Sorry if it wasn’t clear, but this is just a run-of-the-mill desktop variable DC power supply. Not a charger per se.
There should be some documentation about what conditions would cause a shutdown.
I’m leaning toward either a thermal shutdown or an overcurrent protection trip, but that’s just guessing.
My guess? The variable DC power supply is ‘dumb.’ It only has an overcharging circuit that shuts down current when a set voltage is exceeded.
A charger with a built in ‘float’ or ‘trickle’ mode will sense a fully charged battery and then either switch to a lower but constant voltage and/or amperage or actually vary the amperage to keep things topped off as the battery’s condition subtly changes (with time, temperature, or load).
You may have to ‘ride the gain,’ adjusting the power supply voltage and current manually once the car battery is fully charged if you want it to act as a trickle charger.
Of course, you’d have to switch the settings back when a bigger load comes into play (eg, starting up the diesel heater from OFF).
[Or blow off the whole trickle charger idea entirely with this configuration]
Resource:
In my old RV, the inverter was dumb. I made a habit to always leave the floor lighting circuit on to prevent overcharging. Some small current draw, less than the trickle charge current you’re delivering to the battery, can help.
IMHO, though, it’s not particularly wise to leave this power supply set up ‘as a charger’ over a long period of time without keeping a close eye on things, including the voltage of the battery with nothing connected to it.
Bad Things could happen.
I also think that’s a distinct possibility, even as the battery simply gets topped off (or possibly overcharged).
I come bearing data, though I make no promises it’s useful.
This afternoon I went out to a mildly discharged battery, at just about 12v
I hooked up the PS and it immediately read 12.Xv and 5A
I then connected the heater and started it. The voltage dropped to about 12v (still 5A) and everything ran just fine. I was out there for about 90 minutes.
Fairly quickly the voltage on the PS rose to 14v and stayed a little while at 5A. Then the current slowly dropped to about 2A and just stayed there for the duration. I should note that all my current numbers are approximate averages. They sort of oscillate around the number I’m giving, with a +/- of about 0.1A
When I shut the heater down the current immediately went back to 5A, but then started dropping. As I type this it’s a bit below 2A, and I expect in an hour or so it will be 0
I tried to get some info on the PS, but it was a cheapo Amazon item and has no information on it. And apparently it’s back far enough I can’t get it off my history.
Hm.
Honestly, I’m not getting much more clarity about this one.
But I’m wondering if you have a local auto parts store that will test your battery under load. A resting charge of 12V isn’t particularly good. I wonder if the battery and the power supply haven’t been playing well together … at the battery’s expense.
Or if the battery just wasn’t at its fighting weight when you undertook this diabolical project 
From the usage pattern you’ve been describing, I thought you were getting that battery up to full charge before applying the diesel heater load to it again. Was I mistaken?
I see a lot of 5A measurements in the (very good) narrative which is also the current capacity of the supply. In other words, it’s pegged and is probably going into some overload protect mode.
How did it get down to 12 from 14? I thought you were going to leave the power supply energized across the battery.
Following the previous escapade, when the heater shut down to low voltage, I only had time to partially charge the battery. I didn’t get it to full charge because I didn’t want to leave my goldberg setup unattended, so I charged it sans heater while I did other things, until it was too cold and I went back inside.
Sorry that wasn’t clear- the battery seems to hold a charge just fine, but I didn’t have the time to fully recharge between uses, but it was enough to get the heater going.
I think that makes sense. When the battery needs significant charging, the PS pegs out at 5A. As the battery gets closer to charge, the current displayed on the PS drops, eventually to zero. At least, that’s what it looks like to me
And hey- thanks for indulging me on this. It’s been a pretty interesting learning experience.
Two points:
Deep cycle batteries did get mentioned upthread, but not enough. Batteries for starting engines are designed to deliver huge currents briefly, whereas deep cycle batteries are designed to deliver moderate currents for hours. You may find that a deep cycle battery works better here.
Also – though this is a pedantic point – I don’t think the name “diesel heater” is appropriate, because it isn’t dieseling, it isn’t igniting through compression. It’s just a heater that consumes what’s sold as diesel fuel. But, I get it, nobody cares about this one.
Ok, the measurements make more sense if you started with a low batt. [*see scold]. The pulling 5A while at 14v doesn’t seem right but I think it must just be funky PS threshold behavior. Let’s get the battery charged and cycled through a few heater-hours to reexamine. I’m still optimistic that the battery will stay fullish if the supply is left on while the heater’s in use but otherwise turned off.
Your intuitions are correct. PS current means charging and more current is more charging. The current drops off to zero as the supply and battery voltages converge. Beautiful, really, till a spiky bull of a heater load comes bowling through our elegant charge/discharge curves.
[*Scold=playful]In assessing whether X and Y is suitable for use with Z, please don’t start with a questionably faulty X.[/scold]
I’m sure that’s true, but as I mentioned I’m trying to make do. If this ends up not working out (and even if it does, it’s a bit of a messy setup) I’ll just buy a 120vac to 12vdc power supply that can supply the current. There is power to the shed.
QFT
Hey everyone
Wanted to follow up.
I don’t know what happened before, but everything’s been working perfectly. When I head out there after maybe 20 hours of non-use, the battery is showing about 12.8v. I hook up first the heater, then the PS, and I’m showing 14v and 0.5A
I start the heater and quickly the voltage stays the same and the current goes to 5A. The heater starts running and after a few minutes goes to 3A. After everything stabilizes, we run at about 2A. Then I shut everything down.
If I leave the PS connected we’ll drop to about 0.8A indefinitely. Turn it all off, and the next day rinse, lather, repeat.
So I don’t know what happened before, but a few weeks in all seems to be working.