I have a V6 TDI and want to know if it is ok to charge the Battery whilst it is still connected in the car. I only do very small journeys and I wish to supplement the charging with the occasional trickle charge. Can anyone advise?

You should really disconnect the negative lead before using an external charger. The chargers themselves dont have particularly stable outputs and the voltage surges could kill the cars electrics.

I use a trickle charger in the Winter if I'm away for more than a few days. I just give it an overnight charge via the socket in the boot.

I have had no electrical problems whatever.

rotodiesel.

Thanks for your replies. I really don't want to disconnect the battery as I have heard a few people have had issues after doing this?

I use an old Toshiba laptop adaptor rewired for cigarette lighter socket

Seriously, it is small, has the right voltage and gives out up to 8A

some laptops have 20 volt adaptors ,be aware that if you connected one to your battery ,it would charge at the maximum adaptor output ,until you disconnected it!. A laptop adaptor with a 14 to 15 volt output,at 3.5 to 6 amps , would be OK to use.( Toshiba are 15 volt) Also dont leave it connected to the battery ,with the mains turned off,because there will be a small amount of current flowing back from the battery to the laptop adaptor,which will drain the battery eventually,over a period of time.

The purpose built automatic battery chargers ,are a better bet ,as they do not drain the battery when the mains are off , also the charge will lower when the battery is nearly fully charged. More recent chargers are of the switch mode type.They are smaller ,lighter,and about 80% efficient.( Older chargers with a big transformer ,are about 40 to 45% efficient).

I have never disconnected a battery when charging .It is the large high current Garage Chargers ,which put in about 50 to 100 amps in a matter of minutes which can cause damage if the battery is still connected to the car.

Edited 3 December, 200916 yr by AndyPandy

I have a Schottky diode in series with Toshiba PSU to lower voltage slightly (0.2V) and prevent overcharging. This also prevents current flowing back to PSU when unpowered.

For an occasional top-up (I do once every 4-6 weeks or so), you do want to keep 14.9V-15V for at least 12hr-24hr in order to eliminate sulphur buildup, current will be low anyway even without a "special" charger.

There probably exist decent switched mode battery chargers in sub stlg50 price bracket, but I have yet to come across one. Trickle chargers aside, most "battery chargers" boast the "high speed charging" mode which actually overloads car installation to 16V and more - they are really meant to be used with battery disconnected from the car.

On balance, my stlg10 PSU picked off Ebay keeps the circuits happy with its under 15V voltage. I have been using it for 6 years now on one car, and 4 years on the other without problems.

So hang on, I take it you can use almost any power adapter as long as it outputs around the DC 12v -15v range?

For instance I have a power supply from an old scanner sat on my desk. 12v DC, 1.25A. Amps are a little low but it'd work right? Trickle charge it almost right?