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I have a portable diesel heater which runs on 12V. When the heater starts, it warms a glow plug that draws somewhere in the range of 9-10A until the plug is hot (figure, roughly 1 min), after which the power demand drops and the 12V is only used to run a blower fan.

My power supply--an Ecoflow River 2 Max--claims the DC port can handle 10A (12.6V, 126W max), however, not long after starting it trips the overload protection and shuts down. I have tested that the heater works fine when started from a car battery.

Given that the power specs are just at the limit and this initial glow plug warming phase is temporary, I suspect I might be able to make this work if I can limit the initial current (to prevent the power supply from tripping) or serve some of the draw with capacitors, but I'm at a loss of how best to do this. I do know the heater has a voltage range outside of which it shuts down, but given that the glow plug is just a "dumb" heating element, I am speculating it might not be as fussy about the current.

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  • \$\begingroup\$ It is kind of ironic that your DC-powered energy storage device can only put out 10 A of DC. Maybe if you put a diode in series with the heater it will lower the current enough to avoid tripping. \$\endgroup\$
    – user57037
    Commented Dec 15, 2022 at 4:04
  • \$\begingroup\$ The other option is to just run the heater with an AC-DC converter. I assume the Ecofolow can easily put out enough AC to run the heater. \$\endgroup\$
    – user57037
    Commented Dec 15, 2022 at 4:05
  • \$\begingroup\$ Agreed! I was trying to avoid the pretty severe efficiency tax of having the AC inverter on. This is for camping and all of my other power uses are DC. \$\endgroup\$
    – wjc
    Commented Dec 15, 2022 at 14:45

4 Answers 4

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The glow plug will have a low cold resistance which increases as it heats up. So what you’re looking for is a kind of ‘soft start’ to cover over that time.

A current limiter built out of two transistors would work very well for this. More here: https://circuitsan.blogspot.com/2014/04/current-limiter-circuit-using-two.html

A brute-force solution might be to use a small lead-acid battery with a battery tender to keep it topped instead of the Ecoflow.

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Place a switching voltage regulator between the power supply and the glow plug to lower its voltage to, say, 10 V. It will take a bit longer to work, but work it will. That will reduce the current out of the power supply to be within its capabilities.

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I've never used a glowplug, but from a few web searches it seems that it'll draw a large current spike when cold then settle to a lower value as it heats up. If you measure current with a multimeter, it would be pretty easy to miss the spike.

Instead of measuring current, I'd recommend measuring the resistance of the cold glowplug to get an idea of the inrush current value.

It looks like the high current will last at least a few seconds, so capacitors would need to be way too large to provide it.

A low-tech solution would be to use a 12V lead acid battery, even a small one sized for a scooter or motorcycle will not have any trouble with this sort of current.

Another way would be a simple resistor in series, but it will need to dissipate quite a lot of power. You could use a NTC inrush current limiter instead, which is more compact. It's simply a resistor whose value decreases when hot, so it limits inrush current when cold then heats up and gets out of the way. Its value should be calculated according to the glowplug cold resistance to not exceed the ecoflow's maximum current when both are in series.

A medium-tech solution would be to PWM the heater. Assuming the EcoFlow has enough output capacitance (you can always add more in your device), if the PWM frequency is high enough, a duty cycle lower than 100% will draw a lower average current which may not trip the built-in overcurrent protection. You can use a 555 and a MOSFET (plenty of schematics available) or just buy a "PWM motor controller" online, there are plenty of boards like that.

A more high-tech solution would be to build a switching constant current driver, in other words a buck DC-DC converter that regulates current instead of voltage. Fortunately, these are widely available under the name "LED drivers". This will most likely require rolling your own board.

The glow plug looks like it has only one wire which is the positive, negative being the engine block as usual. So if you intend to have a common ground between the EcoFlow and the diesel heater, you'll need a driver that switches in the high side. If you don't need a shared ground, then you can switch low-side, which is simpler. I would recommend checking if exposed metal on both devices is grounded: if this is the case then you'll need a high side switch, otherwise if they touch, their grounds will make contact, which is a problem if the circuit assumes the devices don't share a ground.

NTC inrush current limiter is my favorite solution because it's only one part, cheap, simple, robust, no headaches.

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    \$\begingroup\$ An NTC current limiter will usually heat up and drop resistance value rather quickly in free air, so you might need to mount it on a heat sink to slow down to better match the time constant of the glow plug. \$\endgroup\$
    – PStechPaul
    Commented Dec 14, 2022 at 18:39
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I've some experience with these heaters and while the glow plug will not be fussy, the heater controller may well monitor the current and glow plug temperature, and the heaters are definitely fussy about any voltage drop when warming the glow plug.

It may be that you can put some simple circuit or component in-line with the glow plug (like a large PTC inrush current limiter for example) or just swap the glow plug out for a version from a 24v heater which would (probably) be about twice the resistance... although the startup kick from cold could still trip you up.

Another alternative may be to add a little smoothing circuit inside the heater that maintains a steady 12v supply to the controller board even if the glow plug drags it down for a fraction of a second - a capacitor with a schottky diode in front of it or somesuch.

Another idea is that you could apply a low current to the glow plug externally before trying to start the heater, thus (possibly) avoiding the kick form cold.

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