Mdthbs

I need to test a battery for its remaining capacity and to do that, I need to generate a constant load to be able to measure how long it can sustain that load.

I tried using a 330W load (290W shoe heater + 85%-ish efficient inverter), but I found out that it couldn't sustain that load for more than half an hour before the voltage was too low. Lower loads seemed to work, though, so I'll need to find a way of creating something that has constant load, but at a lower wattage.

Now, this seems silly-stupid simple, right? Just find something that draws, say 100W and see if that works. Or 50W if that should fail. The problem is, I have a hard time finding anything with a constant power draw in the range 15W-300W. It's impossible to find light bulbs that are not LEDs anymore (1-2W instead of 50W), and my vacuum starts at 350W at the minimum setting. Nothing inbetween.

So I guess I could hack my own? I'm not that great at electro-physics, but I understand it's possible to make some kind of super-basic heater: something with a high resistance causing a lot of thermal heat. I have no idea, though, how to make it work at 12V. Is this something I could hack together?

Alternatively, are there common household appliances that can generate a constant load at 50-150W for hours without inducing mechanical failure of some sort?

Automotive headlamps are the obvious, easy 12V choice, being typically in the 45-55W range (don't get LED headlamps.)

If you ask the right folks you can often get dual-filament bulbs (high/low beam) with one filament broken for free.

For 12V DC, you are probably looking for halogen bulbs, not incandescent. You should be able to get a few hundred watts of 12v halogen bulbs for about $10 online, and you could even get them in units of 20W or so if you want to refine your testing.

You don't need to step up to mains voltage, in fact for experimenting it's best not to.

There are such things as 12V car fan heaters that are typically 120-150W.

Or you can buy wire-wound load resistors. A 3 Ω resistor would dissipate 48 W at 12 V, so wiring 3 in parallel would work and allow you to adjust the load. I'd buy 100 W-rated models rather than 50 W, which would get rather hot. Either way they should be screwed to a big heatsink or lump of metal, and connected using cables/connectors/switches rated for the current.

If you really want to use an inverter, halogen floodlights are still available. The one I've linked is 120W 230V.

(UK links because that's what Google assumes I want)

As you know the battery voltage will fall as its charge depletes. Depending on how constant you want the load to be it may need to self-adjust to hold a steady load (wattage).

Ham radio enthusiasts routinely require a "dummy load" for testing radio transmitters without actually transmitting anything. These are basically high-power 50 ohm resistors.

On its own a 50 ohm resistor won't do much for you. Connected directly to the 12 V supply it'll consume only V^2/R=2.9 watts. However, if you put a dc-dc boost converter between the battery and the dummy load, then you can push some serious power into it.

A boost converter that can reach 100 V output from 12 V input will do two things for you: it'll pump 100^2/50=200 watts into the load, and it'll maintain that power as the battery voltage falls. The power level can be adjusted indirectly by adjusting the output voltage of the boost converter. A search for '100 v dc boost converter' on eBay or Amazon yields many options.