Hello, I have 2 old cars with 6V systems that I converted everything to 12V except for the starter. It is the original 6V but I run it with 12V. I am concerned that its life will be short. I would like to drop the operating starter volt to about 9V. First I thought to use a resistor in series (heating element in a vented cage). I know Ohm's law but could not calculate the watt / ohm value of the resistor to drop the volts reaching the stater from 12V to 9V.
The starter is rated around 1 hp / 750 watt with the original 6V supply. Can you help me calculate the wattage / resistance (hot) to achieve the desired drop to 9V?
Alternative; Can I use a PWM circuit? Can I buy a pre-assembled one that will give the desired drop with volt / current rating for this application.
Many Thanks in advance, Med

 

PWM would be good but it would be a harsh environment for a MOSFET to live in. This would minimise the heat production and waste of battery power.
750W implies 125A but the stall current is going to be a good bit more than that, and it will draw stall current every time it is turned on.

 

The best answer may be different depending on the car's brand and/or details about the starter. For example, Ford 6V systems had several different starters. Some had a coil to limit speed others; others did not.

Several years ago, I built a PWN supply for the Ford 6V long-shaft starter running off 12V. I used 5 mosfets in parallel each for the top and bottom sections. I was not aware of a commercial unit at that time. 4QD in England may have something that would work.

Edit: Just saw this: 6/12 V batteries. http://antiqueautobattery.com/accessories.html
However, I would still give using 12V on the starter a try. We had a '55 T-Bird with a 6V system and small V-8. Even in SoCal, it was hard to start. Considered, but never did just convert to 12V. At that time, the radio was the issue.

 

Hello, I have 2 old cars with 6V systems that I converted everything to 12V except for the starter. It is the original 6V but I run it with 12V. I am concerned that its life will be short. I would like to drop the operating starter volt to about 9V. First I thought to use a resistor in series (heating element in a vented cage). I know Ohm's law but could not calculate the watt / ohm value of the resistor to drop the volts reaching the stater from 12V to 9V.
The starter is rated around 1 hp / 750 watt with the original 6V supply. Can you help me calculate the wattage / resistance (hot) to achieve the desired drop to 9V?
Alternative; Can I use a PWM circuit? Can I buy a pre-assembled one that will give the desired drop with volt / current rating for this application.
Many Thanks in advance, Med

Well they ain't classics anymore for starters - other's have suggested SMPSU converters with MOSFETS, I'd flip a coin for GTO thyristors - they're still gonna be big either way...….

 

The best answer may be different depending on the car's brand and/or details about the starter. For example, Ford 6V systems had several different starters. Some had a coil to limit speed others; others did not.

Several years ago, I built a PWN supply for the Ford 6V long-shaft starter running off 12V. I used 5 mosfets in parallel each for the top and bottom sections. I was not aware of a commercial unit at that time. 4QD in England may have something that would work.

Edit: Just saw this: 6/12 V batteries. http://antiqueautobattery.com/accessories.html
However, I would still give using 12V on the starter a try. We had a '55 T-Bird with a 6V system and small V-8. Even in SoCal, it was hard to start. Considered, but never did just convert to 12V. At that time, the radio was the issue.

The best answer may be different depending on the car's brand and/or details about the starter. For example, Ford 6V systems had several different starters. Some had a coil to limit speed others; others did not.

Several years ago, I built a PWN supply for the Ford 6V long-shaft starter running off 12V. I used 5 mosfets in parallel each for the top and bottom sections. I was not aware of a commercial unit at that time. 4QD in England may have something that would work.

Edit: Just saw this: 6/12 V batteries. http://antiqueautobattery.com/accessories.html
However, I would still give using 12V on the starter a try. We had a '55 T-Bird with a 6V system and small V-8. Even in SoCal, it was hard to start. Considered, but never did just convert to 12V. At that time, the radio was the issue.

The best answer may be different depending on the car's brand and/or details about the starter. For example, Ford 6V systems had several different starters. Some had a coil to limit speed others; others did not.

Several years ago, I built a PWN supply for the Ford 6V long-shaft starter running off 12V. I used 5 mosfets in parallel each for the top and bottom sections. I was not aware of a commercial unit at that time. 4QD in England may have something that would work.

Edit: Just saw this: 6/12 V batteries. http://antiqueautobattery.com/accessories.html
However, I would still give using 12V on the starter a try. We had a '55 T-Bird with a 6V system and small V-8. Even in SoCal, it was hard to start. Considered, but never did just convert to 12V. At that time, the radio was the issue.

The best answer may be different depending on the car's brand and/or details about the starter. For example, Ford 6V systems had several different starters. Some had a coil to limit speed others; others did not.

Several years ago, I built a PWN supply for the Ford 6V long-shaft starter running off 12V. I used 5 mosfets in parallel each for the top and bottom sections. I was not aware of a commercial unit at that time. 4QD in England may have something that would work.

Edit: Just saw this: 6/12 V batteries. http://antiqueautobattery.com/accessories.html
However, I would still give using 12V on the starter a try. We had a '55 T-Bird with a 6V system and small V-8. Even in SoCal, it was hard to start. Considered, but never did just convert to 12V. At that time, the radio was the issue.

