Hello,

I'm trying to find what option(s) there are to have a power inverter in a car/truck that is only on when the car is running.

The use case. I would like to go Vehicle Power ---> 1500(give or take) Watt Inverter ----> UPS ----> My Device

The intent being with the vehicle off, the device runs off UPS battery power until eventually the battery dies and My Device turns off (this would not happen very often, so I'm not too concerned about repeated draining of UPS battery. However I do need to have a battery backup despite its function only being used sparingly).

With the vehicle on, the Inverter turns on, which then powers the UPS and power My Device/recharges the UPS battery.

I don't like the idea of an inverter that is always on, because while that ensures My Device is powered, it also means the car battery will drain and die after a while. I don't want to have to rely of someone manually turning off the inverter when they leave the vehicle.

So, are there inverters that do this automatically? My thoughts are a car battery is usually around 12.8 volts, but when the alternator is on jumps to 14.4. Do they make inverters that either only turn on when the input voltage is say above 13.5? Or ones that you can program a voltage it measures to determine if it turns on or not? What about one that shuts off automatically at say 12.5 volts or so, so that it would drain the car battery for a little while but eventually shut itself off before killing the car battery?

Alternatively, what about a way to tie the input power of the inverter to a car fusebox spot that is only on when the car is ON or RUNNING? This makes sense to me, however I don't think I can put out enough amps/large enough cable on a fuse pigtail adapter?

Thanks in advance!

 

You could use a relay to control your main current to the inverter and take your "cpntrol signal" to the coil from something that comes on along with the engine. There's bound to be a connection in the fusebox that you could use to control your relay that only goes hot when the engine is running or at least the ignition is switched on.

1500 W is surely a huge inverter is it not? At 12 V I put that at 125 A. Can your alternator even supply that sort of current? If it can't, you're going to be running your battery down very quickly whether the engine is running or not.

 

You could use a relay to control your main current to the inverter and take your "cpntrol signal" to the coil from something that comes on along with the engine. There's bound to be a connection in the fusebox that you could use to control your relay that only goes hot when the engine is running or at least the ignition is switched on.

1500 W is surely a huge inverter is it not? At 12 V I put that at 125 A. Can your alternator even supply that sort of current? If it can't, you're going to be running your battery down very quickly whether the engine is running or not.

Good idea! So worst case scenario what you suggested is feasible and would work, appreciate it. I've actually done something similar before I don't know why I didn't think about that. I'm still going to try to find a smart inverter that can do what I want but nice to know I have a backup plan.

1500 W is beefy, but no I'm not actually going to use anywhere near that. My rough math puts the load of My Device at 400W or less. I'm assuming the UPS will have some amount of strain as well, both in losses and in charging circuitry. That is why I was shooting for a 1500W Inverter at least first try before I get some real measurements and perhaps spec hardware a little closer to reality. Also, worst case scenario of upgrading the vehicle(s) alternator(s) is a possibility, but I don't think it will actually come to that thankfully.

 

ok so lets count the conversions:

1. Car battery 12VDC
2. inverter steps it up to 120VAC
3. UPS input steps it down and rectifies it to 12VDC or 24VDC depending on model for battery backup
4. Inverter stage of UPS steps it back up to 120VAC
5. your device (assuming a camera/computer/DVR or other surveillance device - the only thing I can think of that makes sense) probably steps the voltage back down to 5VDC-12VDC inside its power supply.

If all those steps are assigned a generous assumed efficiency of 85%, then 85% * 85% * 85% * 85% = 52% overall efficiency.

So if your device really does draw only 400W, then moving backwards through the conversion steps, 400W * (1/52%) = 770W from the car battery.

770W / 12VDC = 64A drawn from the battery.

That's only steady state, not accounting for when the battery in your UPS is depleted so it is drawing extra current, which will probably be almost all of the time.

This does not sound like it would stay powered on inside your car for very long, and it also sounds like you are going to need an aftermarket alternator since you will be almost doubling the intended load on it.

This would be much more efficiently and simply implemented by giving 12V from the battery straight to the device without all these conversions.

If your device is just stepping 120V down to 12VDC inside its power supply, bypass the power supply. If it is actually using 120V (highly doubtful) then replace it for a 12V version.

Put a backup battery in the trunk. Connect the backup battery to the car battery via a relay which is powered from the IGNition line.

 

Best to fit a smart charge relay, they are designed to only charge / switch for leisure / auxiliary / second battery when the voltage goes above standing volts

Engine off 12.? volts = relay open
Engine running 13.? volt relay closed

Just connect the "out" that's normally connected to the second battery to the inverter

 

Many off-the-shelf inverters have remote turn-on feature. Big power connectors for the main power, then a small connector for a signal wire that turns the inverter on. Just connect the signal wire to a keyed power wire in your car, and the inverter will come on when you turn your key on, and off when you turn your key off. Audio amplifiers typically work the same way.

 

Or as mentioned above, if possible power your accessory from a second battery and separate it from the main car battery using a "battery isolator", available as an off-the-shelf device. The isolator allows both batteries to charge when the car is running, but "isolates" (separates) them so they discharge independently. So the accessory battery can be drained and the main battery will remain charged. Search Amazon, there should be many to choose from.

 

Or as mentioned above, if possible power your accessory from a second battery and separate it from the main car battery using a "battery isolator", available as an off-the-shelf device. The isolator allows both batteries to charge when the car is running, but "isolates" (separates) them so they discharge independently. So the accessory battery can be drained and the main battery will remain charged. Search Amazon, there should be many to choose from.

I think I heard about this set up from a guy who brought his truck to the shop for new brake and worx wheels installation. The extra battery is connected using a battery isolator and charging without affecting the main battery. I can't remember the the device he connecting to the extra battery but does its job.