I can't in good conscience let this thread pass without relating my experience with SatNavs in 6V A7s.
First a little explanation about the make-up of the cigar lighter plug GPS chargers. These are usually simple 3 legged 5 volt voltage regulator. These things are cheap as chips and as common as muck but, there is something that needs to be taken into consideration. These little chips have what is usually referred to as a "dropout voltage"; this is the lowest voltage ABOVE the 5 volt output that will allow the chip to provide the regulated 5 volts output. This is almost always less than 2 volts but can often be around 1.5 volts. In effect this means that all will work fine as long as the applied voltage is in excess of about 6.5 volts (to be safe, lets say 7 volts)

Now, the on charge voltage of your average, lightly loaded, 6 Volt A7 battery is usually between 7 and 7.5 Volts - UNTIL YOU SWITCH ON YOUR (UPGRADED) QH HEADLIGHTS when, in a very short time, the battery voltage will drop to somewhere around 6.0 volts AND YOUR SatNav WILL STOP CHARGING.

I discovered this the hard way on minor roads in rural France some years ago.

So, if you will never be using your headlights and SatNav at the same time, you should be OK. If you want to use the SatNav whilst driving any distance in the dark you will need a "up convertor" to supply the cigar plug charger with never less than say 7.5 volts (or ideally about 12 volts) whatever the voltage across the battery terminals.

This isn't in any way a fault with the SatNav chargers, they were designed to a spec calling for them to work on nominally 12 volt and 24 volt systems. No-one ever envisaged that they might be asked to perform on 6 volt systems installed in 90 year old cars.

Ian Mc.

Location: Shropshire

I fitted a simple cigarette charger from Maplins, cost about £3 if I recall.
The important thing is for it to be isolated, ie in a plastic sleeve rather than bare metal. Then all I did was connect the +6v to the centre terminal and the 0v to the outer case. So the Sat Nav is a isolated from the car and doesn't care whether the supply is positive or negative earth, it just has a 6v drop in the right direction.
Can't comment on Ian's clearly better knowledge on working voltage.
I would say that I have a Tomtom1 for the A7 and have no problems. I also have a new TomTom in my modern and have found that it needs more power to charge than the car USB can provide, so have bought a high current lighter adaptor. Might mean that newer Tomtom's will also pull more power than an A7 cigarette adaptor can deliver.
Andy B
Norf Essex

Just to add about the polarity of the connection, it must be the correct way otherwise the USB connection will burn out and if you are running on +ve then the easiest solution is to be a weatherproof USB adapter (for a motorbike) in place of the cigar lighter socket and connect the wires to the correct polarity. The unit is totally isolated and you can simply plug the USB cable for the tomtom directly in. On my "modern", 1965 Rover P6, which is +ve, I replaced the cigar lighter with a USB adapter, this was the exact same size as the cigar light body, fitted this so the car has modernday electronics, I also fitted an on-off switch as the USB was on all the time otherwise. I will be fitting another USB adapter to my Ruby shortly so that too will be able to power all this electrickery (Catweazle), I am on 12V on both cars.

Location: Saltdean, Brighton