Can anyone recommend a USB battery warmer to me?

I have one of those 2.5kwh portable battery packs with built in inverters.  its LiFePo4 so it handles cold relatively well but I'd like to be able to keep it above freezing in the winter. I was going to add a DC heater to one of the USB ports for that purpose.

However i am struggling to find one. Should be a USB cable to a combined heater/thermostat, that will generate heat when below a certain temp.  All my searches get drowned out with USB hand warmers and USB chair heaters which aren't what I need and lack the thermostat.

Hi @ll,

I think the reason you're struggling to find it is that you may be better off separating the thermostat from the heater.

There are small 5V USB digital thermostat controllers with an external temperature probe. You can set a start and stop temperature, then connect an ordinary 5V USB heating pad to the output.

So basically:

USB battery output → thermostat controller → 5V heating pad

Try to found it at Walmart or other similar: USB 5V digital temperature controller with a probe and programmable start/stop temperatures.

I would probably use a 7–10W heating pad, put the battery in a lightly insulated enclosure and heat the enclosure rather than putting the heater directly against the battery. Put the probe against the battery case and perhaps set it to switch on around 2°C / 36°F and off around 5°C / 41°F.

That should do exactly what you're describing without needing to find a specialised “battery warmer”.

Personally I'm leery of anything that would stay plugged in to the battery pack. If the thermostat fails it might cause damage.

Can you describe your larger system? How are you charging the battery pack? If it's solar with a controller, could it power a small 12V incandescent bulb directly?

Howdy, I think USB cables have a max 5 Amp limit and most not rated that high?  So charging USB .5 - 3 Amp max usually.  Like Douglas said “Leery”, I would be as well as cable failure is a fire.  Might be looking for a different system, like solar recharge buffering/dump which changes loads to secondary systems (like heaters) once charge reaches max.  While your charging heat is generated in any battery system, then dump to secondary heater.  If you have a central heated area for battery storage this increases efficiency of the entire system.  A life of integrated technology requires large structural change… I’ve decided to place it in a separate building not a central living space.  Cheers

Douglas Alpenstock wrote:Personally I'm leery of anything that would stay plugged in to the battery pack. If the thermostat fails it might cause damage.

Can you describe your larger system? How are you charging the battery pack? If it's solar with a controller, could it power a small 12V incandescent bulb directly?

It's charged with 200w portable solar, and yes I could put a 12v DC bulb on it, but then it's drawing that amount nonstop forever.

I could protect against thermostat failure by running two thermostats in series to the heater. Then it would only turn on when both agree and a single failure wouldnt cause an issue.

Daniel Andy wrote:

Douglas Alpenstock wrote:Personally I'm leery of anything that would stay plugged in to the battery pack. If the thermostat fails it might cause damage.

Can you describe your larger system? How are you charging the battery pack? If it's solar with a controller, could it power a small 12V incandescent bulb directly?

It's charged with 200w portable solar, and yes I could put a 12v DC bulb on it, but then it's drawing that amount nonstop forever.

I could protect against thermostat failure by running two thermostats in series to the heater. Then it would only turn on when both agree and a single failure wouldnt cause an issue.

Daniel, with only 200 watts of solar you might not be able to generate enough power to run a heater to keep it warm. Is it constantly charging and discharging or does it just sit in wait? I ask because they can discharge cold just cant charge that way. On my larger systems I build abox around the batteries and install a thermostat and 200 watt heater plugged into an AC outlet. AC gear tends to have to pass safety standards, is cheaper and more readily available.
Just a thought.  

Daniel, one way to put numbers on David's energy-budget concern: a 10 W heater running continuously uses 240 Wh/day. If its thermostat keeps it on for half the day, that becomes 120 Wh/day. That excludes the power station's own overhead and conversion losses, and it doesn't establish whether 10 W is enough to keep your particular enclosure warm.

With a nominal 2,500 Wh pack, 240 Wh is about 9.6% of the nameplate capacity each day, before your other loads. The useful comparison is with the actual Wh your 200 W panel brings in on a poor winter day, rather than its rated watts.

Which power station model is it, and what overnight temperatures are you planning for? Before buying anything, I'd check its manual for USB output auto-off behavior at low loads, whether that can be disabled, and the manufacturer's charging-temperature limits. A thermostat can request heat but won't help if the USB supply has gone to sleep. That detail could rule out an otherwise promising heater/controller combination.