 

Hi, Many Thanks for answer and info. The 2 cars are: Citroen 1954 with an old Bendix type drive (no separate solenoid). The other is a 1961 Volvo with a regular Bosch starter w/ solenoid. If you can send me a circuit diagram with values, also sources for the components, I would be grateful. Best Regards, Med

 

You might be able to hunt up a starter/generator/alternator shop that does rebuilds and ask them if they can rewind it for 12V. Shops like these are becoming hard to find in today's toss-it economy but I used to haunt them for maintaining old cars and boats. Planes too but you didn't hear that from me..

 

@collector55

Since you asked for the designs, I have attached them as zipped folders to avoid limits on attachment size.

First, some explanation. Those designs I referenced were not built to convert 6-volt devices to 12V devices. They were to control the speed of DC motors used in the launch of model sailplanes (similar to winch launching full-scale sailplanes) and retrieve the launch line from wherever it fell. The launch winch was a 6V Ford starter and was intentionally run at 12V for launching molded carbon sailplanes (F3B). For sailplanes of lighter construction, the operator needed to tap the foot switch ("pedal PWM") to avoid pulling the wings off. I thought that a controlled winch would be an advantage. It wasn't. That controller was a "5-mosfet" design. We could actually stall the winch for a short period without blowing the mosfets. A heavy duty Marine battery with 2-0 welding cables was used. Current was high, but I have no idea what it was. The design was not "legal" for competition, so its max current was higher than allowable.

Second, having learned about controllers from that first experiment, I decided to attack an inherent problem with "retrievers" that were in use. A retriever recovers the winch line from wherever it falls down field after a launch, because the starter motors in use cannot be easily reversed, and reversal would add a serious element of danger. 12-year-old boys work well but tire too easily. The ordinary mechanical retrievers worked by running a second, lighter weight line with the launch line. They fed from the front the the spool during launch and retrieved to the side. That introduced one twist in the line per revolution, which eventually leads to problems. I designed my retriever based on a spinning reel used in fishing, but much larger. There, the out-feed and rewind are effectively from the face and twisting is avoided. For that, I used a 12V DC brushed motor rated at about 2 HP. That led to the "4-mosfet" design.

Both designs used a LT1158 to drive the mosfets. The only differences were the method of construction and number of parallel mosfets. The winch used 5 mosfets in parallel, and was constructed on buss bars made from 0.025 copper sheet. The 4-mosfet design used a 4-oz copper PCB. It was easily packaged in a cast project box as shown in the pictures.

The main difference between the two designs was that the winch used an LM3524 to drive the LT1158 and the retriever used a TPIC2101 for the same purpose. Both designs included discrete logic, as at that time (2000), I was unfamiliar with MCU's. The zipped file shows the logic used for the retriever, but it should be easy to dissect out just the driver.

With all that said, I do not think that is the best approach. Are you sure those starters will not tolerate 12V? What if you reduce the cable size or increase its length to purposely add a little resistance (milliohms)? What about a couple of high-current diodes in series? Or two, 6V batteries in series (as in early MGB's)? And finally, rewinding as suggested by JohnInTx.


Retriever:

Retriever power supply:


John

 

Hello, I have 2 old cars with 6V systems that I converted everything to 12V except for the starter. It is the original 6V but I run it with 12V. I am concerned that its life will be short. I would like to drop the operating starter volt to about 9V. First I thought to use a resistor in series (heating element in a vented cage). I know Ohm's law but could not calculate the watt / ohm value of the resistor to drop the volts reaching the stater from 12V to 9V.
The starter is rated around 1 hp / 750 watt with the original 6V supply. Can you help me calculate the wattage / resistance (hot) to achieve the desired drop to 9V?
Alternative; Can I use a PWM circuit? Can I buy a pre-assembled one that will give the desired drop with volt / current rating for this application.
Many Thanks in advance, Med

Citroen changed over from 6V to 12V for the Traction Avant in 1953. You don't mention what car you have or what engine is fitted to it, but the starters on the "mainstream engines" were all very similar. The 2CV was a bit different.
However starter motor parts are still available for these cars and just changing the armature,field coils and solenoid will sort your problem . Have a look here:https://www.citroen-traction-avant.com/en/shop/article/S041/ and see if you can recognise your motor. The parts, as vintage parts go, are quite reasonably priced.
I assume the starter on the Volvo is almost certainly Bosch and it is almost certain that there is an equivalent starter motor available in 12V. Do you have the Bosch part number ?. If you can post it I will cross reference it for you.

 

It looks quite likely that your Volvo is either a PV444 or PV544 if it was built in 1961. Cross referencing it, I get Volvo part number 204284 or 240360 for the 12V version fitted to the B18/B20 engine.. The 6V was fitted to the B16 engine. I think they are very likely interchangeable. Looking at the drawing for the B18/B20 starter it looks to be the standard Bosch starter, with 2 hole mounting. The holes are at 2 O'Clock and 8 O'Clock. I am pretty sure that this is the same starter fitted to the 200 series and 700 series Volvos right up to the 1990's