I've got a GSM and arduino hooked up in a remote area. I want to add a solar panel so I dont have to change the batteries every week.
Most solar tutorials with 18650 batteries seem to be aimed at making a solar power bank or fully charging the batteries for use elsewhere. I just want the battery to be backup for when there is no sun. As I see it I have 2 options:
Use a TP4065 charging unit that will control charge the batteries.
Use no charging unit, but just have a solar panel that provides less voltage than the max charge of the batteries. ie 4V solar panels.
Both are relatively easy to implement, but which will provide longer battery life?
What batteries is the project running off now?
Solar Panels behave like current sources and are not suitable by themselves for running anything.
To use Solar Panels the normal approach is to use the Solar panels to charge a battery, and to run the project off the battery.
The battery is always powering the load.
The TP4065 solution would work provided that the OC voltage of the Solar panels in full sunlight doesnt exceed the maximum input voltage of the TP4065.
You can get inexpensive modules on eBay based on TP4065. These modules also have protection circuits for over-discharge (especially useful if your 18650 batteries are unprotected). This should minimize risk of the dangers @Robin2 is concerned about, but the advice he gives about precautions you should take is good. Don't put the battery in an airtight box, drill a small hole for pressure equalisation somewhere where the weather is not going to get in.
Thanks guys. These are samsung 18650B batteries with an inbuilt protection circuit. It's currently running off that. Bursting into flames is one of my concerns but I thought with either the TP4056 (or equivalent) or running the solar panel voltage lower than the max charge of the battery I would be able to avoid that. Would you recommend any other batteries instead? I want to keep the unit small so lead acid is out. Everything else seems to have a lot of life span trade offs. I was researching supercaps for a while, but the balancing circuit is a bit beyond me.
Using a Laptop they got 18650 batteries inside and if you use that laptop everyday like an engineer student
you charge the laptop at least 2 times a day and the laptop can hold more than 2 years with the same battery.
My point is you can use 18650 battery, as PaulRB said a TP4065 a Li-Ion balance battery charger with
a protection circuit on it already installed and there is no danger of explosion or something like that.
Connect the solar panels to the balance charger and to your batteries they gonna have a really nice
balance charging.
Extra option:
To charge the battery only once a day you can add a servo as a switch or a small relay to control with
arduino connecting the solar panels with the charger only once per day for few hours so the batteries
gonna be charged once per day (if you don't want to add a voltmeter and control the process of charging the batteries you can just control the process by letting the panels connected for 2~3 hours to charge the batteries), the batteries will be fully functional while charging so there is no worries to unplug them.
My point of this comment is you are afraid to fall of the bicycle because that's what your parents told you it's dangerous but your friends already ride the bicycles without falling, you just need to get experience and know how to use it, there is no danger.
I love your positive attitude D.60! I guess I was more asking if I should just do away with the charger entirely. If I have a 4V solar panel I'll never be able to overcharge the battery so I don't know if I actually require a TP4056 as long as I have battery protection for over discharge.
According to this article (BU-808: How to Prolong Lithium-based Batteries - Battery University) charging to 4V will increase battery longevity. I just wanted to know if anyone had done this in an application which will cut down my trial and error time.
So get that fear away let's calculate what you have.
*. How much power your full systel/electronics will use, like 5v 50mA per hour?
*. How many batteries do you want to use?
*. How many solar panels do you have? Just one? and what the solar panel statistics? 4v 50mA?
With all the data we can calculate how much your batteries will need to be charged (hours) calculating
the solar panels efficiency and how much your system/electronics will take per hours, in that way we
will know if your system can handle full cycles without going and doing something every week but
going and only checking once per month.
If you have only once solar panel I would suggest you to buy at least 2 or 4, because in my experience with
solar panels without direct sun light or a cloudly day the solar panels go down bellow 50% the voltage
can be high but the amperage will be low and the charging time will be more than usual.
Let me give you an example:
Full sun light 100% efficiency of the single solar panel 5v 100mA it will take to charge a 3.7~4.2v 2000mA
about 20+ hour because some of the mA and voltage will be lost in the charging procedure.
If you got 4x solar panels you will have 4 in parallel that will be 5v 400mA, the TP4065 can take input voltage max 5.5v, in that way your 3.7~4.2v 2000mA Li-Ion battery will be charged in 5+ hours.
You see my point?
1 solar panel charging time 20+ hours
4 solar panels charging time 5+ hours
Thanks for the reply D.60. After some experimenting I can't get the power I need out of a small step up module.
After some experimenting I found out my step up DC-DC converter can't provide enough current so I have another on the way from china, so it will be a while.
In the mean time I'm thinking of using 2 x 18650 batteries in series with a step down module to 5V. This becomes difficult when hooking the batteries up to solar panels. But can you see any problem if I hook up 2 seperate solar panels to 2 seperate TP4056 controllers to essentially charge each battery separately while the batteries are in series? As long as I have the correct diodes in I can't see why that wouldn't work. Below is a poor attempt at a text schematic that will probably just confuse people...
_+ 7.4V Stepdown to 5V
^
|
5V Solar panel - > TP4056 - 3.7V 18650
|
5V Solar panel - > TP4056 - 3.7V 18650
|- 7.4V Stepdown to 5V
blimpyway:
Does the GSM module can go 3.3V (with an 8MHz arduino) ?
Most GSM stuff is made to be powered directly from a single LiPo.
That's what I thought! I bought a few off ebay that had 2 power options available but all only came in with 5V options. Finally I got one that was 3-4.5V but my arduino uno can't interface with it. I had it working with my arduino mini 3.3v, but at some point I must have fried a component on it because it stopped working and I haven't been able to get it to work since.
