New Battery Voltage Readings

[Westy65]

I've just had a new Bosch S5 11A AGM battery fitted to my Cayman 981 GTS and I thought I would test it with a multimeter, just to take stock.

The car has not been started for 5 days and from what I have read, I was expecting a resting voltage of >12.6 - the reading was 12.3.

At crank the voltage didn't drop below 10 and when running it was 14 or so, so these seem OK.

As I am no expert on this topic I wonder whether I should be worried about the resting voltage after only 5 days?

NB the battery is now connected to my CTEK charger.

Thoughts appreciated - thanks.

[Hereward]

Although the battery is "new" it may have been sitting in a warehouse for months. Your car may have also drained a bit of its juice over the last 5 days.

All the parameters look fine. I expect the voltage will perk up after the Ctek charge cycle.

[Westy65]

OK thanks - was just wondering how "new" it is.

In all other respects it seems fine, Eg starts easily and stop/start kicks in without delay, etc.

Will see what the charge says after the CTEK has done its thing.

[Panamax]

It's not obvious to me why anyone would need to use C-tek on a new battery unless the car never gets driven.

What voltmeter are you using? Some are less than accurate, so make sure it's a good one. 12.3v sounds very low for a new AGM that's been correctly handled/installed and the car driven since installation. Whenever you check battery voltage it must be several hours after disconnection of the charger, not as soon as it's disconnected, and don't forget battery voltage changes with temperature. It is winter, after all.

That 14v reading has very little to do with the battery itself and is simply what your charging system is chucking at it.

Was your new battery coded to the car? It should be done at every battery change but isn't really necessary so long as your new battery is absolutely identical to the old one. Not just the same shape and size but absolutely identical in every electrical specification. If not, the car's fancy AGM charging system can get confused. And even if it's absolutely identical, the charging system will need a period of use with the new battery to learn that it's in better condition than your old battery.

[Westy65]

Yes I guess the multimeter might not be accurate - £20 and just purchased - model is Crenova MS8233D.

The battery was fitted and coded by the Indy Porsche garage I use but is a different brand - a Banner was in situ before.

So it is interesting point on the system needing to "learn" the new battery, which I hadn't considered and might have bearing as you say Panamax.

NB the charger was put on the battery when the resting reading was taken and showed less than the "standard" I Googled of 12.6v.

I will take another reading after some more driving and see what it says then.

[Panamax]

That Crenova voltmeter should be fine.

You mention 12.6v but that's the 100% charge voltage for a standard lead-acid battery. For a new AGM battery at full charge you should see 12.9v. At 12.3v a new AGM is only at 70% capacity (UK summer temperatures)

The Banner battery should be fine and as it's been coded there shouldn't be any issues at all.

I'd make sure the car gets driven for a decent time, then leave it parked overnight and check battery voltage next morning. Don't forget that if it's cold (i.e. winter) battery voltage readings can be lower, because chemical reactions tend to slow down with temperature.

Here's a link to a relevant article on the Banner website,
https://www.bannerbatterien.com/en-gb/Battery-know...

[Westy65]

Just to confirm the old battery was a Banner one and the new one is a Bosch.

I will do another test after my next drive, as suggested.

Thanks

[eliot]

also worth noting that cars “wake up” when doors/boots are open and will start drawing a load, my l322 range rover and my 7 series before it will draw 15amps just being “awake” which will pull a fully charged battery down to the lower end of 12v and should return to above 12.5v once it falls asleep again.

I use the following battery monitors (which of course do draw a few millamps) to keep any eye out for phantom drains, normally caused by the car waking up - which will show on the logs as voltage drop.

Battery Monitor BM2 device for... https://www.amazon.co.uk/dp/B08VHMJYXY?ref=ppx_pop...




My battery slowly goes flat due to lack of use.


edit: it also automatically captures the last crank too, these are from a different car of mine:


This particular car takes about two weeks to get low, with a 50ma drain for the car alarm between recharges:

Edited by eliot on Thursday 13th February 06:09

[Westy65]

The voltage doesn't take long to drop a reasonable amount - looks like it's gone from 12.5 to 12.20 in about 6 days. That's probably what has happened in my case.

That gadget looks useful & I will have a look at this - can't really go wrong for £22!

[paul_c123]

Its not a resting voltage, so in itself its a pretty meaningless figure, certainly not possible to say anything about the battery health or condition.

I use a Topdon AB101 to check batteries: https://www.topdon.com/products/battery-tester-art...

[eliot]

paul_c123 said:

Its not a resting voltage, so in itself its a pretty meaningless figure, certainly not possible to say anything about the battery health or condition.

I use a Topdon AB101 to check batteries: https://www.topdon.com/products/battery-tester-art...

who are you replying to?

