Solar yard lights; battery options?

[Dialtone]

Hello all:
I have some small solar yard accent lights (LED style) that are originally equipped with 2 AA size Ni-Cd batteries. They are about 2 years old and the batteries are starting to die.
My question involves what type replacement I can use other than Ni-Cd in this application. Would Ni-Mh or Li-ion function suitably as replacements?. I can still buy Ni-Cd's but would like a more environmentally friendly battery as a replacement. Newer technology and longer lasting power would also be beneficial if attainable. I have read some articles on the different technologies, and am aware they are different specs on each type, but am confused by all the data as to whether they could interchange in this particular application.
Any help / advice appreciated.

Dialtone

 

[audioguru]

Ni-MH cells would work the same as when the Ni-Cad cells were new and will last longer and might operate the lights longer because they won't overcharge on sunny days, they might charge more.
The capacity of Ni-mH cells are at least 4 times more but the charging current probably won't be that much higher.
Li-Ion cells are completely different and would need a different charging circuit and maybe a voltage regulator.

 

[Sceadwian]

NIMH's self discharge up to 50% faster than Nicad's do especially high capacity versions. Make sure the package is well ventilated as NImh's are more sensative to high temperature than Nicads are and it's outdoors in the sun. Neither NiCad or Nimh's appreciate shallow discharging and then recharging, but solar lawn lights are pretty good for either chemistry. Even a 'low' capacity Nimh cell is going to be higher or the same capacity as a high capacity Nicad. So the low capacity NiMh is probably your best bet.

 

[audioguru]

Replacement AA Ni-Cad cells are 600mAh, Ni-MH cells are 2500mAh. A huge difference in capacity that can't be used since the charging current is limited.

 

[Hero999]

The early Nicads were 600mAH but most modern nicads I've seen are 1300mAh.

 

[audioguru]

I don't know if anybody makes Ni-Cads anymore. Cadmium is deadly. The Ni-cads that are sold as replacements for solar garden lights are only 600mAh, the same as the ones they came with.

 

[Sceadwian]

audioguru, NiCad's are still widely used, virtually every cheap cordless phone still uses NiCad packs, they're also still widley used in RC cars because of their extreme ability to dump current especially in C and D cell sizes. High capacity NiMH's are 2500, but standard NiMh's are 1800ma, the walls and internal electrodes are thinner on the higher capacity cells. Lower capacity cells are more durable because the electrodes are thicker and less prone to oxidizing.

 

[audioguru]

The big C and D size Ni-MH cells made by Energizer have a little 2500mAh AA cell inside.

My RC airplanes use Sanyo AAA Ni-MH cells to power the high current electric motor and the RC. They have more capacity than old AA Ni-Cads.
The same power and much less weight.

 

[Sceadwian]

RC cars not airplanes, in cars having the battery pack be heavy is actually an advantage because it puts weight low in the car reducing it's center of gravity and increases traction, and cars needs significantly more torque than aircraft do starting and accelerating so the lower cell resistance of NiCad's allows for much higher stall torques and power draw under heavy load. In order to get the same possible currents from NiMH's you'd have to parrallel the cells.
Most aircraft are now using Lipoly's because they're so much lighter and higher energy density. Modern packs and advanced chargers in various serial/parallel configurations allow for customizable packs depending on how much current you need to draw and the desired airtime.

 

[audioguru]

Hybrid real cars use modern Ni-MH cells, not old Ni-Cads.
I wonder what happens when the car is fully loaded, is going up the long side of a mountain then the battery charge runs out. Get out and push?

 

[Sceadwian]

I thought we were talking about solar yard lights here? 1800mah NIMH's are probably the best bet for price/performance and simplicity, they're just drop in replacements.
If you want to get technical what the guy really needs to do is run a current drain and charge monitor for a few months and find out what the operating point in his area is on average. He should probably be using something like N sized 50mah cells in whatever parallel configuration allows them to on average fully charge under daily light and fully discharge at night.

 

[audioguru]

My solar garden lights are showing the signs of Autumn. They don't shine very long at night. The sun rises late now and sets early. It isn't high in the sky at noon. In 11 days it will be halfway between its highest in Summer and its lowest in Winter.