Measure Ultimate GPS Logger Shield battery voltage

I'd like to have my sketch measure the battery voltage and and report when the battery needs to be replaced (I don't get anywhere near the 7 years claimed in the "Adafruit Ultimate GPS Logger Shield - Includes GPS Module" description. My 1 year old battery is down to less than 1 volt. The GPS took more than 2 minutes to get a position.

Can I just solder a wire to one of the battery holder solder points to use to measure the battery voltage and connect that to A0 (or A1, A2, A3)?

What voltage should I use for the "Replace Battery" threshold?

Thanks. Joe

szedula

Posts: 31
Joined: Tue Jul 07, 2015 10:51 pm

Re: Measure Ultimate GPS Logger Shield battery voltage

Most of the time when you use batteries, you use a 3,3V board. Is that correct?
And since a fully charged battery is close to 4V, it would be outside the range for the A0-A3 pins. So what you need is a resistor-divider, for example 2x10kΩ resistors. Put them in series between V-bat and GND, and connect the middle to A0. Then double the measured value;
Vbat=analog value*3.3/1023*2

blnkjns

Posts: 663
Joined: Fri Oct 02, 2020 3:33 am

Re: Measure Ultimate GPS Logger Shield battery voltage

I'm using the Ultimate GPS Logger Shield on a Mega2560.

szedula

Posts: 31
Joined: Tue Jul 07, 2015 10:51 pm

Re: Measure Ultimate GPS Logger Shield battery voltage

blnkjns

Posts: 663
Joined: Fri Oct 02, 2020 3:33 am

Re: Measure Ultimate GPS Logger Shield battery voltage

I know how to convert A-to-D reading to voltages.

Wat I was looking to get answers for;

1. Where is the "best" place to wire to the battery Vbat?
Is there a nice place on the shield other than surface soldering a wire to the battery hold-down?

2. What threshold voltage should I use for the "replace the battery" message?
Is it 1V, 1.1V, 0.9V or some other value?

Thanks, Joe.

szedula

Posts: 31
Joined: Tue Jul 07, 2015 10:51 pm

Re: Measure Ultimate GPS Logger Shield battery voltage

A Li-Ion should not get that low. Stop using it at 3.3 volt.
https://lygte-info.dk/info/BatteryLowVoltage%20UK.html

blnkjns

Posts: 663
Joined: Fri Oct 02, 2020 3:33 am

Re: Measure Ultimate GPS Logger Shield battery voltage