2003 Ford F-250 Powerstroke won't Crank, won't start?

When I turn the key, I get a rapid clicking noise, and the indicator lights on the dash flicker. Glow plugs also sound like their barely getting enough juice. No crank at all.

I hooked up my 100w solar panel with a automatic12/24 volt charge controller, which said batteries had a good charge. Went ahead and charged them fully, followed by cleaning the post, and battery connections of corrosion.

Result was the same, won't start.

Any ideas what could be wrong?

You need a new battery. Just because it takes a charge, doesn't mean it has the amps to crank the engine.

You seriously think you can float charge a set of dead 850 CCA batteries with a solar charger, then you won't be moving soon. Get a real 10 amp battery charger. The second standby battery is on a battery isolator so it won't charge both at the same time as the main battery unless you charge on the isolator alternator B+ post on the battery isolator.