They are using a 168Wh battery and a 30W panel, so a naive first approximation would be 168Wh / 30W = 5.6h to go from 0% to 100% capacity. In reality:
* The panel won't be pointed squarely at the sun most of the day
* The sun won't be high-noon-bright most of the day
* Lead-acid is about 80% efficient (perhaps 90% at low state-of-charge, dropping to 60% at high SOC)
* Lead-acid chemistry is sluggish. It can charge pretty quickly to 80% (a 168Wh battery will easily accept the full 30W up to that point), but the acceptance rate decays asymptotically toward zero as the battery approaches 100% capacity. No matter how powerful the charger, going from 80% to ~98% takes about 6-8 hours.
* 0% state of charge is about 10.5V, but lead-acid cycle life falls dramatically with deeper discharges. To reduce wear on the battery, they designate 12V as 0% charge and shut down at that point. This means they're only using 50% of the 168Wh capacity, but it also means they hit the sluggish part of the charge cycle sooner.
Summing up the above, with perfect-noon-sun conditions generating the full 30W, the charge curve will be roughly: 3h from 50% to 80% SOC (0%-60% indicated); 6h from 80% to 98% SOC (60%-100% indicated); a few more hours for the last 2%.
Realistically it will get the majority of the charge by early afternoon, and the slow trip from 80%-98% will complete sometime in the early evening. At that point the panel will only be producing a few watts, but it's still enough to finish trickle charging to 100%.
* The panel won't be pointed squarely at the sun most of the day
* The sun won't be high-noon-bright most of the day
* Lead-acid is about 80% efficient (perhaps 90% at low state-of-charge, dropping to 60% at high SOC)
* Lead-acid chemistry is sluggish. It can charge pretty quickly to 80% (a 168Wh battery will easily accept the full 30W up to that point), but the acceptance rate decays asymptotically toward zero as the battery approaches 100% capacity. No matter how powerful the charger, going from 80% to ~98% takes about 6-8 hours.
* 0% state of charge is about 10.5V, but lead-acid cycle life falls dramatically with deeper discharges. To reduce wear on the battery, they designate 12V as 0% charge and shut down at that point. This means they're only using 50% of the 168Wh capacity, but it also means they hit the sluggish part of the charge cycle sooner.
Summing up the above, with perfect-noon-sun conditions generating the full 30W, the charge curve will be roughly: 3h from 50% to 80% SOC (0%-60% indicated); 6h from 80% to 98% SOC (60%-100% indicated); a few more hours for the last 2%.
Realistically it will get the majority of the charge by early afternoon, and the slow trip from 80%-98% will complete sometime in the early evening. At that point the panel will only be producing a few watts, but it's still enough to finish trickle charging to 100%.