24v step downs

[Bandit_guy]

Scored a Pyle 720 off ebay for $20! So this weekend I am digging into the 24v to 12v step-down. I've been reading through a ton of threads on this, and most of them involve a 12v stereo. Since I don't need that, here is what I am thinking, basically following @mongoose2231 's diagram below.

24v off the high side battery and using the lighter into a relay so the step-down functions when the ignition is on. Running that to a 6-fuse Blue Sea fuse panel for a 12v cig lighter and ham radio. Thoughts? Should I ground the step-down separately or run it back to the battery?

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If you use this set up to power your radio, you will damage your batteries. A 5 amp draw from a radio is sufficient to ruin your batteries faster than you would think. The one battery where the 12 v draw is from will never fully recharge and the other one will eventually over charge. If you set it up this way already I would add a 5A capable battery equalizer to compensate for the unbalanced draw and you won't need to change anything.

 

[Irish11]

Interesting. I've seen a few other builds and they don't include a center tap. I've read mixed reviews on them as well. Seems like everyone has a very different approach to this process. I've been keeping a collection of setups:

• Installing new 12V stereo head unit in 24V HJ60 | IH8MUD Forum
• Running a fridge on 24v | IH8MUD Forum
• Converter/Radio install in BJ71 | IH8MUD Forum
• HJ61 and a 12V radio.. | IH8MUD Forum
• Builds - JDM 73 | Page 4 | IH8MUD Forum
• 1986 BJ73 - Spanish Import to US - Under New Management | Page 5 | IH8MUD Forum

 

[HujSGory]

Whats the problem with simply pulling 12 volts from one of the batteries?

 

[jblueridge]

The one battery gets drained more often and the charging system is not meant to compensate for that.
Many people have terrible tales to tell.
I have a 12V portable compressor that I run off just one battery. I only use it 6 times each year so I hope no calamity will befall me.

 

[HujSGory]

Surely as long as you have 24 volt still connected and running it will not matter?

 

[Bandit_guy]

Surely as long as you have 24 volt still connected and running it will not matter?

It matters quite a bit actually. The alternator has no circuitry in it to see the individual batteries like a fancy computer or a controller. It has no idea whether you have 1 24V battery, two 12V batteries, or four 6 volt batteries to make the power bank. The vlotage regulator senses the entire system and turns on the Alternator everytime it drops below the threshold. If one of your batteries has an evil centre tap (as it's known on this forum) drawing power from it the other battery will not transfer power to it to make up the difference. The battery with the extra draw will be depleted quicker than the other because the charging system only reacts to the total voltage. After a while one battery will always be low and eventually die and the other will be overcharged and eventually die. In Vehicles with dual batteries in parallel (2 batteries to add amps but not increase the voltage) you don't have this problem because the batteries balance each other out. In vehicles wired in series you need a balance/equalizer to do the job.

 

[HujSGory]

Yes but in any double bank 24v battery system you have no controller for discharge, its by the grace of God, you are simply assuming the batteries and the cells in the batteries discharge at the same rate, which they do not.
So I can't see why borrowing a couple of amps is going to make a huge difference, given that its fundamentally most likely unbalanced anyway.

 

[Freewheel]

Many of us with 24 volt trucks have gone to pains to do things to reduce the difference between batteries, like getting similar age and consecutively serial numbered battery pairs. And not tapping anything off one battery and not the other.

Anyone who has had a single Canadian 12 volt centertapped headlight fail knows how quickly a mismatched battery load can mess up your batteries.

(Canadian headlights keep a balance by tapping each side headlight off the same side battery. Works great until you blow a headlight, and don't notice 'cause you use your headlights for daytime driving...)

 

[Bandit_guy]

Can confirm the drama of blowing one headlight. Run like that for too long and it will cost you a head light and two batteries.

I installed a Solar Converter battery balancer 12 years ago and it's kept my two optima blue tops within 0.1 V of each other even with a radio and trailer hook pulling from the 12 Volt lead. I think SolarConverters went out of business or were sold to some else, I can't seem to find them anymore but when the box packs it in I'll definitely be replacing with something else that can push at least 10 amps from across from the high battery to the low. There's lot's of other brands out there usually found where they sell solar panels and such for people who have camps and off the grid housing.

 

[HujSGory]

Yes headlights is madness, due to the fact that the cells in the batteries making the battery will differ.

I still cannot see the problem with tapping 12 volt from the battery, for a radio or your USB charger etc.

 

[Irish11]

Those who have installed a step down, where did you ground it to? I've been reading threads for an hour on it and there is not really consensus. Looks like I can follow @elkaholic 's method and diagram to add a negative terminal 1986 BJ73 - Spanish Import to US - Under New Management - https://forum.ih8mud.com/threads/1986-bj73-spanish-import-to-us-under-new-management.823346/page-5#post-9467821 and that might work, but is there a preferred location, or should I run it back to the battery?

 

[jblueridge]

I grounded mine to a bolt 6 inches away.
Works fine.

 

[Bandit_guy]

Ground where you want to the chassis is what I would use. The step down is just another item that's hooked up. It's like a radio or light. It maks it easier to wire things up if you use the chassis for ground.

 

[Freewheel]

+3

15 years of success with chassis ground for converter.



Quote <<<<I still cannot see the problem with tapping 12 volt from the battery, for a radio or your USB charger etc.>>>>


Yes, it seems like a few amps shouldn't matter taken off one side of the batteries. Actual use over the last couple of decades has shown that the opposite is true. If you are interested in replacing batteries way before their usual lifespan, you are free to ignore our hard won advice.