Battery use in a 24v system

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Tennessee Jed

Tennessee Jed

I can quit any time I want...
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I have a 70 series with a 24v system. Assuming a fully functioning electrical system/alternator/good batteries, does anyone have an idea of what sort of run time I can expect for a 45 quart fridge/freezer? I know there are a lot of variables here (how often you open the door, playing the radio, etc). But a ballpark figure would be handy.

Could I let the engine run for a few minutes at bed time then let it cool all night on battery, then start the truck OK in the morning? Or should I be looking into a solar panel?
 
Solar, with a dedicated battery. 24v system with dual batteries needs to charge and be drawn from equally, else batteries will fail early and at odd intervals. It's a balanced system.
 
Thanks. Yes I have a 12/24 v fridge running off the cig lighter. I know not to tap one battery.
 
On my (24V) BJ74 I have a dedicated 12V system with a group 24 gel cell deep cycle mounted in the cab charged by a 24-12 charger and run the fridge off it. It works really well... but it eats up a fair amount of space (at a premium in the 73/74 as you know) and adds some weight.

On my (mostly 12V, 24V start) HDJ81 I run 2 RV batteries, they are a starting/deep cycle, with a 100W mono solar panel on the roof, and an ARB fridge lives in it full time. The solar will keep up to the fridge nicely, and it's a real treat to not think about it... at all.

I like that system enough that I'm considering gutting the whole 12V system in my 74, running the few 12V accessories I have (stereo, ham radio, cb) off a 24-12 converter, and throwing a similar solar set up on it as well to keep the batteries topped up when running a fridge.

@Greg_B has the same RV batteries in his (12V) 1HD-T 80 and runs a fridge off them - same in his 74. He could probably chime in on your run time situation.

FWIW if you have regular starting batteries I don't think they'll like being drawn down by the fridge on a regular basis. I would also run a good harness from the batteries straight to your fridge and run it on 24V (not knowing what fridge you have I assume it'll do 24V?) if you're not already.
 
Thanks BJ70guy - great info. This is a temporary situation, and I want hard wire coming to the back from the batteries in the next year or so, powering a small inverter for USB and 110 power. A small roof rack with a solar panel on a slide might be the perfect answer to keeping the fridge in the truck 24/7. The fridge is a Whynter and it's 12/24 v, which I guess means it doesn't care which voltage it gets. It certainly runs and cools well on the cord from the lighter. I know the lighter is not optimal.
 
I have not run the fridge in my BJ74 for a long while now, however it has seen regular use in my 80, which as @bj70_guy has two two batteries too, but in parallel so 12V. With two Crown DP batteries on the afternoons of the 2nd day (fridge full) my batteries have shown 12.54V twice on trips this year. Pretty darn good. However new batteries in January of 2017 and when not in use I have them on a maintainer. No solar on those trips. On one trip to Wyoming for the eclipse I took a 40 watt panel and cheap controller, as we were planning to be parked for 3 days. Weather was good so the panel had no problems topping up the batteries each day. I do want to go full time solar in the 80. There are some excellent threads on solar in this section. My fridge is an older ARB (metal sided) with an Engle cover I've had since 2005. I would think the 24V would be slightly better as it is more efficient. But as you say...depends on a number of factors.

If I was to set up my 74 for a fridge full time I would be looking to a 24V charge controller solar system. @Glenn-BJ74 set up his BJ74 very well and included a solar setup which you can read about in this thread. Make sure if you go solar that the controller has a lead you can put beside the batteries for temperature compensation.

hth's
gb
 
I done the nerd math a bunch of times. I have an Engel MT45 I think. As you said it depends on a lot of things. But assuming 75 degrees F, getting in the fridge 20 times a day, not adding warm beer, etc I use about 25 amp hours a day. More in hotter weather, less in cooler weather, blah blah.

So if you have a 100 amp hour deep cycle battery you only want to discharge it half way. So under those circumstances you have 2 days (50 Amp Hour) safely with a deep cycle battery. Longer if you must but personally I'd run my truck for a bit before I stress the battery.

