Which Battery to Use NiCd or NiMh?

[SilverFox]

I have some solar powered walkway lights with a yellow LED for light. The units came powered with 2AA 550mHr NiCd batteries. I have had the units for about 4 years and the batteries are wearing out.

The lights have a small solar panel that charges the batteries during the day and a photocell that turns on the light at night. In the summer, the lights are still on in the morning, but in the winter they don't last all night.

My first thought is to go with a higher capacity NiMh battery, but 1460 recharge cycles on a set of NiCd's seems pretty good.

Any suggestions?

Tom

 

[Empath]

The charging circuitry is probably set up specifically for the 2AA 550mHr NiCd system. NiMH cells, particularly of a higher capacity have different recharging needs.

 

[Tater Rocket]

That is assuming there IS a charging circuit. The cheapie white ones we have have no circuit, the batts are just hooked up to the solar cell and led. If there is a circuit (other than maybe a diode) I'd say you probably can't switch them, but if not....you could try and see what happens.

Spud

 

[SilverFox]

This afternoon I found some at a hardware store that were close to what I have. I took them apart at the store and found that they have gone up to a 1000mHr NiCd battery.

Are NiCd batteries cheaper than NmHd in quantity? These are not top quality lights and I think that manufacturing costs would be a major concern.

There is a board inside the light. I figured that it had to do with the photo cell, or limited the open voltage to something that would not burn up the batteries. I did notice that when I pull the old cells out of the light, they are at about 1V, so there must be some sort of voltage cut off also.

It seems to me that NiMh batteries are more tolerent of partial charges and would be better for this type of application. I don't know if either type of battery can handle overcharge conditions well (summer cycle), but by increasing the mHr capacity, overcharging may be less of a concern. The other side of this, as Brock mentioned, is that the higher capacity cells may never get fully charged.

I have 4 lights. Maybe I should do a test. NiCd in 2 of the lights, and NiMh batteries in the other 2. I could report back in another 4 years...

Tom

 

[Brock]

I tried this with a set I had, threw in some old 1600mA, the problem is they never got fully charged. If I left the unit off it would charge them in two days of good sun.

I think that might damage the NiMH's if they never get charged? I don't know, but since they were old I just left them in there and it seems to be working ok.

 

[Albany Tom]

Nicad's are less expensive, but I expect to see this change. I'd be willing to bet that within 10 years NiCads are taken off the market in the US, because of the cadmium. It's toxic, and gets dumped into the waste stream, and NiMH is a better replacement for most applications.

 

[Steve-at-Springboard]

It may not apply for this use since they recharge daily but the major downside for NiMH in flashlights is that they self-discharge much faster than Nicads. A quality Nicad will hold half its charge for five months but a NiMH will probably be dead in a month. Not a problem for notebook computers and cellphones that get recharged every few days but not ideal for a flashlight that sits for weeks at a time.

 

[Entropy]

I think in this application, they're using NiCd because it's much cheaper.

As to notebooks and cellphones - Almost all notebooks/cellphones are Li-Ion these days.

 

[carbonsparky]

I found Nicad replacement batteries packaged for use as replacement for these solar lights at Wal-Mart. I do not remember the price. They had them with the solar lights.
Changing the batteries to a much higher capacity will have little effect since the charge capacity of the solar cells is somewhat limited. You also will find that cold weather and the shorter days of winter will limit the run time of these lights. I bring mine in during the winter. Its one less thing for me to find with the snow blower.

 

[php_44]

My first thought is to go with a higher capacity NiMh battery, but 1460 recharge cycles on a set of NiCd's seems pretty good.

Any suggestions?
Tom[/QB]

<font size="2" face="Verdana, Arial">NiCd's tolerate frequent overcharge just fine, and one study showed negligible loss in capacity after two years of continuous overcharge. NiMH cells do not tolerate overcharge like this - it shortens their life.

NiCd's are said to be capable of a greater number of charge discharge cycles than NiMH. This might be related to NiCd's ability to tolerate overcharge, and the difficulty of detecting full charge in an NiMH cell.

Charging for NiCd's is endothermic (consumes heat) till you hit overcharge. NiMH charging is exothermic (produces heat), and in overcharge is very exothermic. This might be a consideration for something that bakes in the sun all day since neither cell type likes heat very much.

For your application I'd probably pick NiCd.

In fact, although I have some NiMH cells, I'm looking to buy a bunch of NiCd cells for a rechargable flashlight mod I'd like to do. I'm building a light that will remain in a charger till needed - and my NiMH cells would quickly die if charged continuously.

 

[SilverFox]

Hello php_44,

Good information on the charge cycles. In this neck of the woods, we worry more about staying dry in the rain than baking in the sun, though on ocasion we see the sun.

Your comment about NiMh cells quickly dieing if charged continuously leaves me puzzled. The TigerLight uses a NiMh battery pack and there are reports of them being in use for 4 years or so continuously on the charger between uses. 4 years does not seem like a quick death.

Tom

 

[Vikas Sontakke]

Are we talking about dumb or smart chargers here? I am assuming Tigerlight charger is NOT a dumb charger and it switches to trickle charge after detecting charge complete condition.

- Vikas

 

[SilverFox]

Hello php_44

WOW... the plot thickens... Not only do we have smart chargers, but it looks like there are smart cells too.

I just purchased some NiMh cells and sprang for the smart charger. At first I was put off by the price, but justified the price by the cost of the batteries. If I ruin a batch of batteries, a cheap dumb charger just got a lot more expensive.

