Can a battery discharge 'back' through the mains charger?

[davidej]

Those who are foolish enough to have been following my recent posts will be aware that I managed to flatten both my house and motor battery.

The boat is a Bene361, with house and motor battery, each with a separate isolator on the positive side and a common earth with a third isolator. it also has a Dolphin mains charger with two outputs -one for each battery. I accidentally left the house circuit on with a small current draw for a matter of a couple of weeks which managed to flatten both batteries. This seemed to confirm my long-held suspicion that the two circiuts were not properly isolated.

I went back on board (swinging mooring) with a recharged motor battery and a brand new house one - the old one was 5ys old. I connected the motor battery first and some of the lights on on the main switchboard lit up - these should only be fed from the house battery so there is something going on there. Even stranger was that the led on the mains charger lit up - I thought it would only come on when shorepower is connected and it switched on. Interestingly this led did not light up when I disconnected the motor battery and connected the house battery.

Almost the only wires connected to the battery side of the isolators are those from the mains charger - which leaves me wondering if current could leak back up to the charger then into the other circuit. If so is it possible to fit some kind of diode to prevent it?

Any better explanation for all this would be welcome!

 

[VicS]

The outputs should be fully isolated. It might not be a bad idea to check just to be sure nothing has gone wrong.

Some thorough investigation of the electrics would be in order.

A diode would introduce an unwelcome volts drop

 

[davidej]

PS

I also have a VSR which is fitted as per instructions to the motor side of the moror battery isolator and the battery side of the house battery isolator.

Could this stick closed and pass the current?

 

[elton]

It depends on the charger. I left a Halfords charger permanently connected to the house battery after I checked the current draw by the charger on the battery with shore power off, and found it was less than 2mA. So easily made up by my solar panels and wind turbine.

 

[pmagowan]

IIRC the VSR will parallel the batteries as long as the 'start' battery is above a certain voltage so perhaps this is the case with you as the new batteries are charged up. I am afraid I don't really know much more about the magic that goes on inside these devices. Some chargers have a little light to tell you that the polarity is correct and that you have not attached the black wire to the red terminal.

 

[Boo2]

Those who are foolish enough to have been following my recent posts will be aware that I managed to flatten both my house and motor battery.

The boat is a Bene361, with house and motor battery, each with a separate isolator on the positive side and a common earth with a third isolator. it also has a Dolphin mains charger with two outputs -one for each battery. I accidentally left the house circuit on with a small current draw for a matter of a couple of weeks which managed to flatten both batteries. This seemed to confirm my long-held suspicion that the two circiuts were not properly isolated.

I went back on board (swinging mooring) with a recharged motor battery and a brand new house one - the old one was 5ys old. I connected the motor battery first and some of the lights on on the main switchboard lit up - these should only be fed from the house battery so there is something going on there. Even stranger was that the led on the mains charger lit up - I thought it would only come on when shorepower is connected and it switched on. Interestingly this led did not light up when I disconnected the motor battery and connected the house battery.

My car battery charger has a LED that lights when the battery is connected whether or not the mains connection is plugged in and I think that is fairly normal. From what you say about the wiring then it does seem as if the charger is bridging the circuits, but maybe more likely is a wiring problem that you are unaware of ?

What model of charger is it, and is the manual available for download from the web ?

Boo2

 

[halcyon]

This seemed to confirm my long-held suspicion that the two circiuts were not properly isolated.

I went back on board (swinging mooring) with a recharged motor battery and a brand new house one - the old one was 5ys old. I connected the motor battery first and some of the lights on on the main switchboard lit up - these should only be fed from the house battery so there is something going on there. Even stranger was that the led on the mains charger lit up - I thought it would only come on when shorepower is connected and it switched on. Interestingly this led did not light up when I disconnected the motor battery and connected the house battery.

Almost the only wires connected to the battery side of the isolators are those from the mains charger - which leaves me wondering if current could leak back up to the charger then into the other circuit. If so is it possible to fit some kind of diode to prevent it?

The battery charger probably has one regulator feeding two diodes to split the output to two battery banks. If one of the diodes fails short circuit, it will allow it's battery to back feed the regulator, the second diode is conducting from the regulator thus providing a circuit, allowing the service battery if under a load to discharge both batteries. This could also be the reason for the LED on the charger.

VSR fault, depends on the design / spec of the relay, basically they should not be able to weld contacts, but cheap low spec one could. But if a battery is dicharging the voltage will fall, and the relay drop out, so this should not be the problem.

Have you tried turning on the cabin lights, then turn the service isolator switch off ? if either items are passing current, the lights will stay on.

Bian

 

[davidej]

What model of charger is it, and is the manual available for download from the web ?

Boo2

It is a Dolphin - not a current model, circa 2000, so their website does not list it.

When i get a chance, i will connect an ammeter in series with the motor battery, then disconnect each circuit one at a time and try to work out where this stray current is going.

 

[davidej]

I emailed Dolphin and they suggested that the blocking diode is faulty.

Needless to say they suggested I buy a new one.

What chance is there of me identifying the component and replacing it myself? otherwise, who do you recommend on the East coast

 

[VicS]

I emailed Dolphin and they suggested that the blocking diode is faulty.

Needless to say they suggested I buy a new one.

What chance is there of me identifying the component and replacing it myself? otherwise, who do you recommend on the East coast

It should not be difficult to identify and test the blocking diode(s). Certainly worth having a look, esp if you can get an electronics bod to take a look too, before buying new.

 

[halcyon]

I emailed Dolphin and they suggested that the blocking diode is faulty.

Needless to say they suggested I buy a new one.

What chance is there of me identifying the component and replacing it myself? otherwise, who do you recommend on the East coast

Can you open it up and post some photos ?

May then be able to point out what to check.

Brian

 

[davidej]

Can you open it up and post some photos ?

May then be able to point out what to check.

Brian

What a kind offer.

will try to do so over the w/e

 

[ostell]

One of the chargers I have quotes a leakage current of "< 100mA " when connected. How much less I don't know. It's a pain as I can't really leave it connected but it does charge well when plugged into the mains.