# Stopping Vehicle Mounted Units from stopping abruptly

I have installed a touchscreen computer in my old car. The computer runs on 12V DC power supply. I have installed a standard 24V DC to 12V DC converter to power the computer. The 24V DC input to the DC-DC converter is from the car's battery.

Now, my car is having some starting problems, the battery voltage gets low and at times I have to jump start the car. Or in some cases when I start the engine the voltage across the input to my DC-DC converter abruptly falls to 0V, and rises to 24V DC within a fraction of a second.

Due to this sudden fluctuation my computer restarts. Is there a way to ensure that the voltage doesnt fall to prevent this sudden restart ?

I am new to EE-SE please let me know if the tags I have used are appropriate or not. Thanks. I know one of the obvious solution is to get a good battery and get some work done on the alternator and all. But is there any circuitry that can be designed (using easy to find objects at electrical store) at home to achieve this ?

• Use a big capacitor, like those required by stupid-high power car audio systems? – Dan Jun 5 '15 at 16:08

Since you're new to EE, I'm going to have to say that the answer is no. What you are asking is pretty straightforward, but it's not something you can do with hardware-store parts.

Basically, what you need is an energy storage device that you can insert between your car battery and your computer. This is complicated by the fact that you need to isolate this storage from variations in the battery voltage. That is, when you try to start the car and battery voltage drops, you don't want your storage device to try to backfeed the battery, as this will exhaust the storage device and your computer will die. In principle, you can do this one of two ways:

simulate this circuit – Schematic created using CircuitLab

Note that I'm showing your storage device as BOTH a capacitor and a battery. This is not necessary - you can use either. Using a battery instead of a capacitor is in some ways easier, but you have to worry about keeping it charged, which can be a royal pain in the whatzit. On the other hand, you don't have to worry about making sure the capacitor is charged before starting the car, which can be a problem, too.

Also note that the second approach, storing energy after the converter, seems simpler because you don't need a diode to keep from backfeeding the car battery, but this may or may not be true, depending on the converter. It assumes that the converter will not let current flow from output to input when the converter is not working. This may or may not be true. If you select a battery, it also assumes the converter will turn on properly with a humongous capacitor hanging off its output. This also may or may not be true.

Using a capacitor will work, but you have to size the capacitor properly. To do that you have to know a) the current it supplies when needed, b) how long it needs to provide the current, and c) what voltage drop is acceptable during that period. If the answers are a) i amps, b) t seconds, and c) DV volts, then

                           C = 1,000,000 x i x t / DV  (in uF)


So, if you're supply 2 amps for 3 seconds, and you can accept a voltage drop of 2 volts,

                           C = 1,000,000 x 2 x 3 / 2

= 3,000,000 uf, or 3 F.


You'll have trouble finding a 3 F, 12 or 24 volt capacitor.

So, like I say, this may be rather more trouble than you can handle at this stage in your education.

• thank you for the clarification. will try to look for the solution as mentioned. otherwise i guess i ll have to go with an engine work and a new battery. – RicoRicochet Jun 8 '15 at 3:10

Depends on the exact car parameters, but the best is probably to use a supercap battery as first stage. You will need some current limit circuit, but first find a way to add several supercaps. Note, that they will hold the power some time after the car is switched off.