It's nothing new. A battery was/is sold for 6 volt Porsches that made 12v for starting then switched back to 6v for running.
Newer cars with electronic equipment could have problems with a 24 volt start, so I would be very cautious about such a device.
Most non-electronic 12v equipment can safely handle 24v for a short time.

Sure, I've jump-started the '42 Woody with a 12v car, it'll start it, just better to use a good 6v source.

That car has no electronics, the only thing I have to remember is to NOT push the brake pedal with teh 12v source hooked up or it will toast the brake-light bulbs. 24v is not recommended for starting a 12v engine.

__________________

Gone to the dark side

- Jeff

Nowadays most people just use Viagra.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Not if you already have the woody, ...

__________________

Gone to the dark side

- Jeff

I think you're right. If there were a way (beyond what the ignition switch does) to ensure that ONLY the starter motor was receiving voltage during this process then it wouldn't be such a bad idea.

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

Woops...i'll try keeping my series and parallel straight

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1985 300TD-euro 352,000 mi
1974 240D (1?)52,000 mi - has a new home now

Loads of people convert old Farmall Tractors to 12v and use the old 6v starter, its a rugged beast and lasts just fine.
I'd guess that the same is probably true of the starters in our cars, just disconnect the normal wire to the starter and replace with one from the battery combiner, the starter is the only component that should see that voltage.
Thats a guess though, it could also be that 24v hits the starter and the magic smoke comes out and it never works again.

Now that I think of it I *think* I still have the starter from my 240D that didn't work when it got hot, might be interesting for a test...

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Sadly Benz-less

If you put two batteries in parallel, be sure that you have a fuse between them. Without a fuse, a shorted cell in one of the batteries could cause a fire.

CHAD300TDT, Finally, something I can possibly help you with. Go to Backwoods Home magazine's website(backwoodshome.com) and look in the archives for articles by Jeff Yago. He put a battery in the back of his pickup that was charged by the alternator. His setup included a solar panel on the roof but you can omit that part. This was meant for camping but should work just fine for you. He even had wiring diagrams also. Believe it or not but Walmart has pretty good deep cycle batteries and inverters for a decent price. FYI Black and Decker brand inverters are made Vector. Hope this helps. Jeff


http://www.backwoodshome.com/articles2/yago93.html

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1981 240d aka "The rust bucket"

My block heater pulls 420 watts of power as measured by a KILLOWATT meter. That might help you do the math for your battery size/alternator charging time. HTH, JEFF

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1981 240d aka "The rust bucket"

Thanks Jeff, that link is very cool. I haven't moved forward with this project yet because I'm having issues with my wagon again. Hopefully I can get back on track with this after today.

I put 5w40 syn in both vehicles last weekend and it made a HUGE difference in ease of starting for the CD, but the glow plug system on the wagon is acting funky. Today I'll be testing the wires, plugs, replacing and reaming and hopefully all will be good after that.

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Chad
2013 Jeep Unlimited Rubicon
OBK#44
"Pleasure in the job puts perfection in the work." - Aristotle (384-322 B.C.)

SOLD
1985 300TD - Red Dragon
1986 300SDL - Coda
1991 - 300TE
1995 - E320
1985 300CD - Gladys
2006 Nissan Pathfinder LE
1998 Acura 3.0 CL

Well they may sell a switching device that connects the batteries in series to put 24v to the starter, and the starter MAY handle it for short bursts for a little while.

BUT, I'll also bet you they put all kinds of disclaimers on their device saying they aren't responsible for any damage to your car or its electrical system/components. And if you are putting 24 volts on a device (starter) designed for 12 volts you WILL burn it up. Maybe not the first, or even fiftieth time, but it will happen.

Volts / Resistance = Amps and Volts x Amps = Watts. The starter windings are sized for 12 volts and have a certain resistance. Doubling the volts means twice the amps. Twice the amps AND twice the volts means FOUR times the watts. That power has to go (be dissapated) somewhere and where a lot if it goes is HEAT. Heat in the windings of a motor breaks down the insulation on the wire - and the winding wire is only insulated with a thin coating of "varnish" (plastic like coating) in the first place. Heat can cook the varnish off the windings causing a short.

Since the insulation on the windings is designed to withstand 12 volts and you are putting 24 volts to it, any thin or weak spots in the insulation are twice as likely to burn through and become a break in the insulation - again resulting in a short. Needless to say, a short in the windings equals a dead starter. Unfortunately, it only takes seconds for the starter to get really hot when cranking, even when connected to 12 volts, but it takes several minutes for one to cool down. Connected to 24 volts it will get hot four times as fast - or four times hotter in the same amount of time (remember: four times the Wattage), but won't cool down any faster.

So, while you may be able to get away with hitting the starter motor with double the voltage it was designed for, it is a crap shoot and you won't get away with it for long. It may work, or it may fry the starter the first time, but even if it doesn't, sooner or later you're going to crank it too long, or too many times without sufficient cool-down time, and fry your starter. I know I wouldn't do it except in an emergency - certainly not as a regular means of starting my car. Unless of course you happen to have several spare starters and like changing them

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown

All this talk about 12V, 24V batteries, series etc etc took me back about 30 years to a friends Sunoco station at Tenley Circle in D.C. The old Sears store/auto center was next door. A lady had them replace the battery in her 6V VW bug. She drove into the Sunoco station and asked the mechanic on duty to listen to her car. The jockey at Sears installed a 12V battery, the starter spun so fast it sounded like an air starter, the gauges were pegged and the lights were lit up like a Bally's pinball machine. Alan

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DO IT RIGHT THE FIRST TIME, DON'T BE A HACK
Political Correctness is NOT part of my vocabulary
and finally FIGHT CRIME...SHOOT BACK

'82 240D Stick
'85 300D Auto
Drove my first MB at age 16 1960 300SL W190

I would have never guessed MG would have a starter for an old Mercedes. It would almost be worth it just to hear that gear reduction work. I have one on my old Ford 460 (gear reduction, not MG brand) that was the answer to a hot start problem.