'back flow' into chargers

[pcatterall]

Not wanting to side track the current discussion on leaving chargers switched on but............I recall that in 'the olden days' leaving a battery connected to a charger and the charger mains off would result in the battery trying to charge the charger.I assumed that with modern technology this would be a thing of the past.I recently purchased a 'smart' charger to keep my workshop battery fully charged but the instructions said " don't leave charger connected to the battery when the mains supply is off or 'back charging' would result".My concern now is that if the mains trips or my dear wife switches it off then my battery will be flattened.Is this 'back charging' still a factor then in modern chargers or is my 'smart' charger not smart enough.??Can I just wire in a ( diode?) to prevent this?Thanks

 

[earlybird]

Manufacturers do sometimes quote a "back current" drain, < 1aHr/month for a CTek I looked at, so pretty minimal.
A diode in series will only reduce charging voltage for no other gain.

 

[VicS]

Not wanting to side track the current discussion on leaving chargers switched on but............I recall that in 'the olden days' leaving a battery connected to a charger and the charger mains off would result in the battery trying to charge the charger.I assumed that with modern technology this would be a thing of the past.I recently purchased a 'smart' charger to keep my workshop battery fully charged but the instructions said " don't leave charger connected to the battery when the mains supply is off or 'back charging' would result".My concern now is that if the mains trips or my dear wife switches it off then my battery will be flattened.Is this 'back charging' still a factor then in modern chargers or is my 'smart' charger not smart enough.??Can I just wire in a ( diode?) to prevent this?Thanks

FWIW

Of the two very old chargers I have, both of which predate silicon diodes having old fashioned plate type metal rectifiers , one passes a reverse current of 1.5mA and the other 0.15mA. If they were a bit more modern with silicon diode bridge rectifiers I' expect the reverse current to be even less.

Of the two modern smart chargers I have one passes a reverse current of 12mA while the other only passes 0.7mA

I'd expect a charger designed to be permanently connected to pass a negligible reverse current. If you use any other check it before you leave it permanently connected.

A good brand, intended to be permanently connected, like Ctek states the reverse current drain. For a M200 <1Ah /month

 

[rob2]

Quite a difficult one to answer as the manufacturers of chargers build to so many levels of complexity. I am surprised that yours has this warning. I remember finding that a very old charger that my dad used was only half wave rectified so it wasn't too surprising that it would load the battery if switched off - I thought that they were all full wave rectified now and mostly have fancy switch mode electronics, which I guess is what acts as the load.

I guess the answer is to use a permanent supply - much like a cooker point that the wife won't be able to plug the Hoover into. You could always also insert a relay between the charger and battery so that it disconnects if the power is off, but reconnects automatically after the outage.

Rob.

 

[elton]

I have a Halfords "smart" charger, which I permanently connected to the house battery after measuring the drain by the charger on the battery when disconnected from the mains. It drains at 3mA, which is more than compensated by a small solar panel.

So I suggest connecting an ammeter in series between the charger and the battery, to check the drain. Start on the 10A range, and decrease gradually until a meaningful reading is achieved.

 

[rogerthebodger]

I fitted a selector switch between my permanently installed battery charger to select either my domestic or engine battery the included an off position to overcome the 'back flow' problem.

 

[pcatterall]

Thanks guys for the general info and the advice............measure any drain then act accordingly if necessary !!

 

[ianj99]

FWIW

Of the two very old chargers I have, both of which predate silicon diodes having old fashioned plate type metal rectifiers , one passes a reverse current of 1.5mA and the other 0.15mA. If they were a bit more modern with silicon diode bridge rectifiers I' expect the reverse current to be even less.

Of the two modern smart chargers I have one passes a reverse current of 12mA while the other only passes 0.7mA

I'd expect a charger designed to be permanently connected to pass a negligible reverse current. If you use any other check it before you leave it permanently connected.

A good brand, intended to be permanently connected, like Ctek states the reverse current drain. For a M200 <1Ah /month

+1. Leakage was due to the old rectifier technology.

 

[VicS]

+1. Leakage was due to the old rectifier technology.

But the greatest reverse current is with one of my modern smart chargers ( 12mA). Both of the old fashioned chargers with a plate type rectifiers are less than this, one being the lowest of the four ( 0.15mA)

 

[ianj99]

But the greatest reverse current is with one of my modern smart chargers ( 12mA). Both of the old fashioned chargers with a plate type rectifiers are less than this one being the lowest of the four ( 0.15mA)

12ma is a lot for a modern charger.

However self discharge is possibly a bigger factor depending on the type of LA battery, than reverse leakage.

 

[lw395]

12ma is a lot for a modern charger.

However self discharge is possibly a bigger factor depending on the type of LA battery, than reverse leakage.

I've seen 'modern' chargers where the connected battery lights an LED, even with no mains.
So that's probaly about 20mA minimum.
As you say, self discharge can be easily as much.

Personally I use a 'dumb' float charger to keep my bikes charged over the winter, I move it from bike to bike about every week or two and it is regulated at 13.6V. It works very well.