Any real problem getting 12V from 24? (1 Viewer)

Joined
Dec 7, 2019
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438
Location
North Carolina, USA
I have a 24 V BJ73. I am putting in some 12V accessories such as new climate control system, radio, etc.. I have a 40 amp voltage converter to step down to 12V, but I am doubting whether or not it is really necessary. I noticed relays between the 2 batteries sending 12V to the lighting. I have read some post that 24 V is only for the starter/glow plugs etc... I understand that If I tap between the batteries to get 12V that "theoretically" I will be pulling and charging 2 unbalanced batteries and again "in theory" this will cause problems.......Has anyone ever had/caused any real problem doing this?.....I'm wanting to run my electric radiator fan and AC clutch this way....I'm guessing this would cause one battery to have a slightly shorter life than the other.....Again I'm looking for real world experienced problems or not.........
 
Joined
Dec 5, 2006
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Vancouver BC
Hiya Rob,

Welcome to the wacky world of Canadian 24 volt trucks! (Im making this assumption based on the fact that yours is 24 volts, but has 12 volt headlights, which is a feature that only exists on the Canadian trucks.)

You should probably spend some time reading the threads on battery balancing. There is a lot to know about the 24 volt electrical system. First of all, I'm pretty sure that unless you have an upgraded high power alternator, a 40 amp voltage converter is about 20 amps too much. If you are going to run a 12 volt converter, it should just be for stuff under about 20 amps. I don't know what an electric radiator Fan and AC clutch pull, but I'm going to go put on a limb and say that they probably draw too much power for the system. (put some amperage numbers up if you have them)

As for the unbalanced battery scenario, the trouble is real. Ask anyone who has a single headlight go out for a week and not notice it. That's enough time to screw up a set of batteries. In a BJ 73, the truck is running everything on 24volt (except for the Canadian headlights), so the idea that it just starts on 24 and everything else is 12 volts is false.

So to sum up, It's a real world problem, and people have dealt with it in a number of different ways (battery balancers, or always using a battery tender that charges each battery separately), but by far the simplest solution is to never tap 12 volts off one battery or the other.
 
Joined
Dec 16, 2017
Messages
113
Location
Northshore, Auckland NZ
I have some low amp stuff running on a 12v centre tap. Fuel gauge/sender, temp gauge/sender, windscreen wipers.

My truck was like this when I bought it over 2 years ago, and no reason to believe it wasn't like this for at least a few years before that, probably since at least 2013.

Doesn't seem to make any difference to the batteries. My alternator shat itself 18mnths ago, found out when the batteries went flat. The battery supplying the 12v read 0.1v more than the other. I checked them again a few months ago when I installed a winch, the 12v tap battery was reading 0.1v less than the other. The batteries are different brands and possibly CCA as well. I'll probably check them again in another 6 months or so.
 
Joined
Dec 7, 2019
Messages
438
Location
North Carolina, USA
Hiya Rob,

Welcome to the wacky world of Canadian 24 volt trucks! (Im making this assumption based on the fact that yours is 24 volts, but has 12 volt headlights, which is a feature that only exists on the Canadian trucks.)

You should probably spend some time reading the threads on battery balancing. There is a lot to know about the 24 volt electrical system. First of all, I'm pretty sure that unless you have an upgraded high power alternator, a 40 amp voltage converter is about 20 amps too much. If you are going to run a 12 volt converter, it should just be for stuff under about 20 amps. I don't know what an electric radiator Fan and AC clutch pull, but I'm going to go put on a limb and say that they probably draw too much power for the system. (put some amperage numbers up if you have them)

As for the unbalanced battery scenario, the trouble is real. Ask anyone who has a single headlight go out for a week and not notice it. That's enough time to screw up a set of batteries. In a BJ 73, the truck is running everything on 24volt (except for the Canadian headlights), so the idea that it just starts on 24 and everything else is 12 volts is false.

