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Ford Tractors Discussion Forum
:

24 volt to 12 volt starter resistor value for start solenoid

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Bart CA

03-31-2020 12:58:44




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1965 Ford 2000 tractor 3 cyl diesel

I would like to put 24 volt (2X12 volt batteries in series)
to turn the engine faster (tired engine)
I want to put a drop resistor to feed the solenoid
in order to have 12 volt on it.

I have several big adjustable resistors different wattage
what would be the resistor value to have 12 V at the solenoid?

I tried to feed the solenoid tapping it on one of the two 12 V batteries
while feeding starter motor with 24 V... solenoid is activated but
jerks badly. If I feed the solenoid with a separate battery all is OK.

Thanks in advance

Bart

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MarkB_MI

04-01-2020 14:02:53




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 Re: 24 volt to 12 volt starter resistor value for start solenoid in reply to Bart CA, 03-31-2020 12:58:44  
One last comment: You mentioned using a 24 volt battery tender. I don't recommend you do that, as the original battery will get overcharged and your auxiliary battery will be undercharged. (Or vice versa, if the tractor's charging system isn't working.) Just use a 12 volt battery tender on the auxiliary battery. You can rotate it between the two batteries if you want to charge them both.



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Bart CA

04-01-2020 14:06:36




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 Re: 24 volt to 12 volt starter resistor value for start solenoid in reply to MarkB_MI, 04-01-2020 14:02:53  
Good suggestion Thanks



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JMOR

04-01-2020 13:48:23




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 Re: 24 volt to 12 volt starter resistor value for start sole in reply to Bart CA, 03-31-2020 12:58:44  
Quoting Removed, click Modern View to seeSorry Joe F that you saw me as picking. Long thread and present/past tense, etc. left me confused. Some posts sounded like it was a done deal with a all is well, complete and thank you guys. Then a following that made it appear that it is still a work in progress . So all I was asking was to try to understand where things were, what worked and didn't. I'll just sit back and do a "read-only" and probably will ultimately see a "final conclusion". Have a nice day, all.

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JMOR

04-01-2020 11:51:32




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 Re: 24 volt to 12 volt starter resistor value for start sole in reply to Bart CA, 03-31-2020 12:58:44  
"The only change is the solenoid activating coil to a resistor in series with 24 volt." ???? I don't understand. Yesterday, you said you were activating the solenoid with a 12v battery & it cranked up in one revolution????



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MarkB_MI

04-01-2020 13:09:39




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 Re: 24 volt to 12 volt starter resistor value for start sole in reply to JMOR, 04-01-2020 11:51:32  
> "The only change is the solenoid activating coil to a resistor in series with 24 volt." ???? I don't understand. Yesterday, you said you were activating the solenoid with a 12v battery & it cranked up in one revolution????

Re-read his original post.

He put a an extra 12 volt battery in series with the original, but it is only connected to the starter solenoid contact. Everything else (e.g. alternator, lighting, gauges, key switch and start switch) is still connected to the original battery. But when he hits the starter switch, the solenoid won't stay closed, presumably because the voltage of the original battery is dropping too low to keep the solenoid energized. But he was able to get it to work if he used a THIRD battery to energize the solenoid.

So, what he wants to do is to energize the starter solenoid off the extra battery so he has 24 volts to work with, but drop it down with a resistor so he doesn't burn up the solenoid coil.

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Bart CA

04-01-2020 13:15:09




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 Re: 24 volt to 12 volt starter resistor value for start sole in reply to MarkB_MI, 04-01-2020 13:09:39  
Thank you Mark

It is exactly what I done
very well explained

Bart



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Joe Ferguson

04-01-2020 12:40:38




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 Re: 24 volt to 12 volt starter resistor value for start sole in reply to JMOR, 04-01-2020 11:51:32  
JMOR please stop picking



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Sean in PA

04-01-2020 06:28:16




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 Re: 24 volt to 12 volt starter resistor value for start sole in reply to Bart CA, 03-31-2020 12:58:44  
The correct answer is "If the engine is so tired that it won't start on 12 volts then it is time for the engine to be rebuilt".



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MarkB_MI

04-01-2020 04:28:30




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 Re: 24 volt to 12 volt starter resistor value for start solenoid in reply to Bart CA, 03-31-2020 12:58:44  
The reason it didn't work when you tapped off one of the batteries is there was enough voltage drop that the power to the solenoid dropped below what was required to keep the solenoid activated. I'm guessing this is because the 12 volt starter is pretty much a dead short to the 24 volt source. Even though there's twice as much voltage available with the two batteries in series, internal resistance in the batteries limits how much current they can source.

If you measure the solenoid coil resistance, that's the resistor value that you'll need to put in series with it to get the 50 percent voltage drop, but you're still going to run into the same problem you had before, since the voltage across both batteries is double what you have across a single battery. When you half that voltage with a resistor, you're back to where you were with the single battery. So the resistor would need to be a little less than the solenoid coil resistance.

The required wattage rating can be determined using the formula P = V2R, where V = 12 volts. You can cheat a bit on the watt rating, since the actual voltage will be less than 12 volts and the load will be intermittent. I'd say look for a resistor with a rating about half the calculated value.

