I am working on a solar Wisblock repeater. I have something working but there I have a concern about the solar input. The Wisblock datasheet says the solar input must not exceed 5.5V. The problem is that most solar panels I can find have an open circuit voltage of 6V or more. My first question is, whether a 6V panel will damage the Wisblock? I see several projects where people are doing this so apparently it’s not a problem?
I had a couple of solar panels left over from another project and these have an open circuit voltage of about 7.5V so definitely worried about over voltage. I currently have 2 of these in parallel with 2 18650 batteries connected. I had some 5.1V zener diodes that I added in parallel but I’m not sure this is going to work. The battery charge isn’t going higher than 3.9V. Anyone else had and/or solved this issue?
couldnt you put a buck converter inline. something super simple like this:
Needs to be a buck-boost? When the battery is low it will draw lots of current which brings the solar voltage down. It’s only when charging the last 20% that the voltage gets close to 5.5.
FYI, solar panels act like current sources when in full sun. They will try to maintain the short circuit current by increasing the voltage up to the open circuit voltage.
as far as i understood, it also depends a little bit on mA of the solar module:
you should not put a 7V module on the solar input,
but if you put a 6V (open circuit) module on the solar input, that only delivers a maximum of 200mA, then the open circuit voltage will not be reached until the battery is fully charged.
still then, the device will consume a little bit of energy, and maybe a a few mA will still go into the Battery, thus preventing open circuit voltage of a small module.
and even if battery is totally charged, then there will be almost zero mA going thru the tubes of the solar controller, so that 6V 0mA (realistic 4-6mA) will do it no harm.
Other if you install a bigger 6V 12 W module, that can provide 2A… this will give the full 200+mA thru the solar controller and reach open circuit voltage 6,xV at the same time. 6,xV 200mA that maybe too much for the controller.
my experience is:
i have had 9-cell , 10-cell, and 11 cell Modules (all below 210mA !) runnning on Rak19003-solar input without problems. the 11-cell perfoms best, but is a bit above written-limits (up to 7V OC)
so a 10 cell module would be advised:
that is also what Rak-wireless sells : Solar Panel – RAKwireless Store
10 cell 90mA…
i would prefer a slightly bigger 10 cell 200mA module.
or you can attach 2 90mA modules in parallel.
that’s because the batteries are limiting the voltage. A solar cell is like a light-dependent current source into any load that’s enough to pull it down from the o/c voltage. Try with your 5.1V zener in the noonday sun and measure the current. If it’s < imax for the diode then you’re OK.
A 5V three terminal regulator could give you your peace of mind back. You’d get a miniscule amount better efficiency with a buck converter, the Chinese sell small boards for this on Ebay, but it’s probably cheaper to simply buy a slightly larger solar panel 
The Chinese also sell you MPPT boards but at the power levels of a Meshtastic board I’ve found these a waste of time, the MPPT part probably takes more power than the meshtastic board.
I looked at the Wisblock schematic and there is already a 5.6V, 1/2 Watt zener (D3) between solar (+) and ground. The charge controller (U2) max input is 7V so the only concern comes from this zener.
At 5.6V and 1/2 Watt, the max current is 89 mA. So, if the solar panel short circuit current is less than 80mA, no worries. If greater, then add another 5.6V zener in parallel with a larger power rating.
Hats off to the RAK Wireless people for providing a datasheet that includes a full schematic! You don’t see this very often.
If the last two replies didn’t explain it enough for someone, maybe a graphic will help.
Check out the yellow line in the characteristic curve (IV curve) of a solar panel:
You will find that as the panel provides more current, the input voltage will drop.
Use a Zener diode rated same, or higher, as panel’s max power and current and connected as described in first post.- ted
What you’re looking for is an MPPT solar battery charger module. That’s the correct answer. Diodes and switching regulators (buck converter) all introduce inefficiencies or issues.
A solar module will fix solar input voltage at the best voltage for your panel VMP (maximum power voltage, the voltage at which the panel produces the most power, the yellow dot in the above graph). This allows you to extract as much power as possible. It regulates the input.
Lots of people use buck converters, and they do work in some situations, but you’re not harvesting as much power as you could.
I should clarify, a solar module would handle battery charging. Then you run power from that unit to your Wisblock which would be straight battery voltage or regulated to a specific voltage. The Wisblock never sees solar panel voltage so it’s no longer an issue.
can you reccomend any that have a footprint similar to that of a buck converter?
MPPT circuits are a bit more complex than buck converters so they are typically larger. Here’s a list of compact modules that do MPPT and provide varying levels of protection and features. For all of these, the solar panel and battery are plugged into the module and the output from the module is plugged into the Wisblock.
