Why will my solar panel not charge my battery?

When charging a battery pack with a solar panel, several factors affect charging speed and time. These include solar exposure, panel capacity, inefficiency, and real-life conditions.

When using your solar panel to charge a battery pack, there are many factors that affect charging speed and time. Let's examine a few:

Solar exposure: for optimal solar power, your panel should sit perpendicular to the direction of the sun's rays. Sunlight coming through at an angle will reduce the efficiency of your solar panel, as will shade, even partial shade. You'll need to move your panel a few times during the day to follow the arc of the sun across the sky. 

Capacity: your battery has a capacity measured in watt-hours. For example, the Ecoflow River Max has a capacity of 576 watt-hours. Solar panels vary in their watt rating (e.g. 50 watt, 160 watt, etc). The Boulder 50 (2022 and earlier GO All Out), has a rating of 50 watts of output. The Ecoflow 160 panel has a 160-watt rating. At 50 watts of output, you'd need around 11.5 hours to fully charge a 576 watt-hour battery, and with a 160 watt panel, 3.6 hours of charge time. 

Inefficiency and real-life: You won't ever get a full 50 watts from a 50 watt panel - probably 40-45 in optimal conditions. You lose power between the panel and battery in resistance from the charge cable. Once you get to the battery, the rate of charge depends on the state of the battery's charge. A very drained battery will be slow to charge at first, then charge more quickly, and then once you get above 80%, it will slow down again. Think of stuffing the last part of your sleeping bag into it's stuff-sack. The last bit takes more effort to get in there. 

So how can you optimize using your solar panel? A 50-watt panel is great for keeping a battery topped off, or charging it up from 50% to 80%, though it may not have enough "oomph" to get it over the last hump. A 160 watt panel will be more effective for longer periods of time away from "shore power" (120v AC charging). However, one great feature of the Boulder 50 solar panel is that you can plug other panels into them to increase capacity. You can also use the 12v DC power output of your tow vehicle to charge your battery while you drive around. It's not as fast as plugging into a wall outlet, but it's faster than solar charging in almost all cases.