Solar panel battery charging


#1

I was inspired by another post to do some general testing. Here is the panel another member is using and it seems to work well for him.

I started doing some math assuming that the panel is running at 5 Watt as was mentioned.
The panel is 5v and 5 watt. That means it should give us 1 amp while there is good sunlight.
In my area an average day with that solar panel should deliver about 4 amps.
Look up your own area if you like: Hours * 1amp = Total amps
https://www.nrel.gov/gis/solar.html
Of course some days will be more and others less.

Next I tested the Gotenna mesh to see how much of that energy it could actually use.
With the power off the Gotenna will charge at 0.27 amps when the battery is low.
In 2 1/2 hours the Gotenna was full.
It took about 607mah or about 0.6 amps total.

Keep in mind Lithium-ion batteries charge more slowly when they are close to being fully charged. I think they typically start slowing the charge rate at around 80% capacity.
Example: After 2 hours charging the Gotenna reduced the charge rate to only 0.14 amps. That’s just over half the speed it charged at when the battery was below the 80% mark.

I then tested the Gotenna to see how much power it could take well turned on.
It still only accepts 0.27 amps which was not surprising.
This time with the Gotenna was still accepting 0.19 amps at the two and a half hour mark.
At 3 hours it accepted 0.08 amps.
At the 4 hour mark it accepted around 0.07 amps average.
This is with the unit on but not sending any messages.

My primary conclusion at this point is that the panel @ShootAnyAngle recommended should work for a great many of us and any bigger panel would probably be a waste unless you added another battery into the equation. Under peak conditions that panel produces 0.73 amps more then the Gotenna can use.

I will have to do some more testing with simulated traffic to see how much energy message passing will eat. I plan to test over a 24 hour period.
A Gotenna could theoretically pass around 4320 messages in that time if it were very busy. I won’t push the Gotenna that hard however since I just want a baseline to predict overall power consumption.
EDIT: Some typos… oops.


#2

If you get cloudy skies for more then a couple days in a row I suggest a USB battery pack inbetween the panel and the gotenna.


#3

I’m on the fence about which would be better. Replace the internal battery or add an external one. An external battery can bring some of it’s own hassles. For the sake of keeping things simple I was considering replacing the battery in the Gotenna with a larger battery. I’m not sure if the charge controller in the Gotenna can work with another battery yet. I’ll have to look up the specs of the onboard chip that does the work.


#4

I’ve used a couple different external battery options and they seem to work just fine.

Make sure any external USB battery pack you use does not have any button or switch to turn on. It should have the ability to auto turn on by simply plugging a device into it. Also make sure it is able to charge a device and be charged simultaneously.

As long as it meets this criteria you should be fine.


#5

It’s “least risky” to add an external one—charge controllers generally have embedded firmware that matches them to their battery, mostly for the fuel gauge function that determines what is “full” so using a larger battery may throw your reported battery status off. If that’s ok though, just make sure your new battery is the same chemistry as the stock battery because the charge controller will have been matched to the battery chemistry as well. Li battery charge curves are mostly the same but do vary depending on the exact composition of the potion they’re made of.

The other gotcha with replacing the internal battery is the battery’s protection circuit which is there to help prevent rapid disassembly with loud report. At the very least for safety’s sake make sure your replacement battery has a protection circuit!


#6

That’s the main issue. Some are matched to a specific capacity but others use voltage and resistance to find their way. I have been looking at a teardown to get an idea of what I might be dealing with. I would absolutely use a battery with it’s own protection circuit. I’ve been looking at some datasheets. I think one chip might be doing processing, USB comms, and charging for the units. In case anyone has not plugged their Gotenna into a PC lately the new firmware allows them to be detected as an unknown USB device. Now we are just waiting for Gotenna to release drivers/SDK for USB.


#7

Thank you. This is really useful. Because the goTenna is unpaired, is there anyway to tell if the unit is functioning properly? Aside from reconnecting to a paired mobile device/app to it.