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Discussion Starter · #1 ·
Hi all

Bit silly really, I bought a 900NG with three keys, it's a UK P-reg meaning 1996 or 1997, and being an old car I never realised it might once have had a remote alarm key. None came with it.

So I live in a very safe area and not really worried about the thing being stolen, but what I am worried about is accidentally activating the alarm then not being able to disable it or use the car

I normally change car batteries every few years. If I disconnect the car battery on this car - is there any risk it'd turn back on with the alarm engaged? As if it did I probably wouldnt be able to open or drive it due to the immobiliser...

What I am hoping - what the sane design choice is - is that when the car loses electrical power, it comes back on with the alarm not armed...

Thanks in advance for your advice
Chris
 

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My concern would be the radio because when you disconnect the battery the radio will loose its code. Then, when you connect the new battery you will need to input the 4-digit code.

If you didn't get a card with the code you might pull the radio unit out and look to see if someone wrote the code on the radio somewhere.

You can also connect a battery charger to the battery cables to keep the electronics powered up when you change batteries.

My car came with one Fob and I have never used it for 7-8 years that I have had the car. It's still at my son's house and that's where it's going to stay... Ron
 

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Mine never has but it's never been armed by me either. If the alarm does go off it's located in the left front fender well behind the molded liner.

Some have said that when the units internal batteries begin to loose power that it will sound but I have never experienced it... Ron
 

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IMO, a legit question. I like the idea of temporary battery power during main battery downtime(disconnected) . I have my radio code written down in many places ..this is one of the defects in the owners manual and the dealerships. I do not even know if the old 900 even has an active alarm system .. nor would I wish to test it .. here I could be wrong in my attitude ..I strongly dislike alarms and associated security measures .
 

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With the key off there is no continuity between the contacts of the cigarette lighter and the battery.

Personally I would use the +ve distribution block by the maxi fuses and one of the chassis grounding points, that way you are free to move the battery cables with nothing attached to them.
 

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I doubt that a 9v battery has enough current to keep the electronics alive let alone the voltage. I can just picture someone using a 9v battery and opening the car door to get get something from inside the car and when the interior lights try to light everything goes dead.

That's why I suggested a trickle charger. Everyone should have a $5 1-Amp trickle charger for emergencies, if they don't have a 6 Amp charger or larger... Ron
 

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Always off!!
I just checked my schematic diagram for fuse #6 which provides power to the cigarette lighter and it is only hot when the key is in the RUN (or On) position.

I also verified the same thing in the car with my cell phone cord, so plugging into the cigarette lighter won't work with the key in the OFF position - at least in the NG900s... Ron
 

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Discussion Starter · #12 ·
a 9v cell would work to keep the electronics powered up, its well within the current/voltage envelope they require at least for a few hours (not connected via cig socket of course)

Of course it'd roast the moment you connected it, unless you also had a silicon diode connected to prevent back-current. Personally I'd prefer 2 9v cells in series and an LM7815 voltage regulator to keep the output at 15v, but in practice I'd just use a trickle charger like anyone else if I felt a need to keep the systems alive.

I didnt buy a trickle charger. I charge cars with a 1 amp 18v AC transformer that came with a long-deceased wireless router, connected through a bridge of 4 x 1.5a silicon diodes to get a steady DC voltage from it. Been working for years.....
 

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I can see where a small battery would back-feed through the small fuses and Maxi fuses, but current is the question. When the other poster said a 9v battery. I immediately thought about the small kind that's in a fire detector and not a string of six or seven 1 1/2 volt ni-cads in series. or even a drill battery pack and a couple clip leads... Ron
 

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I doubt that a 9v battery has enough current to keep the electronics alive let alone the voltage. I can just picture someone using a 9v battery and opening the car door to get get something from inside the car and when the interior lights try to light everything goes dead.

That's why I suggested a trickle charger. Everyone should have a $5 1-Amp trickle charger for emergencies, if they don't have a 6 Amp charger or larger... Ron
I do it all the time. My Snap-On memory saver uses 5v and at 1Ah. Dont let your doubt keep you from using logic.
 

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If the cigar lighter isnt powered with the key off, then it wont work, of course. There is no consistency with my fleet of Saab/Audi/VW/Lamborghini.
Furthermore, electronics dont blow up in cars with <12v and fewer than 1amp. Thats less than Any circuit on the car excluding the clock. Diodes are placed where necessary to prevent this on all cars. How do you think you dont melt all the electrics when you jump-start or fast-charge at 15v?
 

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What I meant was, if you connected a 9v battery to the 12v feed without a blocking diode, the 9v battery would explode or catch fire due to an inverse voltage charge going through it
Its DC. Does your battery explode when it charges at 15v? Ay over 55amps? 9v is overcome by the 12v battery once connected. If power back feeds into the 9v, rather than just increase its voltage, youre doing it wrong!
 

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Discussion Starter · #18 ·
a 12v 80Ah rechargeable battery wont explode from a 15v charge at 55a

A 9v non-rechargeable battery will probably catch fire if exposed to the power than a car accumulator can provide. Thats why it needs a diode in series to prevent the car's power flowing back through it

the internal resistance of a 9v cell is low enough that a significant current would flow, that it is in no way designed to cope with, its a sealed case full of toxic chemicals - basically a little bomb.

If you want to try connecting a 9v battery to a car battery, positive to positive, without a diode that only lets current flow from the 9v side to the 12v side, you go for it but wear safety glasses and do it outside as thats positively dangerous.
 

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I shouldnt have said anything. Ive been doing it for the 25 years I've been working on cars without incident.
Buy real tools and get a voltage maintainer for this purpose. I have a good Snap-On unit.
 

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IMO, a legit question. I like the idea of temporary battery power during main battery downtime(disconnected) . I have my radio code written down in many places ..this is one of the defects in the owners manual and the dealerships. I do not even know if the old 900 even has an active alarm system .. nor would I wish to test it .. here I could be wrong in my attitude ..I strongly dislike alarms and associated security measures .
I have a 1995 900s, and believe me it has an extremely loud alarm system. I have found out the hard way many times;oops:.
 
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