(1954) Do I need to add a resistor to my battery gauge for a 12V conversion?

[Mariiboops]

I got a resistor for the fuel gauge, but I keep trying to find info about the battery one. Every other part of the car is ready electrically, so a quick answer would be great. I got a secondary voltage gauge just in case there's nothing I can do.

Edited January 1 by Mariiboops (see edit history)

[Bloo]

Does this have marks "D" and "C" or something like that and a needle that swings both ways? If so, it is an ammeter not a voltmeter, and does not care one whit about voltage.

 

If that is the case, it will work fine, unless the new charging system is capable of way more current (amps) than the old one. If the new system is a lot bigger (in amps) it will still work fine 90+ percent of the time, but there is a good possibility that it might burn up the gauge suddenly on some cold morning if the battery is about exactly half dead.

[OrtonvilleBob]

The battery gauge in your '54 is an ammeter and as Bloo pointed out, it doesn't care about voltage. It merely indicates relative amount of current flowing to and from the battery.

[Andy J]

In a subject related to this,is it possible to put a resistor in line with the wire from the starter that supplies power to the entire car to reduce 12 volts to 6 volts?I have had so much trouble with starters I was wondering how 12 volts to the starter with everything else 6 volts would work,if that is possible.I would like to keep everything 6 volts but this starter problem is kicking my butt.

[Bloo]

In theory, yes. In practice, no. Half the power produced by the battery and/or alternator would have to be dissipated (wasted) as heat by the resistor. Keep in mind that 6v items use double the current, so when using them on 12 volts the total wattage needed is double what a native 12 volt device needs. In the case of a starter, that number is a bit less than expected, for reasons way to long and drawn out for this post, but would still be a few kilowatts.

 

If one completely separated the starting and charging system, the situation is better, but the answer is still no because there is no resistor that will drop a constant 6 volts. It varies due to how much current is being drawn (see Ohms Law) and would result in completely unpredictable voltage in-between charging voltage (about 14-1/2 volts) and zero. If you are going to convert, the best way is to deal with accessories individually. Some will work OK on individual resistors and some won't. The resistor will have to waste the exact amount of power that the accessory uses. The resistors are going to get hot, so allow airspace so they cant catch anything on fire. For bulbs, It's better to just change them to the correct voltage. On accessories that aren't a constant load, something more sophisticated than a resistor is usually needed. Linear regulators (Runtz, etc.) work but must waste the same amount of power as the accessory, just like a resistor. The difference is they can hold a constant 6 volts. Little DC-DC buck converters could also work, and waste far less power as heat compared to the accessory they are powering. They get around ohms law (sort of) by changing the power to AC, using a transformer, then changing it back to DC. In my opinion they are unlikely to be reliable over the long term in a car. They also generate a bunch of electrical noise that is probably going to get into the radio if you have one. There are plenty of affordable Chinese buck converters all over Amazon for those who want to experiment. In my opinion it is all way too much screwing around, and for otherwise stock cars, no benefit. I'll never do it again. Some people just want to, or have some 12 volt accessory they can't live without that forces them down this path. To each his own.

 

Edited January 1 by Bloo (see edit history)

[Bloo]

Getting back to Mariiboops question, a little more in depth, it *IS* possible to change the range of an ammeter. at least if you need to change it in the upward direction, with a shunt.

 

All accessories need to be on the Generator/Alternator side of the ammeter, except the horn. The horn is an exception probably because if it stuck it is (in effect) a dead short and might burn up the ammeter. The battery has hundreds of amps available. The stock Generator, maybe 40 amps or less.

 

OK, a lot of guessing follows, so no guarantees about what might happen. The stock charging system was probably capable of 30 or 40 amps, and the ammeter was wired as flowmeter as normal, showing current ONLY in and out of the battery. ALL accessories are wired to the Generator/Alternator side, and NOTHING extra to the battery post. A typical alternator from years ago makes about 50-60 amps maximum. That's too much for a typical 50s ammeter, but not that much too much, and it can probably take it, at least for short periods of time. The only thing deciding how much current to draw at a given moment is the battery. On a system with a voltage regulator, which these are, when charging, a battery will draw very little current when completely dead, draw a whole bunch when partly charged, and taper off to almost nothing as it gets fully charged. In normal operation with a charged battery the ammeter would normally never see the 50-60 amps such an alternator can produce. It will see a high spike when the engine is first started, as it replaces some of the energy used by the starter, and then taper off quickly. The only time the ammeter would see a sustained 50-60 amps (in theory) is when the battery is way down in charge but not completely flat. I am *guessing* that the ammeter would be pegged, but could probably take it for a little while without damage as the battery gets charged.

 

Alternators of 160 amps 200 amps or more have become common. I doubt a car exists that needs all that. If used in a conversion, it would still work, and the ammeter would normally not burn up because there would be a spike right after starting, and it would taper off just like normal. What would happen if the battery was partly dead? I don't know. That would depend on the battery, and how much current it decided to draw while partially charged. It would depend on the battery size and a bunch of other factors I cannot estimate. The point is there would be 160+ amps available to possibly burn, literally burn, the ammeter. Also the associated wiring. On a cold morning with a low battery, it is a good possibility. Also there is now 160+ amps for a shorted horn to burn up the ammeter if it happens to be stuck while the engine is running.

