PV Gap

Ken Said:

How do I know when my battery is fully charged?

We Answered:

a 12V wet cell lead acid battery at a 100% state of charge measures 12.65 volts at 80º F, after the "surface charge" has dissipated. Alternatively, the average specific gravity of the cells is 1.265 at 100% state of charge.

"surface charge" usually measures around 1 volt, results from the uneven mixture of sulfuric acid and water along the surface of the plates, and will dissipate over 4 or 6 hours after you stop charging; on a car you could turn the hi-beams on for 5 minutes, turn 'em off then wait 10 minutes.

An automatic charger knows when a battery is fully charged by measuring the current in the charging circuit. As the battery recharges, it's "internal resistance" rises; as resistance increases, current decreases if voltage is held constant. When the current stops decreasing, the charger shuts off.

If you measured voltage across the battery posts at sundown and get 14.4, then measure again at sunrise and 12.65 or more (at 80ºF, or 12.606 at 40ºF or 4.4ºC), then the battery is behaving normally. This is with no load on the battery. If the voltage is below that in the morning, then either the charger is drawing current when it goes dark (which the controller should prevent), or else the battery isn't taking a complete charge, or else it's rate of self-discharge has accelerated, both of which are symptoms of normal wear & tear.

Keep the electrolyte level high enough to cover the plates; add only distilled water when needed.

Dirt on the battery case attracts moisture, which can form a conductive path and become a parasitic drain, so keep the case clean & dry.

Alicia Said:

does small solar panel can improve car gas millage?

We Answered:

Where on that web page does it say that it improves fuel economy?!

It is a solar trickle-charger for the battery. Would be handy on a vehicle that doesn't get driven much.