Heater not heating when connected to battery - is it broken or am I dumb?

Question

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...

Answer 1

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

^{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.