I'm having problems with the battery backup circuit.

Thread Starter

ford.man.one

Joined Apr 3, 2016
11
I'm hoping somebody can look at my crude schematic and tell me what my problem is. It's a battery backup circuit to keep a couple of things with volatile memory alive when the main battery is cut off. It's a 12 volt system in a semi truck and both batteries are lead acid. I've built the system as drawn, and it mostly works. The problem is a minor one but if I cut off the main battery before I turn the ignition switch off, somehow the backup battery keeps the relay on. Once I turn the ignition switch off, everything except what's being backed up goes off as intended and turning the ignition switch back on does nothing. My intent was for the relay to kick off if the main battery was cut off, and from looking at my schematic I can't figure out why it's not working that way.
PXL_20220702_212253295.jpg
 

crutschow

Joined Mar 14, 2008
30,102
If it's wired as shown, I don't see how the backup battery could keep the relay closed.
Are you sure the top diode is connected in the right direction (cathode band on diode towards backup battery)?
 

Thread Starter

ford.man.one

Joined Apr 3, 2016
11
If it's wired as shown, I don't see how the backup battery could keep the relay closed.
Are you sure the top diode is connected in the right direction (cathode band on diode towards backup battery)?
Yes, I'm sure. I quadruple checked that. Plus, if that were the case, turning the switch back on would engage the relay, wouldn't it?
 

sghioto

Joined Dec 31, 2017
3,315
I agree with crutschow either the top diode is in backwards or shorted assuming the circuit is wired as posted.
Why do you need that diode and what is the other diode with the resistor for.
 

Thread Starter

ford.man.one

Joined Apr 3, 2016
11
I agree with crutschow either the top diode is in backwards or shorted assuming the circuit is wired as posted.
Why do you need that diode and what is the other diode with the resistor for.
The big diode is to isolate the backup battery so that it can only charge from the main battery and not feedback into the circuit and keep it live whenever the ignition switch is off. The second diode and resistor are there and sized properly to allow about 15 milliamps around the relay to keep the memory in the things I'm trying to back up alive while the relay is in between poles during switching. It may be unnecessary, but I don't think it's hurting anything.
 

Thread Starter

ford.man.one

Joined Apr 3, 2016
11
Are you sure the contacts on the relay are wired correctly? What relay are you using?
It's an automotive relay. Just your standard 30 amp relay out of any older car. I'm confident that it's wired correctly because it does what it's intended to do whenever I turn the switch. It clicks on when I turn the accessory switch on and clicks off when I turn it off.
 

sghioto

Joined Dec 31, 2017
3,315
Here's your schematic redrawn as how I see the relay connected.
Shown with the ignition switch OFF.
1656806373200.png
 
Last edited:

Thread Starter

ford.man.one

Joined Apr 3, 2016
11
Only in so much as closing the ignition switch does nothing if the main battery is cut off first. If the diode were in the incorrect direction, closing the ignition switch would power the circuit from the backup battery even if the main battery were cut off. It doesn't behave that way. Also, if I were to disconnect the backup battery and check the voltage on the leads going to it I should see no voltage when the ignition is on and the main battery is on because the diode would be blocking it. That is also not the case. I see voltage on the leads going to the backup battery no matter what if the main battery is connected as intended.
 

sghioto

Joined Dec 31, 2017
3,315
The only way I can see the relay staying ON when the main battery is disconnected is the relay contacts are wired incorrectly as shown in this schematic. With the ignition switch ON the relay coil is also connected to the backup battery through the closed contacts #30 and #87. Disconnecting the main battery has no effect on the relay coil but when the ignition switch is open the relay drops out and the contacts now close #30 and #87A. This explains when the ignition switch is closed again the relay does not activate, until the main battery is reconnected.

1656811302384.png
 
Last edited:

Thread Starter

ford.man.one

Joined Apr 3, 2016
11
The only way I can see the relay staying ON when the main battery is disconnected is the relay contacts are wired incorrectly as shown in this schematic. With the ignition switch ON the relay coil is also connected to the backup battery through the closed contacts #30 and #87. Disconnecting the main battery has no effect on the relay coil but when the ignition switch is open the relay drops out and the contacts now close #30 and #87A. This explains when the ignition switch is closed again the relay does not activate, until the main battery is reconnected.

View attachment 270585
No, that didn't solve it. When I switched 87 and 87A, turning the main battery off caused the relay to oscillate. Turning the ignition off stabilized the relay but the backup battery was powering the circuit. I'm going to switch it back and leave it as is. It will serve the purpose I designed it for.
 

MisterBill2

Joined Jan 23, 2018
12,399
Here is a guess based on some experience. The power that is keeping the relay on is probably not from the backup battery, but from some other load item that holds a charge for quite a while, and might even include a battery of it's own. Also, that relay may have a much lower release voltage than you think it has. There may even be a bit of leakage current through some item that also has a constant battery connection, like a radio.
Evidently I did not adequately examine those last few posts..
 
Last edited:
Top