Battery charger cables

[alanporter]

Looking at my battery charging system a thought has just struck me. When the charger is working, power is flowing to the batteries. However, when the charger is "off", power must flow from the batteries towards the charger. Is there a potential problem here ? I presume there is perhaps a diode that prevents this current from entering the charger. Do I need to instal a switch in the cables ?
Advice appreciated.

 

[savageseadog]

Most chargers output through a protective device to prevent backfeeding. A basic charger is only a transformer and rectifier (diode) assembly so they are no problem. An electronic charger needs to be designed with this requirement in mind. I have in fact charged off a laboratory pwer supply that didn't have a protected output and when the mains power went off it fried.

 

[William_H]

To confirm there is no current being fed back into the charger with the charger turned off you need a multimeter with a current range. Typically this will be 200 milliamp, 20 milliamp and 200 microamp.choice of current ranges.
You need to tempoprarily disconnect one of the battery leads and insert the multimeter on current range into the break. ie batt charger +ve to meter -ve. Meter +ve to battery +ve. Don't do this with the charger powered on mains. You should not be able to measure any current flow even when switched down to 200microamp range. start at 200milliamp range.

If you get any current flow from the battery back into the charger you need to decide if it is significant.

To give you an idea 10milliamps flow from the battery will waste 1 amp hour in 100 hours of being connected. Which is of course 4 days worth. So in 40 days you may loose some amount of charge depending on your battery size. Bear in mind that a battery can loose 1% (ie 1Amp hour from a big battery) aprox in a week.

So a 10 milliamp loss might equate to natural wastage of a large battery bank.

So you figure it out as to whether it is significant olewill

 

[elton]

I hard-wired in an 'intelligent' mains battery charger last year and considered this as a potential problem. So I checked the current flowing back through the charger when shore power was disconnected. It was just 2mA, and given that I have a solar panel and wind turbine, I decided it wasn't an issue.

It will depend on the make and type of charger of course, so it's definitely worth testing the reverse current flow.

The simplest solution (if it is a problem) would be to fit an in-line switch in the positive lead from the charger to the battery.

 

[jollyjacktar]

In simplistic terms the charger converts high voltage AC to low voltage AC of about 15V via a transformer then it is converted to DC via generally a bridge rectifier set of diodes and fed to your battery. When you turn the charger off the battery tries to send a current in the reverse direction through the charger leads but the diodes in the bridge rectifier act as a blocker to any current flow and would possibly require typically about 400V to overcome this blocking effect. This not going to be produced by your boat batteries. Thus the only power loss would be due to short circuits or leaks in the insulation of the electrics or through other devices in the general circuit of your vessel.