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