that much damage.
If you're at 12Vdc and the unit is going to work all the way down to 9V , then you're OK at 2Watts inrush you're only going to see about a 1V dip at most (I'm using mental math, so beware )
Try out the 50Wh/day I'm pretty certain you'll be happy with the outcomes.
Do you have a PV that you will be using?

BTW: You're still light years above my programing skills!

Cheers!
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DamonHD

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Re: Has anyone built a BMS for NiMH (AA) cells to use with eg Solar PV?
« Reply #23 on: March 22, 2017, 07:45:20 PM »
I have >500Wp of off-grid PV now!  But I'll allow the feed via the existing MPPT controller if I do this, which is another reason to limit surge current, since I don't want to fry my main controller.

Rgds

Damon