Charging NiMH Bank

[Particle]

Hello-

Does anyone happen to know if the Kid charge controller can be customized to properly/safely charge a bank of nickel metal hydride batteries?  I know nickel chemistry is unusual in solar applications, but since it is an experiment I'd like to perform I thought I'd ask.

I saw today that the controller has a "custom" charging option, but I didn't know how custom that can get.  Delta-V is probably out since there will be a load on the bank while being charged.  I'm not sure what a controller would be expected to do for NiMH.

Thanks

[inetdog]

From what I have read about the characteristic voltage versus charge curve on the NiMH chemistry it would not be safe to try to charge while there is a variable load on the battery.
NiMH really demands a positive end of charge, not float or trickle current to avoid damage, and unless you very carefully and precisely measure the battery temperature, the end of charge indication is either the inflection point in the V versus time curve or an actual decrease in voltage with time (and that one is subtle and easy to miss even when there is no load on the battery.)
If the CC is able to measure battery current only via a shunt, and uses an algorithm to maintain constant current into the battery even though the load is varying, you might be able to use the voltage signatures.
But the other problem is that if the charger starts out from scratch with a fully or near fully charged battery, the voltage signature may not even be present in a form the CC could recognize. It needs the baseline of the earlier stages of charging as a starting point.
You can see lots of discussion of NiMH charging algorithms on www.candlepowerforums.com.

The alternative, I suppose, would be to use a very low current and terminate based entirely on voltage. But since this would be a slow charge people tend not to do it.

[Westbranch]

you might look into the \blue\sky line's algorithm.  I have an old blue sky 2000 and it has a NiCd setting.  Do not know if it will help but a start...?

[mike90045]

NiMh is not a very robust cell.   I'd hazard to guess that an accurate temperature measurement of cell case, would be the best option, since voltage will be wonky with loads and varying solar charge current .
NiCad settings (and NiFe) are voltage based, but since flooded cells are used, makeup water can always be added.

[Particle]

It's unfortunate to hear that NiMH probably won't be doable, at least in an automated fashion.  I really like the chemistry.  Perhaps I could use two banks--charge one while the other is under load.  I'll have to consider the options for a while.  Though, if someone does have an answer please do chime in. 

Westbranch, how do you like your Cotek inverter?  Which precise model are you using?  I was thinking of buying an SK series 1500.

[tecnodave]

Particle

I have been using a Cotek SK1500 24 volt unit for a year now. Very good unit, good surge capability, apparently others like this too as it is relabed as the Samlex SA 1500 K , sold by the Vanner Company as a heavy duty bus and ambulance inverter and others , Go Power of Tiawan is the same unit.
Designed by Cotek of Tiawan and built in China.

This model may be phased out soon as the SA 1500 is a bit more technologically advanced and the
SK1500 24 volt units have been appearing on Ebay and Amazon in the $225-250 range. USD

I use it my shop for power tools and it handles reactive loads better than most high frequency inverters. If your loads include large electric motors then the low frequency types are better.
I have a Dynamote Brutus Inverter which handles my big motors ( it is a high/low frequency hybird much like the Outback and Magnum Design) but these units are in another price category all togather.
($1200-2400 USD)

The Cotek SK1500 handles inductive loads (motors) better than my Xantrex 1800 watt Pro-Sine!

That said I do not like Chinese products and much prefer American technology,......I'm patiently awaiting the MidNite inverter

David

[tecnodave]

Particle and others,

I have been using my kid to charge NiMh and NiCad power tool batteries which the supplied charger will no longer recover. I charge using a solar panel and a simple series current regulator to get them warmed up (up from zero volts to about 60% of normal voltage) then switch to the kid with current limiting set down to 2 to 4 amps but I use an industrial temperature control to shut off all power when cell temperature exceeds 125-130 degrees F.

I have successfully recovered Makita 12 and 18.2 volt 2.4 a.h. NiMh  (  , DeWalt 12,18.2 and 36 volt NiMh (4 so far)  and Panasonic 15.6 volt NiCad batteries  (4)

After about 4-5 cycles of manual charging the batteries can be recharged by the computer based charger which is supplied by the tool manufacturer which are very conservative in charging reluctant batteries.

NiMh and NiCad power tool batteries rise in temperature rapidly when fully charged as they throw off excess power by heating. All manufacturers of power tool chargers use thermal feedback as all these batteries have thermistors built in to them.

I reverse engineered the factory chargers to come up with my profiles.....temp is most important, current must be controlled and voltage does not seem to matter to much. One Makita 18.2 volt NiMh needed 90 volts to " wake it up".

