Help with building a battery back up.

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Neyd3400

Member - Posts: 7

Joined: October 14, 2002

Location: United States

Posted: March 25, 2004 at 10:13 PM / IP Logged

Since spring finally hit the midwest, I am looking to upgrade my truck. I would appreciate help with building a backup battery (BUB) to my POS BW FM7150 with 3 sirens. I could buy one, but thats 3X more expensive and not nearly as rewarding.

I figure a 12V 7ah battery placed between the alarm's 12V constant and the car battery would work. I am worried about the BUB discharging immediately from lights and other stuff once the car battery is disconnected. I read that diodes are one way valves fro current. Is this what a diode would be used for.

Javelin3o4

Copper - Posts: 239

Joined: August 22, 2003

Location: United States

Posted: March 25, 2004 at 10:26 PM / IP Logged

its_radio_shop

Copper - Posts: 76

Joined: March 16, 2004

Location: United States

Posted: March 25, 2004 at 10:41 PM / IP Logged

Yes thats what a diode is for. You could hook your battery up and put a diode inbetween it and the rest of the vehicle. Because of the cost of a large diode (you would need more than just a 7amp diode) I would wire up a diode and a relay. The diode would allow the battery to charge and you wouldnt need a diode with the higher current rating. Connect the relay to the ignition line in the car and the back-up battery will only pull current when the vehicle is running so it wont be draining your main battery while charging. When the vehicle is off the back-up battery is disconnected so if you leave the headlights on it will drain your main battery but your back-up battery will be fine.

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auex

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Joined: December 23, 2002

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Posted: March 25, 2004 at 11:27 PM / IP Logged

IMO it would be worth it to spend the money on a pre-made BUB system, they can't be that expensive on ebay or other sites. The isolation ciruit and voltage drop alert would be worth the extra money alone. If you wire a relay for this it would pull current possibly draining your battery over time and could cause a dead battery in a week.

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Neyd3400

Member - Posts: 7

Joined: October 14, 2002

Location: United States

Posted: March 25, 2004 at 11:55 PM / IP Logged

Wow, I didn't expect such fast replies, Thanks. I am little confused about diodes. I did a google search and got 1000's varieties. How would I know what kind to use, especially for car applications. Why would I need more one rated a 7amp.

its_radio_shop

Copper - Posts: 76

Joined: March 16, 2004

Location: United States

Posted: March 26, 2004 at 11:46 AM / IP Logged

The relay setup wouldnt drain the battery because the relay installed as I mentioned would only connect the battery to the charging circuit when the vehicle is running. When the vehicle is off, the relay would be at rest and disconnecting the back up battery from the system.

The reason you would need one higher than 7 amps is because the actual output of the battery far exceeds 7 amps. 7ah only means it will put out 7 amps for one hour before it is drained. So lets say you left the headlights on in your vehicle and say they draw 15 amps. The battery will provide the 15 amps to run the headlights just not for as long. A 7amp diode will be toast trying to isolate a 15amp current draw.

Neyd3400

Member - Posts: 7

Joined: October 14, 2002

Location: United States

Posted: March 26, 2004 at 6:13 PM / IP Logged

Thanks, for the Help.

xetmes

Silver - Posts: 586

Joined: May 18, 2003

Posted: March 26, 2004 at 6:15 PM / IP Logged

wow there was a topic exactly like this a while ago, i think it was for a car stereo try searching to see if it is still around.

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Help with building a battery back up.

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