Appears to be a battery spill...

[Missed Approach]

Background- 1986 911 coupe 3.2 now with 160,000 mi. The battery was replaced in January of 2021 (150K mi.) with an O'Reilly AGM group 48. I've always made it a habit to put down an acid-absorbing mat. I was late in pulling the battery this year for her downtime. Just got around to it tonight. That does appear to be a leaky battery, but I always assumed AGM batteries kept their liquid encapsulated in their internals, making it impossible to leak. It does appear the acid mat did its job, but I'll still wipe it down with a baking soda solution. I've always monitored the voltage, especially during the run to Rennsport Reunion, and there never appeared to be a charging problem. Glad the mat appeared to work, any insight if this is an actual AGM failure/leak is greatly appreciated.

[PeteKz]

Have you checked to be sure the white deposits are acid? Could they be dried soap or something else?

[darrin]

Missed Approach -- also, have you been monitoring your Carrera's voltage for transient voltage spikes? I.e. could your voltage regulator be failing. Warning signs include "dimming" dash lights/headlights and random seat belt warning light illuminations. A failing voltage regulator will allow the alternator to overcharge the battery and "boil out" battery acid. An unfortunately common occurrence on our Carreras.

[AlBackus36]

I'll say what you shouldn't do - don't just pour water and baking soda all over it. You will wash any residual acids into the cavities you don't have access to and have no way of draining out.

[EC900]

Agree that the substance can be something other than battery acid. Instances of a leaky washer pump/line could be the issue.
My first approach would be find out what the stuff is.
There’s battery post cleaners that will change color indicate battery acid. I use Noco Battery Cleaner and Acid Detector regularly to clean and keep clean posts, even sealed batteries have to vent corrosives and off-gassing over time.

Put a straight edge along the sides and bottom of the battery, any bulging with indicate an overcharging issue.
There’s also possibility the battery case had a small crack when you bought it, maybe it was dropped on the corner at the store?
Although there are many brands of batteries all made by but a few manufacturers and rebranded, I stay clear off such off brand store labels.
For what it’s worth, I’ve found unused AA Duracell batteries leak in the package, so every now and then malfunctions are going to happen.

[Missed Approach]

Hi Everyone- Thanks for all the responses- I took EC900's advice and grabbed a can of Noco ($3.17 from Walmart). I sprayed the battery, pad, and Battery Tray area with no change in color (it changes from yellow to red in the presence of battery acid.) I'm not sure what caused the white residue on the pad- it could have possibly been a fluid leak from the intensive washer tank. I'm still taking the AGM back to O'Reilly and have them check the health of the unit. Again, thanks for the responses.

[EC900]

Hard water deposits?
Not sure if you use a full strength washer fluid or a combination of tap water and a cleaning product.

Fixed my non operating washers, found the reservoir (from California) was full of sediment, old and used water(?) calcified the spray nozzles and back-flow connections on the lines, plus some evidence of sludgy mold slime.

The stuff here (IL) is formulated to prevent freezing and to cheap out one can use a 50/50 mix with tap water in the summer. I don’t, always full strength.
I have jugs of dedicated washer fluid over two years old, never seen deposits in the jugs.
Just lay out some of those blue shop towels around the pump and resevoir, operate the washers and look for leaks, even leave the towels there overnight for few days/weeks to monitor.

[bkreigsr]

....the brake fluid overfill hose also empties into that area. - unless they changed it for 1986 model year.