battery – Should I Carry extra batteries in the field or bring a solar charging unit? 30 day trek in Nepal

If having a solar charger, you should note the limitations of law of energy conservation.

Sunshine at best produces 1000 watts / square meter. However, best single junction solar panels are around 20% efficient so you get out 200 watts / square meter. Add 95% efficient charger (you need DC-DC converter somewhere, you can’t just plug a raw solar cell into a raw battery) you get 190 watt-hours of charge per square meter per hour.

A 20 cm x 20 cm square solar panel thus provides 7.6 watt-hours of charge per hour. A Canon LP-E6NH battery is 15.762 watt-hours. To charge it at 95% battery charging efficiency (which is a different thing than DC-DC converter efficiency) you need 16.592 watt-hours of energy. Your 20 cm x 20 cm solar panel produces it in 2 hours 11 minutes. So, not even 2 hours is enough to charge the battery!

If you think a smaller solar cell let’s say 15 cm x 15 cm is enough, it needs 3 hours 53 minutes of charging.

To make matters worse, this requires the solar cell placed at an optimal angle towards the sun. Most likely, the sun is at 45 degree angle on the average and your cell at 90 degree angle (you lay it flat on the ground) so the charging time is multiplied by 1/sin(45*pi/180) or the square root of 2. So your 15 cm x 15 cm panel now charges the battery in about 5 and half hours. That’s nearly half of the average sunlight hours of 12 hours (of course some seasons have more and some seasons have less sunlight hours).

Is that fast enough for you? Most likely you won’t want to spend half of your sunlight hours staying standstill charging one camera battery!

Also note that as is well-known among photographers, clouds can reduce the exposure by 2 stops. This means energy is one fourth of what it is without clouds. So the 5 and half hours in cloudy weather is 22 hours. The trouble is, there’s no 22 sunlight hours in a single day.

So, if selecting a solar charger, please do make sure the panel is large enough that the charging time is acceptable. I would say 20 cm x 20 cm is minimum, and even then you can reasonably use it during sunny hours unless you consider it acceptable a charge takes the whole day. Clouds? No way.

Because you need to consider that there can easily be 5 consecutive cloudy days, you most probably need not just the solar charger but also few spare batteries.