I worked for Toledo Scale (now Mettler-Toledo) in service then in design engineering for 34 years. I am working for a load cell manufacturer in Scottsdale now (you know who it is). Small world.I would guess the solar panels are the source of the EMF either through damaged internal circuitry, or potentially external static (nearby thunderstorm) especially since the charge happened "suddenly". Are the panels securely grounded?
My day job is industrial weighing and I have seen static charges throw off readings and scramble electronics. Once a nearby 3-phase AC motor zapped my tech through the loadcell shield wire.
Static charge is a huge concern with my golf cart and I hesitate to use onboard 48V inverter without being first grounded & bonded to my house breaker panel.
Setting a battery on concrete has been debunked on the internet however I have no personal experience.
When I was tech in Los Angeles CA, I had a customer that filled boxes with plastic pellets for injection molding. The scale had a wooden pallet sitting on the deck with a large (4' cube) corrugated (cardboard) box lined with plastic. The scale was used to cut off the pellet feed when the box was full. Every so often the static charge would get so high it would jump from the box to the scale deck (4.5") and reset the scale. The operators treated that scale with the highest respect (nobody wanted to get near it). 4.5" sparks is over 135 kV and it had to penetrate the plastic liner of the box too!
This arcing happened over and over again. I was amazed this didn't damage anything, they just had to empty the scale and recapture zero after every ZAP!
I fixed the problem by adding braided ground straps around the load cells, plus I also had to add a 100' extension cable between the floor scale and the indicator (extra coiled up under the instrument). The impedance of the longer cable slowed down the rise time of the induced surge in the cable to the point that the instrument was able to handle it without resetting. Toledo Scale made some tough instruments (8142 was the model, I recall).