Solar, Li-Ion, potential brownout or battery decay. - Page 1
Forum section: electronics / beginners

author: J
author_rank: Newbie
timestamp: March 13, 2018, 06:19:24 pm
content: Hello everyone,after some research, I couldn't find an solution to the following (admittedly beginners) problem:The project :I'm working on a wireless (433MHz) temperature/hygrometry/soil moisture sensor. It is powered by a solar panel (those cheap chinese 15x15cm ones), through a battery charger, to two 18650 Li-ion batteries. The resulting energy is fed to a boost converter to up it to 5V, then it powers a clone of Arduino Nano and the bunch of aforementioned sensors.I've made a test last night, because it bothers me since some days, and discharged the batteries (I plugged a raspberry pi on the boost converter) to see what would happen, and if the contraption would work again after a day under the sun. Admittedly, it still winter here and the sun is not at its best, but I thought maybe (once the raspberry was unplugged) it would charge the batteries enough to make it work again after some time.The result:It failed utterly. After a whole day on my balcony, the batteries are measured at ~0.85V.So I've two theories, one is that the solar panel doesn't output enough, and that the Nano is in a constant brownout, constantly stealing the small energy from the batteries before they have time to charge. The second one is that the batteries have been killed by the boost converter, reaching a point of no-return.Can anyone explain to me 1/ What is the real problem 2/ What is the solution to this, I guess quite common, problem ?Thanks a lot for any link I could learn from !

author: Buriedcode
author_rank: Super Contributor
timestamp: March 13, 2018, 06:46:14 pm
content: Without numbers (measurements, specs) a schematic - or at least a diagram of how parts are connected - and actual part numbers, who knows.Firstly, if I've understood you correctly, you're just transmitting data over 433MHz, infrequently (how long?) and it is powered by a battery that uses a boost converter. And you've also plugged a raspberry pi on this? A RPi would draw significant power, and would drain the battery pretty quickly.As it is battery powered, you want to use as little power as possible. This means turning everything off taht you do not need, only powering things up to transmit your packet, then back down - this includes your boost converter. I would have thought a fully charged 18650 would last for weeks in a system that transmits small packets every 10 minutes over 433MHz.So to get any handle on it, you need to work out your power budget. The current draw of each part. Parhaps also decide whether you really need a system at 5V or whether it can run off 3-4.2V (the cell discharge voltage). Remember, boost converters are no 100% efficient, and for low current draw can actually be pretty bad, as in <50% because the converter itself requires power to operate.Essentially you need to do the maths. There is no way around this. You need to measure draw currents, total power consumption, and total energy (power x time = joules = watts * seconds).With all that said, lipo cells do not like the cold, so even if you are only drawing a few mA, the cold would reduce the capacity significantly.

author: J
author_rank: Newbie
timestamp: March 13, 2018, 06:59:47 pm
content: Thanks for your reply. I did not include schematics or measurements because the problem seems (to me) a quite general one.The Raspberry was here only to deplete the batteries, and was unplugged after some hours.I'm a embedded computer engineer, I'm well aware of the power management topic from the software side. That is, when the electronics are designed by competent people, not meI'm mostly an ignorant on the electronic side, so my question is basically : How do you ensure your battery isn't stealing the power needed by the payload (MCU and all that), and/or how do you balance that ?

author: Buriedcode
author_rank: Super Contributor
timestamp: March 13, 2018, 07:44:55 pm
content: So you have your solar panel powering your charger, hooked to the battery. And the battery is powering your circuit. And you wish to know how you stop your battery being charged over powering the circuit? I'm afraid if your load is in parallel with the charge across the battery, then it becomes quite complicated trying to work out what the solar panel powers.Also, you mentioned an Arduino nano and a 5v boost converter. And given you said you aware of power management you put the nano to sleep, but it will still draw a few mA since the nano has a USB-TTL converter on board. A nano isn't the best device for low power usage - great for development, but perhaps a "pro mini" would be better, as that is essentially the bare MCU with a voltage regulator.I guess I mentioned " we need numbers" because for all we know you could be drawing 200mA from the battery, the solar charger only provides 10mA etc.. Without making measurements or providing diagrams, who knows where the current is going.Back to "load sharing" - there's a fairly simple (MOSFET, two diodes and resistor) load sharing circuit that allows an input to power a load, whilst also charging the battery. It is used so the battery gets fully charged because the load draws power form the input rather than from the battery (stealing).However, this assumes that the input can provide both the current drawn by the charger (its charging current is set by a resistor) and the current drawn by the load. Solar panels as you know vary in both output voltage and current capability, and for larger systems use MPPT to optimise efficiency, but this is over kill for smaller systems.Again, maybe I'm missing something, but I can't really understand the question. I think you are asking why the battery drains in the system when it has a solar panel, and believe this to be because the load is "stealing" power from the charger, so the battery never gets charged. If the solar panel can provide more power than the load (your system) then the battery will slowly charge. If not, it will drain. So either your load is drawing too much current. Or your solar panel isn't providing enough.