[paul_c123]

eliot said:

who are you replying to?

OP

[Westy65]

paul_c123 said:

Its not a resting voltage, so in itself its a pretty meaningless figure, certainly not possible to say anything about the battery health or condition.

I use a Topdon AB101 to check batteries: https://www.topdon.com/products/battery-tester-art...

This is not something I am particularly familiar with, so please can you clarify why this is not a resting voltage?

The Topdon looks to be another good bit of kit - very reasonable for £37 - although it looks like it is not permanently attached to the battery, unlike the BM2 device.

[paul_c123]

Westy65 said:

This is not something I am particularly familiar with, so please can you clarify why this is not a resting voltage?

The Topdon looks to be another good bit of kit - very reasonable for £37 - although it looks like it is not permanently attached to the battery, unlike the BM2 device.

To get the bonnet open you'd have needed to unlock the car, open the door and pull the bonnet release. The first two would have undoubtedly "woken up" a number of electronic modules, which draw current, thus its no longer a resting voltage that you're measuring but a voltage with a bit of current draw. To obtain a true resting voltage, you'd have needed to actually disconnect the battery and leave it for a number of hours - not very practical.

In any case, resting voltage gives no indication of battery health and is an approximate indicator of battery state of charge (which are different things). I'd say these days, SoH is a key measurement because a car is inevitably going to sit with various modules making small quiescent current draws, then be woken up, then started. So its important it has the capacity to supply energy for those days/weeks, then enough voltage to start the car without dipping too much (which upsets modules).

[M11rph]

If you'll not be driving the car for a week or two and don't have easy access to trickle charging, drive it in Sport mode. This quite dramatically increases the charging voltage supplied to the battery.

Typically normal mode will charge at around 14v, whilst Sport will show anything up to 15.3v. (Normal is minimising loads/demands to maximise fuel economy, similarly engine temp is allowed to run higher than in Sport).

You can also reduce the alarm's draw whilst parked by disabling interior alarm monitoring, double clicking the lock button should do this. (RTFM).

[a340driver]

I don't think there's any problem putting a trickle charger/conditioner on your battery ever. My Z4M has been garaged for a month or so but whenever it goes in the garage I plug it in via the quick plug connection without thinking about it.

[ATM]

Westy65 said:

Just to confirm the old battery was a Banner one and the new one is a Bosch.

Porsche approved batteries are Banner

[Westy65]

paul_c123 said:

To get the bonnet open you'd have needed to unlock the car, open the door and pull the bonnet release. The first two would have undoubtedly "woken up" a number of electronic modules, which draw current, thus its no longer a resting voltage that you're measuring but a voltage with a bit of current draw. To obtain a true resting voltage, you'd have needed to actually disconnect the battery and leave it for a number of hours - not very practical.

In any case, resting voltage gives no indication of battery health and is an approximate indicator of battery state of charge (which are different things). I'd say these days, SoH is a key measurement because a car is inevitably going to sit with various modules making small quiescent current draws, then be woken up, then started. So its important it has the capacity to supply energy for those days/weeks, then enough voltage to start the car without dipping too much (which upsets modules).

Thanks for that explanation - useful info.

[Westy65]

M11rph said:

If you'll not be driving the car for a week or two and don't have easy access to trickle charging, drive it in Sport mode. This quite dramatically increases the charging voltage supplied to the battery.

Typically normal mode will charge at around 14v, whilst Sport will show anything up to 15.3v. (Normal is minimising loads/demands to maximise fuel economy, similarly engine temp is allowed to run higher than in Sport).

You can also reduce the alarm's draw whilst parked by disabling interior alarm monitoring, double clicking the lock button should do this. (RTFM).

That's interesting - if I needed an excuse to drive in sport mode this is it! I hadn't read the alarm section of the manual and I think I will use this feature.

[Westy65]

Just thought I would provide an update.

I purchased a BM2 battery monitor and once fitted it showed that with the Ctek charger fitted, the charging voltage wouldn't really get above 12.6v, which seemed low (doors locked, etc). When I removed the Ctek extension cable, the charging voltage immediately increased to over 14 volts, so clearly there was an issue with this lead. This is now no longer in use.

I then noticed on my next two trips, that the stop/start function wasn't working - it had been working perfectly well since the new battery was fitted. It was only when I was discussing this with my local indy porsche garage, whilst having the oil changed, that I realised that it had stopped after I fitted the monitor. Diagnostics revealed an error "000401: Current Distributor Terminal 30 Reset" which would cause the stop/start to stop working apparently.

I have subsequently removed the monitor - stop/start still not working - but I suspect that the fault code will need to be cleared first. Hopefully once cleared, this will then work.

So, I am not going to use a monitor in future, just the volt display on the dash, and the occasional test with the multi-meter.