Volt meter, power drain?

[rich]

I have wired up my volt meters, but they are on while the batteries are switched on, how much drain will they put on the batteries? is it ok to leave like this. Cheers Rich.

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[jkim1]

I have recently done the same its probably best if you switch them to the voltage you want to monitor. Be warned however that if you use car type voltmeters they do take current and they also take time to stabilise. With DVM,s being so cheap now its better to have a switched system and just turn the DVM on when required.

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[discovery2]

When I re-built our previous boat, I wired the voltmeter via a switch that was normally 'on' whenever we were on board (the fridge circuit or fresh water pump circuit, for example) on the basis that if the boat was unattended these circuits would probably be switched off and the voltmeter would not run the batteries down unnecessarily.

David

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[dick_james]

If you are using car-type voltmeters, they are 'hot wire' meters and take somewhere between 100 & 200 milliamps. Should not be left on all the time.

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[oldharry]

Volt meters like any other electrical appliance do use a small amount of current - even DVMs! It is not good practice therefore to leave them permanently on.

There is little point in leaving them 'on' while other appliances are running anyway, as they give a false low reading as the load on the supply battery increases. The only useful readings are a) checking the charging rate when the alternator is running, and b) when the battery is not discharging, and has stood for at least 5 minutes without a load. Any load on the battery will result in a meaningless reading: just watch the voltage dip when you start the engine! The heavier the load, the lower the voltage reading.

A bit of a waste of time fitting them really, except as a charge monitor! I use a battery and charge state monitor (Maplins) which has a row of 6 LEDs permanently wired via a 'press to make release to break' switch, which I can flick for an instant battery state check before switching anything else on, and when the engine is running just to ensure the alt. is charging properly.

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[halcyon]

Reading the last post, reminds things don't change.
In around 1982 we bought out a battery monitoring sytem that used 6 / 8 LED's, gave you battery state on battery switch on, gave a read out as battery was being used based a load curve, gave engine battery state, battery charge status, controlled split charge relay, and turned the fridge off to stop it draning the battery flat, thus maintaining a reserve for nav equip. This used less than 1 amphour a day, a small amount when compaired to any other load.
For a system to be of use it must allow uaseable monitoring when in use, and run at a load that will allow this. After all, what is the point of knowing you have low power supply only when the lights go dim.

Brian

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[jkim1]

I agree with Halcyon. Maplins used to sell a three outlet car type socket with a battery monitor built in. I have been told by the Southampton branch that they no longer stock it. Stupid really as it was the same price as a unmonitored unit. I would still go the DVM route as it will read to one hundredth of a volt. Yes you will have to turn it off and buy batteries for it. but it will only cost £2,99. Check out the price of the Maplins analog one its £15 and reads to half a volt.

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[William_H]

Hello rich I disagree with ole harry's comment that it is a misleading reading of battery voltage when the load is on. This is just the time when you can get a usefull indication as often a really stuffed battery will give a reasonable voltage reading with no load but dies with a load. Any way in answer to your question you did not say what kind of voltmeter you have fitted. As another contributor noted car type voltmeters can draw a lot of current. A moving coil type old fashioned meter draw quite low current typically 1 milliamp. Fixed digital voltmeters if it is a Liquid crystal type will be quite low wheras a Light Emitting Diode type can draw quite a lot of current to operate the LEDs. Your best bet is to use a digital multimeter on current range to check the actual current used, Bear in mind that a largish battery will have a natural self discharge rate of several milliamps so anything around one milliamp will not make much difference. (milliamp is 1000 th of am amp) regards ole will

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[oldharry]

Quite right William, the level of voltage drop on load is one way of telling whether your battery is shot. But thats not what most people are looking for most of the time on their boats: usually you are wanting to know the state of charge - is the battery going to let you down half way through cooking dinner? And if you use it for starting the engine as well, is there enough life to do that tomorrow?

Unfortunately voltmeters do not make good 'state of charge' meters, because voltage varies with load - often by more than the 1.5 volts or so open circuit voltage difference between a fully charged and a discharged battery. Also many deep cycle batteries have a different discharge voltage characteristic, being designed to maintain a fairly constant voltage as they discharge - again giving confusing and misleading voltmeter 'charge level' readings.

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