 

You can raise the scale on an ammeter with a shunt. Crimp and solder some uninsulated ring terminals on some short pieces of small solid bare copper wire, I think I would start with 20 and 22 gauge or so, in a configuration so it bolts down solid between the posts of the ammeter. The ammeter posts are electrically hot, so make any changes with engine off and battery disconnected. Make all shunts exactly the same length so you can compare. This rasies the full scale value of the ammeter. How much? Well you need a charging system tester for that, like a Snap-On AVR, or a Sun VAT-40, or some cheaper equipment that does the same thing. With the inductive clamp abound the alternator wire, and the battery clamps on the battery posts, engine running at a fast idle or cruise speed, increase the load on the battery while watching the ammeter on the dash. Read amps off of the tester when you get the dash ammeter to full scale. This is your new full scale value. Only do this for a few seconds, long enough to read. The tester is going to get HOT. You'll probably need to make and try a bunch of shunts of the same short length with different gauges of small solid bare wire. Smaller wire raises the full scale value less, bigger wires raise it more.

 

The downside to this is if you make full scale say 160 amps, it becomes almost impossible to see the maybe 2 amp normal charge that you see when going down the road with a fully charged battery. Also you are going to need bigger wiring. These are a couple of reasons why modern cars with huge alternators use voltmeters to monitor the charging system instead of ammeters.

 

Edited January 1 by Bloo (see edit history)

[Andy J]

Thanks for your answer Bloo.I should have known this wouldn't work because it's just too simple.I just need to do what everyone says about 6 volt stuff:fix it right because it worked just fine when it was new.I certainly don't need a fire or ruin a gauge or two trying to rig up something that may or may not work.

[Mariiboops]

On 1/1/2024 at 4:36 PM, Bloo said:

Getting back to Mariiboops question, a little more in depth, it *IS* possible to change the range of an ammeter. at least if you need to change it in the upward direction, with a shunt.

 

All accessories need to be on the Generator/Alternator side of the ammeter, except the horn. The horn is an exception probably because if it stuck it is (in effect) a dead short and might burn up the ammeter. The battery has hundreds of amps available. The stock Generator, maybe 40 amps or less.

 

OK, a lot of guessing follows, so no guarantees about what might happen. The stock charging system was probably capable of 30 or 40 amps, and the ammeter was wired as flowmeter as normal, showing current ONLY in and out of the battery. ALL accessories are wired to the Generator/Alternator side, and NOTHING extra to the battery post. A typical alternator from years ago makes about 50-60 amps maximum. That's too much for a typical 50s ammeter, but not that much too much, and it can probably take it, at least for short periods of time. The only thing deciding how much current to draw at a given moment is the battery. On a system with a voltage regulator, which these are, when charging, a battery will draw very little current when completely dead, draw a whole bunch when partly charged, and taper off to almost nothing as it gets fully charged. In normal operation with a charged battery the ammeter would normally never see the 50-60 amps such an alternator can produce. It will see a high spike when the engine is first started, as it replaces some of the energy used by the starter, and then taper off quickly. The only time the ammeter would see a sustained 50-60 amps (in theory) is when the battery is way down in charge but not completely flat. I am *guessing* that the ammeter would be pegged, but could probably take it for a little while without damage as the battery gets charged.

 

Alternators of 160 amps 200 amps or more have become common. I doubt a car exists that needs all that. If used in a conversion, it would still work, and the ammeter would normally not burn up because there would be a spike right after starting, and it would taper off just like normal. What would happen if the battery was partly dead? I don't know. That would depend on the battery, and how much current it decided to draw while partially charged. It would depend on the battery size and a bunch of other factors I cannot estimate. The point is there would be 160+ amps available to possibly burn, literally burn, the ammeter. Also the associated wiring. On a cold morning with a low battery, it is a good possibility. Also there is now 160+ amps for a shorted horn to burn up the ammeter if it happens to be stuck while the engine is running.

 

You can raise the scale on an ammeter with a shunt. Crimp and solder some uninsulated ring terminals on some short pieces of small solid bare copper wire, I think I would start with 20 and 22 gauge or so, in a configuration so it bolts down solid between the posts of the ammeter. The ammeter posts are electrically hot, so make any changes with engine off and battery disconnected. Make all shunts exactly the same length so you can compare. This rasies the full scale value of the ammeter. How much? Well you need a charging system tester for that, like a Snap-On AVR, or a Sun VAT-40, or some cheaper equipment that does the same thing. With the inductive clamp abound the alternator wire, and the battery clamps on the battery posts, engine running at a fast idle or cruise speed, increase the load on the battery while watching the ammeter on the dash. Read amps off of the tester when you get the dash ammeter to full scale. This is your new full scale value. Only do this for a few seconds, long enough to read. The tester is going to get HOT. You'll probably need to make and try a bunch of shunts of the same short length with different gauges of small solid bare wire. Smaller wire raises the full scale value less, bigger wires raise it more.

 

The downside to this is if you make full scale say 160 amps, it becomes almost impossible to see the maybe 2 amp normal charge that you see when going down the road with a fully charged battery. Also you are going to need bigger wiring. These are a couple of reasons why modern cars with huge alternators use voltmeters to monitor the charging system instead of ammeters.

 

The wiring has been replaced, yes. I might have overdone it by using mostly the larger wires from scrapped car battery chargers, and the thicker trailer wires for simple lights minus the headlights. I believe with the information available, the ammeter should be okay. If it is not, I can edit a standard voltmeter from Bosch to resemble it and use that instead.