I get paranoid about preserving Herbie's batteries. I am determined not to over discharge them (ie more than 50%) at any time because that's what makes them degenerate. The best I can do up until now is to measure the voltage regularly. 12.7v is fully charged and 12.2v is 50%, 12.3 is 60% etc.. However such readings are often meaningless, because if the batteries have just finished charging the volts will read misleadingly high, and if the battery is under load e.g. if the fridge is on, the volts will read misleadingly low. I have been known to turn the fridge off overnight becuse the voltmeter is reading 12.1v, then checking again in the morning and it reads 12.48, so I needn't have unplugged.
I manage OK by being ultra conservative, but it's unfair on the others aboard when I won't let them use the telly because I'm unsure how the batteries really are.
So I've splashed out on a Smartgauge which is a gubbins that claims to give you true readings of battery capacity at all times no matter whether charging or under load. As I understand it this clever little box constantly measures battery voltage and uses a computer model to work out what is going on and what the state of the batteries is. So when Claire and family borrow the boat, as they will do tomorrow, I can just say look at the gauge and if you are above x% you can keep the telly on. I have yet to work out what x is though because I have to allow for how much more the fridge will take overnight before the engine recharges the batteries next day. One night observing the Smartgauge should tell me the answer to that.
As we're delivering the boat to Greenford today to give Claire a head start tomorrow, I hope to fit the gauge this evening. In theory this is dead simple. Just attach a couple of wires to the batteries. However I'm old enough to know that it'll probably be a pain because I have to run the wires back to the cabin which means crawling around in the engine hole with bits of cable tie, then I have to find a hole to pass the wires through to the cabin, and then cut a hole somewhere inside the boat to mount the box.
I'll let you know how I get on. Note to self - take the camera!