backchannel: “my [200w] solar refills my batteries by 10:30 am nearly every day!”

I think it’s worth talking about so onlooker solar newbies don’t get confused about the abilties of their solar config, worry about whether or not it’s working, and/or murder their lead batteries.

From this post on the Promaster forum.  This is the system in question.  The author and I are running the same battery bank.  He charges it with 200w of panel and I charge with 570w;  I still don’t get my bank fully charged “by 10:30am nearly every day”.   It can happen (a night of shallow cycling, early AM driving, etc)  but it’s not normal.   I am not calling BS;  rather, I suggest critical information is either being left out or not understood.   Either way it’s bad for solar newbie onlookers.


I think 200w of panel is a real sweet spot for vandwellers. I like that amount and recommend it as a sane default. > my solar refills my batteries by 10:30 am nearly every day! Depends on what "refilll" means.  It's pretty easy to hit (an incorrectly measured) "100% SoC" by voltage ~12.8v or even Vfloat.  It's pretty tough to get deep-cycled Pb fully charged (ie, completed Absorption down to battery [manufacturer spec]( -- endAmps) with solar-only charging by 10:30am. Given that lead-chemistry batteries take 2-3 hours [to finish Absorption]( after deep cycling, **Vabs, usually in the mid-14v range, would have to be attained by 7:30am-8:30am**. Not too many solar-only systems can do that, even significantly overpaneled ones charged by MPPT under excellent [ouput conditions ]( and insolation). **Caveat to onlookers:** the controller switching to Float does not mean that Absorption stage was successfully completed. It means the controller switched to Float.  The way to know for sure that Absorption is being completed is to watch your meters during late Absorption and verify that the battery is accepting something like a typical  [C]( cutoff (or whatever your battery mfg recommends). For a typical 200Ah lead bank this would be something like 1-2A. If your controller allows you to set endAmps of C/200 or whatever, go ahead and set it. Most consumer controllers don't do this (or can't separate out battery charging and loads to measure accurately) so you may have to just max out any Absorption duration setting that is available. Once you know your system is finishing Absorption correctly you can let it fly on autopilot. If you are offgrid and your controller refuses to finish Absorption, you might choose the **nuclear option** and set Vfloat == Vabs (check water levels more often). **Example with data:** I am putting ~175w into a 220Ah bank this partially-cloudy morning in NV and at 10:30am local I still hadn't kissed my 14.6v Vabs. This puts me at a fully charged ("filled", IMO) between noon and 1pm. My system has more panel but because I am running loads on my system (~200w worth of laptop and crypto mining) this is a pretty good approximation of how a 200w:215Ah system with no loads would charge the bank on a good day. [![](]( If I stop being an idiot and don't run the crypto rig until later in the day I can typically get fully charged by noon assuming my preferred level of snowbird insolation. :-) This is with an [overpanele]( system and not the usual experience with solar. Having said all that, banks cycled lightly to ~80% or [assisted by alternator]( (or other) charging in the early morning would have a decent chance of being fully charged by 10:30am.