MacBook 7,1 randomly shutting down (shutdown cause -102)
On a ten-year-old laptop with a non-removable battery, I suspect that the battery chemistry has aged to the point that it can't reliably provide an appropriate supply of power. I'm vaguely surprised that it's over voltage protection, but the chemistry of an aging battery is a complex topic.
iFixit indicates that you can get a replacement battery for $95 and that the replacement process is easy (only five steps!). Their guides are generally excellent, and this one looks like no exception. They'll even sell you the weird trilobe screwdriver you need to pull it off.
Even if this doesn't solve your problem, it will almost certainly improve your overall experience with this aging laptop.