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Lights still on after disconnecting battery

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Hello everyone!

I've been doing a lot of work on my 91 this summer (steering wheel upgrade, ABS upgrade, replacing all coolant hoses, installing hugabuga's window repair kit, realigning windows). Because I've been stacking my projects, I've been disconnecting and reconnecting my battery several times. There has been a few times when I've disconnected the battery and the lights inside have stayed on. But once I press the seat adjustment, they kick off and there's no more power. One of those times I decided to turn the key to II then everything started flickering (lights and dash). I also heard the main relay engaging and disengaging several times. I've never experienced this type of issue before. Why would this be happening? Is it possible the main relay is going out? Or is this something unrelated to the main relay?
 
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"There has been a few times when I've disconnected the battery and the lights inside have stayed on."

Which lights have stayed on? The interior lights or some / all of the dash lights? With the battery completely disconnected (disconnecting the negative terminal is the safe way to do this) it would be pretty hard to power up the interior lights unless you have developed some sort of inter dimensional energy transfer mechanism or you have some aftermarket device such as energy storage capacitors for an aftermarket stereo system. If you had some energy storage capacitors, trying to operate the power seats would immediately drain them.

"I also heard the main relay engaging and disengaging several times. "

Relays typically chatter if there is insufficient voltage to drive the required current for the coil to hold the contacts closed. If this occurs while the battery is connected, read the voltage on the dash voltmeter. If the voltage is 12 volts then you have a problem in the circuit which energizes the main relay. Ignition switch failure would not be uncommon on a 1991 car; but, you could have failed battery post clamps, a wiring / electrical connection problem or an actual problem with the main relay (make sure that it is the main relay that is chattering). If the dash voltmeter reads less than 12 volts then your battery is probably due for replacement.

If this problem occurred when the battery was disconnected - stop turning the key on. If you have some aftermarket energy storage capacitors in the car, ditch them. The way you have them wired up they are potentially going to cause some problem in the future.

The only non main 12 v battery energy storage device in a stock NSX is the emergency back-up capacitor in the SRS controller. Its relatively small and your SRS would have to be fairly screwed up for it to back feed the vehicle 12 v system.
 
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Is the car stock?

Electrical gremlins are a huge time suck. If it's not causing the car to behave dangerously, I wouldn't worry about it! My guess is that there is some sort of built in capacitance somewhere and it is discharging immediately once you use the current. Are you disconnecting your battery properly? Sometimes dirty contacts can cause the symptoms you describe, although I have only ever experienced it when cranking.
 
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"There has been a few times when I've disconnected the battery and the lights inside have stayed on."

Which lights have stayed on? The interior lights or some / all of the dash lights? With the battery completely disconnected (disconnecting the negative terminal is the safe way to do this) it would be pretty hard to power up the interior lights unless you have developed some sort of inter dimensional energy transfer mechanism or you have some aftermarket device such as energy storage capacitors for an aftermarket stereo system. If you had some energy storage capacitors, trying to operate the power seats would immediately drain them.

"I also heard the main relay engaging and disengaging several times. "

Relays typically chatter if there is insufficient voltage to drive the required current for the coil to hold the contacts closed. If this occurs while the battery is connected, read the voltage on the dash voltmeter. If the voltage is 12 volts then you have a problem in the circuit which energizes the main relay. Ignition switch failure would not be uncommon on a 1991 car; but, you could have failed battery post clamps, a wiring / electrical connection problem or an actual problem with the main relay (make sure that it is the main relay that is chattering). If the dash voltmeter reads less than 12 volts then your battery is probably due for replacement.

If this problem occurred when the battery was disconnected - stop turning the key on. If you have some aftermarket energy storage capacitors in the car, ditch them. The way you have them wired up they are potentially going to cause some problem in the future.

The only non main 12 v battery energy storage device in a stock NSX is the emergency back-up capacitor in the SRS controller. Its relatively small and your SRS would have to be fairly screwed up for it to back feed the vehicle 12 v system.
Thank you for the quick reply! Yes, all lights. I always disconnect from the negative terminal first. I do have an aftermarket amp powering my speakers. I bypassed my radio with a bluetooth receiver because one of the previous owners completely botched the radio harness trying to repair the radio. I'll try to track down what's causing the issue. I only put the key in that one time just to test what was going one and never did it again because I knew something wasn't right.
 
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Is the car stock?

Electrical gremlins are a huge time suck. If it's not causing the car to behave dangerously, I wouldn't worry about it! My guess is that there is some sort of built in capacitance somewhere and it is discharging immediately once you use the current. Are you disconnecting your battery properly? Sometimes dirty contacts can cause the symptoms you describe, although I have only ever experienced it when cranking.

Yeah, the car turns over perfectly fine when the battery is connected. I haven't noticed anything odd since that incident. Other than it still happens occasionally when i remove the battery. I'm currently running an optima yellowtop D35 with the optima maintainer. There was some buildup on the negative terminal previously, but the issue started happening AFTER i cleaned the terminals soooooo[emoji2375]
 
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In the 1991 NSX, the big back-up capacitor in the SRS is only 3900 UF (0.0039 F). It is the largest capacitor in a stock 1991. The energy storage in a capacitor is defined by 1/2CV**2 so if the back up capacitor is fully charged to 12 volts it has a whole 0.28 joules of stored energy. For comparison, the capacity of your generic AA battery is around 2500 Ma.Hr. Playing a little lose with the formulas a penlight battery has about 3 joules of stored energy so the big capacitor in the SRS isn't going to do much.

If you have a big aftermarket power amp (particularly a class D amp) I would definitely be looking at how it is wired up. If it is powered up through a dedicated switched relay (like the NSX power amps) the relay should prevent the capacitors in the amp from back feeding the NSX 12 v system. If the amp is connected directly to the switched power supply the input capacitors could definitely be back feeding and causing strange behavior.
 
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My old ‘02 TL had a short of sorts that backfed 12 volts from the tail light into the 12 volt switched ckt for the “brake” warning light system. It would keep the engine running if the lights were on even when the key was turned off and removed; but that was a very different circumstance. Presuming you are shutting off the engine before disconnecting the battery ;).

Perhaps an aftermarket alarm has some lighting capacitors to keep the lights on for 30 seconds after shutting off the engine and closing the doors?
 
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