So to sum up, It's a real world problem, and people have dealt with it in a number of different ways (battery balancers, or always using a battery tender that charges each battery separately), but by far the simplest solution is to never tap 12 volts off one battery or the other.
Hiya Rob,

Welcome to the wacky world of Canadian 24 volt trucks! (Im making this assumption based on the fact that yours is 24 volts, but has 12 volt headlights, which is a feature that only exists on the Canadian trucks.)

You should probably spend some time reading the threads on battery balancing. There is a lot to know about the 24 volt electrical system. First of all, I'm pretty sure that unless you have an upgraded high power alternator, a 40 amp voltage converter is about 20 amps too much. If you are going to run a 12 volt converter, it should just be for stuff under about 20 amps. I don't know what an electric radiator Fan and AC clutch pull, but I'm going to go put on a limb and say that they probably draw too much power for the system. (put some amperage numbers up if you have them)

As for the unbalanced battery scenario, the trouble is real. Ask anyone who has a single headlight go out for a week and not notice it. That's enough time to screw up a set of batteries. In a BJ 73, the truck is running everything on 24volt (except for the Canadian headlights), so the idea that it just starts on 24 and everything else is 12 volts is false.

So to sum up, It's a real world problem, and people have dealt with it in a number of different ways (battery balancers, or always using a battery tender that charges each battery separately), but by far the simplest solution is to never tap 12 volts off one battery or the other.
Thank you for the reply. I will search to find out about battery balancers. The voltage converter put out as much as forty amps it doesn't mean that it draws 40 amps all the time. Not sure how much the air conditioner fan and compressor clutch will pull. Electric radiator fan is labeled 80 Watts which is slightly less than 7 amps at 12 volts. stereo will be minimal.
voltage converters wired directly to the battery terminals right now. I worry a little bit there may be a small idle current drained by the converter even without a load. I may connectit to a ignition switch relay later on.
Bj73 is from Spain. some point I'll go around the vehicle with a multimeter to determine what is running on 12 and what's running on 24.
 

TLC Norway

woodwelder
Joined
Apr 3, 2009
Messages
2,376
Location
North of Norway
Hiya Rob,

Welcome to the wacky world of Canadian 24 volt trucks! (Im making this assumption based on the fact that yours is 24 volts, but has 12 volt headlights, which is a feature that only exists on the Canadian trucks.)

You should probably spend some time reading the threads on battery balancing. There is a lot to know about the 24 volt electrical system. First of all, I'm pretty sure that unless you have an upgraded high power alternator, a 40 amp voltage converter is about 20 amps too much. If you are going to run a 12 volt converter, it should just be for stuff under about 20 amps. I don't know what an electric radiator Fan and AC clutch pull, but I'm going to go put on a limb and say that they probably draw too much power for the system. (put some amperage numbers up if you have them)

As for the unbalanced battery scenario, the trouble is real. Ask anyone who has a single headlight go out for a week and not notice it. That's enough time to screw up a set of batteries. In a BJ 73, the truck is running everything on 24volt (except for the Canadian headlights), so the idea that it just starts on 24 and everything else is 12 volts is false.

So to sum up, It's a real world problem, and people have dealt with it in a number of different ways (battery balancers, or always using a battery tender that charges each battery separately), but by far the simplest solution is to never tap 12 volts off one battery or the other.


Amen to that. I've had 24v in all my life... First rule is to avoid center tap.
That goes for all 24v machinery, excravators, earth movers, big trucks and or lovely 24v cruisers.

Get 24v stuff for high amp draw. (Fan, lights, fridge and so on)
Run 12v low amp stuff on the converter. (Gauges, small stereo etc.)

I also have my CB and BMW seats on the converter.

Embrace the 24v, and think of the two batteries as one. 6+6 cells linked with a cable.
Therefore the cable is important, keeping it clean and have little resistance in it.

Some get lucky with low amperage 12v tap, I still recommend doing it properly. Especially in colder climates.
 

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