I need to change a car battery on a modern car, and don?t want to lose any settings.
I?ve left it a bit late to buy a cheap ODB memory saver.
I am considering making up a cable with some 1.5mm Mains flex and a cigar lighter plug (5A fused??) at each end. Plug one end into another car and start it?s engine.
Plug the other end into the car I want to work on and switch on its ignition (so it?s cigar lighter socket is live) and carefully disconnect it?s battery making sure I don?t short the car?s +ve doesn?t short and then connect the new battery and remove the link cable.
Most cigarette lighter sockets are fused at 15 or 20A.
I don't think there is any harm in trying this method. I wouldn't start the engine as you want a passive 12V, not 14.4V or more from the car running and charging its battery.
If you do short the lead all that's likely to happen is blow the cigarette lighter fuse.
My Toyota and both my Hondas lost radio settings, clock, trip computer etc. Since then all cars have not been affected.
The cigarette lighter method is fine. First you need to check if the lighter stays live when the ignition is off. Then I used a cigarette lighter plug to a small 12V batter. If you don't have such, then wires to another car will do. Make sure you get them wired correctly!
You don't need the other car running. Connect to its battery, and turn on ign/acc if needed in your car, plug in lighter, disconnect your battery. Fit replacement. Remove lighter plug, disconnect etc. Job done. Did it many times on the Hondas.
I've done this a couple of times, and I didn't resort to any "high current capability" to get the job done. Leave key out of ignition. Turn off internal lights so they can't overload the (weak) backup source.
On my previous car, I used a wall adapter and a diode to prevent backflow. On my current car, the last battery swap was done with a homemade adjustable supply and a diode for backflow prevention. The supply in these cases, is only good for around 1 amp or so.
No need for a second car, revving to give lots of amps. That's reserved for "jumping something".
If your battery is already flat-as-a-pancake, then the settings are already lost.
I verified current flow while the battery was still connected, with a clamp-on ammeter, just to see if there was any significant load. For example, on some previous car repair exploits, I could detect the dome light that I forgot to turn off :-) 0.25A or so. If your battery is holding up well, there probably isn't a big drain there. Maybe the computers are asleep.
Because my battery goes flat very quickly, I have fitted a knife switch to disconnect the battery when left for longer than a couple of days. I have disconnected the battery many times and the only noticeable dashboard failure is the clock. All the engine, gearbox and suspension settings are maintained.
Would it not run from a reasonably sized 12v psu from the mains while the battery was swapped? I'm sure I remember somebody doing this once. Not sure why you would need to start the engine of the other car though, the change would only take about 10 minutes. Brian
Some sockets in modern cars are not meant for lighters which are crude in any case, and may have some kind of electronics behind them so do check that first
In message <rr4ith$be8$ snipped-for-privacy@dont-email.me, "Brian Gaff (Sofa)" snipped-for-privacy@blueyonder.co.uk> writes
In my 1954 Ford Prefect, I once swapped a battery (which wasn't flat) by starting the engine and letting it run while I did the swap. However, these days, I'm not sure if this would be a good idea.
The Disco 2's would default the ECU, so they "didn't drive right" for the next 100 or so miles. Entering the radio code was a bit non-intuative and difficult if you don't know the code. B-) Not that more recent cars have a radio code as the radio isn't quite so easy to remove...
Not had to disconnect the battery in the FL2, though I have a feeling it's on it's way out. Starting when cold (approx 0C, it's not go really cold yet) isn't quite a zippy as one might expect. I'll dig out a 12 V SLA and ciggy lead when I do, probably spend more time working out how long the ciggy lighter is live without having the car fully powered up (aka ignition on). No physical key so no "accessory" position, things are just turned off one by one at various times after the "ignition is switched off".
Codes tend to be on older radios that are universal fit? Mine includes all the comms package including the sat nav, so would need matching to the car electronics if swopped even without the battery being disconnected.
I'm curious why an ECU should take 100 miles to 'learn'? If the driving style changes, isn't that rather a long time before it reacts?
I had a curious thing with the old car this week. It's hardly been used due to Covid - no old car shows to go to, etc. So have had to charge the battery regularly. Did so last weekend, then needed it to pick up materials from Wickes - too long for the daily driver. Drove normally. Not that far - only a couple of miles each way.
Needed to do the same again on Thursday. Wouldn't start - slow turning over and soon stopped even that. All the symptoms of a flat battery.
