Need help, second battery in vehicle!

[mrfrostyman]

Im looking for some help on this. I have a unit that runs off of 12 v. I also have a small .8ah 12v battery hooked up to this unit. Also i have a constant 12v source going to this battery and unit. The battery has a diode installed so that the 12v source from the vehicle only goes to the battery and unit and not the battery back to the vehicle. When the vehicle is running, will the amps from the 12v source kill the battery, or will the battery automatically draw only what it needs. Heres my crappy diagram of what i am talking about.

http://img145.imageshack.us/img145/9792/batteryandunitmu7.th.jpg (http://img145.imageshack.us/my.php?image=batteryandunitmu7.jpg)

Thank you

[rc51]

Im looking for some help on this. I have a unit that runs off of 12 v. I also have a small .8ah 12v battery hooked up to this unit. Also i have a constant 12v source going to this battery and unit. The battery has a diode installed so that the 12v source from the vehicle only goes to the battery and unit and not the battery back to the vehicle. When the vehicle is running, will the amps from the 12v source kill the battery, or will the battery automatically draw only what it needs. Heres my crappy diagram of what i am talking about.

http://img145.imageshack.us/img145/9792/batteryandunitmu7.th.jpg (http://img145.imageshack.us/my.php?image=batteryandunitmu7.jpg)

Thank you

You may risk overcharging the smaller battery. Ultimately, you should have some sort of battery isolator in the circuit to allow for charging the batteries separately and only when they need it. However, most iso's I know, typically 'assume' or state the the 2 batteries be as equal in size as possible, unless you opt for one of the 'intelligent' (microprocessor based) units out there, they might do the trick.

[mrfrostyman]

im guessing that it is pretty expensive then eh. So say i hooked this up to a vehicles cig lighter. And that lighter has a 15 amp fuse, what determines how many amps go into the battery? Cause i am only running this unit + battery spliced into the car circuit. The 12v source could be a cig lighter or even the 12v constant going to the ignition switch or a 12v pole in the fuse box(in a truck) Does this have any bearing on this?

[mrfrostyman]

anyone? Is this in the wrong topic section?

[ah802]

The recharge voltage is a bit higher... ~14 volts, this is what enables recharge. Less than 12 volts when running is a diagnostic for charging circuit failure. If both batteries are topped up, and your charging system can handle the flow... you could be good to go. The issue is what do you want the second battery for? If you try to actually use it in place... (as a starter system) the smaller gauge wires will not be able to handle the current flow (read melted wiring). If you want it to just augment the system to listen to your sound system, you might be asking for trouble as the draw will not be equal unless you have a sophisticated balancing system (although at a lot of car shows, I've seen this used). Over all, this extra battery will be a larger drain of the system and will inevitably cause problems.

If it ain't broke, don't fix it.

[mrfrostyman]

The extra battery is a very tiny addition. Basically, it will be inside a box with a unit(lets say gps) The box itself is portable so the battery and the unit are detachable. The battery is there to keep the unit powered at all times, even when the box is disconnected. So to the box there is power, ignition and ground. The power is spliced to a 12v constant on the vehicle. The unit itself draws no more than 400 ma and will run at 10ma when ignition is nolonger introduced. So the battery can last a couple of hours which is fine. I wanted to use a diode so that the battery itself will not send any current to the vehicle and the vehicles circuits will charge the battery(the battery can charge at 14.4v) and run the unit. Now my concern is how does the battery know only to draw what it needs to charge itself. The voltage will be fine, its the current im worried about. The cars batteries is prob 65 amps at least. The battery in the box is .8ah, so will the battery damage itself if it is connected in parrell with the cars circuits, or will it run perfectly fine the way its supposed to. Basically i want it like a cell phone. When the cell phone is plugged into a cig lighter, the car charges the phones battery and lets you use the phone, when the lighter is disconnected the battery takes over.

[dealhunting]

You will need some kind of isolator. Most "plug in" accessories have a built in charger that will stop charging when the battery is fully charged. This goes for cell phones, laptops, etc. Depending on the car, most modern cars will use some sort of voltage differnetial to determine charge rate. This will be controlled by either a voltage regulator or control unit, maybe both.

Since it appears you are contructing a simple parallel circuit, your "control unit" will most likely overcharge the smaller battery as it will attempt to charge the larger battery to capacity, unless you add some kind of "intelligence" to the circuit. Picture a shot glass and a large pitcher. Place one under the tap and then the other. Which one fills up 1st, and what happens to the shot glass if you try and fill it with the same amount of water??

[mrfrostyman]

Thanks for the help, im off to try to find somekind of regulator, Usually it doesnt have a battery, but this time they want it portable, and it needs to have this battery. Good example, any idea of how much a regulator like that would cost?

[dealhunting]

To be honest I don't think an automotive application will work for you since most applications involve amperage not milliamps. You need to visit an electronics parts store. Start with the type of battery you're trying to charge and go from there. Li-ion requires a special charger. Ni-Cad is fairly standard but will have memory issues.....

What you are asking gets further complicated as most chargers convert 120v to the required voltage. You need a 12v source......

Would be much easier to simply hook up an external battery source that will need to be charged in house, unless you want to include a volatage inverter into your plans......

[mrfrostyman]

im just using a regular lead acid sealed battery. Thanks for the help.

[mrfrostyman]

this seems like it might now work out now, hmm, i would love to have an external charger but i would need a relay then, and it just adds steps that the people need to take like making sure that they plug it in etc... Thanks for all the help hough.

[stealth]

anyone? Is this in the wrong topic section?

No, I think it might just be the worng board for this kind of question. A lot of boats have 2 batteries, saw a show last week called Ship Shape where they were talking about this very thing. Might want to check some boating/marine forums.
Or some high end car audio forums.

[Tinted]

it will work just fine, all u need is a battery isolator.