Battery dropping voltage

[JimmyC Advanced Member Find all posts View Profile Close JimmyC]

G'day all.
This may end up being a follow on to my thread regarding my optima battery, or may be a separate issue.
Tonight I checked my battery voltages, car had been sitting there for 9 hours without being driven, no fridges or aux stuff running off the batteries. Main battery was 12.35V and aux battery was 12.58.
Main battery is a Century heavy duty approximately 8 months old and the aux is an Amaron which is less than a week old.
The car starts fine and doesn't seem to crank slowly at all. My multimeter seems to be accurate as I have a permanent voltage gauge connected to my aux battery and that was reading 12.6 (only reads to 1 decimal place). The water levels in both batteries are ok.
Can anyone think of what might cause this?
About 1 week ago I installed a voltage booster (Piranah Alternator Output Compensator) but even prior to that I had noted sometimes that my main battery would be on the lower end of voltage.
For the last few months the car has done trips of maximum 30 minutes (usually runs for 10 mins, twice a day to/from work)

Cheers.

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

Hi Jimmy and you pretty well answered your own question.

Those types of mid range battery voltage levels are most likely caused by your short driving times.

Even if you were to do a long drive now and then, you are still going to end up with those voltage levels.

You can fix the problem by either charging your batteries on a regular basis with a battery charger.

Or if you park in the open for long periods of time, try fitting a small solar panel inside your vehicle. Up to a 10W solar panel can be fitted without a solar regulator and this will charge and maintain your batteries and remove the need to use a battery charger.

[JimmyC Advanced Member Find all posts View Profile Close JimmyC]

Sounds like a fair explanation. Going to buy a ctek tomorrow when the sale at Repce starts. $99 for a 5 amp charger which seems sufficient.
Will look into a solar panel. Not too familiar with how to set it up so it will charge both batteries. Would it work hooked up to the main battery and I rely on the voltage to be above 13.2 when charging so the SBI12 I have installed will click over allowing power to both?