Battery. Why is it so?

How did you determine your SOC ??

Hi Ted , did you add any water to the cells?doc

One possibility to explore is that one cell cold be getting close to failure, while on charge it is "ok" but when you remove the charge, it drops its charge very quickly. When reconnected to charge, it eventually makes it back up to an acceptable state? So perhaps, a lot of the solar charge is being wasted in leakage of this cell?
Easy to test, get the batteries load tested and it will fail if this theory is correct.
Good luck

surface charge is what you are reading first off, and one or both of your batteries are on their way out.

hi
I.m with Jim B8 and Dozer on this
imho you have at least 1 failing cell in each battery all the charge rate will be constantly going to them to try and keep them boosted and as soon as you disconnect charge will rapidly drop it,s a no win situation time for new set
been there done that scenario
cheers

Maybe the batteries just need equalisation. Does your battery charger have an equalisation stage on it? If so is it enabled or disabled?

Read more here - 9. HOW DO I CHARGE (OR EQUALIZE) MY BATTERY?. When you have that sorted out, read the rest of the site.

First and above all ... adding water to your batteries WILL effect the state of charge.
Lead Acid batteries are a water / acid reaction .... adding water will dilute the acid and effect the state of charge.

This is why when doing battery maintenance you do you observations and measurements before you top them up, THEN you put them back on charge till they are fully charged.

Second remember reading voltage ( which is what your battery managment system does) is not a bang to rights measure of state of charge and may be effected by errors.

Third ..... every cell in a battery is an individual ...... they will all have differences ....... as batteries are charged and discharged normally the differences between these cells my widen.
This is why it is wise to equalise batteries from time to time ...... basicly you give em a good flogging.

Your batteries may be on the way out ..... remember the basic life span of a Lead acid battery is 2 years in service ........ by some minor chemical trickery from the manufacturers the practical service life can be extended to 5 to 7 years ...... if treated very well.

If ya batteries are in the 3 to 5 year age ...... yeh you getting bonus life and should be thinking about replacement sometime soon.

It is very much worth running a couple of equalising and resting cycles on your batteries ...... looking at your hydrometer readings ....... this my bring you some more life out of your batteries ...... or it may show the suspect cells to be on their way out.

Full points for looking at your batteries in the first place.

cheers

Once you've done all that then do a drain test to see what the banks can actually deliver. It's the only way to be sure about their health. Voltage doesn't tell you much.

Thanks everyone for your input. I'm curious as to why Coromal fitted a PSU instead of a charger in the first place.

My thoughts when the time comes is
http://www.supercheapauto.com.au/online-store/products/Calibre-Battery-Charger-Multi-Stage-12-Volt-20-Amp.aspx?pid=213976#Recommendations
charger with SLA battery.

Regards
Ted