How to monitor your 12v battery

June 15, 2014 at 11:41 am

Now I’ve got some progress with Duet’s interior refit, my focus is starting to move towards her electrical systems.

She currently (no pun intended!) has one 70AH leisure battery fitted, which was new last year as I fried the predecessor charging my phone and using the autopilot one sunny afternoon, totally discharging the battery and it wouldn’t re-charge.  I learnt a lesson there, and am keen to make sure that it doesn’t happen again… mainly because batteries are expensive!

I have already done a quick calculation of what power I need for cruising and have recently bought a 20w solar kit to replace the knackered 10w panel I removed earlier in the year.  However, because I’m not really sure how much juice I need I’ve gone for just a 20w panel, and I still suspect that I might need another battery but haven’t decided where I will put the bank as there’s currently only space for one.  I have no experience of boat 12v electrics so I’m just going to try it and see where we get…

Anyway, all this needs careful monitoring as if you discharge a battery below 40% it will fry it (or at least shorten its life).  I’ve been looking at the NASA BM-1 which retails at about £90 though it does show the battery draw when something is turned on (which helps with calculating power needs I suppose) and it shows the current voltage and percent charge.  However, like I said, it’s £90 which is more than the cost of a(nother) new battery, and it needs wiring in which is a bit intimidating as to be honest I’ve never done electrics.

However, there is a drastically cheaper alternative: I’ve bought a plug in 12v voltage meter, on eBay for about £4 delivered.  It looks like this:

12v Voltage Meter hurley 22 duet

Really easy to use, you just plug it into the cigarette lighter and it tells you what the current voltage is, by which you can work out the percent charge of the battery.  Couple of caveats though; always test batteries when they have stood without charge or use for at least 30 minutes. Batteries just taken off charge will have significantly higher voltage until the surface charge decays over 30 mins or so.

I have done a chart on what the voltage means, that I will be printing off and laminating for the boat.  It’s pretty self-explanatory… basically try not to discharge below 12.42v (80%) and never discharge it below 12.06 (50%). Please help yourself to the image if it would be useful to you too!

12v Voltage Charge table hurley 22