There are also led lights that tolerate 12 and 24v. I've got a couple of sets of those in two of my utility trailers and they work great with either 12 or 24 volts tow vehicles.
I bought them from ebay or something, Chinese made of course.
There is a fly in the ointment in that the tow vehicle is supposed to charge the trailer battery when traveling. I suppose I could simply not attempt to "activate" that feature. Otherwise I tow it with my perfectly satisfactory supercharged 80 but I really want to be able to use the 74.
I am an electronics geek with no experience with 24V vehicles, but here goes:
I'm guessing that those two 12v batteries are connected in series.
That means that the positive cable of one of them runs to the negative post on the other.
If you take a volt meter and use an alligator clip to connect the ground lead to a good body or frame ground, you should be able to figure out which positive post is half the voltage of the other.
This is where you would want to splice in a large wire for trailer battery charging, and maybe a smaller circuit to run to radio and cig lighter.
The lights would be perhaps another question? I don't actually understand trailer wiring all that well. You could use a bank of 24v relays to switch power from this between-the-batteries situation to the various lights.
Trailer brakes can be done the same way. You'd use a 24v brake controller and then run it into a 24v relay using the half-of-your-alternator-voltage source.
What I'm unclear on really is the grounding scheme for trailer lights, specifically 4 pin trailer hookups vs. 7 pin confuses me just a little. Also, are trailer brakes actuated by delivering or removing power? I don't have a clue. well, I'm guessing by delivering.
You would also want a relay that disconnects the trailer power when the alternator isn't spinning, because drawing off of only the 'bottom' battery in a non-charging scenario could age that battery faster than the other, their internal resistance will begin to differ, and that middle point will move.
It might turn out that the trailer light power is in no way associated with the trailer battery power, in which case you can just put 24v bulbs in the lights, or as others have mentioned LEDs that are tolerant of 24v. Those will cost more, because they have an active power converter in them instead of just stringing 4 3.3v LEDs across 12v.
My limited research (I have been a 24v guy for a couple days now.... ) has lead me to believe that tapping one battery for 12v power creates a situation where the batteries become unbalanced and the service life is dramatically reduced. This BJ74 has had a big old 24v to 12v converter (it looks like about a 2500w inverter in size) fitted to power radios and other sundry things. I have not yet had the opportunity to study the execution in detail.
The last thing you want to do is using the 12V center tap aka evil tap. Not even for a simple radio.
Why? The discharge on the lower battery (12V) is bigger than the discharge on the higher (24V) battery. However the charge current is the same for both because the alternator sees one 24V battery and thus creating an unbalance between the 2 batteries. The High battery is the first one to be "full" so the "low" battery is under charged. In the end the Low battery gets never a full charge and dies while the High battery is overcharged and also dies. This subject pops up about 3 times a month in this forum. Do a search for "center tap" , "evil tap" or "12V tap".
Replace the bulbs on the trailer for 24V ones or buy LED bulbs. LED bulbs work on any voltage between 12V and 24V.
If there is other stuff on the trailer that needs 12V use a 24V to 12V converter.
The last thing you want to do is using the 12V center tap aka evil tap. Not even for a simple radio.
Why? The discharge on the lower battery (12V) is bigger than the discharge on the higher (24V) battery. However the charge current is the same for both because the alternator sees one 24V battery and thus creating an unbalance between the 2 batteries. The High battery is the first one to be "full" so the "low" battery is under charged. In the end the Low battery gets never a full charge and dies while the High battery is overcharged and also dies. This subject pops up about 3 times a month in this forum. Do a search for "center tap" , "evil tap" or "12V tap".
Replace the bulbs on the trailer for 24V ones or buy LED bulbs. LED bulbs work on any voltage between 12V and 24V.
If there is other stuff on the trailer that needs 12V use a 24V to 12V converter.
The short answer is: NO
12V from the converter is not enough for a full charge for the battery.
The battery needs to be a little over-charged to get fully charged.
In a 12V car the alternator output is max. 14.4V. All the 12V stuff is designed to work with this (over) voltage.
In a 24V car the alternator output is max. 28.8V. All the 24V stuff is ..........same story.
The converter output is a steady 12V and can only keep the battery alive.
You'll need an extra 12V alternator with separate wiring to the trailer to solve this problem. Or use solar panels with a 14.4 Volt output. They are available in camping and boat shops and designed for this.
When my truck was wired for the travel trailer, I needed to feed 12v trailer brakes and a coach battery in the trailer. To accomplish this, 2 x 24v - 12v converters were used and all of the marker/signal/brake lights were changed to 24v.
One converter was dedicated to trailer brakes and was triggered by the brake switch and the other converter fed the coach battery. I never noticed any undercharging on the coach battery with this arrangement.
The last thing you want to do is using the 12V center tap aka evil tap. Not even for a simple radio.
Why? The discharge on the lower battery (12V) is bigger than the discharge on the higher (24V) battery. However the charge current is the same for both because the alternator sees one 24V battery and thus creating an unbalance between the 2 batteries. The High battery is the first one to be "full" so the "low" battery is under charged. In the end the Low battery gets never a full charge and dies while the High battery is overcharged and also dies. This subject pops up about 3 times a month in this forum. Do a search for "center tap" , "evil tap" or "12V tap".
Replace the bulbs on the trailer for 24V ones or buy LED bulbs. LED bulbs work on any voltage between 12V and 24V.
If there is other stuff on the trailer that needs 12V use a 24V to 12V converter.
I think that is what I have in the truck now. I have not taken delivery yet but I did see it and it's a big boy. It looks to be about 10 x 15 inches and is at least three inches thick.
I think that is what I have in the truck now. I have not taken delivery yet but I did see it and it's a big boy. It looks to be about 10 x 15 inches and is at least three inches thick.
Hard to tell from physical size. Switching power supply technology has advanced a lot over the last 20 years. The 15A 12V supply i bought for my buddy's RV was physically half the size of the 10A it replaced.
The thread that Kim referenced above is a good one. I made a switching box based on Lowenbrau's diagram and it works great. I was using a series of rented trailers and didn't want to be switching bulbs all the time.
Some 24 to 12 volt converters have adjustable outputs, though it sounds like yours putting out 13.8 volts is exactly what you need. Some converters put out exactly 12 volts, which will not charge a battery properly.
Good luck with the project, Dan, and welcome to our crazy 24 volt world!
The thread that Kim referenced above is a good one. I made a switching box based on Lowenbrau's diagram and it works great. I was using a series of rented trailers and didn't want to be switching bulbs all the time.
Some 24 to 12 volt converters have adjustable outputs, though it sounds like yours putting out 13.8 volts is exactly what you need. Some converters put out exactly 12 volts, which will not charge a battery properly.
Good luck with the project, Dan, and welcome to our crazy 24 volt world!
Yes it is crazy somewhat. I have been in the car businees for 35 years and this is my first venture into something other than 12 volts. I hope to disprove the saying that an old dog can not be taught new tricks.
) has lead me to believe that tapping one battery for 12v power creates a situation where the batteries become unbalanced and the service life is dramatically reduced. This BJ74 has had a big old 24v to 12v converter (it looks like about a 2500w inverter in size) fitted to power radios and other sundry things. I have not yet had the opportunity to study the execution in detail.
lol
