The “Myth” of 100% by Midday

[noelex 77]

It is often stated on this forum that it is impossible for solar panels to achieve 100% charge by noon using solar alone. When members post that they regularly achieve these results, it is suggested that their battery monitor or solar regulators are misleading by falsely indicating 100% or by dropping to float too early.

So I thought I would post an example showing 100% charge before noon to prove this was achievable.

At this time of year we are close to the summer solstice in the northern hemisphere. With long daylight hours, 100% before 9am is usual. By noon the batteries have usually been sitting on float for several hours.

This morning I photographed the battery monitor app (a Pico). The solar regulators actually dropped back to float before I could take the photo. So I started another absorption cycle to show that the end amps had been reached.

So here the battery monitor is showing 100%. The solar controllers are regulating. The battery voltage is at the absorption voltage for these gel batteries. The end amps are at 4.36 amps, which is 0.7% of the nominal 600AHr battery capacity.

There are many definitions of 100% full lead acid batteries. The reality is actually that the battery SOC approaches the 100% in such a gradual way that a realistic definition is hard to standardise. The most common definition is when the battery charging drops to float, providing this is done at the correct point.

Most battery manufacturers suggest when the end amps reach 2-3% is the correct point. Many people suggest charging more aggressively holding the absorption phase for longer until the end amps reach 1 or even 0.5%. For gel batteries 1% is a good number so the 0.7% suggests the absorption phase has been held a little too long for this battery chemistry, but in practice the difference is only slight. At this SOC the end amps drop very rapidly so the difference in the time when these various definitions are reached is only small.

Note the time is 8:17. Noon is still over three and half hours away.

This will not be achieved all year. We spent winter in Scotland and at this time of year and these lattitudes, 100% was only achieved late in the solar day (although as sunset was around 4pm, this was not really late ) and of course there are days where 100% is not reached.

So solar can charge the batteries to 100% by noon. This is easily achieved by many marine solar systems, at least at the time of year when conditions are good.