Not used for a couple of weeks = Flat Battery

I went out to move the car out of my way, so that I could sort the garage.

Guess what?

No starter, no lights, no anything.

Only an idiot would forget to keep his car charged during the Lock in!

It’s been in there for, about, 3 weeks!


1 Like

If it was dead in three weeks, it was probably shagged anyway.

May just be the battery terminal connections if there are no lights whatsoever? First thing I’d do is disconnect them, clean them up and try it again…

Not necessarily, some modern cars have a higher parasitic drain than the older ones, and can drain a battery in 2-3 weeks of no use.

My car is fairly modern with all sorts of electronics. My wif and I had a six week stay in USA with the car parked close to the airport. I have to admit I had concerns about the battery. It was fine.

Had to fit a new battery to daughters car the other day after two weeks, it was ten years old. I recommend going out every day and giving them a short turnover…

That’s fine BUT you must let the car run at idle speed for 10+ minutes every time you do it.

Does it charge the battery if you just leave it ticking over same as when you’re defrosting icy windows? If so, how long does it need to run for?

It is funny how for years manufacturers would fit smaller and smaller batteries, My Mitsubishi Starwagon had a tiny battery but along came engine management computers and suddenly batteries got bigger and bigger. My son’s Nissan Navara has two large starter batteries though my similar Isuzu Dmax gets away with one. (I don’t know the Amp hrs but that is generally just a function of size.)

I do agree however that if it completely discharged in a couple of weeks it needs changing assuming, as someone else pointed out, it is not corroded terminals.

Yes it does. I was told that it takes about 10 minutes to replace the energy used by starting but I would double that if the battery is a bit dodgy.

Having said that a good battery should have many starts in it without recharging.

I’ve been starting mine up. I remembered to put petrol in before lockdown thank heavens!

Fuel is much cheaper now though. 88c/L in some servos.

All well after recharging the battery.

Ran a quick check, on the Hyundai i40, to see what the current usage is when switched off.

Even though it’s a huge battery, the drain is quite high due to what’s still connected when switched off, i.e. Alarm system.

I’ll need to trickle charge it for the rest of the lockdown.

Luckily no loss of key codes, radio setting,s, etc. so just keeping it charged will be enough.


Our Sainsbury’s had petrol at £1.059/lt this morning. I can’t remember the last time it was so cheap.

Re: drained batteries. Friends of ours bought a brand new BMW 523i, delivered to their home by BMW, superb looking motor. They put it in the garage, locked it up, and went on a 2 week holiday as previously planned. Returned home, and the next day D went out to the garage to go for a spin and, clunk, battery was flat. During the 2 weeks they were away, the alarm system, immobiliser and sundry other electronic gizmos within the vehicle had drained the battery because they are NEVER switched off. Needless to say, D and H were not best pleased and had a real go at their BMW agent.

No, letting a car tick over will not charge the battery, to replace the amount of power taken from a battery on one start needs a run of about 30 miles, an alternator only puts out power above 1500 rpm, so a car ticking over at say 900 rpm will achieve nothing, except leave you at risk from some scrote pinching it.
I have a device on my landrover, identical to the one’s fitted to police cars, a runlock, Basically, you start the engine and push the runlock button, then remove the ignition key, the engine remains running, however releasing the handbrake or pressing the brake pedal causes the engine to shut down, it’s great in winter when it’s on the drive defrosting.
If you are not using your car then leave a battery charger attached, modern battery chargers will charge the battery, then go into what is called ‘float mode’, they don’t put anything in, so no risk of overcharging, they simply monitor it and top up as required. Another thing to remember if it’s not being used is to leave the handbrake off, either leave it in gear or put a brick behind one of the wheels, there’s nothing better for strtching the cable, or causing a sticking handbrake than leaving it on for several weeks.

It’s all to do with what is called the ‘body control system’ , modern cars have several computers each doing a particular job, my landrover has two, my previous car, a Citroen, had three! They handle various things, from traction control and engine management, controlling fueling and emmissions and such, right down to locking and immobilising the vehicle. One of them controls power management and in mechanic speak, ‘puts things to sleep’ at various times. initially, when you turn off the car it shuts things off like aircon and heated seats, as time passes it shuts off other devices.usually if you leave a door open, it will turn off the interior light after about twenty minutes, as another part of the system goes to sleep,eventually, only the security system is left active, and the clock and radio memory, even the central locking is put to sleep, it’s only reactivated as you open the vehicle, trying to open it, turns off the alarm, a split second later it ‘wakes up’ the central locking.
One important word of caution about jump starting a modern car, do not leave the keys in the ignition when you connect the jump leads, the car will automatically lock as it senses the power!
A mate of mine had a new Mercedes, he’d not used it for a while as it was winter and it stayed nice and dry in the garage. When he went to start it the battery was flat, so he borrowed my leads, I have some really long heavy duty ones, he connected them to the Merc and took the other end out of the garage to connect to the other car, to get out he had to shut the Mercs door, he attached the leads and heard a clunk, he then found out that it had locked, with the keys inside. it took him ages to find the spare set as he’d put them away ‘safe’. he was lucky he had a spare set.

That is not correct most modern cars will charge the battery at idling (unless the battery is already almost fully charged or the car is very old).

I’m with you there, Bruce. I thought it was the old dynamo system that needed the engine running at more than 1500 rpm to charge the battery.

Sorry Bruce, I’ve only just seen your reply.
Thanks very much.

Not true,

A rough rule of thumb is six miles will put back the energy used for one start, and they do charge on idle.
All cars have parasitic drain as well - the more gadgets the more drain. However, the bcu (body control unit) shuts down anything not needed within a few minutes.
Most cars have a parasitic drain of circa 60 mah, but 150 mah is not unusual on some. 60 mah will drain the battery at a rate of around 1.4A per day.
If a car is not used for 2-3 weeks, it’s not unusual or a fault if the battery becomes flat.