I do not do this on an automatic basis as these battery types can explode when abused, they are very expensive.....$107.00 USD for a Makita 18.2 volt 2.4 a.h. and not very available.....this extends the life of my power tools which I use in the field in the electrical construction trade.

These power tool batteries are C cell format and I suspect are much different than what you describe.
I have never seen large format NiMh batteries.

David

[Particle]

Large format cells continue to be patent-encumbered, unfortunately.  Instead, high-power applications tend to use lots of small cells.  The Prius is a good example of this, and my bank of cells is the same.  It's a bunch of C-sized cells tied together.  They're also built with D and F sized cells, but I couldn't afford them.

Is the MidNite inverter going to be available in a 48V model?  Any idea of its rough feature outline?

[tecnodave]

One bet on the MidNite inverter is that all the features of the Outback's, Magnum's, and Schnieder inverters will be matched and exceeded. Communications with the Classic and share WBjr data.
24 volt and 48 volt is assured as this is the most common voltage used in larger solar applications. Maybe there will not be a 12 volt version as this voltage is too low and limited for serious solar applications.

My wish.......load sharing with the generator....AND......utility power much like Magnum has done with the new MSH4024RE

There are many comments and wishes in various threads about the inverters and I am sure that MidNite will acknowledge these and incorporate peoples wishes and desires in their design.

David

[tecnodave]

Particle


Just curious how big is your NiMh bank?  I have several 12 volt 12 a.h. Banks which are the good cells from failed power tool batteries which I use to power portable lights in construction. All of the failed power tool batteries that I have disassembled for this use had only one or two dead cells. These are available at recycling centers as most people buy new ones or have moved on to Li ion batteries.

For your info....Trace C series controllers do have a NiCad option and I have modified one for my NiCad and NiMh but this controller is only 12,24 or 48 volt........most of my power tolls are Makita 18.2 v.NiMh , with a few other oddball voltages thrown in.......DeWalt 36 volt....Panasonic 15.6 volt
Etc.  The Kid is the only controller that I know of which can charge these.

David

[Particle]

I'm doing a preliminary test with 40 cells right now, but there would be 400 cells total (48V, 60 Ah / 2.88 kWh).

[tecnodave]

Particle,

When I recover dead batteries I string them up long strings and read the voltage on each one to weed out weak ones, current at 2.0 amp and keeping an eye on the temp all the time.I recover 85% of the cells that I remove from dead power tool batteries and match them up in equal voltage sets to use in other portable applications. When I get stable 12 volt sets they last about 3-4 years before failing to charge again.
I'm interested to hear about your experiment.

David

[Particle]

I managed to start testing the first batch of cells today.  I knew they'd be overrated since they were cheap imports, but man.  The cells weigh 45 grams each which would put them on par with a quality 2 Ah cell.  Charge and discharge testing will commence tonight hopefully, and I'll post back with results in case anyone is interested.

I've found that not all chargers for NiMH batteries seem to rely on dV or dT.  Some chargers like the well-regarded MAHA MH-C9000 simply charge to a fixed voltage point.  The target point presumably changes based on charge current is all.  Once it hits about 1.52V at 1C, it starts a topping charge at say 0.1C for two hours.  This makes sense as most cells can tolerate 0.1C nearly indefinitely.  In any case, I don't know how cells would behave with a variable load in parallel with the bank, but I figure I may as well see what happens.

[zoneblue]

Years ago i worked in battery lab servicing wet NiCd cells. We had two methods of charging them, constant 0.1C  current until 1.6V pc, and the big pulse charger which would deliver a 1s pulse of 400A, then a short discharge pulse, and repeat. I cant remember how it terminated. But it was huge, noisy, and expensive.

These days the best algorithum for both cells is to use a microcontroller and wait for the rate of voltage increase to peak then start to fall. It does this prior, (actually quite a bit before) minus delta v/delta t. If you wait til the voltage starts to drop, or heats up then youve overcharged the pack.

I have no idea how youd do any of that with a variable load connected.

[Particle]

One option might be to size the battery bank to be 10x larger than the max charge current.  Cells can tolerate 0.1C for as long as peak solar would be available during a day even at full charge.  Unfortunately, with as much research as I've done I've found that NiMH costs more than LiFePO4 at this point.  Up-front cost for AGM SLA is about $0.18/Wh, LiFePO4 is about $0.45/Wh, and NiMH is about $0.65/Wh.