100 watt panel question.

[PfloydBham]

I have a battery with 18650 cells that is 7s4p or 25.9 nominal volts. I have a 100 watt panel which is about 18-20 voc. I also have a 12/24v PWM SCC that has a lithium setting. The manual indicates that it will charge at 28.8 volts if I connect to the battery first then connect to the panel. Will the charger increase the voltage to match the battery? or, do I just need to use two panels in series to input 36 volts.
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[tictag]

Your charging requirements are determined by the battery chemistry, not your charger, or PV panel for that matter. Typical 18650 cells use LCO/LMO/NMC chemistry and typically require a CC/CV charge profile to 4.2V per cell or 29.4V for your battery and at no more than 1C charge rate. PWM chargers are very simple devices, they just switch your panel(s) to the battery using a controllable mark/space ratio. So in your case, you'll be connecting a 20Voc panel to a battery that requires 29.4V to charge i.e. it's just not going work.

Will the charger increase the voltage to match the battery?

No.

do I just need to use two panels in series to input 36 volts.

Ish. Firstly two such panels in series would generate 40Voc, not 36V, but, and here's where my knowledge is stretched, a PWM charger does not output a CC just pulses of current depending on prevailing sunshine and measured voltage so I'm just not sure this is the right solution at all. Also, even if it did work, with a PWM charger your panel voltage would just collapse to the battery voltage on the 'marks' then recover on the 'spaces' meaning that it would be a very inefficient charging solution. PV panels have a 'sweet-spot' for power production (Vmp) which would likely never be hit in such a solution.

If I'm honest, I'm probably stretching my knowledge too far here, other much more cleverererer people reside on this forum, save to say that my 'spider sense' is tingling and you might need to investigate this further.

 

[PfloydBham]

Thanks for your input, it is very helpful. I have assembled a system for my travel trailer using a Tesla "S" battery pack but I still don't know most of the ins and outs of other components. I have several odd components I have picked up that I am trying to put to use. I am trying to provide LED lighting for rural/agricultural shelter we use for equipment storage and feral hog hunting.
I have about 30 Boston Power type cells that run in the 3.7v to 4.2 range and will charge at -20 centigrade. From your discussion it looks like a 4s config and a MPPT SCC with user set charge profile capabilities like Epever 1210 or larger is probably what I need. with a single 100 watt panel.

 

[SolarRat]

You either need one more panel in series, or to use a BOOST MPPT charge controller with your single panel for a ~24v battery.