Can I sub NIMH for ni-cad?

[stump]

My wife has a lot of those solar powered garden lights. It's finally the time of year to check them out in preparation to deploy them. We put them all out in the sun for 2 days and about a third of them don't hold a charge. They use AA ni-cad batteries. I'm finding cheap AA ni-cads dificult to find. Can I put NIMH AAs in them or won't they charge right?
Thanks.

 

[lyrrag]

The ni-mh should work for you did the same for a solar post light.

 

[Mr Happy]

Nicads generally stand up better to the uncontrolled charging of solar garden lights. NiMH will work but their lifetime may be shortened.

I have found nicad AA cells available in the landscape lighting section of places like Home Depot and Lowes. Cheaper nicad AA cells are also available from Harbor Freight Tools if you have one nearby. Given the application I'd be inclined to try out the Harbor Freight ones. Their quality is a bit variable but you can't beat the price ($4 for 4, sometimes $2.50 on sale). Buy a bunch and throw away the duds.

If you do try NiMH I'd suggest LSD ones like Eneloops. They seem to be more robust than many other cells.

(Incidentally, remember to properly recycle old ni-cads. Don't put them in the garbage because the cadmium is a heavy metal poison.)

 

[yellow]

ni-mhs will work and are better suited for such a charge

get a few newer bright and "sun colored" 5mm leds instead of the mounted dim-blue ones and mount them in.

(PS: I dont think such small solar thingies ever succeed in topping off AA cells)

 

[gSPIN]

Strange, but I have just the opposite impression.
I'd say NiMH are probably the worst suited for this app becuz they are least able to tolerate heat.

OP wants cheap which I guess rules out LSD.
So with respect to generic NiMH, their higher self discharge would mean wasting a lot of power (which gets worse when hot) so yeah, the battery is probably never topped up when you need it.
Also to compound the problem NiMH charge efficiency drops to something like 25% @ 45*C (stand to be corrected on that) plus charging a hot NiMH cell will results in a shortened lifespan.
Suffice it to say NiMH don't do very well in a heated environment which is unavoidable in a solar application.

- NiCd fares better on both counts.
- LSD/Hybrid definitely addresses the first issue, but I don't know if they are substantially more robust on the second, probably a little better.
- Although it's not up for consideration, the ideal choice is RAM, the manufacturer of which is specifically targeting solar.

RAM have even better charge retention than LSD NiMH, in particular at elevated temps, plus their lower cost.
http://www.pureenergybattery.com/solar.html

However their higher voltage may mean the existing charge circuit in the garden light may not be high enuf to fully charge them.
But it would cost very little to try, worth probing around with a voltmeter in any case.

In any event I'm sure stump isn't as interested in my technical hair-splitting & would just like an answer to his question.
Replacing with NiMh will work, just how well & for how long is another matter if they are in a position at risk of being cooked.

 

[SilverFox]

Hello Stump,

Here you go...

Tom

 

[shadowjk]

I thought you needed some sort of asymmetrical AC to charge RAM? Or maybe that was just when charging regular alkalines..

 

[Mr Happy]

My limited understanding is that the difference between rechargeable alkalines and regular alkalines is physical rather than chemical. When an attempt is made to charge an alkaline some gas may be produced and the pressure increases inside the cell. A regular alkaline is not constructed in such a way as to contain the pressure or to vent excess gas in a controlled manner and so leaking may occur.

The primary way to control leaking when charging an alkaline is to charge at a very slow rate so that the rate of gas production and increase in pressure may be balanced against the charging rate. I understand that an asymmetrical AC charging current is another, perhaps more effective, way to regulate and limit the amount of gassing and overpressure.

Purpose designed RAM cells have a physical construction with strength, sealing and vent arrangements that permit charging without leaking. Anecdotally it has been suggested by someone that Duracell copper top cells are (were?) constructed in almost exactly the same way, but do not take this as verified.

Even with purpose designed RAM cells however, they should only be charged on the appropriate charger (or the charging current should be limited to a small value say below 100 mA). With an uncontrolled connection to a solar cell that is likely in a cheap garden light, I do not know if the charging current would be limited to a low enough value (but it might be).

 

[SilverFox]

Hello Mr Happy,

I seem to remember that the charge rate was supposed to be around 60 mA, and it needed to be pulsed and not constant current.

I also think that the cells had something added inside to help with gas recombination, but I don't exactly remember what they used or how they did it.

There used to be a bunch of plans for home built chargers for charging Alkaline cells. At lower current draws, it seems to work somewhat, but there were a lot of complaints about leaking cells.

Tom

 

[Mr Happy]

The alkaline charging circuits I've seen did employ pulses. I am not sure if that was because it was better for charging, or because it was easier to design a circuit that way. However it often happens in engineering systems that pulses work better than continuous flows. So it might be that the ease of design and the advantage for charging is a happy coincidence.

