The battery situation for older laptops can be tricky. The PCG-71811M is compatible with Sony battery series (and its variants). Original batteries have a rating of 10.8V / 4,000 mAh / 44 Wh . However, many third-party replacements offer higher capacities:
Since the PCG number is not specific enough for software or driver searches, you need to find the "Product Name". sony vaio pcg-71811m specs
The PCG-71811M is a surprisingly capable platform for upgrades, making it an excellent project machine. The battery situation for older laptops can be tricky