evandude
New Member
I have a computer in my car for playing music, and soon to be GPS navigation, etc.
I want to put in a simple timer to keep the thing powered up when the car is off for a preset amount of time (say, 10 minutes) to allow me to run in to a gas station or something without leaving my keys in the car. That part I have no problem with.
The problem arises when starting the car again. due to the high current draw of the starter motor, the voltage in the car dips well below the nominal 12v of the car battery. Not surprisingly, this causes the computer to reset.
So I need some way to keep the computer running for a few seconds. Preferably, automatically.
I originally thought of using a decently large capacitor but by the normal calculation of I=C*dV/dT, with a conservative estimate of 2A supply to the computer (more like 0.7-1.2A), a maximum voltage droop of 1V (down to 11V from 12V) and a dT of 3 seconds for starting the car, I would apparently need a 6 Farad capacitor. Obviously not a cheap solution.
I also considered using a battery setup. I have a small 12v gel-cell battery that I can use but I am at a loss for a good way to hook it up. I was considering connecting it with a simple power diode OR arrangement with the battery and car 12v line as the two inputs, so that when the battery is charged to slightly below the car voltage level, it will only be drawn from when the car voltage droops, and will then keep the computer supply voltage at one diode drop below the nominal 12v level.
The problem with this arrangement is how do I charge it? i am not all that familiar with gel cell batteries, so I don't know what is and isn't safe. If I connected a resistor between the car 12v and the battery +, it would act like a trickle charger for the battery. attached is a crappy ms paint diagram. the biggest issue I see is that when the car is on, the 12v line goes to more like 13.6v. thus the battery would gradually charge to that voltage when the car was running, and then fall back to 12v when the car was off. I could put a 12.1v zener diode across the battery to keep it at that voltage instead, if the voltage cycling would be harmful.
Will that work? ie - will the battery survive as long as the resistor value is appropriate? what value of resistor should I use? should I use a zener diode as well?
thanks in advance. I spend way too much time just working with microcontrollers and such and I always get hung up when I have to deal with a power issue...
I want to put in a simple timer to keep the thing powered up when the car is off for a preset amount of time (say, 10 minutes) to allow me to run in to a gas station or something without leaving my keys in the car. That part I have no problem with.
The problem arises when starting the car again. due to the high current draw of the starter motor, the voltage in the car dips well below the nominal 12v of the car battery. Not surprisingly, this causes the computer to reset.
So I need some way to keep the computer running for a few seconds. Preferably, automatically.
I originally thought of using a decently large capacitor but by the normal calculation of I=C*dV/dT, with a conservative estimate of 2A supply to the computer (more like 0.7-1.2A), a maximum voltage droop of 1V (down to 11V from 12V) and a dT of 3 seconds for starting the car, I would apparently need a 6 Farad capacitor. Obviously not a cheap solution.
I also considered using a battery setup. I have a small 12v gel-cell battery that I can use but I am at a loss for a good way to hook it up. I was considering connecting it with a simple power diode OR arrangement with the battery and car 12v line as the two inputs, so that when the battery is charged to slightly below the car voltage level, it will only be drawn from when the car voltage droops, and will then keep the computer supply voltage at one diode drop below the nominal 12v level.
The problem with this arrangement is how do I charge it? i am not all that familiar with gel cell batteries, so I don't know what is and isn't safe. If I connected a resistor between the car 12v and the battery +, it would act like a trickle charger for the battery. attached is a crappy ms paint diagram. the biggest issue I see is that when the car is on, the 12v line goes to more like 13.6v. thus the battery would gradually charge to that voltage when the car was running, and then fall back to 12v when the car was off. I could put a 12.1v zener diode across the battery to keep it at that voltage instead, if the voltage cycling would be harmful.
Will that work? ie - will the battery survive as long as the resistor value is appropriate? what value of resistor should I use? should I use a zener diode as well?
thanks in advance. I spend way too much time just working with microcontrollers and such and I always get hung up when I have to deal with a power issue...