Full disclosure, I designed and manufacture an MPPT solar module. I’ll try to be fair but now you know I am biased. I designed it for my needs so obviously I think it works best for my use-case (Meshtastic). This first one is mine:
Voltaic Enclosures: Multi-Chemistry Solar Battery Charger and Sensing Voltage Regulator (MCSBC-SVR)
This module accepts solar input from 7V to 20v (panel VMP). It offers regulated output at 3.3v, 4.2v or 5v (it has a buck/boost regulator). The onboard INA3221 sensor means you can monitor solar/battery voltage and current remotely. It also includes selectable charge voltage so it works with multiple battery types: li-ion, Lifepo4 and LTO. The LC filter on the output makes the voltage smooth and clean which is important for powering radios. It also includes battery overdischarge protection with 3 threshold options: 1.5v, 3.35v and 4v. It’s designed to be efficient in low current applications. There’s more but you can read about here (currently out of stock).
Waveshare: Solar Power Manager Module (D)
This one works with li-ion batteries and includes a boost circuit to provide 5V output. It accepts solar input from 6v-24v. It has built-in battery protection including an overdischarge threshold at 3v. This is the only other module I recommend because it includes all the important features and does not have crippling flaws. Waveshare has another variant of this module that also works but it’s larger because it has a battery holder built-in.
DFRobot: 900mA MPPT Solar Panel Controller - Solar Power Manager Module for 5V Solar Panel - Support Solar and USB Charging
This one works with li-ion batteries and accepts solar input from 4.5v-6v. It comes with full battery protection and a boost regulator for stable 5v output. However, it has a glaring flaw where it will not restart if the battery fully drains (it requires physical interaction to reboot it).
https://a.co/d/au1Jfax
Walfront: LiFePO4 Lithium Battery Charger Module MPPT Solar Panel Controller
This is a simple MPPT battery charger. The input voltage is fixed so it only works with some solar panels. The charge voltage is adjustable so you can choose what battery type you want to charge (set to 4.2v for li-ion). This is probably closer to the size you are wanting. However, this module is missing important features. It doesn’t appear to prevent battery power from draining into the panel when it’s dark out. It doesn’t have any battery protection like overdischarge protection. Also, the output is straight battery voltage (it doesn’t do buck/boost on the output).
https://a.co/d/dTpaayy
There are other cheap modules you can find on AliExpress or Amazon but they offer less than any of these. If you find a different module that works well please share.
i dig that waveshare model. Ill pick one up for simplicity sake.
I appreciate your honesty in supporting your designs. As a fellow maker i highly reccomend Tindie for your items over etsy. Its a much easier interface to deal with imho. Id consider picking one of your units up if it werent for the price or out of stock. Nothing against your decision for your price point (I know pcb’s are a pain to get made and shipped) but your module is almost the price of a completed node i make. (Rak wisblock, 6db antenna, battery, solar panel, case). Even with a parasitic draw that a buck converter would introduce i feel like it would be nuanced given the function. Not an expert at any means but its a game of cost justifying the means.
You could make one of these and test it for a day or two just checking voltage output with a multimeter.
I would like more information regarding the MPPT solar battery charger module. I am trying to build an enclosure with solar using RAK. I’m thinking a 4 battery setup
TIA
Hi, you can find more details on the Etsy page linked above. It will be back in stock soon. I can’t upload the PDF here but here’s a screenshot of the main points.
Nice idea but adding zeners in parallel is not advisable. Even 2 from the same batch will not have exactly the same breakdown voltage. The RAK one could take all the current.
Also your power calculation is incorrect. You have to take into account the difference between the input voltage and zener breakdown voltage to calculate the zener power requirements.
Well, duh, you have to buy more zeners than you expect to use then measure and bin them out. As far as the power calculation, power is Volts x Amps. You know the zener voltage and you can measure the short circuit current. Looking at the diagram posted by ted, it turns out that the max power point would be less current than that for open circuit so there’s a safety margin.
However, there’s a problem. I removed the radio module from the Wisblock and raised the voltage up to 6 and didn’t see any zener breakdown. Schematic is wrong? D3 (LBZT52B5V6T1G) is between solar + and ground.
Haha good one. You NEVER even mentioned buying several and then measuring them. You might have to buy 100s in order to get EXACTLY the same bd voltage. Who is going to do that???
Also you have failed again as you HAVE to take into account the input voltage to the zener not just its bd voltage to work out power.
I won’t explain it to you again as you obviously don’t have any idea of electronics.
Your suggestions could easily blow someone’s kit up so maybe read up about a subject BEFORE posting.
You are confusing a zener with a resistor. When a zener is in breakdown, the voltage across it is equal to the zener voltage. That’s how they work. Current increases until the input voltage drops to the zener voltage due to internal resistance of the source. BTW, this applies to all diodes. Power dissipated = bias voltage x current. For example, a forward biased Silicon diode has voltage drop of about 0.6V. If this diode is conducting 1.0A, it will be dissipating 0.6 Watts.
Whatever. I have a degree in Electrical & Electronic Engineering and used to market diodes (signal, zener, rectifier) for Philips Electronics. Believe whatever rubbish you want, just don’t mislead people.
So using your knowledge, if you subject a 5.1 zener to a reverse voltage of say 20v it will dissipate the same energy as if it is subjected to 5.2v. Lol…you’ve got a long way to understanding the subject.
Oh by the way, it’s reverse VOLTAGE that puts a zener into breakdown, NOT current. So AGAIN you have no idea what you’re talking about.