Shore power and a dead battery

[demonboy]

One thing that has puzzled me is why some of my electronic gear cuts out when there appears to be enough voltage to power them. For example the fridge is supposed to cut out at 11v; the power-hungry auto-pilot hasn't worked for ages; the windlass isn't turning; my stereo cuts out when the volume is turned up... all of these things indicate low voltage from my batteries.

And yet, even though my batteries are dead (I've left two standing for a couple of weeks without charge and they sit at around 12.3 - 12.4v) I have the shore-power charger supplying a healthy charge, giving the batteries a reading of 13.4 or more. The lights don't dim, my smaller fridge works ok, the bilge and sink pumps work fine, etc etc.

My question is this: even if there is enough power coming in from the charger, do the batteries still under-perform and act as a kind of resistor to the current, making my larger electronic items under-perform or not perform at all? Or should the charger be enough to power these items irrespective of how dead my battery is and so these are all just coincidental problems that appear to have the same cause but don't?

 

[the_branflake]

Your battery charger only has so much power - Depending on its output its trying to recharge a dead battery first and then as you turn more and more stuff on its also trying to power that.

If it can only output a few amps say 10 then you can soon be asking for more power than it can deliver.

 

[whipper_snapper]

And yet, even though my batteries are dead (I've left two standing for a couple of weeks without charge and they sit at around 12.3 - 12.4v) I have the shore-power charger supplying a healthy charge, giving the batteries a reading of 13.4 or more. The lights don't dim, my smaller fridge works ok, the bilge and sink pumps work fine, etc etc.

That does not seem to make sense. Are you saying that with batteries on charge and showing >13.4V, your high power systems are not working? If so something is wrong with the distribution; it is possible to imagine a battery so dead that it can appear to take a charge but then not deliver anything when asked, but not likely.

If I am reading this right, the 1st thing to do is to check the voltage at the battery terminals when the fridge fails or the windlass sulks. I am prepared to bet it stays at 13.4V. THen test the voltage at the equipment itself. I expect it to be at 13.4V when the kit is off but crash when you switch it on.

If that happens, you have a duff connection somewhere and you need to find it because it may be getting very hot!

 

[lustyd]

they sit at around 12.3 - 12.4v

What are you measuring that with? This is the reason the clever electrical people on here suggested adding a resistor to cheap multimeters on another thread since the voltage won't drop if you don't draw power at the same time.

 

[demonboy]

Your battery charger only has so much power - Depending on its output its trying to recharge a dead battery first and then as you turn more and more stuff on its also trying to power that.

If it can only output a few amps say 10 then you can soon be asking for more power than it can deliver.

If this is the case then this makes sense. The battery charger is a 45amp unit, so if it's using X amps to try and charge a dead battery it is not going to find the additional 60 amps to power my windlass, correct? Does this mean then that it is forever using all 45amps to charge the battery, never achieving full charge and never allowing other electrical items to run properly?

@whipper-snapper - does your statement...

That does not seem to make sense. Are you saying that with batteries on charge and showing >13.4V, your high power systems are not working? If so something is wrong with the distribution; it is possible to imagine a battery so dead that it can appear to take a charge but then not deliver anything when asked, but not likely.

... still stand with branflake's comment?

@lustyd - I'm using a normal digital multimeter and was not aware of the resistor suggestion. What does this achieve?

 

[concentrik]

.

If you take some simple step-by-step measurements you can find the cause of this quite easily.

You will need a meter otherwise you're just guessing. The 'resistor in parallel' trick is just to overcome a characteristic of modern digital meters, which don't impose any demands on the circuit under test by drawing current from it (well not much anyway). It might be easier for you to get hold of a car light bulb, say a sidelight bulb, wired to a couple of aligator clips. Attach these to the points you want to measure and read the voltage across the bulb with the meter. This will put a load on the point under test and you'll have the additional indication of he bulb - is it lit?

I would remove the shorepower before you start. It will only confuse the issue. Whatever the problem, it'll still be there with or without shorepower.

At 12v4 your batteries are reasonably charged, certainly sufficiently to start testing. So start with everything turned off and measure the battery voltage. Put your bulb across the terminals with the croc clips. Do a little diagram as you work and write down your readings. It's easy to become lost after a few tests.

When you've established your battery "no-load" voltage its time to begin adding loads. Don't get too bogged down in absolute values and ignore the x decimalplace on your meter, like "0.0x". You are more interested in relative readings because you're looking for a fault. Write down your results.

Go to the equipment which you feel is having problems functioning. Measure the voltage at the equipment without it turned on (remember the bulb). Measure again with the equipment on.

