On Batteries Problems 485 Any help gratefully received.

I have hardly used my house batteries off shore power since the whole bank was replaced. The new batteries are Rolls - so should be of very good quality.

I find running the DC system off just the batteries the volts drop very quickly. For example at 75% charge the volts read 11.6V under a load of 30 amps, and fall away further. If the load is reduced it doesnt make much different to the volts. The volts are measured on the main panel and also on a BMV-501. The position is similiar when the gauge is showing 100% charge albeit the volts are just a little higher. At 75% the AIS trips out due to low voltage.

As soon as the charger is switched on the volts recover almost immediately.

My feeling is there is a poor contact somewhere. Could this explain what I am seeing?

I should have checked where the BMV-501 is measuring the volts (I didnt install this). It would be interesting to know which side of the main DC switch it is in case there are any poor contacts in the switch. I have a main rotary switch for the DC power, and a main DC swith on the panel.

I would be very interested if anyone has any thoughts please?

The 11.6 V is way too low for 75%. That is more consistent with low % charge. Several questions and comments:
If you have a digital volt meter, measure the voltage on each battery, directly at the terminals. The batteries should all match.
If you have 6 volt batteries in serial/parallel combination, the voltages should match and add up to the 12v nominal.
If you don't have a digital volt meter, buy one to keep on board. Almost any brand ($20 to $30 at Amazon) should be good enough as general debug tool.
If you haven't checked levels and specific gravity of electrolyte (battery acid) do that for each cell. Review your battery manual, if this isn't familiar to you.
The voltages at the battery (top of battery stack) should be very close to whatever your BMV-501 is measuring - whether charging the batteries or discharging via DC loads. If they don't match, then you likely have poor connections somewhere.
Let me know what you find, and if you have further questions. You certainly have some problems some where.
Fair Winds,
Drew