I have an electrolyzer that works with car battery (12 volts DC) and draws current around 15 amps. I have a digital multimeter that can measure 10 amps at most. So how can I measure the current? Maybe I should use a resistance to measure its voltage but I can't think of a resistance that would tolerate 15 amps.
I'd get a precision 0.01 ohm power resistor and measure the voltage across that. At 20 amps you'd only have to dissipate 4 watts at the resistor.
Alternately, you could find yourself a Hall Effect sensor. The resistor is simpler though.
Power resistors usually look like this:
The easiest way (other than buying a DC-capable clamp-on meter) is to use a shunt with Kelvin connections and your multimeter (on a voltage range).
The one shown provides 75mV output with 50A flowing, so you can measure the voltage on the 200mV range of your meter. Take the reading in mV and multiply by 2/3 to get amperes. Chinese shunts are available for a few dollars each.
The power connections go to the LARGE (outside) screws. The meter connections go to the SMALL (inside) screws. This is called a Kelvin connection (named after William Thomson), and it means the connection and wire resistance and variations of those resistances do not appreciably affect the reading. Consider the schematic below:
The exact values of R2-R5 do not affect the current reading hardly at all, provided the meter is high impedance. So even if the screw is a bit tighter or looser, or the wire going to the connections heats up a bit and changes resistance, the reading will stay steady.