In this application, it sounds like NiCd's are the way to go, because I am sure the solar pannel does not incorporate smart charging in it. NiMh may work better in the winter, but NiCd's might be able to handle overcharging during longer daylight periods a little better.

I suppose I should measure the charge current and try to figure what is going on.

Tom

 

[php_44]

Originally posted by SilverFox:
Hello php_44,

Good information on the charge cycles.....

Your comment about NiMh cells quickly dieing if charged continuously leaves me puzzled. The TigerLight uses a NiMh battery pack and there are reports of them being in use for 4 years or so continuously on the charger between uses. 4 years does not seem like a quick death.

Tom

<font size="2" face="Verdana, Arial">If you treat them right - NiMH can last a long time. Here's a quote from an engineering application manual for energi#er brand NiMH cells:

Proper charging of nickel-metal hydride cells is the key to satisfaction with their performance in any product. A successful charging scheme balances the need for quick, thorough charging with the need to minimize overcharging, a key factor in prolonging life.
....

In general, the nickel-metal hydride cell appears to be more sensitive to charging conditions than the nickel-cadmium cell.
....
The way the nickel-metal hydride cell is designed into an application can have dramatic effects on the life of the cell. This is especially true of the design of the charging circuitry for the application to ensure adequate return of charge while minimizing overcharge. In fact, effective control of overcharge exposure, time and charge rate is the way of enhancing cell life.

<font size="2" face="Verdana, Arial">According to them - a three step smart charger charge scheme is best for NiMH (high current 1C, lower current C/10 as end of charge is reached, then trickle C/40). For "dumb chargers" or timed chargers they recommend a rate of no more than C/10 to effectively charge the cell and yet not cause damage if left in continuous overcharge.

The TigerLight (nice light

)appears to use a dumb charger with an effective rate of C/10 - so you need 10 hours to charge an empty pack, but after that you could leave it in the charger and not damage the cells.

Interestingly one of the major companies (I forgot who) wanted a foolproff and cheap way to enforce proper charging of their NiMH cells. They patented a method of putting a pressure activated switch in every NiMH cell. The switch is closed all the time during discharge. During charge when the cell gets "full" it begins to generate a lot of hydrogen gas and the pressure inside the cell builds. This gas pressure opens the switch and shuts off charging to the cell. This charger is meant to sense the cell literally disconnecting itself and turn off power to that charge port. This scheme is meant to work for individual cells and probably wouldn't work if a bunch of these were charged as a pack.

 

[dude]

my 2c:

Nimh batteries have TRIPLE the capacity of your nicads. If the solar thing has such a simple circuit and has 550mah batteries, chances of significantly overcharging the nimh batteries seems unlikely. Keep in mind, it is dark EVERY DAY.

Admittedly unscientific reply.......

SPEAKING OF BATTERIES:

I found a spec of lithium 123 batteries at only 1300 mah. If this is typical, they are ONLY good for storage life, not long light. Any takers out there? for an EDC light, NIMH seems to rule.

 

[SilverFox]

Hello dude,

I am still working on this.

I am currently doing a little test. Two of the lights have NiCd batteries and the other two have NiMh. I charged both sets of batteries up and then installed them in the lights. At first, the NiMh batteries kept the lights on longer than the NiCd (due to their higher capacity). I get up in the morning and check on them. After about a week, things evened out, neither battery types are keeping the lights on through the night. I need more sunlight...

This one may be answered by which is the cheaper type of battery to purchase.

I think several of the forum members utilize NiMh batteries in their every day lights. The only downfall to the NiMh batteries is that they loose 1-2% of their remaining capacity per day. That means that in a little over a month (35 days figuring 2% loss per day) they would be at half capacity. If you can keep track of your batteries - no problem.

Tom

 

[spyder]

Silverfox,

Any update on your testing? What size NiMh batteries did you use? I started another thread about this before I found yours. My lights are Intermatic solar powered Malibu lights for along my walk. They use 550mah NiCd's and don't work for squat at this point (1-2 years old). Thinking about replacing them with the Dynacells due to cost.

 

[Ray_of_Light]

NiCd batteries have lower capacity than NiMH, but also a much lower internal resistance. This because the negative electrode of a NiMH battery is a "sponge" the physically releases hydrogen during discharge. Under very heavy current draw, NiCd shows better performance than NiMH... this is why high end flashlights, like SureFire and Streamlight, uses NiCd batteries, not NiMH.

A word about charging. In NiCd AND NiMH, at end of charge, oxigen and hydrogen develops respectively at positive and negative electrode. The battery is designed to allow those to recombine and form water and heat. This is possible only at trickle charge rate, otherwise those gases vents out. NiMH are more tolerant of trickle charge than NiCd; if not properly designed (with a ticker glass fiber separator), NiCDs gets shorted, due to the growth on cadmium cristals in the separator. NiMH, on the other end, gets easily damaged from heat, due to the intrinsic delicacy of the nickel-palladium negative electrode.

Anthony

 

[freewheelin']

I have two lantern type solar lights now, 1 $29.95 new with the low capacity NCAD's and 1 garage sale $1.00 that I put new Duracell 1800 NIMH's in. I've only had them for a week (bought the garage sale 1 on the way home from Home Depot purchasing the other). Both have 3 LED's and so far they both last all night (lots of sun this time of year in Texas) but the $1.00 NIMH light is brighter by a long shot. Not very useful since I've no idea what LED's are in them but thought I'd mention it anyway.