Have a Lincoln Guardian Model G121 Series A grease gun 12 volt. Battery number is model G915.

I see lots of aftermarket batteries on Amazon for 12 volt models but don't see this gun or battery numbers referenced anywhere. It's probably at least 10 years old and the battery does not hold a charge anymore.

Does anyone know if the 12 volt batteries for these are pretty much universal same physical size with perhaps just different mAh ratings.

Looks like aftermarket batteries are 1/2 the cost of originals but wondering on compatibility.

I know the Lincoln's seem universally hated on Ag Talk but this one has been reliable and trouble free for a lot of years.

Wondering if anyone knows the 12 volt particulars & whether they are interchangeable and can be used with original chargers ?

Also any other success stories /links with online sites selling batteries for these ?