How long will a car battery run this stuff?

Football season approaches and tailgating parties with it. We’ll be heading down every Saturday (well, and some thrusdays) to celebrate the victorious local college team. Since we like to watch as much football as possible and throw some great parties, we’re going to be bringing along some more stuff this year.

A tv, DirecTV dish, and receiver.

I have an extra dish, and an activated receiver. In addition, I have an old 21" CRT that we’ll be toting along. For power, we’re planning on running a car battery. It looks like we’ll be drawing about 100Watts with the receiver and TV (through an inverter).

Does anyone know how I can figure out the time I’m going to get from a standard car battery?


IANAn electrician, but I think you need to know what amperage you will need as the car battery has an amperage rating per hour that will linit the time it will run all that. You may have to run off the battery in your car and start and run it now and then to recharge.
Perhaps and electrician will chime in and tell you how to wire the extra battery into your car’s system, or you could get a power inverter installed in the car.

100 watts using 12 volts would run 8 1/2 amps. Of course, there wil be variations on those figures, but that should be ballpark. Now you need the ampere-hour rating of the battery. I’m pretty sure that once the voltage dips to a certain level, the equipment won’t work, but I don’t know what that level might be nor how long it would take to reach it. My guess would be you’d get 3-4 hours useful power here.

I will suggest that a deep-cycle (marine) battery is better suited to this use than an automotive battery. A car battery probably wouldn’t last the season being used this way.

Instead of using the battery you might consider buying a very small generator and a long power cord so you can place it where you won’t hear it run.

Exactly, you can get a small Honda (quiet) for a couple hundred bucks. They’re really quiet at that size too.

But as far as using the car battery goes, you might want to consider a backup deep cycle marine as GaryT suggests. You can connect it in parallel to your car’s charging/run system when you’re not using it and when you want to run your electronic equipment you’d take it out of the circuit so as not to drain your primary auto starting battery. Just a couple of cables is all you’d need to make the connections. Of course, you’ll have to route them from the engine compartment to the trunk and then mount the reserve battery in a safe and rated enclosure.

Don’t connect batteries in parallel in a vehicle.

You can install a second battery using a battery isolator that will allow you to have a second battery hooked to the car alternator for running auxiliary equipment.

I’ll leave the other options to other persons. I don’t want you to do the parallel battery thing, thinking it’s a good thing to do.

My F-350 Diesel has two batteries in parallel…

Unless it’s been beat to death or stolen, those little red ultra-quiet “EU” series Honda generators will cost about a buck a watt, and the smallest unit is 1000 watts. If you’re lucky, you might find it on sale for $850-900. They’re excellent machines stuffed with all manner of technology and careful engineering to make them run near-silent and the price reflects that.

It is possible to retrofit some generators to be less noisy. I’ve got a “Supertrapp” muffler on my 5700 watt generator, and “sound board” (compressed sugar cane) panels stood up around it absorbs a lot of noise as well.

If you’re running things off a vehicle’s battery, the bigger concern will be "How long can I run this stuff and still have juice to start the engine later? As **Gary T ** said, you’ll be better off using a “deep cycle” battery. Car batteries are built to deliver a huge wallop of power to the starter for a few seconds, rather than be steadily run down to near flat over a few hours and re-charged. Do your vehicle a favor - get a deep cycle battery, charge it up at home, then when you want to fire up the TV, hook it up and watch TV until it runs out.

Both your batteries have been placed there at the same time in a system that is meant to run the car. One bad battery will kill the other. The car isolator directs charging to the batteries individually as they need it, and you can not discharge the car systems battery , by leaving the television on. One bad battery doesn’t ruin the other.

How exactly, does one bad battery ruin the other?

Yeah… I thought about stealing one of my Dad’s marine (deep cycle) batteries, but decided it wouldn’t be worth the heartache.

We’re not actually going to be running off a car. Our tailgate spot is a couple of hundred yards from where we park, and unless we can find someone else with keys to the forestry building this year, we won’t have an outlet nearby.

The idea is to run this stuff for about 4 hours a day once per week for about 7 weeks (spread out over the season). In between games, I’d bring it home and trickle charge it for the next week. I know that constant drain and charge cycles aren’t really great for a battery, but I didn’t figure it’d be too bad for those games.

The price difference is the real reason I didn’t automatically go for a deep cycle, but I may end up there. I’ll head down to Les Schwab (local tire and battery place) today and see what they’ve got.

As to the generators… I went camping last weekend with the inlaws and they had one of the whisper Honda generators there… just a really nice, quiet piece of machinery, but a little more cost than I want to absorb when I won’t really use it for anything but this.

The other thing is that we’re trying to keep this as cheap as possible. Mostly becuase a college campus isn’t the greatest place in the world to have something of value. We’ll have our box locked down and chained to a tree… but college kids have ways of getting that stuff.

I don’t doubt that for the high end name brand, but cheap knockoffs can be had in the $100-$200 range, I’m pretty sure I saw one for $119.97 not so long ago. I have no idea about the noise level.

The dead battery will be a lower voltage, depending on the number of dead cells. Take off 2 volts for each dead cell. The good battery will discharge into the dead or dying one. During charging the good one will be over charged. All this shortens the good ones life. You can have an explosion or or melted battery in the extreme cases.

You must connect two batteries with the same discharge and charge characteristics if no isolating device is used, or you risk the same consequences.

You can count on the noise level being HIGH. Making a portable generator quiet is a lot more difficult - and expensive - than making it powerful.