Would This Setup Survice Crank?

**Blinky13** · 03-07-2005, 06:38 PM

Hi pr3d.

You are on the right track with your circuit. Just place the diode on the positive line and make sure the cathode (the side with line on a diode schematic) faces towards your backup battery. One other thing, make sure you have a diode that is rated for the type of current you are going to need. I would recommend no less than a 20A rated diode. Remember that when diodes are rated, they take into account that a great big honkin heatsink is attached to them.

Hope that helped. Good luck.

**bluehcc** · 03-07-2005, 10:36 PM

That setup with diodes on definely works. I have used that same setup with a 9Ahr battery, for a month without any problem. But now i have a different setup since i am worrying about forgot to turn off the inverter and the main battery will go dead. Rather than putting a diodes in between two batteries, I use a relay. The relay is tapped to accessory 12+. In case someone doesn't know, the purpose of accessory wire is to cut off the power to radio, amp and other miscellaneous device so starter motor get all the current to crank the engine. In other word, the power is cut between main battery and small battery when it is during cranking state. This way, computer is running from small battery for few second. After you release the ignition key, the accessory get 12+ again, and your relay connected the main battery and small battery. But keep in mind, don't use too small wire connecting all the device. When your computer run off the main battery, once your small battery is drained. The next time you put your key in, you are charging up the battery with infiniti current as well as powering up your computer, so your small wire will get hot might result in fire. My 10A fuse almost melt, so don't run off your small battery from main battery for too long, unless you use all big thick wires...

**happpyg** · 03-07-2005, 10:49 PM

Yep, thats the exact circuit I have been using for about 2 months now, battery out of a UPS & works great. Just tied about 4 6A diodes in paralell from a local electronics shop, one by itself got pretty warm so spread the load a bit.

Mine is on the positive line though

**MSUC5Vette** · 03-08-2005, 06:06 PM

You are better off putting the diode on the positive line.

I would also suggest the relay idea, especially if you plan on running the computer when the engine is off, this will prevent a dead main battery.

If you do plan on doing this you may need even more than 20A rating on the diode.

W/O knowing how efficient the DC-DC power supply you have is, you are pulling in the neighborhood of 90 watts / 12v = 7.5Amps for the power supply assuming it were 100% efficient and only supplying 12v. If you run the backup battery down and need to charge it, then that battery becomes a load instead of a supply, and will add current consumption to your power supply. 20A is probably enough, but always better safe than sorry. Also, make sure you have a fuse very close to the main battery on the lead going to the second battery, and another fuse close to the second battery. This way if the cable shorts you won't blow up the battery or start your car on fire. If the fuses are the same rating as your total diode that would also protect the diodes in the event of an over current draw.

**Curiosity** · 03-08-2005, 08:52 PM

I think KyferEz got me started on low voltage drop doides. I got this one at Mouser:
http://www.st.com/stonline/books/pdf/docs/5512.pdf
Have only tried charging a battery with it so far and didn't see much drop, but did notice as the battery reached capacity the voltage drop decreased to 0.