One example circuit I've seen used a current of about 80 mA for an AA cell, so I do agree that the current should be low. This means that to replenish a lost 1000 mAh into an AA cell would take 12 or more hours, and this does seem to be a significant drawback to rechargeable alkalines. For the garden lighting application it would be hard to obtain 12 hours of direct sunlight on a solar cell even in the height of summer.

 

[Darkpower]

My limited understanding is that the difference between rechargeable alkalines and regular alkalines is physical rather than chemical. When an attempt is made to charge an alkaline some gas may be produced and the pressure increases inside the cell. A regular alkaline is not constructed in such a way as to contain the pressure or to vent excess gas in a controlled manner and so leaking may occur.

The primary way to control leaking when charging an alkaline is to charge at a very slow rate so that the rate of gas production and increase in pressure may be balanced against the charging rate. I understand that an asymmetrical AC charging current is another, perhaps more effective, way to regulate and limit the amount of gassing and overpressure.

Purpose designed RAM cells have a physical construction with strength, sealing and vent arrangements that permit charging without leaking. Anecdotally it has been suggested by someone that Duracell copper top cells are (were?) constructed in almost exactly the same way, but do not take this as verified.

Even with purpose designed RAM cells however, they should only be charged on the appropriate charger (or the charging current should be limited to a small value say below 100 mA). With an uncontrolled connection to a solar cell that is likely in a cheap garden light, I do not know if the charging current would be limited to a low enough value (but it might be).

My belief is that there is a slight modification to the chemistry and or the electrodes that acts as a hydrogen gas absorbing agent.

 

[MorePower]

My limited understanding is that the difference between rechargeable alkalines and regular alkalines is physical rather than chemical. When an attempt is made to charge an alkaline some gas may be produced and the pressure increases inside the cell. A regular alkaline is not constructed in such a way as to contain the pressure or to vent excess gas in a controlled manner and so leaking may occur.

The primary way to control leaking when charging an alkaline is to charge at a very slow rate so that the rate of gas production and increase in pressure may be balanced against the charging rate. I understand that an asymmetrical AC charging current is another, perhaps more effective, way to regulate and limit the amount of gassing and overpressure.

Purpose designed RAM cells have a physical construction with strength, sealing and vent arrangements that permit charging without leaking. Anecdotally it has been suggested by someone that Duracell copper top cells are (were?) constructed in almost exactly the same way, but do not take this as verified.

Even with purpose designed RAM cells however, they should only be charged on the appropriate charger (or the charging current should be limited to a small value say below 100 mA). With an uncontrolled connection to a solar cell that is likely in a cheap garden light, I do not know if the charging current would be limited to a low enough value (but it might be).

Compared to normal alkaline cells, RAM cells have a thicker composite separator (non-woven backed with a polymer layer), a different electrolyte concentration, different anode additives, different particle sized distribution of the zinc, higher graphite to EMD ratio in the cathode, and usually some sort of silver oxide added to the cathode as well.

RAM cells are designed to vent at a higher pressure than normal alkaline cells, but they will still vent and leak if abused.

 

[MattK]

Hello Stump,

Here you go...

Tom

We continue to recommend NiCD cells for solar lights. The NiCd's are significantly less expensive and the solar panels cannot fully charge the higher capacity NiMh cells fully which means the cells never get fully charged or discharged which leads, inevitably to the formation of 'memory' (yes, even NiMh cells can get memory - they're just less prone to it) - also because of the limited output of the solar panels you'll never really take advantage of that extra capacity.

I also recommend buying, if you don't already own one, a charger with discharge function so that you can 'refresh' the cells by fully discharging then charging them. This will potentially allow you to recover some of the cells you already have and if you cycle the new cells 1-2 per year even this will add to their lifespan - something like one of these:
Good:
http://batteryjunction.com/tic8chniandn.html
Better:
http://batteryjunction.com/vav10baysmch.html
Best:
http://batteryjunction.com/8800.html

 

[mdocod]

I was just reading the thread title and OP and went and grabbed the link from BatteryJunction to the $0.85 AA NiCad to throw up here.... looks like Tom already beat me to it

Anyways, for that price, you really can't go wrong.

 

[TorchBoy]

Re: Can I sub NiMH for NiCd?

ni-mhs will work and are better suited for such a charge

NiMHs will work (for a while) but NiCd are better suited.

... the solar panels cannot fully charge the higher capacity NiMh cells fully which means the cells never get fully charged or discharged ...

The cells can certainly be fully discharged. It might take only one cloudy day.

 

[MattK]

Re: Can I sub NiMH for NiCd?

I don't believe that they do a full discharge like you would get on a refresh cycle on a charger.

