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  1. #11
    Join Date
    Apr 2016
    Location
    Chichester
    Posts
    711

    Default Re: What voltage do you let your batteries drop to? Or Smartgauge percentage?

    Quote Originally Posted by GHA View Post
    That's probably the more important bit to look at, takes ages to get lead acid back to really fully charged - very unlikely just under engine. And must be the no1 way batteries get killed, general consensus seems to be best to get really 100% about once a week or less.

    You need some solar
    Smartguage seems good at percentage discharge of capacity but is over-optimistic on giving a measure of 100% charge. We use the fridge as a nice-to-have and will use it if we can be sure of getting the batteries up to full charge - which either means an overnight on the mains charger within a week or leaving the boat on a mooring with 100% showing on the Smartguage and with solar to bring it genuinely fully up to charge. In three years, the batteries have never been less than 80% discharged and usually not less than 85%. I know this is conservative but we lived without a fridge on a boat for 15 years without distress and I like the idea of getting my money's-worth from the batteries.

    We also have a Victron BMV 700 which, because it is user-programmable, gives a better reading of full charge than the Smartguage (which, in turn, seems more reliable for percentage discharged).
    Last edited by Poey50; 11-09-19 at 11:49.

  2. #12
    Join Date
    Feb 2013
    Posts
    971

    Default Re: What voltage do you let your batteries drop to? Or Smartgauge percentage?

    Strangely, in this magical world of battery science I have read those who say that frequent but small use of the batteries without ever a reasonable discharge can be equally damaging. In other words running down occasionally to say 80%, but at all other times on the grid. I think the logic is that the charger rarely comes out of float, and when it does, its is running for a very short time on bulk. I have done several complete cycles on my bank from around 50% charge to as close as to full as possible. At the lower end of charge I can put around 54 Amps into the battery running two large chargers simultaneoulsy - but it is interesting the amount this backs off as the batteries come up to "full" charge, and also how little charge the batteries will then start to accept. At the top end, this means very little charge is going into the batteries, and I guess the argument is this is exactly the area in which they are likely to sulphate, and without ever being cycled, the sulphation gradually increases. On this point I am not sure whether the science really supports the argument or it is just another myth and the extent to which it depends on the construction of the battery.

    Also, I think it is well rehearsed that bring the batteries up from 50% to say 80% is relatively quick and easy - the problem with any lead acid battery is that the last 20% to 10% is really hard as the battery accepts less and less of the charge. I can run my bank down to say 55% overnight, and either on solar by before lunch they will be back to 80%, or, on the Genset possibly a couple of hours, but the last 20% will take until the evening, which is a really pain. It isnt shortage of power, but the inability of the batteries to absorb the available power.

    Perhaps, if there were room, for the serious liveboard, and in an ideal world, you would have two banks so one could be on charge, while the other was in service!

  3. #13
    Join Date
    Nov 2009
    Location
    Southampton
    Posts
    35,519

    Default Re: What voltage do you let your batteries drop to? Or Smartgauge percentage?

    For what it's worth, and with no particular science behind it, my answer to the OP's question is 50% on the Smartgauge. I rarely see that, but if I do I will start thinking of motoring (or motor-sailing) for the batteries' benefit where I might otherwise sail.

    Otherwise, I don't really make any decisions (engine on, marina vs anchor, etc) with the batteries in mind, and my normal pattern of use keeps them reasonably topped up.

    Pete

  4. #14
    Join Date
    Mar 2005
    Location
    East Sussex.
    Posts
    21,345

    Default Re: What voltage do you let your batteries drop to? Or Smartgauge percentage?

    Paul Rainbow's reply and others have reassured me. I did see 67% discharged and 2.55 volts before re-connecting to mains and turning on the charger. The reason the voltage was higher is that I now have 4 x 50w flexible solar panels on the bimini and between writing my first post this morning and arriving at a jetty with mains power this afternoon the sun had been providing enough power to balance the fridge load. I hardly ran the engine so the alternator has contributed very little. It now appears to me that I can run the fridge for two days or slightly more "off grid" without doing damage to the batteries, which is the result I wanted from adding the solar panels. Where I am now there is no ice available, but it seems clear that if I can start the fridge off with a couple of ice blocks I can stay off grid and still have cold drinks for about a week even in the 30+ temperatures here, and aim to keep the Smartgauge percentage at 60% or higher.

    EDIT. The batteries are fairly new, bought in May this year and are 3x110AH plus 1x110AH starter battery. When I recharge on the mains charger I keep it on until it goes first into float mode and the turns itself off. Its a 30A pro Charge Ultra, and on connecting today it charged for a time at just over 30 amps, is now down to 16 amps and will gradually drop its output until it is doing no more than balancing the 12 volt load from lights, fridge, and this computer.
    Last edited by Norman_E; 11-09-19 at 15:18.
    Working on immortality - One day at a time.

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