I have a 100W solar panel which allows me to go off grid pretty much permanently.

@Greg_B I live like 2 miles from the Crown factory and can go there and get cosmetic blem batteries or possible ones with repairs. I've got a couple 6V monster golf cart batteries in series. 70 lbs/piece.
 
Fantastic - thanks for that. So even better with a 2 battery 24v system I'm assuming. Not that I would risk a deep discharge for no reason...but it would be nice to comfortably go overnight without worry.
 
Prairie Swamp said:
@Greg_B I live like 2 miles from the Crown factory and can go there and get cosmetic blem batteries or possible ones with repairs.
Click to expand...

You aren't coming to Vancouver any time soon are you? :hillbilly:
I have Crowns in my 81 and would be happy with a set in my 74... but I'd be happier with blem pricing!
 
The cigarette lighter socket in my BJ74 is only hot when the key is in the on position. What is the preferred method of either redirecting constant feed to this socket or wiring another outlet? If wiring another outlet, does one pick up power from the fuse block or direct from the second battery via fused heavy gauge wire? I ran mine for several days on a hastily wired set up from the first battery (12v so I could charge electronics too) though I knew better. It ran just fine for the first four days (some drive time each day) but then caused a no start condition. Oops.
 
From the fuse block, or some other source OTHER than one single battery. As you can see tapping 12v from one battery is bad news. My 73 has a small fused panel in the engine compartment although I don't know the specifics of what is switched, what is constant, etc. I assume it's all 24v. Any electronics I buy are 12/24v - they're not hard to find. I charge phones, batteries and tablets with the cigarette lighter with no issues and the cig lighter will also run the fridge if wanted. I will eventually run power to the back compartment, it just won't come from a single battery.
 
Yes, the ARB fridge (and probably others) will certainly run off the cigarette lighter but only when the key is in the on position. Not very practical given that limitation. One option would be to re-wire the cigarette lighter to a constant hot, another would be to run wiring from a suitable fused location in the fuse box (assuming there is a blank/unused constant "hot" 24v source available, and a third would be to run fused wiring from the second(24v) battery to a new accessory plug in the location of your choice. As you say, running off of the first battery, resulting in 12v and an imbalanced situation, is probably not a good long term solution.

The issue of balance on the 24v system escapes me somewhat, however. In a two battery 12v system (batteries run in parallel) I can see where balance is quite important. In a two battery 24v system they are run in series and it is simply voltage 1 plus voltage 2. I'm not quite sure why it matters. I am sure someone much smarter than me will set me straight. I hope so anyway :)
 
"The issue of balance on the 24v system escapes me somewhat, however. In a two battery 12v system (batteries run in parallel) I can see where balance is quite important. In a two battery 24v system they are run in series and it is simply voltage 1 plus voltage 2. I'm not quite sure why it matters. I am sure someone much smarter than me will set me straight. "

You have it somewhat back to front in terms of what the problem is...

24V system with 2 batteries can be very problematic if you draw from one battery. This creates a state of charge imbalance - i.e. one battery charged and one battery discharged. Now, when the alternator kicks in it wants to push out 2 x 14.4V (nominal) so the already charged battery will immediately go to 14.4V and then higher since it is already charged. The 28.8V (alternator/regulator output) doesn't know anything and continues pushing 28.8V out, but the charged battery will now climb higher and higher (say 16 or more volts) and that means only 28.8-16 = 12.8V is going to the discharged battery which will NOT charge it. So, now you are boiling the charged battery and not charging the discharged one. BAD BAD BAD... VERY BAD...

Parallel batteries have NO problem if one is fully charged (isolated) and then the discharged (been running the fridge for a day or two) is placed in parallel and the alternator kicks in with 14.4V. The already charged battery sees 14.4V applied and draws negligible current while the discharged battery also sees 14.4V and starts taking a whole bunch of current to charge up.

cheers,
george.
 
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