Working on my truck. I'm planning on installing an old school "driving" computer. Without going into a lot of detail about the unit, I need some help. In wiring up the unit, I need constant batt power, and ign switched battery power. The constant power is for "keep alive" memory. I would like to add a backup keep alive power source, in case of a dead battery or needing to disconnect the battery while doing other work on the truck. I'm thinking of a rechargeable battery connected to the truck battery positive with a diode in the wire. That way if I lose truck power the backup battery will power the computer, but the diode will prevent discharging the backup battery, and will allow charging the backup battery.
My question is: What kind/size battery can be used for backup (min 10 volts- max 12 volts nominal)? What size/type diode would be best?

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Ordinary AGM security system/server backup battery like this, but you'll need to figure out a way to comply with its max charging current limit.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Charlie I am the last person to give electrical advise but here goes! When I had an inverter mounted in my company truck, it was set up to do exactly what you said. I had 2 large batteries mounted in the back to run my 110V inverter and they were charged off the vans alternator. But there was an isolator mounted between the trucks battery/charging system and the inverter batteries. I think the isolator came from the local RV shop. So they should be available most anywhere.

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

I don't think I need that much AH. I only need to power the Keep alive memory, in case of a short term main battery issue. I haven't checked the amount of currant draw, yet. I was thinking something like a rechargeable battery from an existing piece of electronic gear. But I don't know what is available. This is similar to what I am going to power.
https://www.ebay.com/itm/395078725110?_ ... %3A4429486

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

I don't need any think that "heavy". I'm guessing the current drain from the keep alive memory, is just milliamps. I have to wire it up first then I can check the actual current usage.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

I knew I would be no help!

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Oh! That's all? One of these might do the job fine. Takes a regular 9-volt battery.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Think I have this figured out. The "keep alive" circuit will be powered by this aux battery (don't need this large but is the smallest SLA i can find) https://batterysharks.com/12-volt-5-5-a ... ttery.html The positive of this battery will be connected to the vehicle battery through a 1N5404 power diode (400 volt@3 amps). The negative to ground. That will allow the vehicle to keep the aux battery charged, but the diode will prevent the aux battery from discharging through the vehicle electrical system. easy/peasy simple and inexpensive.
Bought the diode today, 50 cents, and ordered the battery.
PS: I was told by several sources, that I really should use the lead acid type battery, as other types don't play well with automotive charging systems.
PPS: Dan, your first battery suggestion was correct. But I am using one as small as I could find

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

That should work great! Make sure the diode is rated for enough current.

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Matt Cramer
1966 Dodge Dart turbo / EFI project

The diode is rated for 3 amps, but I got a "2" pack, If necessary, I can put two in parallel. But I don't think the battery can handle that charge rate.
I just wish the weather would warm up so I can install the new windshield. Then I can go to work installing this driving computer and backup battery system.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

3 amps is probably plenty.

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Matt Cramer
1966 Dodge Dart turbo / EFI project

You can warm up quick... Come to the Midwest and go back... Florida conditions will seem better to ya.....

It's not that cold that I can't work. The problem is the metal gets cold overnight, and makes it hard to get the urethane sealer to flow properly (I'm told you should heat the urethane to about 140 degrees while still in the tube). Plus the rubber gasket is harder to work with.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

When I had my new windshield installed about 7 or 8 years ago, the guy said he was not going to install any sealer. I thought he was crazy but he promised to fix it if it leaked! As soon as the truck was back on the road I took it to the car wash and ran it through. It was perfectly dry then and hasn't leaked since. Go figure!

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

I have never installed a windshield, so just going by this video. Made by the same people as my gasket. I did have a lot of rust at the top around the drip rail (above and below). Now repaired, but want to make sure no leaks.
https://www.youtube.com/watch?v=I4uqkEak-Qg&t=721s

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225