ok electronic engineers, heres a ?? for you

[cammeddrz]

i need to charge a much smaller battery on board my truck that is 12vdc but can only take .5 to .75 amps or so.

what diode, or resistor or whatever else do i need to install on the wires that are charging the battery? i need to keep the 12v of the existing charging system, i just need to add a leg to the charging system that has .5 to .75 amps or so

 

[Surrey]

What is this battery for, exactly?

 

[cammeddrz]

What is this battery for, exactly?

the answer you seek is irrelevant.

 

[Legend]

If you hook a 24 ohm resistor across the 12v battery, .5 amp of current would flow. So if you put that 24 ohm resistor in series with the small battery you are going to charge, the current would be limited to .5 amp max (if the small battery were a complete short). if the small battery were sitting at say, 10 v then the current would be limited to (12v - 10v) / 24 ohm = .083 amp
so i guess a generic formula would be:
12v - <small battery voltage>v / .5 amp = value of limit resistor

 

[cammeddrz]

so....a 24ohm resistor connecting + to - of the leg attatched to the battery? would i need to put a diode to prevent some kind of back-flow issues?

 

[Ranger Sport Honduras]

a diode would be a good safety addon yes

 

[cammeddrz]

does the diode go in line with +? or across + to - ? when i walk into radio shack what do i ask for?

 

[doorgunner]

orrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr......go to radio shack.....look for a phone car charger that puts out the charging volts/amps that your battery requires

 

[Ranger Sport Honduras]

inline with + diodes usually have a one way only flow. and the negative terminal is marked by a paint line. that way u know ur connecting it for voltage to flow to the battery, but it wont flow back from the battry to your trucks system. draining your battery or causing a back flow short circuit.
a 1 amp diode would be enough. probably 1/2 or 1 watt.

 

[cammeddrz]

orrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr......go to radio shack.....look for a phone car charger that puts out the charging volts/amps that your battery requires

12v is much tougher than you'd think, almost no portable device operates on that voltage

 

[adsm08]

If you hook a 24 ohm resistor across the 12v battery, .5 amp of current would flow.

If you run a 24 ohm resistor across a 12v battery .5 VOLT will flow. The amount of amperage that can get through will be higher because car batteries and alternators are rated in hundreds of amps.

Ohms Law: It takes 1 volt to push 1 amp through 1 ohm of resistance.

Resistors don't drop amps, and neither do diodes. A diode is an electrical check valve. It will keep voltage from traveling in one direction on the line it's installed on.


In many cases a load will only draw the amount of amperage that it wants. If you can find a .5 or .75 amp fuse, install it inline with the battery. That way the circuit can't handle more than the battery.

 

[doorgunner]

If you run a 24 ohm resistor across a 12v battery .5 VOLT will flow. The amount of amperage that can get through will be higher because car batteries and alternators are rated in hundreds of amps.

Ohms Law: It takes 1 volt to push 1 amp through 1 ohm of resistance.

Resistors don't drop amps, and neither do diodes. A diode is an electrical check valve. It will keep voltage from traveling in one direction on the line it's installed on.


In many cases a load will only draw the amount of amperage that it wants. If you can find a .5 or .75 amp fuse, install it inline with the battery. That way the circuit can't handle more than the battery.

forget what I said....I like his answer better....bring down the volts--then fuse it...(I think)

 

[adsm08]

forget what I said....I like his answer better....bring down the volts--then fuse it...(I think)

Actually, I'm saying fuse it and see if the battery will only draw what it wants. Don't put in anything to drop the voltage at first.



The key issue here is voltage. Voltage is a form of pressure. If you have a 3 gallon air tank with 70 PSI and you have another 3 gallon tank that is empty, and you hook them together, air will stop flowing when they both get to 35 PSI.

Similarly, trying to charge a 12V battery with a .5 volt line, while not impossible, is in feasible. This is going to work best if the full 14 volts of the alternator can be delivered.

 

[robertc1024]

Wait a sec, Surrey's question is extremely relevant. Depending upon the type of battery you are trying to charge, there is a huge difference. If it's a lead acid and you pump too much current into it, you can boil off the electrolyte and fry it. If it's a nicad, you can make it go "BOOM", if it's lithium-ion you can make it turn into a fire. The issue isn't so much the voltage but the current. You have to limit the current going into the battery for the specific type of battery.

If you want a good answer, you must tell us the type of battery, it's size (mAH), how fast you want it to charge etc. There are a lot of variables in properly designing battery charging circuits.

 

[gribly]

NO. DO NOT PLUG IT DIRECTLY INTO YOUR CAR FUSED OR UNFUSED.

DC to DC charging is more complicated than that. You can set up a little fused harness and have it work great for a year or so, and one day you're going to plug your little 12v battery in - and have it blow up in your face. Now, while I might find this hilarious - especially if it happens to a guy who stuck a Chevy frame under his Bronco, I don't wish that on anyone.

The reason it may work has already been stated by adsm08. What he left out is what will happen if the voltage is too low on your smaller 12 battery. The amperage might spike, the battery will get real hot real fast, too much gas will form too quickly - BOOM.

Here is a link to build your own - http://ludens.cl/Electron/12Vcharger/12Vcharger.html

Here is a link to buy one - http://www.powerstream.com/DCC.htm