10 watts of.solar panel can easily overcharge some batteries. The no charge charge controller advice might or.might not be valid depending on the battery., Capability of hot solar panel through glass. And parasitic loads. As well as the state of charge of the battery when solar panelstarts producing.
Lots off dried up failed batteries out there because.of.unregulated.small solar panels whose.owners were.told no controller was needed. It's just not true in All cases. Too many variables.
can't argue with that- though in my experience a 10W panel doesn't truly output 10 Watts at 12V. Open circuit they'll put out 18-20V, and at a short circuit they'll put out 0.75-1 amps, and into a 12V load they'll put out maybe half an amp. Into a 14V battery and a through a blocking diode (0.7V loss, so it looks like a 14.7V battery or 3.3V difference), the current quickly tapers to a quarter amp or less. For that reason I won't even bother with something that small.
On our travel trailer I installed a 150W 2'x4' panel with a 15amp (?) charge controller. the weak spot for so many of the controllers is that the SCRs/xFETs used aren't properly cooled. In mine they "hope" that once the back plate is screwed on that the angle of the devices press their backs to the aluminum back plate for thermal transfer. Short of installing heat sinks and a different rear casing, I re-bent mine and liberally applied a gooey silicone thermal grease (not paste like CPU coolers use, the grease flows easier). It's been working now for 3-4 years, with a daily load, as the controller also turns on the porch light at sundown nightly for a few hours. For rough math, that 150W panel seems to be good for about maybe 4-5 amps tops into 12V during peak sun.
So all in all, the wattage rating has been a little misleading for PV in my experience.