Please don't try that, you could end up damaging the cars electrics as well as your new toy.
By connecting it with the cars battery also on line, the welder output will be smoothed and it will assist the battery for starting.
Measure the output of any battery charger off load and it will probably be way above the nominal battery voltage.

The rectified output waveform of a charger can confuse meters somewhat.

There were basic rules for current, final voltage etc for optimum battery charging but no maintenance batteries have muddled things.

Lengths of resistance wire from an old heater element, toaster etc, perhaps doubled, can be used to reduce volts and current. Considerably high voltage may be reduced through a high wattage bulb, or two in parallel. Amps can be measured using a car ammeter.

Ideally batteries should reasonably regularly be brought to full charge, but to do this correctly requires attention to detail. High and low currents and too much of either can be detrimental. Anything which will provide 7v across the battery terminals for a day or so will do no harm. A regulator in car may maintain 7.3 but charging does not continue for hours on end.

Location: Auckland, NZ

Sorry Bob, but I don't understand what you are trying to say.
There's no way that lengths of resistance wire or car ammeters can work operating a starter. The current is way too high.

Hi Jim

The 19 June 8.35 post questioned use as a battery charger.

Location: Auckland, NZ

Bob, as usual, is talking a lot of sense but failing to explain in sufficient detail.

Ignore the starting situation for now and consider using this 10 volt supply as a battery charger. as others have said, it is the voltage and current while actually connected and charging that matters. But if the voltage is really too high when connected, then Bob is right, headlight bulbs can be used in series with the battery to provide a lower voltage to the actual battery.

If one bulb leads to too low a voltage at the battery then use 2 bulbs in parallel with each other, in series with the battery.

I have used this method to charge 6v motorbike and 6v car batteries from jump leads to my 12v van. This was in a break-down situation when the 6v charging system on the broken down bike/car had failed.

When a student in Dundee in the 1960s I bought two Austin 7s from a great character, Walter Vogelsanger, who lived in Abernyte.
He had a shop in the Hawkhill where he restored and worked on bubblecars.
For music while he worked he used a pianola which he powered from a cylinder vacuum cleaner. He adjusted the speed at which it played using varying wattage bulbs in series with the vacuum cleaner as suggested by Andrew.
His shop doorbell was a set of Austrian cow-bells operated by strings.

Hi Jim

Sorry for use of minimum words. But I have been repeatedly rubbished for full comments. Those that know it all are bored, but not primarily intended for them. Recently when a specific question which others had not attempted was answered in reasonable fullness a sarcastic count of the words was applied.

Location: Auckland, NZ