OT: Car battery drain current

A typical figure would be 30 mA.

Most cars with a decent fully charged battery should start OK after being unused for 3 weeks. Many a lot longer.

Roger Hayter presented the following explanation :

It might not work with all vehicles. Some cars (like mine) monitor for sudden changes in voltage, to trigger the alarm. There will be a difference in voltage between the main battery and a spare battery, plus volts drop in the different cable routings. If not enough to trigger the alarm, it could be enough to wake up your systems from sleep mode.

Yeah, I'm not sure 'just' matches the OP's skillset here but ... ;-)

Ok ...

R i g h t ...

;-)

Cheers, T i m

Yes, typical car battery is 40 to 60 AmpHours, at 600mA a 60Ah battery will be totally discharged in 100 hours, say 4 days. It will last a

Dave Plowman (News) expressed precisely :

Mine can sit unused for much longer than that, which can eventually wreck a battery. I used to have a regime of putting it on charge every few weeks, for a couple of days on a smart charger.

I recently changed that to a better system, so the battery is brought up to full more frequently. I have the car permanently attached when parked, to a less smart charger, via an extra ciggy lighter socket I fitted in the cabin for charging. The extra socket has a more direct connection to the battery, than the built in one, which I found suffered volts drop.

The 'less smart charger' simply outputs a controlled 14.4v if it 'sees' a correctly connected 12v battery across it. I have the charger powered from a plug in timer, which powers it for 15minutes every 24hours.

To confirm I have made a good connection when plugging into the ciggy adaptor, I made up a CMOS 555 with a flashing LED, one flash every 10 seconds, but only when it sees the battery volts.

LED flasher, plus the discharge of the charger when plugged in, add up to around 10mA. The cars own systems discharge around 20mA, so a total of only 30mA x 24 hours = 720mA hour per day to make up.

The charger is rated 6amps, so capable of putting 1.5amp hour back in in 15 minutes, once per day if necessary.

No, 600ma and it won't crank after a week of standing.

That sounds high. How long did you leave the connection through the meter? Did it stay at this level for more than say 10 minutes.

I knew someone with a Golf with discharge issues but these were to do with the radio and required a software update to correct a known fault.

I thought so but you have to make allowances for timmy bouy.....

Ah, an OBD plug with crock clips, not an 'OBD2 Unit' that implies it's an active device of some sort and I thought you were trying to do something clever!

So, once he's bought one of these bits of kit, he puts the plug into the OBD port and connects it to an external battery to effectively power the car from the second battery (assuming he has one).

Best if he didn't do this when he just gets home and with the fan still running. ;-)

Then he can disconnect the main battery, puts the multimeter (?) on mA, 10A (?) and then connects (crock clips) between the end of the disconnected battery lead and the battery post and once done, disconnects the internal stuff, allowing the std car battery to continue powering the car but now though the multimeter. All the above is to minimise the power up current potentially overloading the DMM?

I guess he could have just used the 12V battery and a pair of jump leads to do the same?

Or disconnected one battery terminal (ideally the -ve), re-connecting it via a jump lead, putting the meter across the jump lead (one on the jump-lead clip on the battery, the other on the battery terminal clamp) and then after a suitable delay, removing the jump lead from the end of the lead.

No 'OBD2 unit'., no 'extra 12 battery', just the jump leads from the boot. ;-)

Cheers, T i m

timmy timmy timmy...you are a pain in the arse.....

No, it's to get a reading of the quiescent current of the car without waking it up and it going through another several hour/day phased shutdown which is likely if you actually disconnect the 12V supply.

You can use real ammeters!

My wife's Mazda 2 (2009) draws about .05amp and if we go away for more than 3 weeks the battery will be flat.

I've not checked my Ford Focus but it has gone flat (ie not enough to start car) once during lockdown.

spot on Roger I thought timmy was trying to be a smart arse and challenge my knowledge of car electrics there but he was just showing up his lack of knowledge on the subject.......... nice to see him squirm and try and cover himself though

Would this be the sort of person like me, Spike and NT you seem keen on calling your "mate"?

Yup, so like I said then: "All the above is to minimise the power up current potentially overloading the DMM?" ... and to then read the background current on a low value range as there has as yet, been no specific mention (by Jim) of leaving it other than by me and others in general?

See, I 'know' why the OP wants to do it and why you might want to do it that way (along with not losing radio codes etc). I was just curious what Jim was talking about as he didn't make it clear with first the "tick your multimeter in line and tell us..." and then the "Plug in a £5 12v battery powered OBD2 unit into the OBD2 socket".

'A battery powered' OBD2 unit? What's being powered in the 'unit' he sent the link to then?

Cheers, T i m

jon expressed precisely :

That suggests you have a fault somewhere, I'm not surprised it will not crank after a week.

It's the equivalent of a 3w lamp left on all of the time, check interior and boot lights all go off.

The initial wake up from cold and self test might well be 600mA for the first five minutes or so until the cars systems time out and shut down. Modern cars are increasingly like computers with an internal combustion engine attached as an afterthought.

Mine if you cycle the power like that will wake up with the car radio on and wanting its keycode re-entered, bluetooth hot and various other security features like immobilisers in a state of near panic.

They can take a while to go into a quiescent state from a hard powerout.

Even 100mA would be on the high side for a modern car so it might be worth isolating fused circuits to find where you have an electricity leak. The boot courtesy lamp stuck on used to be a favourite for this sort of thing. Out of sight out of mind but still using electricity.

It happens that Roger Hayter formulated :

Presnt car is fully into sleep mode in ten minutes, last one took thirty minutes undisturbed, its final act took a month, where it would power down the remote central locking radio receiver. You can see the current draw fall, as various items gradually shut themselves down.