battery monitoring

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Would a digital multimeter, attached permanently to my battery, be an accurate and cheap solution to battery monitoring?

If so, what readings are the vital ones to look for?

Thanks

Pat

Pat O'Donovan
 

tgalea

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hi Pat,

Just a humble opinion!

Essentially a multi-meter performs the job of several instruments combined in one with a selector. So of course in voltage mode this instrument would read battery voltage however i would think that a simple voltmeter (which you could possibly buy from these electrical shops) should cost much less than a digital multimeter attached permanently as a voltmeter.

As regards to readings... well i would say you should be after an approx 12 volts with you battery master on, engine off and no load. About 14 volts when the alternator is supplying power to the system and a reading between these two when you turn on any systems.

i would say that you will in time get to know your own electrical system and you will get used to the "standard" readings. One day you will / might notice a difference which will tell you that something is not as it used to be or needs attention !

consider installing a little Ammeter. These two instruments together then can give you quite a good picture of your electrical systems.

Just my opinion however...

cheers

Tyrone.
 

EdEssery

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I too would be interested in knowing the voltages that indicate charge state of a battery - I've read them somewhere and now can't find them. The one I can remember is that a 100% charged battery reads 12.60 Volts across the terminals.

Does anybody have the other voltages (80%, 60%, 40%, 20% and flat) and a reference for where they are documented. I thought it was in Nigel Calder's book but can't find it in there.

Thanks,

Ed
 

tjc

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I too am in the market for something similar. Heart Interface make a battery monitor which is about £150, it seems quite good and is easy to install. They are on http://www.heartinterface.com/.

Blue sea systems make a seemingly good multimeter for approx the same price and it seems slightly better spec'd. They are on the West Marine site at http://www.westmarine.com or http://www.bluesea.com/index.htm. Heart are at most chandlers.

Having said all that every one I know has a simple ammeter and voltmeter (again Blue Sea) for approx a quarter of the price. It depends how much info that you want. Me I went for the Blue Sea meter.

Hope this helps
 

ean_p

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Rough figures are......100%---12.80-----1.28
90%----12.65-----1.26
80%----12.5-------1.24
70%----12.4-------1.22
60%----12.3-------1.20
50%----12.2-------1.18
40%----12.1-------1.16
30%----12.0-------1.14
20%----11.8-------1.12
10%----11.7-------1.10
0%-----11.6-------1.08

These are rough and 'off load' figures the battery having rested a good while after charging and they are also very dependent on temperature.......to me a better test of battery condition, which is what you really need to know is the 'on load ' terminal voltage...this i think tells you more about the state of the cells and would show up potential weakness......anyway hope it helps....
 

oldharry

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Any electrical gadget connected across a battery will eventually drain it. Although digital meters take very little current, an analog moving coil voltmeter (such as those sold by the car shops) and often found on after-market switchboards take enough current to cause problems if left on for weeks at a time without recharging. I suspect even a digital meter left on for the winter could seriously deplete battery energy, though I never checked the figures.

Unless you have permanent charge top-up either by wind or solar or mains power, its best to have charge voltage monitors switched through the engine or domestic circuits.

I have at least twice come across unexectedly discharged batteries caused by analog voltmeters being left permently connected.
 

robp

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oldharry, you might be right but if it's as simple as Ohm's law, I'm surprised. One should be able to expect a modern analogue Volt meter to have a DC resistance of 50,000 Ohms per Volt. If I = E/R, then at 12V the current drain should be 0.00002 Amps. (0.00024Amps at worst). Or am I missing something? I'd like to do this..
 

ParaHandy

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Don't do that - they're far too useful elsewhere. The meter has to be a digital volt meter for precision measurement and Halfords have one at £25 or so.

Many helpful posts give you the open circuit voltage (anything below 70% and you're flogging a dead horse). Find a convenient place on your boat to measure the voltage and monitor it. You'll soon get to know what's going on. This web site might help:
www.uuhome.de/william.darden/
 

brian_neale

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I have bought a cheap (cost me £10 in a sale) digital multimeter in Maplin. I think the full price might be as much as £15! Includes a simple circuit tester for checking wiring, which could be useful (if only to check fuses).
 

