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
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.
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...?
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.
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.
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
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 (8) , 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
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?
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
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
I'm doing a preliminary test with 40 cells right now, but there would be 400 cells total (48V, 60 Ah / 2.88 kWh).
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
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.
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.
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.
Particle,
Have you looked into the efficiency of the cells themselves........I'm talking about how much you put in to how much you get out? NiCad and NiMh are not very good at that being less than 60 % at best for the C cell form factor. They have very good power to weight ratio, that is why they are used in power tools. And very fast charge times....45 minutes for a 2.3 a.h. 12 v. NiMh.
Compare that charge efficiency to 95% or so for a very high quality Flooded Lead Acid such as the Rolls Surette. And maybe even better for Lithium Ferro Phosphate.
I am only using NiCad for my older power tools like the Panasonic for which there are no NiMh available. All my Makita now have NiMh where available. They are environmentally cleaner and have better power to weight ratio but they need to be cycled.......you cannot use only 10% of the charge and pick it up weeks later.......it will be dead and won't charge........you need to run them down and fully charge them to full every week or you will have dead cells months.
td
Charge efficiency for NiMH depends largely on the rate of charge. At 0.1C it's only around 60% but by 0.5C it's around 95%. They don't suffer from the memory effect you're describing but they do have relatively high self-discharge. The discharge isn't terribly concerning though given that they wouldn't be used to store power longer than a day.
Can the Kid be made to stop charging based on battery temperature with a custom set point? If I could set it for say 50C and install a probe, that'd be an easy way to keep from overcharging regardless of the input current.
If the kind of LSD technology involved in the AA eneloops ever makes into bigger cells, that would be kind of cool. Very very high charge efficiencys there.
On balance what i would do is chart out a charge curve for your particular cells, at your typical charge current. Locate the point just prior to peak voltage, and use that as your absorb setpoint. Set absorb to 1 minute, and float to something low enough to get the charge acceptance down to 0.01C.
Repeating my point above, waiting for the cell to heat up is a mistake, never mind that heat reaching a large poorly heat bonded sensor like that. The internal pressure rises exponentially, starting from somewhere around the point where delta v/delta t peaks.
See http://batteryuniversity.com/learn/article/charging_nickel_based_batteries
That's an interesting idea. If I did charge at say 58V, that should be sufficiently before the peak that it would be safe. My concern though is that at lower charge currents that peak will occur at lower voltages.
Interesting. These cells I'm charging are actually cooling with respect to ambient. I wonder if these cells are actually NiCd with a NiMH wrapper on them. Crafty Chinese importers. heh
It has been a while since I last posted on this topic, but I haven't abandoned the project. I have however abandoned the generic Chinese cells and have instead settled on Tenergy 4/3 AF NiMH cells instead. I have done some experiments with 40 of the new Tenergy cells configured as 20S2P (20 in series, 2 series in parallel). I've got another 60 cells coming in a week's time since the pilot has progressed well. Currently, the reserve capacity is small at about 180 Wh, but that will increase to 450 Wh with the extra cells.
I've learned much about how NiMH chemistry behaves during charge by having manually charged the prototype pack a few times. The first is that I'm not sure that there is a suitable CV (constant voltage) or CC (constant current) target that can be set if the goal is to do a fast charge. The reason for this is that charge current doesn't naturally taper off at the end of the charge cycle. I'll describe what instead happens. If both a CV (1.425 Vpc) and CC (1C) limit are set (and they should both be), the pack will be limited by the CC limit for the first 60% or so of the charge. As it charges, the voltage gradually increases until it hits the CV limit. Once hit, charge current will gradually decrease through about the next 20% of the charge until it hits a minimum of around C/10-C/12. After this point, the pack will begin to warm for the first time and will do so quickly. Through the high-current and tapering-off parts of the charge cycle, the pack temperature will only have risen a few degrees. This increasing temperature combined with the dropping current is indicative of the pack having reached about an 80-85% state of charge. The tricky, unfortunate bit is that as the temperature rises the pack's internal resistance begins to drop again, and it will start to accept greater charge current even though it has been at the CV limit for quite some time without changing. Charge current will quickly begin to exceed safe limits if left untended, so it's necessary to reduce the CC limit to C/10 or below once this stage has been hit.
A solar charge controller would seem to need to charge this chemistry somewhere along these lines:
Temperature < (Room + 15C)
- Voltage < 1.400 Vpc : Charge at 1.425 Vpc or 1C max (fast charge)
- Voltage < 1.425 Vpc : Charge at 1.425 Vpc or C/100 max (maint charge)
Temperature > (Room + 15C) & < ((Room + 25C) or 55C)
- Voltage < 1.400 Vpc : Charge at 1.425 Vpc or C/10 max (slow charge)
- Voltage < 1.425 Vpc : Charge at 1.425 Vpc or C/100 max (maint charge)
Temperature > ((Room + 25C) or 55C)
- Do not charge until temperature falls to < (Room + 20C) or 50 C
All
- Do not switch to a higher-current charge mode within 5-10 minutes of having switched to a lower-current charge mode so as to prevent oscillating when at an edge.
- Do not charge when voltage exceeds 1.450 Vpc until pack voltage drops to 1.400 Vpc or below.
Correctly charging the pack with a variable load attached would still be possible I believe, but the temperature probe would be critical.
I've also given thought to what is essentially a bypass system for the battery pack with discrete charge and discharge ports. The charge side could allow or limit the charge current itself based on the rules above and cut off the charge port when out of the defined operating area. The discharge side would have no such regulation but have a diode and fuse in line. The charge controller could then also be attached to the discharge side after the diode, perhaps with an EDLC capacitor in parallel. This would allow the batteries to charge when it's safe to do so and the charge controller to continue to contribute extra current to the system when more than the pack's charge current is available from solar and the load exceeds that charge current.
A preferred alternative would be to use no bypass system but instead have a charge controller that compensates for a variable load by continuously reading the voltage drop across a standard shunt and adding the instantaneous discharge current to the charge targets.
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The graph below shows the condition that a charge controller would need to avoid. When charging at a constant voltage, charge current will eventually start to creep back up. Charging past this point (current minimum) can be dangerous unless the charge cycle is being actively managed by a person or controller. Left to themselves the batteries will self-induce failure due to internal pressure and temperatures exceeding safe operating limits. Prior to I-min, the batteries do not warm appreciably even at high charge currents of 1C.
This graph shows me manually controlling a charge cycle. The cycle starts with a mostly depleted battery and charge conditions of about 28.5 CV and 7.4 CC. Once I saw I-min had been passed and temperatures had risen to about 52C, I turned down the CC limit to about 0.8 A before dropping it further to 0.6 A. This continued for a while as a means to finish the charge and keep the pack in balance.
(http://content.quadninja.com/image/autodate/2014-11-15/Battery_Charge_Profile_-_2014-11-15_-_Small.png)