Showing Battery Level With LiPo Module

Hi All,

I have only been doing Arduino projects for about 6 months so this a probably a stupid question so I appologise in advance.

I have been looking for an inexpensive solution to powering an Ardiuno project I am working to the following rough requirements:

  • Small in size not much bigger than 9v
  • 5v output
  • Rechargable, its in a sealed unit with USB to charge

I found the following Item on Aliexpress


They are cheap LiPos (500mah) that fit in 9v package and have built in USB port for charging.

The plan is you connect to to mini buck converter to give my Arduino, LCD, RTC etc life.

The problem:
I had planned to use a Voltage sensor on the 9v battery to measure the drop in voltage and provide battery level information back to the user based on measured results etc. However when testing it to gather stats about voltage over time I found that it stayed at ~9v until the second it was dead then dropped to ~0.3v in less than a second. This obviously means I cannout use voltage to predict battery levels.

What I have found:
I took onr of the batteries apart to see what was happenening and it seems to be a ~4v liPo with a buck converter up to 9v plus charging ciruitry. So it is 4v Converted up to 9v then into my Buck converted down to 5v :(.

The work around:
To make any sense of the real voltage I need to measure the LiPo before any converters. I have soldered to wires directly to the internal battery and drilled a hole to expose them for my voltage sensor I have not tested this yet but I have attached the picture.

This will probably work but it feels like I am doing things the hard way, what do other people use to determine and show the battery levels in there projects when it needs to be rechargable?
Are there better modules out there which include battery status or am I missing somthing obvious?

Thanks in advance for your help.

Get a LiPo battery.
Get a 8 MHz, 3.3V Arduino (Pro Mini has this option).
Forget about any voltage conversions as the device will be perfectly happy with any voltage coming out of that LiPo. Any halfway modern RTC (so NOT the DS1307) and most LCD displays will be happy as well.

Thanks I will try that as it will no doubt help with getting more life out of it. But the main issue I am trying solve is knowing when it is going flat as I want to show the battery level on screen or at least a warning led. I was tracking the voltages in a hope that it would give me a roughlt linear formula based on voltage that indicates remaining battery time.

So I thought I am sure I am not the first person to want a battery indicater in my project so why not ask the forum in case you are doing it the hard way. :slight_smile:


No linear relationship. No easy/useful relationship in the first place.
Best way to get battery level of a LiPo or Li-ion battery is to measure the current draw, and calculate the charge taken based on it, and knowing the battery capacity you can estimate the amount of charge left.

Do a Google search on;

'lipo discharge graph'

To see how the voltage on a Lipo varies as its discharged.