# How to drop the voltage of a power supply?

I have a DC power supply which outputs 21V 2.2A.

I want to use it to charge a car battery so I need to get the voltage down to 14V.

I have a 3.9Ohm 100W resistor but am am I correct that that would draw around 5A.

What resistor should I use to drop the voltage to 14V?

• You cannot and should not charge a car battery this way. The voltage is too high (more than 13.8 V) so you'll overcharge the car battery and damage it. The resistor will not limit the voltage to 14 V when the battery is full. Also explosive gasses could escape. Car battery chargers are cheap, just get one of those. Commented Aug 12, 2020 at 12:39
• Resistors don't work that way. At the very least you need a voltage regulator, which could be a single component or a circuit of multiple components (some of which may be resistors). But a single resistor will not do what you want. Commented Aug 12, 2020 at 12:47
• (21-12)/2A=4.5 Ohm. (21-11.5V)/3.9 = 2.43 A . overcurrent on low battery, manual cuttoff needs monitor in real-time. Commented Aug 12, 2020 at 12:57
• Try to limit the charge current by connecting a car lamp or a combination of lamps between the power supply and the battery. For example, try with two 21 W lamps in parallel or two 60 W in series... or something else (as a bonus you will also have free lighting:) You can make simple calculations but keep in mind that the lamp resistance is nonlinear (it increases with temperature). I have done it many times in the past when I was dealing with cars... and not once did a battery explode... You can enlarge this idea with some else useful ballast load (e.g. a fan:) Commented Aug 12, 2020 at 13:26
• I'm with @Bimpelrekkie. This is NOT a suitable application for a resistor, don't even try it. Or if you do, wear a full hazmat suit and eye protection so the battery acid doesn't permanently disfigure you. Dude, you can literally buy a "real" battery charger for like \$10. Sometimes you just gotta suck it up and take the pre-engineered solution. Commented Aug 12, 2020 at 19:14