[rikur_]

I've got a 2 year old car, with a 70Ah AGM 12v battery (aka 'stop start' battery).

Pre-COVID, car driven daily, battery happy .... normally around 12.5v getting into the car, and no complaints from the car.

During COVID the battery discharged a bit through lack of use for 2 months, resulting in a 'low state of charge' indicator from the car. (At this point the car behaviour changes, doing things like not leaving on the courtesy lights after parking, turning off the radio immediately with the ignition, etc to preserve remaining battery)

A few long trips later, and it was toggling in and out of 'low state of charge' .... so I put the battery charger on it for 12 hours. Before the 12 hours was up, the smartcharger was reporting the battery at 100% / 12.9v. However, this seems to just be surface charge, as any small amount of electrical use, and it's back down to 12.3v and complaining about 'low state of charge' again.

So I put the smartcharger back on again: Initially up to 14.6v, 8A flowing into the battery, but quickly (~ 10 minutes) tails off to about 0.7A flowing into the battery, and then reports fully charged again.
8A for less than 10 minutes is barely putting 1Ah back into the battery.
Assuming 12.3v is ~ 50% discharged ...... how do I get the remaining 35Ah back into the battery?
I've done 20 hours of driving so far this month, which also seems to have failed to get the charge back in.

The cheap car, with cheap battery, which has been driven less seems to have no problems .... grrr!!

[Rob P]

Not helpful I know, but does anyone with Stop-Start not override it?

It's the first action on the pre-pulling away from the kerb checklist.

Sorry for such early threadrift

Rob P

[jrp]

I also disable the electric windows and the central locking. Never use the heater and I've got a really good sort of plastic stick on double glazing thing for the back window to stop it misting up. I can stop the windscreen from getting too misty by rubbing it with a potato.

[nallen]

Is it one of those batteries that needs to be programmed to the car? Could something have gone awry with that interface?

[rikur_]

Is it one of those batteries that needs to be programmed to the car? Could something have gone awry with that interface?

I don't think so. I've had the smartcharger directly on the battery, and taking the voltage readings from the smartcharger. There is a serial connection to the battery, but I think that's just a battery temperature sensor.
The car seems to go through a learning cycle to learn the state of charge from the battery. It appears to take 3 cycles of the engine for it to learn that the battery has been recharged.
It also adjusts the alternator voltage based on the state of charge, ranging from 13.6v for fully charged battery, up to 14.8v for 'low state of charge' in cool conditions.

[rjc101]

Some cars say to charge from a specific charge point, so the car can monitor the charge and adjust itself accordingly - what does the handbook say?

I needed to top-up the battery in my car once, whilst it's in the boot the charger was connected to a couple of points under the bonnet.

[Flyin'Dutch']

Is the ashtray not full so you can get another car?

Seems a tedious problem.

[ChrisRowland]

Isn't a two year old car still under gaurantee?

The battery on my car started getting low capacity when it was about 18 months old. It was OK if you used it every day but would fail if you left it for two or three days or if there was snow on the ground. A drive to the garage and check showed nothing. Eventually it failed when I was on holiday in Scotland. I rejected offers of assistance from my friends and called the official car service people who turned up, tried it, and jump started it. I was now on the database so went back to the garage. They kept it overnight, decided the battery needed replacing and that was it. All under guarantee, all it cost me was time and inconvienience.

[riverrock]

Obvious question perhaps, but does your smart charger support AGM batteries?
The charger I have has a special AGM setting. There are various different properties between the battery types. A smart charger that doesn't do AGM won't fully charge one.

[rikur_]

Obvious question perhaps, but does your smart charger support AGM batteries?
The charger I have has a special AGM setting.

Good question, yes it has AGM mode and that's what I have been using.

@ChrisRowland not sure about warranty, but suspect either way the dealership will happily take it and put it on charge for 24 hours. They did this with a previous problem car .... but easier just to charge it myself if I can.

@rjc101 owners manual refers you to authorised service agent with no DIY instructions!

@Flyin'Dutch' you jest, but I got rid of the last car due to regular flat batteries from a radio/dashboard software bug that meant sometimes the dash & radio didn't power down.

[Paul_Sengupta]

You may already have damaged the battery. If a lead acid battery gets low, the last thing it needs is a quick charge from the alternator. I try and charge any low lead acids on 1A or less for a few days. I have an Optimate for the purpose (actually a couple of them).

[rikur_]

You may already have damaged the battery. If a lead acid battery gets low, the last thing it needs is a quick charge from the alternator. I try and charge any low lead acids on 1A or less for a few days. I have an Optimate for the purpose (actually a couple of them).

I did wonder if this might be the cause, but the other way round.
The smartcharger certainly starts with a trickle charge, only building up to full charge when it thinks the battery is at (?) 60%, and then moving on into optimise and maintain (like the Optimates do). If the car is also smartcharging (do they do this?), then it might explain why it was only very gradually recovering any charge.

[ChrisRowland]

@ChrisRowland not sure about warranty, but suspect either way the dealership will happily take it and put it on charge for 24 hours. They did this with a previous problem car .... but easier just to charge it myself if I can.

They replaced the battery under warranty.

Subsequently the front shocks as well which were leaking slightly.

[Rob P]

I also disable the electric windows and the central locking. Never use the heater and I've got a really good sort of plastic stick on double glazing thing for the back window to stop it misting up. I can stop the windscreen from getting too misty by rubbing it with a potato.

Why would you disable something functional?

S/S is a workaround by the manufacturers to squeak into emission targets, it provides no benefit to the owner/driver and some small benefit to others in congested urban areas, somewhere I rarely am.

Stupid comparison.

Rob P

[nickwilcock]

At the beginning of April I found myself stuck in Waitrose car park with a battery which didn't have enough charge to start the car. My fault - I'd only used the car once per week to drive to the supermarket and back and had also had to move it out of the garage a couple of times. It was a cold day in April, so I'd used that recirc mode which M-B has to run the fan and keep the interior warm....

M-B/RAC found the voltage was 11.8V. A short recharge was enough to get the car started and I was advised to leave it running for a while. But that was during early lockdown when non-essential driving wasn't permitted.

So I bought a Wolf intelligent charger. The first time I tried it, a battery fault warning appeared. But I ignored that, disconnected and reconnected. No warnings, so left it on for the rest of the day. That sorted things out, but it was only after a few more sessions that my 6 year old battery recovered to its normal state.

I'd suggest you simply keep the smart charger connected for several long sessions?