HaraldS

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A digital multimeter would give you a fine reading of your battery voltage. However voltage is quite load dependent so you need to also see the current draw.
While multimeters can measure current, they will not do this to well in a boat electrical system, since you would have to have it connected in serial and the will not do more than 20A, and that only for a short time.
To measure current, you will need a shunt, which is a calibrated resistor with very low resistance that can take several 100 Amps ongoing. Then the small voltage drop on the shut is measured, depending on the shut you get say 1mV per Amp.
You could buy a shunt and then wire the multimeter so you can switch it between battery voltage and shunt voltage to read the Amps.
A more costly and not necessaryly better (needs some poer and is not accurate at low loads) is a Hall-Sensor which measures current by measuring the magnetic filed around the wire. Clamp on multimeters that can measure DC use that method.

In the end you don't watch the meters all the time and that is what a real battery monitor does, it integrates the measured current over time, this way having a pretty accurate figure of what went into the battery charging and what went out on usage. A mulimter will not do that for you.

I have two battery monitors one for the 24V domestic set and one for the 12V engine starting set. The later is a pure luxury, because this battery is usually fully charged unless something goes wrong.

But I'm mentioning it, because they are of differnt make and both are good.

The cheaper 12V is from VDO and is very easy to install, since the shunt goes right onto the negative battery terminal, and then it has a similar pport where you connect negative battery cable. Thr rest is a couple thin wires to the instrument. It does everything you would expect from a battery monitor.

The second is from Heart-Interface and called a Link 10. (There is a Link 20 if you want to motitor two circuits). It has a high power shunt that can take 500 Amps, and it works with 12 or 24 Volt systems.

I can recommend both.
 

oldharry

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Like I said RobP, I hadn't checked the figures for a DVM, so the internal battery losses are probably greater than the DVM draw.

However, it is still not a good idea to leave any electrics permanently 'on' if the boat is left long term (except auto bilge pumps and chargers which are designed to do it). Damp and condensation on an unsealed PCB can creat havoc with it, and could create a potentially dangerous fault condition.

And yes, any cheap DVM will give a reading - the Maplins ones being particularly good value for money. They need to display to .01 of a volt for accuracy.

But cheap meters like these are not designed for the cold and damp of life aboard, and will not last unless a little care is taken to keep them properly dry.
 

dick_james

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But it's not as simple as that. The analogue meters used in cars etc. are I believe moving iron to give a non linear scale (10 to 15v approx). They take between 100 or 200 milli-amps.
 

oldharry

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You are right Dick, most - if not all - car meters these days are moving iron. They are much cheaper to make, and marginally more robust than their moving coil counterparts. But they are not sufficiently accurate to give readings to .01 of a volt, which is why they rarely have properly calibrated scales, which are not needed anyway in this application.

To gain a clear idea of the actual state of charge of a battery simply using a voltmeter, the battery needs to have been idle (i.e.no current draw) for at least 10 minutes before the test. Monitoring voltage in servoice is largely meaningless. As the current drain increases the voltage will drop accordingly, then recover when the load is removed.

Engine cranking for example usually causes a voltage drop to between 8 and 10 volts at the battery terminals, and the battery can take 5 or 10 minutes to come back up to voltage. However the alternator cuts in once the engine starts, and takes the voltage up to nearly 14 volts, and only drops back as the battery nears its full charge. The battery remain at around 13.5 volts for an appreciable time after charging, even if it is not fully charged, and quiescent voltage only returns after around 20 minutes.

So for most of the time, a voltmeter being used as a 'state of charge' meter will be giving a false reading, and can be dangerously misleading either way, and ought only to be used in conjunction with an ammeter to be of any real use.
 

ean_p

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think I might have to disagree there Harry....the amount that the voltage drops when it is given a good load is a very good indicator of the health of the cells...independent of the previous charing/use regime that the battery has been exposed to...thus a cell given a reasonably good load that drops its terminal voltage to less then say1.5 from a starting point of 2 or more is probably a cell that is weaken, defective or unable to accept a full charge due to sulfation or some other fault....even cells in a poor condition can show a good terminal voltage after a charge for some considerable time....this is why the only true test of a cells condition is to charge it and then discharge it through a resistance and measure the time taken for the voltage to fall
 
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