Friday 11 October 2013

Electrical conundrums

I'm very pleased with the Sterling gizmo installed by Kings Lock Chandlery – it seems to do a splendid job of fooling the alternators into delivering far more juice than they were hitherto inclined to do, and of charging up the battery bank much more efficiently. But there is still the issue of what happens overnight.

Following some comments on this blog and elsewhere, I've taken to switching off the inverter when we go to bed. That turns off the fridge, and since there seems to be nothing else drawing electrons, the battery voltage remains pretty stable overnight. One thread suggests that it's best to keep the batteries above 50% charge, so if I can get them to remain at 12.2 volts overnight, I'm happy.

This is in considerable contrast to when I left the inverter on overnight. The voltage would drop down to 11.6 or something, and I'd be running the engine as soon as I got up to restore it to that magical 50% figure. The voltage would drop, though not as much, even if I simply turned off the fridge, but left the inverter on. So it seemed that it was largely the actual running of the inverter that was to blame. But I've been puzzling over the maths. The Victron website says the unit (a 3 Kw Phoenix MultiPlus) should be consuming about 15 watts when it's on but not servicing any appliances. Let's say that converts into 1.5 amps from the batteries. After 8 hours thats still only 12 Amp-hours. Since the battery bank is nominally 440 Amp-hours, the inverter should make only a minimal impression running overnight. Even if you add in a fridge and assume it's pretty inefficient (let's say 3 amps when it's running), that's probably only another 12 Amp-hours, plus the inefficiency of the inverter. Let's say 20 Amp-hours maximum – that would be a total of 32 Amp-hours to run the fridge overnight. That's less than 10% of the battery capacity and should equate to less than 0.2 volts drop.

So what's happening? I went to the CanalWorld website, but a lot of the threads I followed ended either in nothing very helpful or in people hurling abuse at each other. I've been seriously considering getting a 12 volt fridge so I can do without the inverter much more. But it seems to me that the inverter shouldn't be taking this much toll on the batteries, and some of the comments in the threads I followed suggested people use them as much as I do, with far less penalty.

Any suggestions?


  1. An inline shunt and meter on the battery positive feed removes all doubt, it gives amps in and amps out of the batteries so you can see just what that fridge is doing.
    Our Beco mains fridge takes 10amps for 13 mins every hour. That's where the juice is going.
    What's more, the batteries deliver far less in cold weather, 25% less by my reckoning so winter always looks bad when monitoring battery condition.
    Mo - Balmaha.

    1. Thanks for the info, Mo. My Daewoo is supposed to do better than that, but I see that the Inlander and Shoreline 12 volt fridges are rated at between 0.8 and 1.5 amps. I must investigate the price of a shunt / meter – I much prefer to take expensive decisions on the basis of hard data rather than supposition!

  2. Hello Martin

    Just found this blog.

    I suspect the fact that you have been under charging has had an effect on your batteries and they are now sulphated.

    The batteries were originally 440 amp hours but suspect that they are a lot less than that now.

    How do you monitor your batteries? (rhetorical)

    A voltage reading to give an indication of SOC (state of charge) should be taken when the batteries have been rested, that is not charged or discharged for a number of hours, almost impossible on a narrowboat, if every thing is turned off over night then the reading taken, it will give an indication.

    If the batteries are at 12.2v (50%SOC) then to get them to 100% SOC will take many, many hours, depending on the size of the charger/alternator.

    Your alternator, needs to be spinning at a minimum of 3,000 rpm, that is 1,000rpm on the engine if your pulley ratios are correct.

    At that speed it will probably only be actually giving 50% of its stated amps.

    The batteries control the alternator output, when they are 'flat' they will take all the alternator can give but this reduces as the batteries become charged.

    Batteries can be charged very quickly from 50% SOC to about 90, 95% SOC but to get that last 10, 5% will take as long again plus a bit.

    Two sites for you to have a peruse of:

    The 'Smartgauge' is the simplest method to monitor your batteries, just two wires to the batteries. Costs in the region of £170

    I have no connection with any of the above links just a happy user of the Smartgauge.

    1. Thanks for your comment, Keith. I've seen the Smartgauge before. Though I think if I were going to spend money on something extra, I'd probably go for the remote for my new Sterling AB charger, which seems to offer a whole heap of diagnostics for about that price.