PTC Heater + Sand to heat off-grid cabin's inverter/batt

[JohnV2]

Hi all,

Anyone see a problem with this?

Situation:
Off-grid cabin. BC Canada. Unoccupied for a few months a time in winter, with temps going to-25deg C (-13 deg F) for weeks at a time. 2x6 insulated walls, 160 sq ft cabin.
I've got 2 strings of 4 x 460W panels (one ground mount, one on roof), connected to the separate MPPTs on the EG4 6000XP inverter (which will be "Off" so no parasitic drain, just serving as solar controller & battery charger during winter), to charge 14.3kWh EG4 PowerPro with built-in heater (which pulls 200W-220W apparently, when running). I'm worried snow will accumulate on roof panels (~22deg incline) or not enough sun on my ground mount (45 deg incline, but partially shaded throughout day), and not enough solar will come in on a "weekly" basis to keep the PowerPro's internal heater going all winter. I was going to build an extra insulated box around the battery (to minimize heat loss from internal heater), but am also worried about 6000XP operating in those temps as the MPPT controller.

Plan:
I want to use a cheap 12V PTC heater (THIS or THIS), running off a single separate vertical 200W or 460W panel, through a separate 30A MPPT cheap solar controller's LOAD output (so no 12V battery involved), to heat a small area to help keep my inverter and battery warmer, reducing catastrophic failure if PowerPro drains down, or 6000XP malfunctions in the cold. I'm thinking of putting the PTC heater in a metal bucket of sand, or bunch of bricks, inside the cabin. The idea is that it would increase the air temp in the cabin some amount (or maybe just a smaller "closed" sized enclosure (with 6000XP + PowerPro inside).

Worries:
Cheap 30A MPPT solar controller malfunctions and burns down cabin.
Cheap PTC heater malfunctions, or wires get hot, and burns down cabin.
PTC heater doesn't pull enough load once warmed up, so cheap 30A MPPT controller overheats and explodes and burns down cabin.
Other?

Your thoughts?

Thanks kindly

 

[ConnerLabs]

PTC heaters limit their own temperature, so already have one form of protection built in. You could probably connect a 24V PTC directly to a panel with just a (DC rated) thermostat in the circuit. That would simplify things and eliminate failure modes.

Note that PTCs don’t work well in series, one will always get a lot hotter than the others. So look for a 24V heater or a “12V” panel.

The biggest worry in any of these systems is that a heated battery will drain itself with its own heater and die of over discharge. I’ve never seen any battery or BMS maker spell out the fine details of how their heater control algorithm works and what steps they took to prevent this, and I wish they would.

Personally I would hook things up to guarantee in hardware that the battery heater can only run off the charging sources, not the battery’s own energy.

 

[JohnV2]

Thanks @ConnerLabs!

Directly connecting the 12V or 24V PTC to the panel makes sense to avoid failure points..

I have a 100W panel that nearly matches 24V.. but that's not a lot of energy coming in. I'd like to use a larger spare 460W panel (41 Voc, 34 Vmp).. could I pass that through a DC-DC converter (like this) to get to 12V for the PTC.. would that work, or are these DC-DC converters more designed for a battery's consistent voltage output? Relatedly, I thought anything connected to a panel serving as a "load" would pull as much as its designed for (ie. 100W PTC would only pull 100W), and any "extra" wattage produced by panel is unused/irrelevant/safe...?

 

[ConnerLabs]

Thanks @ConnerLabs!

Directly connecting the 12V or 24V PTC to the panel makes sense to avoid failure points..

I have a 100W panel that nearly matches 24V.. but that's not a lot of energy coming in. I'd like to use a larger spare 460W panel (41 Voc, 34 Vmp).. could I pass that through a DC-DC converter (like this) to get to 12V for the PTC.. would that work, or are these DC-DC converters more designed for a battery's consistent voltage output? Relatedly, I thought anything connected to a panel serving as a "load" would pull as much as its designed for (ie. 100W PTC would only pull 100W), and any "extra" wattage produced by panel is unused/irrelevant/safe...?

With a 34V panel I'd almost be tempted to use a heater designed for 120V AC. It will make about 1/12 of its 120V rating: a 1200W heater would give 100W.

The usual caveats about any thermostat contacts not being rated for DC (but at only 41V they might be fine)