Thread: Maintaining battery while parked

I think you would be better off buying a battery charger

A computer PSU idea will not work. The PSU is regulated at 12 V your battery is at 12.3-12.6V. Current flows from high to low. The PSU won't do squat until your battery drains down below 12 volts. You need to have a power source that has a higher voltage rating than the battery. Alternators are rated at 14.4 when the engine is running. That's how the alternators keep the batteries charged while the engine's running. While the engine is running, a battery becomes an electrical load to the alternator because the alternator need to produce power to charge the battery. Same idea applies to the PSU. You need a power source that has a higher rating than the battery idle. A battery charger is not a good idea either. Chargers are for short term use. They are unregulated. When a battery is fully charged, the charger's voltage will start to clime. Unregulated charger puts out it's rated amperage. In order to maintain the amperage output on a fully charged battery, the voltage goes up, as high as 17volts. Trickle chargers don't help either. They put out 12.6V at 1-2 amps. A computer pulls anywhere from 5-40 amps depending on the computer PSU. high efficient DC-DC (Opus, MPBS1) pulls about 5 amps. Low efficient AC PSU with DC-AC inverters pull more than 15 amps possibly 30 amps. If you're looking to maintaining the battery for short term like an hour or less, a regular charger would be your cheapest way. For mid term solutions, a second battery with isolators. That should last a few hours depending on what accessories you're running. If you wanna go all out, you can get a 40+ amps switching power supply. It has a voltage dial with amperage and voltage meter. set that thing to 13V and you can run your accessories for days. The power supply will put out the amperage required to maintain the 13V voltage. Once you're done running the accessories, undo the power supply and your vehicle will start up like it has a brand new battery.

First off OMG registered back in Oct 2002 and this is your first post!?!?

Originally posted by Trottingwolf
It all depends on the power supply you have for you computer. If you have something like an OPUS then that power supply will work at less than 10V and should be just fine for playing music for at least an hour or more.

He's looking for a way to run the ACC, not necessary just the PC.

If you have a sproggy type powersupply then you have to check and make sure the 12V supply for the spoggy does not get too low, but the 5V should be fine.

Are you talking about the voltage going into the sproggy? or the output? He originally wants to rig up a battery maintainence power supply from an ATX PSU which he cant b/c the 12V line of the PSU is regulated at 12V and a car battery is at 12.2-12.6V The ATX PSU wont do anything for him until the battery runs down to below 12V.

With this setup I was able to run my player for about 3 hours and the battery stayed at 11.8V the whole time.

PC alone will run for at least 1-3 hours. PC and a high powered stereo sytem won't last more than an hour.

Bottom line is that he needs to get a regulated power supply that puts out at least 13V and at least 10 amps. Rat shack had a 25amp 13.8V PSU for about 125. That'll maintain all of your ACC for hours even if you have a high power systeo system.

I think you got the wrong idea on what ATX PSU he and I are talking about. He wants a regular AC style PSU and rig it so that the 12V line could maintain his battery. Im saying that the battery would have to run down to 12V before the AC ATX PSU will start to maintain his battery. I was recommending him to get a AC input with 13.8V output regulated PSU so that he could maintain the battery at a constant 13.5-13.8V. He could listen to his stereo cranked for hours and hours and it would never kill his battery. BTW Welcome back

Originally posted by Zip-Lock
Would it be able to keep it from dropping below 11.5 volts?

It depends on what you are pulling out of it.

If the old power supply you are talking about runs off DC then it depends on what the lowest input voltage the power supply will run off is. If you are using an inverter to give the power supply 120V then it would only matter how low a voltage the inverter will run at, which should be around 10V-11V.
Most car accessories will probably work down to about 10V because all the electronics in them are made to work in the cars power range of about 9V-11V up to 15V-16V. Most computer power supply are made to run in a much smaller range because they are meant to be powered with a more reliable power source. Depending on how the power supply is designed it might give a good 12V output with only 10V or 11V in or lower. You really just need to hook it up and try it to see if it works with that low voltage in or check the specs on the supply if you can find them.
If the power supply works with an input of less than 12V then you should be fine if the car system drops even as low as 11V. The reason you car battery is at above 12V when you turn off the car is because the battery is charged up by the alternator to something like 13V-14V, but the battery can't hold that voltage very long because it is not made to be at that voltage. But when it get down to 12V that is the batteries normal operating volts and it should stay in the 11-12V range for a while, dropping slowly the whole time when you are running stuff off it. If it all works at the lower voltage then the only thing you really have to worry about is that you don’t run the battery down to where you can't start the car.
If the power supply won't give 12V output with an input lower than 12V then the way it is designed just wont work, and in that case you would need to get some sort of power supply to keep the cars electrical system above 12V or another power supply.
I would say just hook a volt meter to the 12V output on the power supply and try it to see if it works. If you can get two Vmeters I would also hook one up to the car power and see what it is at. If it works just run you music until the car voltage starts to drop below 11V. If it runs long enough for you then you should be just fine. If your stuff drains the battery too quickly then you need an extra battery or another power supply to plug into the wall when your car is off.