DC voltage drop.

Multi-meters can be bought for just a few quid these days ..... plenty good enough for occasional use at home by most people. Well worth having one at home.

Thanks RK  and FaW. The meter is as you describe, and is generally reading about 13.5v when we are running heating and anything from the sockets, but when we have the lights on it drops and if we run the pump the lights flicker and dim slightly, although don't go out. We will carry on to the next site and see what's what then. You never know a good shoogle en route might just sort it out, although I doubt it, but one never knows.

ED, I will certainly get one and learn how to use it too.

Anyone using a meter on mains voltages does well to remember that they are potentially lethal. Not just the electric shock but the fright from the bang when something shorts out.

Many of us learned this the hard practical way, and none moreso than the safest electrician I've ever known.  He only worked on equipment with one hand - he had lost the other in an electrical power accident.

Sounds like the hab battery is ready for the last rites to me.

That's my worry, CY, although it is only 3 yrs old and has been used, on EHU, virtually every month since purchase. It has an indication light which is showing green, so not requiring charging, supposedly.

Agree with CY, classic symptoms of carked battery…..volts ok, but not able to deliver enough current. You could ask a battery specialist to perform a load test before buying a replacement.

I would definitely be looking for a poor connection then. 

Will do, SB, cheers.

The green tell tale is only looking at the voltage of just that one cell .... there could be another or others that are goosed!

He wouldn't get the voltage then. As has been said, a poor connection somewhere if NTH has confidence in the battery.