Topics

Charging with solar panels

W6gib@...
 

I have access to 250 Watts of solar panels and a 75 Ahr battery. What charging time might I expect from this setup (on a sunny day of course)?

Robert Stessel
 

Back of envelope calculations:     240 watt / 12 volt = 20 amp.  75 Ahr / 20 = 3.75 hours.   Or about 4 hours.  That would be minimum for a dead battery.   Considering inefficiencies, double it to 8 hours.  If battery is not fully discharged, cut time preportionately.  Also depends upon type of charger used.  MPP (max power point) chargers will charge faster than bulk chargers.

Mikel - KE0AUE
 

Robert. Good calculations.

I had further thoughts about efficiencies.

Are the panel(s) in series? Parallel?  What battery chemistry?  What charge controller type and amperage?  That can affect the rate as well.

If you only have a 12v 5A PWM controller then that "maxes out" at 60 watts of panels, the rest of the panel wattage gets wasted.

My current setup is a 92AH lead acid battery, 100w Panel, and this 10amp PWM Ham friendly solar charger  http://ki0bk.no-ip.com/~pwrgate/LLPG/Site/Solar.html   I'm a bit undersized on the panel with this setup.

Lead acid is very forgiving on charging, but for newer chemistries, we need much better battery management. 

He explains on his blog (http://ki0bk.no-ip.com/~pwrgate/LLPG/Site/Blog/Entries/2019/5/7_LiFePO4_Batteries%21.html) that you can use his charger with LiFEPO4 but it requires a higher charge rate than for Lead Acid, so it needs a secondary battery controller (Bienno - built in battery protection) to manage that lower voltage.  So the charger needs to fit the battery chemistry better for maximum charging. Not optimal, but workable.

Good. For a lead acid battery, then you need a low-noise 20amp PWM controller as a minimum.  This is a on/off situation.  You need to have a minimum threshold of light (no early morning sun, heavy clouds, snow or tree branches shading part of the panels)

Better is a 20 amp low-noise MPPT one that can take whatever power the panel(s) puts out and maximizes that battery charging. 

Best.  If you need to be more portable for EmComms, then LiFEPO4 is the way to go.  Much lighter, 100x more charge cycles than lead-acid, less picky about charge status, and also less "explodey" than regular Lithium Ion.  Also much more expensive up front.

I'm either going to buy a system from Hardened Power Systems (https://www.portableuniversalpower.com/pilot-g2/) or more likely build my own (https://oh8stn.org/diy-battery-storage/).

Mikel

On 10/25/2019 5:09 AM, Robert Stessel wrote:
Back of envelope calculations:     240 watt / 12 volt = 20 amp.  75 Ahr / 20 = 3.75 hours.   Or about 4 hours.  That would be minimum for a dead battery.   Considering inefficiencies, double it to 8 hours.  If battery is not fully discharged, cut time preportionately.  Also depends upon type of charger used.  MPP (max power point) chargers will charge faster than bulk chargers.

Virus-free. www.avast.com

W6gib@...
 

Thanks Michael and Robert,
This is all really good info. Although I am a back-of-envelope-calculation type of person, a more in depth analysis keeps me on my toes and my batteries healthy.

Justin, KG7CNE
 

Do yourself a favor, build your own.