Can you charge the 12V battery as normal?

[davewins]

Help!

So - I cleaned the car on Sunday, changed the wiper blades and left them in service mode - and today (Tuesday night) the car is completely dead. So it's looking like I somehow drained the battery. I suspect when I left the blades in service mode, I didn't turn it off properly!

Can I just charge the battery as you would a normal car battery? Are there any charging points in the bonnet or charge direct in the boot?

 

[Coulomb]

Can I just charge the battery as you would a normal car battery?

Yes, but after completely draining the 12V battery, there can be some wrinkles.

Firstly, some 12V chargers might refuse to charge the battery if the voltage is very low, say less than 6V. I have a lab power supply that doesn't care how low the battery voltage is, but many owners won't have one of those.

Next, when the battery voltage reaches a certain level, all the computers turn on temporarily, and this could cause a drain that overwhelms a typical smallish (e.g. 6A) battery charger. Maybe that's more likely in a pure EV than a hybrid, I don't have experience with modern hybrids. I found I had to charge the battery for a while with the negative cable disconnected, until the battery voltage reached 10 volts or so.

Finally, be aware that lead acid batteries don't like full discharges, and its performance may be permanently impaired. All sorts of weird problems can happen when a lead acid battery gets completely discharged. If the battery was in good shape before the complete discharge and you recharge it within a day or so, it may be fine and only lose a month or so of life. But otherwise, be aware that it may be impaired, and look out for possible problems.

I'm just about to replace the 12V battery on my MG4 after less than 18 months of ownership, and just over two years since manufacture (it was a demonstrator for about 9 months). Ten months ago I found it dead, battery at around 3.5V, and had to disconnect the cable when it reached about 6V. I got to it within a day or so, and it seemed fine for about 6 months. Then the car started throwing about 5 warnings like "ACC system unavailable" every startup, and I could not use cruise control or speed limit detection. Life was hectic then and I didn't get around to fixing it for months. Now I suspect that MG won't know how to fix it, and aren't likely to try under warranty (without taking the car in, just based on posts in this forum). I found a post where someone fixed their problem by replacing the 12V battery, so I'm about to replace it to eliminate that possibility first. I mention all this as an example of things to watch out for after a complete discharge.

As for the cause of the discharge, I have no idea. I'll be installing a battery monitor soon, probably before I actually replace the battery, so I can see some potential before and after changes.

@davewins, hopefully your discharge isn't to such a low voltage, in which case little or no damage may have occurred.

 

[T1 Terry]

If you have a reconditioning type battery charger, I would disconnect the battery and let that do it's thing for a min of 24 hrs.
Lead acid batteries take a long time to fully charge, when the voltage reaches 14V, the battery is anywhere from 70% to 85% charged, this depends of just how fast the charger is pushing in current, the slower the charge rate, the higher the charge % by 14V, but to get to 90% to 95% charged, the battery will have to sit at 14.4V for at least 12 hrs, then 13.8V for at least another 12 hrs .... by then you will have reached around 98% SOC ..... to get that last 2% in, will depend just how long since it was 100% charged, but 48hrs at 13.8V would be required to get as close as is possible without a full desulphation cycle on the battery .....

The wonders of using 19th century battery technology, they were amazing for their time, very out dated by todays standards .....

T1 Terry

 

[davewins]

Oh what fun! Luckily the driver's door is unlocked, but I can't get any of the other doors open from the inside or out and I can't open the boot to get to the battery! I'm not sure I'm capable of climbing in the back to see if there's an emergency pull on the inside.

I suspect there's probably a 12V lug for charging in the engine compartment - probably under the fusebox, whose lid can't be easily removed without taking off the plastic cowling around the front of the car that's blocking the removal of the lid of the fuse box.

scraped knuckles, here we go I think.

 

[davewins]

Ok - had to go mountaneering but I got the boot open and got the charger on - as soon as I turned it on, the car started bonging so that's a good sign. I'll leave it on for a couple of hours

 

[Porsche944]

From flat will be 14 hours or so on a normal domestic charger.

 

[T1 Terry]

From flat will be 14 hours or so on a normal domestic charger.

I don't know why I bothered to one finger type that long post Never mind, someone might get some use out of it ......

T1 Terry

 

[davewins]

Well, it's back up and running. I'll keep it on charge for most of the day now. I did have a whole set of errors come up about various things not being available, but they all disappeared slowly. I also had to reset each of the windows (all the way down, all the way back up again)