DDK - Die Deutschen Klassiker

Got my 3.2 engined 911T going today.
Routine is to charge the 2 batteries, change oil etc and start up. Done this for 30+ years, all ok, but today...
The car has sat still in the garage since last September, batteries connected, car electrically OFF.

The two batteries are Bosche Silver Plus S5 12V 250A 25Ah and are certainly 10 years old. Always been perfect from new.

Put one on the charger, and the charger showed Zero Amps. Voltage was 10.4 V.
Put the second one on the same charger, charger showed 6 amps and I left it charging for 30 mins. = 12.6Volts off the charger.
Put first battery back on, no movement on the ammeter but left it for 30 mins. Voltage now 12.3Volts off the charger. Charger still showed zero charging current.

Put both batteries in the car, started the engine and all normal, usual 3.2 churn and fired onto 6 as it always has.

Left it ticking over to warm up so i can check the oil level etc. This took about 20 mins.

Shut the engine off and checked the volts on both batteries, both 12.6 volts.

I'm puzzled why the charger did not show a charging current on one battery, but a normal 6 amps on the other. Time for new batteries?

Or a new charger?

Well, maybe...
It is at least 25 years old and not quite the same since using it to de chrome some parts recently.
You might have a point.

Got a cheap one off Amazon recently which does the job for very few £

Hi Graham, Could be that one of the cells has gone high resistance. Happened to me this winter.
Cheers

Hello Chris:
Could that explain why my charger shows a charge rate on it's ammeter on one battery and not on the other one?
The same charger instantly shows a charge rate on the Lola battery and the huge traction battery I use for the winch on the trailer, thus performs as expected on other batteries.

Reckon so. Battery has six 2.2V cells in series. If any one goes high resistance you'll struggle to push any charge current through it. It will also struggle to deliver current when starting. I'm guessing the other parallel battery is doing all the heavy lifting.
Cheers

Thanks Chris.
To my surprise you can still by these excellent batteries (Halfords don't do them Noe) on line for £85, cheaper than Halfords 12 years ago.
Will order one today.
Thanks again,
Graham.

Yes, just buy a new one. Over time, and particularly with lack of use, the lead plates become 'sulphated'. I've tried to resurrect batteries in the past but never with any success. A simplistic diy method to check battery internal resistance is to see how much the voltage (measured directly across the terminals) drops when a known load is applied. Divide the voltage change by the load current and you've got a reasonable indication of internal resistance. Do it on a fully charged battery to get a feel for how healthy it is.

Good luck for the forthcoming season. Hope to see you at one of the events soon.

New one on it's way now.
Measured both batteries in the car, nothing connected, both gave 12.7V

Just to wrap this up..
Got the battery from Tayna, arrived 24 hours after placing the order.
Added a 30 min charge (Charger showed 6 amps charge) and into the 911.

Ignition ON and the engine turned over 2 x faster than it has for years. And ran a dream.
The one battery has obviously been 'dead' for some time, the other doing all the work as Chris said.

Took it round the estate for one lap, brakes good, transmission good steering nice as ever, job done.

Hope to take it to Loton Park hill climb test day on Saturday as it's first run in 2023.