Flat battery while on trickle charger

I left my 996 in my garage on a trickle charger (the official Porsche one, bought new 2 years ago), I do this for weeks at a time as I don't use it much. It plugs into the cigarette lighter in the dashboard. Today I tried to use it and found the battery was completely flat, even the interior lights and the small drop in the window when you open the doors were not working. Porsche assistance managed to get it started and charging again, and said to leave it on charge for a few weeks but that I might need a new battery. Assuming that I didn't do anything wrong and set up the charger correctly when I last put it away (given how many times I've done this I think it unlikely), I would appreciate any ideas about what might have gone wrong? Porsche Assistance also told me it's better to connect directly to the battery and showed me how to do this with clips but I would have to leave the front boot open. However, I understand some people fit a CTEK ring adapter to their battery to get around this?

I do the same as you. Some battery’s need topping up with distilled water once a year otherwise no amount of trickle charging will help. Eventually they all die. I connect directly to the battery with the crocodile clips. You can close the bonnet on the wire there is enough of a gap. Or take the wires out of the windscreen washer side of the bonnet if you can’t.

How old is the battery? If it's the non-AGM battery used as OE in 996 and 986, mine gave up after about 4 years even in a long distance daily driver.

if you leave the frunk open.... remove the bulb..

Had this happen in last 2 years with both 986 and 993 despite being on continuous trickle charge and chargers connected directly to batteries. Was advised to upgrade the trickle chargers to 10 amp output models (previous were 5 amp) and told these were less susceptible to fluctuations. Used to store the 993 in a Carcoon which came with its own 10A charger and batteries would last 7 to 9 years with that. For the frunk, I press the latch to closed position that turns out the light and stops the alarm beep

This happend to my 996T, and just meant the battery was pooped, even when left on CTEK. There's no need to leave the bonnet open when connecting directly. You can pass leads through the plastic cover over the battery/wiper area.

Interruption of the power supply discontinues charging with the Ctek charger (Porsche branded). So a power outage following connection would have halted the process. Did you notice the the row of charging lights still illuminated or was there just a single green LED? Not sure about 996, but later models auto-disconnect power sockets after 20 minutes or so to avoid battery drain. Maybe your battery hasn't received a full charge yet. If the battery's completely dead, you will struggle to get the Ctek to operate - it needs a small voltage to start the cycle. Connect up a regular charger in the first instance to give the Ctek some voltage to work with. Before rushing out to buy a new battery, try a full overnight charge and see how it is the next day. You can get the battery tested for a just a small charge and this will set your mind at rest about its life and reliability. Regards, Clive

Thank you for everyone's helpful replies and suggestions. To respond to the various points: Yes, I did take out the lightbulb, Porsche Assistance also suggested that I'm not sure how old the battery is, but it hasn't been replaced recently, I looked at the part number and it does appear to be a lead acid battery rather than an AGM one I will look into getting a different charger and passing the leads around as suggested There was just a single green LED on the charger, but I don’t believe there has been a power outage at the property, and I keep my BMW on charge next to it and it was fine and still charging