electrical/electronic gurus

-
C

Charrlie_S

Well-Known Member
Joined
Oct 17, 2005
Messages
4,651
Reaction score
2,761
Location
Silver Springs, FL
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?
 
Thanks, but I do have a couple of those type batteries. They are much bigger than I need for this application. I'm looking for something I might be able to mount behind the dash. Doesn't need a lot of AH capacity. It would just be to keep the memory alive, so I don't need to recalibrate the computer any time the main battery might go dead.
 
I realize that, but anything other than an SLA will need some kind of charging circuit. a small nicad may also work.
 
Couldn't you just throw one of these in your glovebox with a 9v battery? Or use a NiCad or Li-ion 9v battery on it and just leave it plugged in?
Simple, and works for me for saving radio settings when I have to pull/disconnect the battery.
1703716742018.png
 
Professor Fate said:
Couldn't you just throw one of these in your glovebox with a 9v battery? Or use a NiCad or Li-ion 9v battery on it and just leave it plugged in?
Simple, and works for me for saving radio settings when I have to pull/disconnect the battery.
View attachment 1716183792
Click to expand...
That's basically what I had in mind. However I would like it to be "hard wired" , and I think if using a diode The battery would stay charged, but not discharge if the main battery had a problem. Also I was thinking even if the 9 volt battery was enough voltage for the computer, I would be concerned that the difference between the 9 volts and 13+ charging voltage might be excessive.
 
This could be a problem. The project, I mean. You really need a dedicated charger for the backup, and fully isolate the car system and the backup battery. This is because almost NO batteries, other than lead acid, are natively happy running off a car 14V system, which it really is. (Warm, running, the system is 13.8--14.2 nominal)

I no of no simple way to do this, other than to find a small lead acid battery, which you might be able to do. In that case you could use 1 diode in the main vehicle line. You would want the banded end of the diode towards the unit you want to back up and the opposite end coming from the vehicle power system.

Anode coming from vehicle system, cathode feeding to the backup unit. Backup battery connects on backup unit side of battery.

Diodes-1.png
 
The second strategy would be to obtain a battery close to 12-14v whatever you think will work, for which you can get a designed for purpose vehicle powered charger. Then, easy. Connect the new backup battery ONLY to the new device, and connect the vehicle electrical system to the charger input.
 
What's the minimum voltage the trip computer needs to operate? A small lead acid battery is a good choice if using a diode as the charging demands are simpler. Another option might be to take two USB phone battery banks and wire them in series - they would supply 5 volts each, so that could work if your device works on 10 volts.
 
MadScientistMat said:
What's the minimum voltage the trip computer needs to operate? A small lead acid battery is a good choice if using a diode as the charging demands are simpler. Another option might be to take two USB phone battery banks and wire them in series - they would supply 5 volts each, so that could work if your device works on 10 volts.
Click to expand...
The phone batteries sounds like what I want to do. I don't know the minimum voltage requirement for the computer, but the trouble shooting page lists a possible problem if the voltage to the unit drops below 5 volts during engine cranking. I would assume 10 volts would be ok as an emergency backup. The unit needs to be calibrated to the vehicle.
1: pulses per mile
2: fuel flow meter to actual fuel usage
3: size of the fuel tank
4: clock time
If memory power is lost these all need to be recalibrated
The more you drive the more accurate the outputs become, as you can "tweek" the calibrations.
There are several different versions. I have a ZT-1 with cruise control, a ZT-3, and a ZT-4 which is on my 64 Valiant.
I also have a Direct Connection P4349443 performance timer, that was made by Zempco.
This is similar to the units I have. 1979 Zemco ZT-3 Driving Computer NEW OLD STOCK Complete MPG MPH Fuel | eBay
 
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.
Thanks for the suggestions
 
You must log in or register to reply here.
-

Back
Top