Storm Highway by Dan Robinson
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                   Sunday, November 10, 2013 - 2:39PM CDT

Car inverter "battery drain prevention" circuit

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I, like many storm chasers, have what I call a "dedicated storm chasing equipment circuit" in my car. This consists of heavy-gauge wire connected directly to the battery and running to a busbar in the trunk of the car. The circuit is protected with a fuse at the positive battery terminal (very important if you direct-wire to the battery!). The busbar in the trunk consists of positive and negative terminal strips, to which all of my gear is connected and powered (inverter, WxWorx, mobile internet router, dashcams, battery chargers, etc). This circuit is necessary because cigarette lighter plugs and wires are too flimsy for frequent use, and are way too small to safely handle the current loads that all of my gear would place on it.

My equipment circuit is controlled by a single on/off switch I installed under the steering wheel. It needs to operate seperate from the ignition, since I need the power to stay on to keep data flowing when I make brief stops for fuel or food. I also need it to stay on to keep the dashcams running for other brief stops, since I run those full-time whether I'm observing or not.

The problem with this circuit is that I sometimes forget to turn it off when parking for the night, which eventually drains my battery. The inverter is primarily responsible for this drain, as it draws the most power of anything else on the circuit. From 2004 to May of this year, I used a 750 watt inverter - which would drain my battery if it was left on for about 10 hours. After this inverter died during my storm chase expedition in May, I replaced it with a larger 1200 watt inverter. This new inverter drains the battery in less than 4 hours if I accidentally leave it on.

With the old inverter, I drained my battery about 2 to 4 times a year due to accidentally leaving the power on - an acceptable impact I just dealt with. However, with the new inverter, I've drained the battery like this about a dozen times since May alone, leading to many inconvenient and embarrasing requests for a jump-start. This obviously was a problem that now required a fix.

I brainstormed a few possible solutions:

  1. A dual-battery setup with an isolator (similar to those used with boats and RVs). The second battery powers the equipment, leaving the main battery safe from draining. This is an effective but expensive option (over $200) that would require a second set of heavy-gauge cables running through the cabin of the car (the second battery in my car would need to go in the trunk, as there is no room under the hood).
  2. A 'battery buddy' type disconnect device, which cuts power when the battery voltage drops below roughly 11.7 volts. My concern with this was that the equipment circuit might cause a big enough voltage drop to trip the device immediately when the ignition is turned off.
  3. A transmitter connected to the equipment circuit that would sound an alarm in my apartment when my car was parked outside with the power on. This would require finding some other device that could be repurposed for this application, none came to mind. This would also not be effective away from home.
  4. Flashing lights on the back of the car that would catch my eye in my CCTV system. The problem here is that the lights may not always be noticed away from home.
  5. A timer that would shut the equipment circuit off about 30 to 60 minutes after turning off the ignition.
  6. A buzzer to alert me that the power was on when the ignition was turned off.
I quickly settled on the last option - not only the simplest and cheapest of all of them, but a pretty foolproof one. This solution is simply a buzzer powered by the equipment circuit, which is switched by a relay connected to the ignition-controlled cigarette lighter circuit. When the ignition is turned off, the cigarette lighter circuit shuts off, closing the relay contacts and sounding the buzzer if I've left the equipment circuit on. The parts for this project cost me only $33 from an auto parts store:
  • SPDT (single pole double throw) relay. (The relay must have the option to close its switch with either the coil energized or de-energized).
  • 12v buzzer or chime. I used a 'backup alarm' for my circuit.
  • Wire and terminal plugs.
Here is a schematic of the circuit:

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