Old SLA Battery for a 1.5 Watt Solar Panel

The little solar panel (Harbor Freight #62449) has proven itself to be capable of sending out 1.2 W, within reasonable reach of the 1.5 W announced on the box. However, we’ve also learned its actual power output varies tremendously depending on its orientation relative to the sun and the weather. As a result it’s not terribly useful on its own. We’ll need to add a battery in to the mix.

Enercell SLA

An old sealed lead-acid (SLA) battery from the parts pile is thereby enlisted in the project. We can start the experiment by hooking up our solar panel directly to the battery terminals. It’s not ideal, but a big lead acid can tolerate this abuse, at least in the short-term. (Never do this with lithium-ion batteries of any size.)

The volt meter indicated this battery was overly neglected in storage, because its voltage had self-discharged down to 6 volts. This is far below the recommended range for lead-acid batteries and may have caused some damage. Fortunately it was able to handle a charging cycle and held an open-circuit voltage of 12.5 volt. Good enough to continue the experiments.

Once the battery is in place to cache power delivered by the little solar panel, we can now power a 12 volt USB charger and charge a cell phone on solar power. But the small panel does not track the sun throughout the day, so it could deliver only a fraction of its maximum power. As a practical matter this means the panel need to charge the lead-acid battery over several days before enough power is collected to charge a cell phone for a single day of use.

Based on the latest findings, we can take the solar investigation in one of two directions:

  1. Wring more power from the little panel: build a sun tracker so it can face the sun throughout the day.
  2. Throw money at the problem: buy bigger solar panels.

The sun tracker can be a fun project, but it’ll have to wait. The vote was decided by the arrival of a Harbor Freight coupon for their solar kit. So: option #2 it is!

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