auto memory saver

[Taps33]

After spending the last hour on this site i will apologize ahead of time for my ignorance, I am humbled by the collective knowledge, if not a little frightened. I want to build a memory saver used when changing out batteries that uses a OBD2 connector. I would like to use a 12v power supply, as most available products do. The problem with existing products that run on 12v is most OBD2 systems have a 5a fuse and if a module or component is on while the memory saver is in use it will blow the fuse. How can I keep the current under 5 amps at all times, while maintaining voltage? Is this even a valid question or am I wasting our time? If the latter is the case, i again apologize. Thanks.

 

[crutschow]

Don't see any way to prevent the fuse from blowing if the load draws more that 5A. Could you not apply the voltage directly to the battery terminals to supply 12V while changing the battery.

 

[Taps33]

Thanks for the help, I wanted to try to stay away from the battery terminals, maybe best to try to find another point of entry.

 

[crutschow]

Thanks for the help, I wanted to try to stay away from the battery terminals, maybe best to try to find another point of entry.

How about the cigarette lighter socket? That can handle quite a bit of current. Of course that won't work if that socket is switched by the ignition switch, which I think some are.

 

[Taps33]

Yeah, that's the problem, a lot of times the key has to be in the accessory position. I'm so naive I thought there may be a way to limit the amount of current in a circuit with a set voltage power source, no matter what was trying to draw. Probably wasting my time and should just buy a OBD2 connector to work with a Jump Box. Thanks again.

 

[crutschow]

Yeah, that's the problem, a lot of times the key has to be in the accessory position. I'm so naive I thought there may be a way to limit the amount of current in a circuit with a set voltage power source, no matter what was trying to draw. .........................

You certainly could put a current limiter in the circuit, but it does that by lowering the voltage until the limit value current is flowing, which is probably not what you want, unless a lower voltage will suffice to keep from having to reset anything.

 

[KeepItSimpleStupid]

9V battery and a diode in a lighter socket, so 9V flows out of the socket.

Pull the dome fuse or don't enter the vehicle until the battery is replaced.

 

[Taps33]

Thanks guys, looks like I am making this a lot more complicated than it needs to be. Thanks again for your time.

 

[KeepItSimpleStupid]

The only real issue to this approach is whether or not the lighter is live when not in accessory.

So, I might modify the adpater slightly:
1) fuse or PTC fuse
2) two clips
3) A Schottky diode (lower voltage drop). Wired the same way. Band of diode to red clip.

As for a suggested connection point, the alternator battery terminal and ground.

You will have to be only slightly more careful with the + battery cable. Just put it in a zip lock when you remove it.

So, modified removal instructions:

* Verify 9V battery is good (Could add a button/LED test)
* attach clip to + of alternator (usually easy to get to)
* attach clip to ground.

Remove Negative terminal of battery first.
Remove positive terminal. Any accidental ground connections. i.e. ratchet hitting ground could _______compromise backup.
Put positive terminal in zip lock.

Install battery. Positive terminal first. Same precautions.
Negative terminal. Always do this connection quick.
Remove clip from alternator.
Remove clip from frame.

There are blade fuse adapters that you can use too that can give you access to +12 on the batt side. e.g. https://www.ledcom.co.uk/add_a_circuit_atm_mini_blade_fuse_holder_connector

How many times do you replace a battery? Every 6 years? I recorded by Radio Presets. Now that might not even be an issue on some cars.

Downside of not backing uo: ECM has to relearn AND you may not pass inspection after a battery replacement until after the full test sequence completes. One part is cruising speed for some length of time and at least one warmup/cool down cycle. Codes will be erased. BUT, if you need to replace a battery, it's LIKELY that memory loss may have already occurred.

If it occurs in the driveway and you have another vehicle, you might have a fighting chance. If it occurs 30 miles out of town and you don;t have tools all bets are off. Your insurance company may give you a free tow.

 

[dan longwire]

hi I dan from seattle . I do auto elect circuits as work for 7 years. for mem saver pwr I use cig/power port with 12 volt 7 amp hr sla battery. smaller capacity or lower voltage battery gets very hot when hook up of new battery and u should only have .02 volt drop across mem saver battery when stand alone