Batteries
anybody burns home                      Storing Sunshine for the Future

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The spiffy little battery holders with the snap-on wire leads are from All Electronics. The snap-on leads make it easy to swap out batteries, hopefully though, these batteries will rarely need replacement! Most of the batteries used in this project were from Home Depot: about $5 Oregon dollars (no sales tax!) for four 1.2 volt AA 900mAh Nickel Cadmium batteries.

Rechargeable, recyclable batteries help with some of the environmental issues of batteries: you can reuse them several times. The batteries used in this project are Nickel-Cadmium (NiCd): they have a large, but limited, number of recharge/discharge cycles. You might want to replace the NiCd batteries in your solar lights (if in use year round) every couple of years. 

It is okay to let your NiCd batteries sit around discharged; this doesn't hurt them. But don't let your batteries roll around and touch their ends to each other or other metal. The same with the wires on battery packs. The batteries will short out and be ruined. Tape them together and seal up in plastic bags between trips to the playa.

If you are thinking about designing a solar system that uses a big, lead-acid battery (deep cycle marine), the care and feeding of these batteries is different than NiCd or NiMh batteries: lead-acid batteries need to continually be recharged or they will lose their ability to hold a charge.

About the mAh rating on batteries:
Mike says, "mAh...means milliamp-hours. It's a capacity measurement and a way of thinking about rate. 550 mAh means you can draw 55 mA (milliamps: thousandth's of an amp) for 10 hours out of a fully charged battery. To recharge it you would need to put in 55 mA for 10 hours (or 70 mA for nearly 8 hours). Charging a battery is an chemical process that not as efficient as taking power out of a battery -- but for the purposes of discussion, we'll assume that they are the same."

After discovering the high mA output of the Solar CDs, Mike wrote: "If you add more capacity to the battery pack, it'll 'soak up' all that excess power. Think of the battery like a sponge, and the power the solar panel puts out during one day like a puddle. This is a good analogy. Bigger sponge, no problem."

You have to be careful not to overcharge batteries by applying too much "current" (too much mA) for too long of a time. The Solar CDs produce about 220mAh on a sunny day, requiring four 900mAh batteries to soak up the power over the course of a typical day without overcharging and being ruined.

Mike says: "In the ideal world (new age music plays in the background) you would charge rechargeable batteries at a C/10 rate. The capacity (900mAh in this example) divided by 10 hours. That's what the battery manufacturer recommends. So, again in that ideal world, we would want a 90mA solar panel to charge those batteries for 10 hours. You can charge with higher current for a shorter period of time, but those battery chargers have sophisticated features to detect when the battery is finished charging. The difference between done and "crispy" could be a few hours of overcharging..."  

Anybody know of a web site that objectively compares rechargeable Nickel-Cadmium batteries with Nickel-Metal Hydride and Lithium Ion batteries? email it!

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