I'm not a fan of your idea. I have a tractor that's been converted from six to twelve volts, and it works good. But that's quite different from a twelve to 24 volt conversion. You'll need to convert any accessories to 24 volts; running twelve volt accessories off a single battery will result in that battery being undercharged and the other battery being overcharged. Anyone who's owned a tractor with a 24 volt system, such as a sixties-vintage John Deere will tell you the same thing.

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Bart CA

04-01-2020 08:16:10




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 Re: 24 volt to 12 volt starter resistor value for start solenoid in reply to MarkB_MI, 04-01-2020 04:28:30  
Thank you for your reply

You are right there is a major voltage drop on the first battery.
Very good idea to measure the solenoid coil resistance to figure the same value for a series resistor.

The engine is tired but..ounce started it runs very good no smoke running as a healty diesel and has good power but a good blowby. I use the tractor for snow blowing and few chores on my little hobby farm it may outlast me..but I do not want to rebuilt the engine If it dont last I prefer put money on a newer modern tractor.

For the accessories lights etc...I connected them to the first battery then 12 volts. For charging..I have a 24 volt battery tender which is connected when the tractor is not in use. The first battery is charged by the generator then I can use lights heater ect.

I would like to say: Many thanks to all you guys participating to this forum for their help and to our sponsors that provide us this site. It is greatly appreciated
especially in this time of confinment.

Bart

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MarkB_MI

04-01-2020 08:38:59




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 Re: 24 volt to 12 volt starter resistor value for start solenoid in reply to Bart CA, 04-01-2020 08:16:10  
> For the accessories lights etc...I connected them to the first battery then 12 volts. For charging..I have a 24 volt battery tender which is connected when the tractor is not in use. The first battery is charged by the generator then I can use lights heater ect.

Ah. So you're not installing a 24V alternator. What you're doing makes sense.



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Bart CA

04-01-2020 09:13:09




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 Re: 24 volt to 12 volt starter resistor value for start solenoid in reply to MarkB_MI, 04-01-2020 08:38:59  
Almost nothing changed on the electrical system.. generator and all are connected to the first battery = 12 volt. The only change is the solenoid activating coil to a resistor in series with 24 volt.

The second battery in series is connected the the solenoid big post only = 24 volt

The first battery is charged by the generator when running

When tractor not in use a 24 volt battery tender will keep the batteries charged.

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MarkB_MI

04-01-2020 05:35:03




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 Re: 24 volt to 12 volt starter resistor value for start solenoid in reply to MarkB_MI, 04-01-2020 04:28:30  
Correction:

The formula for power is P = V2 / R !



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Sean in PA

03-31-2020 21:05:07




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 Re: 24 volt to 12 volt starter resistor value for start sole in reply to Bart CA, 03-31-2020 12:58:44  
Quoting Removed, click Modern View to see

Where did you have the separate 12 volt battery connected (+12 volts AND ground connections) and where did you have the dual 12 volt batteries in series connected? Can you provide a wiring diagram?



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JMOR

03-31-2020 14:21:08




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 Re: 24 volt to 12 volt starter resistor value for start sole in reply to Bart CA, 03-31-2020 12:58:44  
Quoting Removed, click Modern View to see"........worked as normal". and starter engaged & cranked engine over? y/n?



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Bart CA

03-31-2020 14:30:51




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 Re: 24 volt to 12 volt starter resistor value for start sole in reply to JMOR, 03-31-2020 14:21:08  
Yes starter worked engine started first turn



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Sean in PA

03-31-2020 13:11:10




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 Re: 24 volt to 12 volt starter resistor value for start sole in reply to Bart CA, 03-31-2020 12:58:44  
Just keep everything wired exactly the way it was, with a wire directly from the plus side of the first battery that is 12 volts above ground to the key switch and then the wire from the "start" position on the key switch going to the small post on the solenoid. That way you know you've got 12 volts everywhere it's supposed to be and don't have to worry about calculating voltage drops across resistors. You want the rest of the tractor running on 12 volts anyway for the warning lamps, gauges and lights (and thermostart if it has it), so you should only wire that second battery's positive post (the one that's 24 volts from ground) directly to the large post on the solenoid, as that only goes to the starter, and only when the small post on the solenoid is energized with 12 volts. As to how you're going to charge that second battery, that's up to you, but I would go with a DPDT switch that would switch it from being in series with the first battery to being in parallel for normal operations so it would get charged by the standard 12 volt charging circuit just like the first battery. And if you forget to switch it for starting you would still have double the normal CCA available at 12 volts.

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Bart CA

03-31-2020 13:21:54




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 Re: 24 volt to 12 volt starter resistor value for start sole in reply to Sean in PA, 03-31-2020 13:11:10  
Thanks you very mutch Sean for your time to reply
I connected the solenoid supply and accesories to the first battery as
you described but at my great surprise the solenoid jerked
badly like it was feeded with AC voltage.
I connected the solenoid supply to an completely independat
12 volt battery solenoid worked as normal.

Thanks again..greatly appreciated

Bart



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