13 volt relay cutoff

[tc0nn]

I'm trying to find low draw circuit that will cutoff when a car battery dips to a less than perfect voltage. I had a one amp draw on my battery at all times which means if I don't drive every other day, its dead...

Even a 60 minute timer relay would do the trick.

 

[buzzp]

Well you will need a large solenoid to switch the battery load. Make sure it has an adequate DC current rating for your starter as well. Place this solenoid in series with the battery cable. Now the question is what are you going to use to monitor the voltage. There are devices out there capable of monitoring DC voltage this low and in turn energize a small relay built into the monitor. This relay will control the coil of the solenoid. Or if you want a timer to auto shut off after 60 minutes you are going to have to rig this into your ignition so it knows when to start the timing.

In any case, I think I would find out where 1A is going rather than trying to make a temporary fix. If its not an amplifier or stereo then I would be somewhat concerned. What kind of vehicle is this, new or old?

 

[tc0nn]

Whoa now.. I'm not wanting to disconnect the battery from my vehicle, but simply disconnect this single circuit (12 ga/20 amp) circuit.

I know where the 1 amp goes, its my VHF APRS rig used for realtime vehicle location. It basically sqawks my GPS location every 30 minutes, every 1/4 mile or at every major turn. The radio, the gps and the modulator doesn't quite draw a full amp, but when it sqawks a 50 watt RF signal, that all adds up.

After a little thought, I may just use a second battery and a bridge rectifier (i.e. four diodes) to limit the current drain direction.

 

[buzzp]

If you need to switch 20A then you can use a slightly smaller relay (found at any automotive store). Why don't you hook this relay into the ignition switch so that it only comes on when the key is on? I guess your not worried about finding the car if it is not running. Or even better yet, find a power source in your fuse panel that only comes on with the ignition and hook this directly to the relay coil that switches power to GPS system directly off the battery. (coil voltage 12VDC with 20A contacts @ 12VDC).

Diodes would not be necessary if this GPS was tied to only this second battery. But still this battery will need to be charged and to use your onboard charging system will require the second battery be tied into the first battery. In order for the battery to get charged, you can not use diodes to block current flow. I would definately not look at a second battery. I do not see how diodes are going to help you in this situation at all.

 

[nbucska]

Since your rig doesn't move if the engine is not running, why don't you just turn off the rig after 30 min-s ?

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[BrianR]

nbuksa's solution would do the job, but otherwise use a solid state relay that triggers from logic levels. Your battery voltage should be a min of 13.7V on charge, fall to 13.2V as charge ceases then rapidly drop to 12.5-12.6V as the surface charge bleeds off. This should let you choose when to set the relay to turn off.