We have a generator charging our 4 x Victron 12V/165Ah GEL Deep Cycle Batteries through our Multiplus II. Charging works, taking the battery charge voltage up to about 57V which is expected. However, when the generator is then switched off, the voltage suddenly drops to about 52V. Is this to be expected and why?
Are you wating long enough for the batteries to finish bulk and absorption and then drip back to float voltage?
If not, the batteries are undercharged.
Check all your batteries individually.
Measure all open-circuit voltages and compare them.
As your 4 batteries are all in series, I suspect a faulty battery with at least one cell. This raises the charging voltage but does not maintain it after switching off.
True, there can be so many other reasons.
Do you have battery balancers?
How fast do you charge them?
We do not know how fast we are charging them other than we keep the generator on until Multiplus shows Float which can be about 4-6 hours. How do we know? Is this something we can control?
What else is connected to the system?
Control unit ? GX Device or something else ?
Have a look at the batteries to see if they have become slightly bulbous.
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Beside the Multiplus, the solar array is connected to the batteries via SmartSolar MPPT Charge Controller. To clarify, the batteries charge up to capacity (approx 57V) and stay at that on Float when charged by solar no problem. My original observation is only when charged via the Multiplus.
You could look in the ‘Charger’ area via VE-config and post the screen for us.
Do you access your system via VRM or even locally via MK3 adapter?
Don’t have access to the Multiplus system, only the charge controller which has Bluetooth
Then consider retrofitting a GX … then you’ll be able to monitor these values and see them on your smartphone. You could then also access the configuration via the VRM portal.
Many thanks for your time giving advice, it’s appreciated. What settings in the charger section should I be looking for?
Looking like this :
Voltage, Current , Time … all you need is this
btw: is this a Mobile Home or a Homeinstallation ?
57.60, 55.20 - guessing they are the settings that should be checked? This is an off-grid setup.
These values, such as charging voltage but also charging current and time, should be checked. The values for the maximum current for the generator under ‘Inverter’ would also be interesting … the feed-in current may already be reduced here. Topic GX is back 
OK time to get the thing that allows me access to that screen. I’ll come back when I have.
You could use a Cerbo or a Venus GX … the Venus has less CPU Power but also a lower Power Consumption … Maybe more important for a OffGridUse than the Cpu Power of the Cerbo
Take a look at the Technical Sides from Victron for these Products …
and a shunt would be a very good choice to really see what goes in and out of the batteries. The Multi itself is a very poor battery monitor …
A simple test in the interim is to measure the voltage of each series-connected 12V battery block. (No need to disconnect them.)
They should all measure the same. If one has a markedly lower voltage reading, its a suspect for degraded cells.
Make a measurement at full charge (not on Float), again when the battery is discharged somewhat under load, and before recharging with the battery at rest (no load.)
Perfect …
How did you get there now 
via the dealer?
The whole thing would now have to be compared with the manufacturer’s data sheet …
Why storage mode ?