Electrical advice needed

[alanporter]

My twelve volt system comprises two deep cycle batteries, a 1 - both - 2 - off switch (30 years old but well protected from salt water), a single output shore charger connected only to battery number one. The alternator is connected to a battery isolator.
Engine off; shorepower on, switch to battery one. Voltage at battery one is 14.4 volts while that at battery two is 13.75 volts. This battery is supposed to be "resting" with voltage no more than 12.8.

I am presuming that somehow power from the charger is getting to number two battery, but at a power less than that to number one.
Can anyone explain this ? How do I go about checking the system ?
Any help gratefully received.
Meantime, Merry Christmas to all on the forum.

 

[Pye_End]

What wre the voltages before you switched on the charger?

Don't know about deep cycling, but leisure can certainly get voltages above 13v particularly after a charge.

 

[bdsweeting]

Is the main battery isolator/changeover switch in the OFF position when using shorepower?

Sounds like there is a feed into battery 2 somewhere which is allowing some voltage through from the feed to battery one.

It might be wise to draw a schematic of the electrical ciruits through the various switch positions. This may make it easier to trace where the power is going.

 

[halcyon]

Sounds a diode volt drop.
Try turning off the charger, and putting a load on battery one, is battery one now lower than batt 2, and batt 2 voltage falls to around 13.2 volt. Try removeing a battery lead of the splitter, could be a short circuit diode on battery 1 output.
If not look for a back feed to the alternator.

Brian

 

[yoda]

As Pye_End says you need to see if the voltage at battery 2 changes when the battery charger is turned on and off. That will give us more to work on!

Yoda

 

[philip_stevens]

As Brian says, it sounds as though one diode in the splitter has gone short circuit, and the half volt difference between the batteries is the volts drop across the good diode.

You can either remove one lead to the splitter - Brian's advice, or disconnect the splitter completely and with a multimeter, see if you get an Ohms reading in both directions from one diode.

From what you say, I would think the diode in the splitter to the higher voltage battery is the one that has gone short circuit - remove that lead to battery and then check voltages when the charger is on.

Halcyon should know, as he developed marine electronics under KDD Powercentre - see his url

 

[Birdseye]

When you alternator is running and the battery is charged, the voltage controller will hold the alternator at something in the mid 13s. This is the float voltage.

When the engine is stopped, the voltage on the battery takes some time (maybe as much as 24 hours) to drop to the fully charged figure of around 12.7. This is because some charge is left "floating" so to speak on the surface of the plates and needs time to leak away. This is much the most likely explanation of your problem because a circuit leakage would most likely keep both batteries at the same voltage

If you still have more than 12.7 after 24 hours then you will have some leakeage somewhere. There is no way that anyone can give you good advice on this without a circuit diagram, so all we can do is give pointers. First thing to look for is any equipment permanently connected to the batteries ie by passing the main switch. It would have to have a resistance or the voltage would be the same. Disconnect any wires at the battery one by one until you find the cause.

Second most likely is something to do with the alternator charging circuit because it would be very unusual if you did not have some provision for charging both batteries when the alternator was running ie a common circuit. These are usually kept seperate by a diode or a relay. I doubt it would be the isolating diode causing the problem because the only way that would affect the voltage would be if it failed in which case the voltage would be the same. More likely there is a leakge through some part of the circuit which gives a resistance and reduce the voltage in this way

 

[dannyboy]

How accurate is your voltmeter? I don't know what type of shore power charger you have, but most are designed to maintain batteries at float voltage after charging, so assuming your domestic battery is fully charged, the voltage across it whilst being float charged/maintained by your charger should be in the region of 13.6 - 13.8 volts. 14.4volts is certainly too high for a maintenance/float charge and unless your charger is goosed, I doubt it will be the case. I would suspect your voltmeter is inaccurate - being in the order of 0.8volts or so out. Which would mean the true voltage across your resting engine battery would be around the 12.9 volt mark - a more realistic figure. Before you go tearing your electrical system apart, try and borrow another half decent volt meter - preferably a good digital one such as Fluke. If yours is an analogue one that hasn't been calibrated since God were a lad or a digital one that cost less than £80, then bin it. Basic ones used by domestic electricians are simply not accurate enough for measuring low voltages down to fractions of a volt. Hope this helps.

DB (electrical engineer for 22 years)