Protecting 12v Battery over Christmas & New Year

[Geoff_W]

Last year when I got back from visiting family in Australia for a month over Christmas, the 12v battery on my Crosstar was flat and the car wouldn't start. The AA replaced the battery with a new, uprated one (guaranteed for 5 years) and that has been fine since.

However, I am going away for a month again this Christmas and want to mitigate any repeat problems. The car will be parked on my drive and therefore exposed to the elements. It's not practical to leave the battery on a charger, as the mains cable would have to run from the garage and the charger would have to sit under the car. Also, the charging cable would be trapped under the closed bonnet. I've looked at those small solar panels which sit on the dashboard and plug in to 'cigar lighter' socket but as far as I can tell, the socket is not connected when the car's switched off.

Do any of you good people have any suggestions, please?

[Kremmen]

All you can do is make sure it's trickle charged to max before you go, then, make sure you 'double lock' the car to ensure as many circuits are shut down to prevent excessive drain

[ColinB]

... I've looked at those small solar panels which sit on the dashboard and plug in to 'cigar lighter' socket but as far as I can tell, the socket is not connected when the car's switched off.

You're correct that the 12v socket is not live when the car is switched off, but the OBD socket remains live and can have a solar panel connected to it. I'm not sure exactly where the OBD socket is on your car, but in the Mk3 it's behind the fascia just above the driver's left knee.

TBH, if you have a battery less than a year old it ought to survive a month of non-use; my Mk3 survived 6 weeks once without issue on a battery that was a couple of years old. You could always buy a battery jump starter (NOCO is a popular brand) just in case it's flat again on your return.

[Lord Voltermore]

The 12v battery should last a month if its fully charged before you go. More important  if you normally only  drive short distances .   

When I first got my 2021 car I read reports on this forum about the  12v battery going flat. As I needed to leave the car unattended for about 2 months in January and February  I bought a 'AA' branded solar  charger   ,which can connect to the OBDll port, (which unlike the  cigarette socket is constantly live.)  . This did give a very slight trickle charge despite the short winter days with little sun. The car started OK  but I suspect it may have  done so anyway 
The advantage of using a solar charger is it should prevent the battery going fully flat ,which might cause damage.    And it could be used on a car thats parked on the road etc away from a power supply.  But  there are downsides  for those intending to use it regularly. It is surprisingly large and heavy ,so not ideal for carrying about  in the car. And its quite a stretch to plug in the OBDll adapter on a Jazz. I had to get on my hands and knees at the side of the car. It may get easier with practice.   TBH I only used it that once.

https://www.ebay.co.uk/itm/388490881336?

I later bought a small  'powerbank' jump starter which I carry in the car for emergencies. I have never needed to use it so  I cant guarantee it would be able power up a Mk4 with a flat 12v battery. But I dont see why not.   Has anyone actual experience of using one for this purpose?   

Here is just one example of the many listings and designs on fleabay 

https://www.ebay.co.uk/itm/205741388681?

[Kremmen]

The issue as I see it though ...... whilst there are jump starters and the like, if the 12v fully discharges it can become irreparable and will discharge far more quickly in the future

Been there, got the t-shirt

[NetworkMan]

Try turning off as much of the data communications as possible.  Stop auto updates, turn off WiFi connection to any router if possible.

I would have said turn off the interior motion detector but I don't think the Jazz has one which was a surprise to me. The intrusion detection alone should not be an energy hog.

[coldstart]

...
However, I am going away for a month again this Christmas and want to mitigate any repeat problems. The car will be parked on my drive and therefore exposed to the elements. It's not practical to leave the battery on a charger, as the mains cable would have to run from the garage and the charger would have to sit under the car. Also, the charging cable would be trapped under the closed bonnet.
...

If you take @Kremmen's advice to charge the 12V-battery to the max before leaving and "double lock" the car, you should be in the clear.

Please see the rather comforting message from @Botak in this thread:
https://clubjazz.org/forum/index.php?topic=17352.msg148189#msg148189
(his MK4 Jazz was still working even after two and a half months worth of absence)

I would rather forgo the purchase of some solar buffering gimmick and save the money for an - eventual - replacement of the 12V battery.

