Question about using battery charger and fuel gauge with 8.4V lipo battery


I am using a 8.4V lipo battery ( to power my project, meaning that I have put a step-down regulator ( between the battery and the Photon. My plan though is to charge the battery using a 3.5W solar panel. Despite spending most of my day researching how to do that, I haven’t really found a solution. My problem is two fold:

My apologies for the newbie question, but for some reason, my google kung-fu isn’t allowing me to find the answers myself.
Thanks a lot in advance,

Most of the charging boards you’re going to find out there are for Li-Ion batteries.

Why are you choosing NiMH batteries?

The fuel gauge will not work right with that type of battery but you can setup a resistor divider on one of the pins and calculate the voltage via some math to keep track of the NiMh battery voltage.

Thanks a lot for the response @RWB. The only reason why I bought NiMH is because I didn’t understand all the battery technologies and thought they were the same as lipo, which I have since discovered that they are not.
My plan now is to use a 2xAA battery holder and put 2 AA lipo batteries in there. The questions in my original post still stand, but with those 2xAA lipo batteries instead of the NiMH:

Thanks a lot in advance,

Yea, ditch the NiMH batteries.

What Particle Device are you using? The Boron and Electrons have battery chargers built in that accept solar input that work great without needing external components.

I am using a Photon. I have a 5V step-down regulator to power and protect the photon ( as the batteries combine to a 7.4V output), but I need to find a way to charge those batteries.

You could use a LiFEPo4 battery along with this solar charging chip and then boost the voltage up to 3.3 or 5v to power the Photon. These batteries can last a lot longer than LiPo batteries.

I apologize, but I am not familiar with LiFePO4 batteries. If I use a 2xAA or 3xAA battery holder, can I charge those batteries with the charger you showed?

It’s just another battery that’s safer and last a lot longer.

Most Lithium batteries do not come in AA size so you would need a different holder.

Ideally you want to use a single Li-Po battery big enough to get the run time you are seeking.

@RWB: Oh ok. That is what I am trying to find. It seems difficult to find companies that ship batteries to Canada. :angry::angry::angry:
Assuming that I can find a 7.4V or a 11.1V lipo battery pack, it still doesn’t answer the question of how am I going to charge it?

You don’t need multiple battery cells.

Just get one LiPo and the SunBuddy MPPT solar Charger.

Then add a 5v Boost circuit and your good to go.

@RWB: Thank you!!! I guess in a round-about way, you’ve answered my question. In my mind a 7.4V lipo battery pack is multi-cell as it includes 2x18650 1 cell battery, but, what you are saying is that I can actually treat the pack as a single cell battery. Am I correct?

Well if you add 2x 18650 cells “which are not the same size as AA batteries” you need to have battery protection PCB’s on both batteries in case one battery fails so it does not short circuit the 2nd battery and cause a fire.

If you use just one battery of the same capacity of 2 of the 18650’s then you have a safer and more simple setup.

@RWB: Thank you for the explanation. It makes complete sense now!!!

I’m happy to help the same as so many others on here have helped me along this journey :wink:

It is great to see things being paid forward. :smile:

One last question: if I also want to use a fuel gauge to report on the battery charge level, do I need to connect the fuel gauge between the Sunny Buddy and the step-down regulator or can I connect it between the step-down regulator and the Photon? Said in a different way, if I connect the fuel gauge to the 5V power rail on the breadboard, which is powered by the step-down regulator, will it be able to monitor the battery charging level through the step-down regulator?
From what I have read, the Sunny Buddy doesn’t include a fuel gauge.

Thanks again for your time,

Yea, there is no fuel gauge on the Photon.

What you can do is setup a resistor divider network to measure the battery voltage and use that as a way to tell battery SOC. Search the form to see how others have done it.

My search for LiPO batteries continues. Apparently, it is not that easy to find LiPO batteries above 3.7V. All I can find are batteries for drones and remote controlled planes and stuff, like these ( so I am thinking of using a 3.7V LiPO battery and a 9V step-up regulator to power the valve and the sensors. That sounds like the easiest way right now, as it will also allow me to monitor the battery SOC easily with a fuel gauge or use something like the Battery babysistter, which is both a charger and a fuel gauge.

The battery babysitter is a really nice combo so do go with that option.

All single cell LiPo are in the 3.7v range with a 4.2v fully charged voltage so don’t expect to see more than 3.7v as the nominal voltage for a single LiPo Cell.

Run the Voltage OUT from the Battery Buddy to the VIN on the Photon. When the battery gets low or below the recommended 3.6v the Photon will start acting up or shut down. Ideally we would not feed the Photon less than the recommended 3.6v in the datasheet.


Can you provide a little more info on your project?
IE, what’s the duty cycle of the valve and sensors?
Will the Photon Sleep, or be continuously “ON” ?
Basically looking for the power requirement for the entire system (mAh over a 24 hour period).
That will give you some hints on battery and solar panel size.

If you can find a pump that requires less than 9V, this project could be simplified.
If not, a 12V SLA and 12V Solar Panel w/ charge controller may be the easiest, depending on your size/space restrictions.

If he goes with a 12v SLA battery he’s going to have to leave behind that fancy accurate fuel gauge he is wanting to use.

The SLA battery will be take up more space also which only matters if space is limited.

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