Removed the battery and charged it indoors. Didn't take as long as I'd expect with a flat battery. Then used my ACT tester, which reported 65 amp.hr capacity which is fine for a 4 year old nominal 70 amp.hr one. Not put it back in the car yet as it's raining. ;-)
After much web searching I found that while Halfords do not sell a ?cheap? ODB memory saver, they do sell a cable with an Anderson connector at one end and ODB at the other, they call it a solar panel to OBD connector £10. The price is right! Particularly as after I?ve cut the cable, I?ll not only have a cable to connect to the ODB, I?ll also have an Anderson connector.
IIRC the DIsco 2's "radio" could be removed with a pair of the normal radio removal prongs. Would only fit another Disco 2 though as it also had the climate control interface and wasn't a DIN size.
100 miles would be about how long it took before I noticed it was "back to normal", it was a gradual change.
Slighly corroded connection some where in the high current wiring? If not in the +V side, the earth straps from engine block to chassis can do the same. Traceable by finding the warm/hot one after trying to start the engine.
And here, with a F5, 20 - 25 mph sustained, "Fresh Breeze" the rain stings a bit and the wind chill is between -5 and -10 C.
Whole job was a pain. Couldn?t find any info as to where to find the OBD socket, struggled with undoing nuts, etc, etc.
I used an old school battery charger to run the OBD thingy (which came with zero documentation) as a memory saver.....
When I?d finished I found the radio had lost its presets (but thankfully it didn?t beep / refuse to play.)
The car started on the new battery and at a quick glance, everything else seems fine.
When I came to pack up, I found the fuse holder on the OBD thingy had melted and welded itself to the carpet. I?ve chopped out the section of wire with the holder and it?s not open circuit, but it?s higher resistance than a direct short on the meter leads.
With hindsight, maybe I?d have done better if I?d pulled the fuses for the interior lights and the boot light (had to get some more tools out).
At least the connectors on the OBD cable look ok, so hopefully the bit in the car is ok too.
With hindsight, I wish I?d gone the cigar lighter route or the jump lead route or the just disconnect the battery route.
When doing a memory holder, I recommend the placement of an appropriately rated diode. diode |\ | (+) (-) Alternate power source -----| >|------- system ----- battery --- GND |/ |
The diode does not solve all the worlds problems, but it does prevent the new battery from backfeeding into the alternate power source and damaging something. If the alternate power source is current limited, the diode should be rated for conduction of that forward current towards the car. That's to prevent the diode from being incinerated if a mistake is made while working inside the car.
You can make your scheme safer.
diode |\ | (+) (-) Alternate power source -----| >|--- fuse ---- system ----- battery --- GND |/ | (protect materials on the left)
At the very least, don't do this. What happens here, depends very much on the class of the power source on the left.
(+) (-) Alternate power source --------------- system ----- battery --- GND
For example, if the alternate power source happens to be another car battery, equalizing currents will flow between the two batteries, when two batteries are present. If there's a fuse in the path, it could pop on you.
Here's my memory holder, made from spare parts and a stick of lumber.
formatting link
If for some reason, you're going to be running an OBD scanner on the car, with the memory-holder in place, that would wake some computer or two on the car, and the current drain will go way way up. You can still use your car battery idea, but if selecting a diode, it's going to need to be pretty stout. For example, this one is rated 35A, and should be fitted to a heatsink plate, if you plan on running 35A through it. The voltage drop would be about 1V (if you're lucky) times 35A is 35W of heat. To give a ballpark idea of "how hot is hot" if you fired up the OBD. Try not to trigger the fuel pump in the gas tank.
formatting link
Using a car battery, say 12.6V when settled, minus 1V at load, means the car sees 11.6V. At lower loads, the drop across the diode is less. The schottky in my picture, the tiny 2A diode, it drops 0.3V at really low currents. My memory holder is adjustable, so I can set whatever DC voltage looks reasonable. I want a value which is less than the new battery, so when the new battery is connected, the memory-holder doesn't try to charge the new battery. Because the diode is there, as long as the diode goes reverse biased when the new battery is there, I should be OK. I probably would not lose my settings if things were mis-adjusted, but because I have the capability, I'd probably measure "old-battery", "new-battery" and set my memory holder so no assumptions are violated.
I've done two car batteries that way, and nothing melted or overheated. I turn off the accessory lights in the car, before beginning work.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.