[NetworkMan]

The Jazz charging system is cleverer than some.  It has a monitoring device in the negative lead that can measure charge current.  After a cold start the nominal 12 volt supply is allowed to rise to about 14.5 volts (assuming the charge current is not too great) which charges the battery rapidly.  When it approaches fully charged the voltage is reduced to, IIRC 13.5, to maintain charge without damaging the battery.

The implication is that if, when you get home, the voltage has dropped to the maintenance level then you are probably OK.  I have a cheap voltmeter plugged in to the 'cigar lighter' socket. If your journey is too short in duration then the battery may not have a long enough time at the higher charge rate.

There will be some temperature compensation so the above figures are approximate.

Older cars tend to just stabilise the voltage at 14 volts or so and take longer to replenish the charge lost during starting.

[Jazzik]

This is what I posted in the topic "12V Battery after long stand still"

Opened the garage this morning for the first time in months. The Jazz has been parked there since February 20.
Due to (medical) circumstances, the car was not used for 71 days (10 weeks, 2½ months).

When I park the car I always turn off the audio, use "dead" (double) lock" and switch off the key fob.

The HV battery should be safe because it has not been used for less than the 3 months Honda warns for. But how did that little 12 volt battery hold up?
With fingers (also toes and eyes) crossed I pressed the unlock button on the key.
Flashing, mirrors unfolding and everything seems ok.
Foot on the brake, finger on power button... and I see 9 (nine!!?) bars on the left.... HV battery almost full? After such a long time? Indeed!
I have no idea what the voltage of the 12 volt battery was, but it was certainly enough to start the car.

After a drive of about 45 minutes, the Jazz was parked again with peace of mind.

[JB]

When I brought my jazz new October 2023 the salesman said to me [if you have to phone
the breakdown cover after about six years all they will do is bring a new battery with them]

[Jazzik]

...the salesman said to me [if you have to phone
the breakdown cover after about six years all they will do is bring a new battery with them]

That would really make you happy if you were stuck in the middle of nowhere with a flat tire and no spare...

[aphybrid]

Kremmen's (who knows what he is talking about) advice of the double lock procedure should be followed.

Last Christmas/New Year period I left my Mk4 in an open Scottish airport carpark for 3 weeks with no ill effect, first time usual prompt start.

[Geoff_W]

Thank you all for your helpful replies. I do have one of those little voltmeters and it always shows an initial charging rate of 14.6V, falling to about 12.6V when charged. I always 'double lock' and switch off the remote when going away, so hopefully I should be OK, especially as I have a relatively new battery.

Edit: 21/10/25 Noted voltage at 12.6V when charged. It didn't take many minutes of driving time to stop charging and I do a lot of short journeys (less than 3 miles).

[NetworkMan]

Thank you all for your helpful replies. I do have one of those little voltmeters and it always shows an initial charging rate of 14.6V, falling to about 12.2V when charged. I always 'double lock' and switch off the remote when going away, so hopefully I should be OK, especially as I have a relatively new battery.

You need to see it drop to substantially less than 14.6 volts with the power system on so that you can be sure that it is almost fully charged.

I asked Chat GPT to search around for some measurements on the power off current for the Jazz (and the Yarris too).  It replied that several users have measured the Jazz at about 25mA while the workshop manual quotes a worst case of 50mA.

Now 50 mA for 30 days works out at 36 amp hours which is roughly the battery capacity so that would seem marginal for you in the worst case.  Typically though you should be fine.

Battery capacities are usually quoted at the 20 hour discharge rate and the one month discharge rate will almost certainly result in more  capacity - just how much more I don't know.

[JB]

...the salesman said to me [if you have to phone
the breakdown cover after about six years all they will do is bring a new battery with them]

That would really make you happy if you were stuck in the middle of nowhere with a flat tire and no spare...

I would have to make sure then when I phoned them that I told them I had a flat tyre.