Much of the work will involve physically finding good spots to take measurements. You may have to peel back insulation, open connectors etc. Do your poking around with the power OFF, then when you're satisfied with the security of the probes, apply the power. That way you will avoid melting things. Even at 12v4 those batteries will punch quite hard. Post your findings.

 

[whipper_snapper]

If this is the case then this makes sense. The battery charger is a 45amp unit, so if it's using X amps to try and charge a dead battery it is not going to find the additional 60 amps to power my windlass, correct? Does this mean then that it is forever using all 45amps to charge the battery, never achieving full charge and never allowing other electrical items to run properly?

@whipper-snapper - does your statement...



... still stand with branflake's comment?

I don't believe the charger has anything to do with it. If the battery is showing 13.5V it really does not matter what the charger is doing. If you disconnect it, what happens to the voltage? Unless it immediately plummets to near 12.0 then it is not an issue and as said, proceed with the diagnosis with the charger disconnected. Does the charger show you are still putting a lot of charge into the batteries ?

@lustyd - I'm using a normal digital multimeter and was not aware of the resistor suggestion. What does this achieve?

It is true that digital meters drawing sweet FA can give spurious readings when connected to downstream wires because you can have an essentially broken connection somewhere that still allows enough charge to trickle across to show a voltage where there is actually nothing useful. But across the battery that cannot be an issue. Otherwise a bulb as suggested by concentrik is a good idea. Actually I find that holding both wires with sweaty salty fingers as I use the meter helps remove any spurious voltages (don't try with mains!)

 

[Ruffles]

I'd start using your meter to measure the voltage across switch contacts and connections. Sounds like there's something amiss. A multimeter with a max/min capture feature is brilliant for this. One connection on the load, one on the battery. Select max and turn the fridge on.

 

[stevensuf]

remember the voltage will drop the further away from the batteries you go, internal wire resistance, corroded connections etc, every connection adds some resistance to the chain dropping the voltage yet again.

My fridge would cut out on low voltage all the time, i cleaned up the fuse connectors then the blade connectors on the compressor, both were dropping a volt or so, even though i had 13.5v at the battery the fridge was only getting 11.5.

 

[the_branflake]

If this is the case then this makes sense. The battery charger is a 45amp unit, so if it's using X amps to try and charge a dead battery it is not going to find the additional 60 amps to power my windlass, correct? Does this mean then that it is forever using all 45amps to charge the battery, never achieving full charge and never allowing other electrical items to run properly?

@whipper-snapper - does your statement...



... still stand with branflake's comment?

@lustyd - I'm using a normal digital multimeter and was not aware of the resistor suggestion. What does this achieve?

Ok that's a fairly meaty charger then. As other have suggested you need to conduct these tests to see where the fault lies.

Does your charger have any sort of amp gauge on it so you can see what its putting in to the battery?

 

[halcyon]

My question is this: even if there is enough power coming in from the charger, do the batteries still under-perform and act as a kind of resistor to the current, making my larger electronic items under-perform or not perform at all? Or should the charger be enough to power these items irrespective of how dead my battery is and so these are all just coincidental problems that appear to have the same cause but don't?

If you are only reading 13.4 volt with battery charger running, you are either in float charge, or the batteries charge cycle is not complete, or you have a faulty charger, or faulty battery.

The charger needs to take the batteries upto 14.4/14.6 volt, if not you are under chargeing your batteries.

You may have a bad connection causing a low voltage to certain equipment.

Take your multimeter negative probe, and a length of 1 sq mm cable, long enough the reach from battery to bow. Strip back insulation and twist wire on to probe end, and then rap with isulation tape.

Take the other end of the wire and attach to battery negative terminal post, i.e. remove battery clampbut wire between clamp and post, refit clamp.

You can now measure voltage relative to a fixed point.

Say the winch does not work, you can put the positive probe on winch, and get someone to operate, you now have actual voltage, now put positive probe on negative terminal, you are now reading volt drop in negative cable.

You can check all positive and negative voltages with your multimeter.

If you find a low voltage work your way back to the battery, when you get a big change, the fault is between there and the previous reading.

Brian

 

[CharlesSwallow]

You first need to test the batteries properly and this means a load test. Worth buying such a tester with the size of battery bank you probably have on your Oyster 435. Disconnect them and clean the terminals before testing. Then, after re-connection and with the engine running and charging, turn on your largest load and after a minute or two (don't use the windlass as it is often not possible to run this on load for a few minutes) put your hand or finger on every terminal you can find. The ones that are hot are your problem and need taking apart, cleaning and greasing before re-assembly. THAT is where I think your problem lies and my guess would be that it is in the negative return wiring somewhere.

Give your engine battery circuit the same treatment. It isn't connected to your domestic system but since it lives in the same environment and has done so for the same time, it will be just as prone to the same environmental diseases!

Chas