extra 12v to alarm

How do I go about connecting an extra battery supply to the alarm when battery dies or is disconnected.

now is this a full size car battery which in turns should be connected to the alt?

thanks

Give it a go, thanks.

What if the alarm had multiple fused battery connections such as: 2 30 amp fuses, 1 20 amp fuse and 1 5 amp fuse coming off the alarm brain itself?  What fuse value should be used from the main battery to the backup battery?  And would the 4 amp diode be adequate for preventing the vehicle's electrical system from draining the backup battery?

Thanks for the info!

Quote: Originally posted by webguy on December 11, 2002

Typcal Backup batteries for auto alarm systems are "gel-cells", which can be mounted in any position. However, the preferable location is inside the car, not under the hood cause these gel-cells do not like the heat...............You should be able to find em on the Net, and you want something in the 1600mah or higher range. Connection is simple using a 1N4004 diode (4 amp rated). Connect the main alarm power lead directly to the backup battery (+) positive. Run a fused (10 amp) line directly from the car's battery (+) to the backup battery (+) positive. Now, right at the backup battery, cut that power lead about two inches away from backup battery, strip both ends and crimp a butt-connector,on each wire, then install the diode with the stripe (cathode) towards the backup battery. This will keep the back up battery charged and in the event the vehicle's battery goes dead (or, heaven forbid, someone deliberately cuts the battery cable), the backup battery will not drain off into the vehicle's electrical system.

Also, is the negative terminal of the backup battery hooked up to common ground or ???

Also, the IN4004 is 1amp rated at 400V (according to digikey), the 1N4001 is 50V at 1amp which is 2 cents cheaper!!!

Now, why are you chosing a 4 amp rating, from my knowledge 1, 3, 6, 12, 15 .... Amp rated diodes are available not 4 amp, is there a specific constraint with a 4 amp rating you are trying to uphold, from the specs on most back up batteries they are usually in the milliamp ratings, so I do not follow the need for the "4" amp rating, since your just preventing electron flow in the reverse direction to prevent the battery from draining.  Just intrested in the clarification, I have not done much with back up batteries. 

Thanks

Craig

Craig, your correct. My ignorance & my bad! The amp rating (1A) is the same for diodes 1N4001 thru 1N4007. The reason I stated such a high amperage rating, (which was an incorrect statement for which I humbly apologize, when I should have said "Peak Forward Surge Current") was for overhead, just in case the backup battery gets drained somewhat. The momentary intial current draw pulled by backup battery upon recharge could be greater than the "Peak Forward Surge Current" ratings of the 1N4001, which is 50 amps @ 8.3 mSec. as it is for the whole series mentioned above.   

So, in flapping my gums, I really should have said that I would use the "physically larger" 1N4004 for durability. Thanks for pointing out my oversight. Someone is paying attention......