 

[Turak]

Definitely the LOWER capacity Ni-Cds for the solar lights.

THe lower capacity (i.e 600-900mA) Nicd's end up working better than the NI-MH in my opinion and experiences. The lower capacity cells means that they get more of a full charge/discharge cycle when used with the solar lights.

Using the higher capacity cells and especially the NI-MH cells are just a waste since there is no way that a typical solar light used for edging sidwalks, flower beds, etc would ever be able to fully charge them given an 8-12 hour 'useable' sunlight day. Most of the solar lights I have tested, typically put out somewhere in the neighborhood of 25mA-50mA charging current at the most and thats in direct sun. Even with a 16 hour day at 50mA, that's only 800mA and at least 10-15% of that will be wasted in the energy conversion process, so you end up getting about 700mA or so.

Stick a 2700mA NiMh cell in there and it will never get a 'full' charge. As said above, because of the constantly repeated shallow charging cycles, it will develop a memory over time. Performance will steady decrease as the shallow charging and complete discharging takes it toll over time.

NI-Cds tolerate the heat better than NiMh.
Ni-Cd's tolerate cold better than NiMh.
Ni-Cd's tolerate deep discharges better than Ni-Mh.
The lower capacities make them ideal for low current solar cell applications.

I have the flower beds in front of my house lined with solar lights. Each one takes 4 AA cells and they will run about 10-12 hours in the summer night time. When winter comes, they tend to drop down to about 6-8 hours or so before running down. In winter there is less overall sun, snow on the solar panel, cold temperatures....they all play a part in the reduced run times in the winter.

Every year, I run the 4 cells in each solar light through a R/A cycle on my BC-900. You could do the same with the MH-C9000 using cycle mode and set it to about 3 or 5 cycles. I have 600mA Ni-Cd cells that are going on 4 and 5 years now and are still working fine. They are all still showing anywhere from 85-95% of their original capacities.

On another note, I received some solar lights with 'supposed' 650 mA cells in them.....sheesh, they were lucky if they could hit 400mA. No amount of Break-In cycles or R/A cycles seemed to do anything to get them any higher. My conclusion was some seriously mis-labeled cells, or just really shoddy cells overall.

I have also tried a set of the Harbor Freight Ni-Cd's (700 mAh's) and so far they are still working fine, although they have only been in use for about 18 months so far.

 

[Darkpower]

Definitely the LOWER capacity Ni-Cds for the solar lights.

THe lower capacity (i.e 600-900mA) Nicd's end up working better than the NI-MH in my opinion. The lower capacity cells means that they get more of a full charge/discharge cycle when used with the solar lights.

That is an excellent point that you make there Turak: Matching capacity to the circuit to ensure a full discharge and charge cycle. Excellent point to ensure longevity. We all suffer from the "bigger capacity, the better" syndrome as we all look at packing more power density and capacity in each volume of space in an "AA".

On another note, I received some solar lights with 'supposed' 650 mA cells in them.....sheesh, they were lucky if they could hit 400mA. No amount of Break-In cycles or R/A cycles seemed to do anything to get them any higher. My conclusion was some seriously mis-labeled cells, or just really shoddy cells overall.

Yeah, probably a last minute penny pinching effort by the manufacturer in substituting the cheapest cell they can find with a fairly decently engineered product. I see that all the time.

 

[TorchBoy]

Re: Can I sub NiMH for NiCd?

I don't believe that they do a full discharge like you would get on a refresh cycle on a charger.

Maybe your solar garden lights are different from the ones around here, but AFAIK none of them have anything to stop them discharging once they get low, apart from the Vf of the LEDs themselves.

THe lower capacity (i.e 600-900mA) Nicd's
...
Even with a 16 hour day at 50mA, that's only 800mA and at least 10-15% of that will be wasted in the energy conversion process, so you end up getting about 700mA or so.

Stick a 2700mA NiMh cell in there and it will never get a 'full' charge.
...
I have 600mA Ni-Cd cells ...
On another note, I received some solar lights with 'supposed' 650 mA cells in them.....sheesh, they were lucky if they could hit 400mA.

mAh. Come on - it's capacity, not current. You know better. And I don't want to hear any dribble about "You know what I was talking about."

I have also tried a set of the Harbor Freight Ni-Cd's (700 mAh's) and so far they are still working fine,

:twothumbs

NiCd does prefer full discharges while NiMH isn't bothered if it's not fully discharged, but that doesn't negate the other benefits of NiCd in this situation that you and others have mentioned, such as deep discharging, which MattK's comments aside, I think does happen here. Those maintenance cycles sound like a great idea Turak.

 

[Probedude]

Hello Stump,

Here you go...

Tom

I think I'll buy some of these to try out in my Mag85 (high current application)