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So, I'm a total amateur, my sum total electrical engineering experience is stuff my grandpa explained to me when I was 6 and playing with Snap Circuits.

I'm trying to build an electric heater in to my jacket: I purchased this battery:

https://www.amazon.com/gp/product/B016BJCRUO/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

and this heating element: https://www.amazon.com/gp/product/B00ZP3XCS4/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

The element is a silicone heat pad designed for 3D printer beds; it's rated for 150 watts @ 12 volts. It was the cheapest thing that seemed like it would do the job in my price range. The battery is a lithium-ion pack that's supposed to output 72 watts at 12 volts. They say it's supposed to be able to handle a 6 amp load continuously but I'm aware battery manufacturers always lie. An electric blanket is supposed to draw around 2 to 3A anyway so that should still be enough in theory to get the job done.

Now I realize these numbers don't match up, but I figured the heater would just... not be able to pull more amps than the battery will put out. I don't want it going up to its design temperature anyway (there's an electronic thermostat involved here too).

The thing is, I've tried connecting the heating pad directly to the battery and nothing happens. Even leaving it there for half an hour, the pad is still cold to the touch.

Do you think it's more likely that there's some overload protection circuit on the battery pack that's cutting off the power, or is it more probable that something's just broken?

I've checked the pad with my multimeter and it gives me an ohm reading, so presumably there isn't a broken connection inside the thing. Interestingly, checking the output of the battery gives 8 volts (at half charge) to 10 (fresh off the charger,) and whether the heater is connected or not doesn't seem to have any effect on that. The multimeter also gives 10 volts checking the charger itself though, so maybe it's just broken. It's pretty old...

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  • \$\begingroup\$ The heater will draw whatever it needs. If the battery can't keep up, the voltage will drop, and it might overheat and explode. If the battery can keep up, it should work as expected. However, your weird battery pack-thingy should have protection. The documentation will tell. It would be good if you could find a link to the actual detailed specification. \$\endgroup\$ – pipe Feb 2 '17 at 17:41
  • \$\begingroup\$ What resistance do you measure across the heater connections? It should be no greater than 1 Ohm. (R = E(squared) / P). If you are measuring more than one Ohm, then your heater has a problem. \$\endgroup\$ – Paul Elliott Feb 2 '17 at 17:50
  • \$\begingroup\$ @PaulElliott I remember it being 80-something. I'll check again. \$\endgroup\$ – Schilcote Feb 2 '17 at 17:51
  • \$\begingroup\$ Well, now I don't get a reading at all, so I guess it is the pad itself. I bought a replacement that's within the wattage rating of the battery pack this time. \$\endgroup\$ – Schilcote Feb 2 '17 at 22:06
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First, get a new, working meter. If you can't trust what you have ("maybe it's just broken"), then you don't have any idea what your problem might be. And if the meter is working OK, your charger is in bad shape. If the battery is only being charged to 10 volts it will be essentially discharged, and there is no way the heater will get warm.

With that said, the battery is really too small for what you are doing. You say, "An electric blanket is supposed to draw around 2 to 3A anyway so that should still be enough in theory to get the job done.", but you're not dealing with an electric blanket. In fact, your heater pad should draw 150/12, or 12.5 amps, which is twice the battery rating. It is entirely possible that the 6 amp limit given in the Amazon page is enforced by a current limiter in the battery pack, and this may work by shutting the battery output off for a period of time. This would also explain why you're not getting any heat - the battery is simply not putting out any current most of the time.

And finally, " I'm aware battery manufacturers always lie". Well, no. Chinese sellers on eBay, maybe. Amazon, no. In fact, lying about a product is a good way to get sued, so you should not assume that it's happening.

What you might do is to get 2 more battery packs, and 3 high-current diodes (like 10 amps or better). Then you could connect them like

schematic

simulate this circuit – Schematic created using CircuitLab

This would limit the battery current to about 4 amps per, which is comfortably within spec. You would also have to provide a modest cooling for the diodes, but since I doubt you're going to try this, let's not complicate matters.

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  • \$\begingroup\$ With those diode-connected batteries, if the batteries aren't very closely matched (age, state of charge, capacity, etc) then you will find that the current through the diodes will not be matched, and one of them will be providing the bulk of the current. With just one diode conducting, that current will be around 12.5A, so for safety the diode rating should be double that. It's probably easier to just get a bigger battery. \$\endgroup\$ – Paul Elliott Feb 2 '17 at 18:41

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