How to Select the Best Used Battery (V1 Motion Sensor example) 🔋


This process requires a voltmeter and a leaded resistor to perform (~300-330 ohms for a V1 Motion Sensor Battery).


I had a V1 Motion Sensor I needed to reactivate, and a batch of used CR2450 batteries. I did a quick no load-voltmeter only test on them and picked the top performer to use (3.119V on a 3V battery). Sounds perfect, right?

Well, the motion sensor wouldn’t even light up (it should blink once when activated). So it was completely inoperative.

Now obviously, I didn’t need to measure the batteries to find the highest-voltage unit. All I really had to do was pop them into the motion sensor one at a time until one flashed the light. But this article is about selecting a battery from several used units to find the one that will last the longest, and to illustrate why testing them unloaded does not tell you that (or much of anything, actually).

So I called up the CR2450 battery spec, and it said in order to test the ‘pulse’ condition (a high short-term load the battery should be capable of) to use a 300-ohm resistor and a test goal of 2.7V.

So that’s what I did. This simulates the motion sensor going from its quiescent state when the sensor is in standby, to a pulse current when the sensor wakes up and transmits.

My sensor was not waking up and transmitting, which is indicated by the light blinking when you install the battery (it is trying to report motion – your body installing the battery).

I didn’t have a 300-ohm resistor, so I used a more common 330-ohm size.

The pulse load is a high load, so you really don’t want to leave it on the battery long. It is not a heat risk; the 300-ohm resistor will only draw 9mA at 2.7V, which would be less than 1/40th of a watt. The voltage will move as the load is measured, so what you want to do is just apply it and a few seconds later take the ‘gist’ of the measurement (for me that was to 2 decimal places). The highest voltage battery under that condition will last the longest. :slight_smile:

The result on the battery I used above was that the output was less than 2.7V when loaded, a red flag.

So I tested all 9 of my used batteries, and 2 new ones. The results are in the table below.


You will notice ALL batteries measured above 3V unloaded, so all might otherwise be considered good (they all actually all measured above 3.1V)! Even loaded with what the battery manufacturer considers a constant load (7500-ohms), they all were still above 3V.

However when pulse-loaded, the 3 batteries that failed to light the light on the sensors were clearly below the ones that worked. They were all also the only batteries below the 2.7V target voltage.

As an example of ‘unloaded measurements don’t work’, notice that battery #3 had the HIGHEST voltage of the used batteries unloaded (this is the one I started with), but failed to run a sensor and measured below 2.7V when pulse-loaded.

Also, battery #1 was the LOWEST voltage of all batteries when unloaded, but still ran a sensor and had a greater than 2.7V measurement when pulse-loaded.

So unloaded doesn’t work as a way to test batteries for a V1 Motion Sensor. Or for many other devices.

This process can apply to most any battery-operated device. Just do an Internet search for the battery’s spec sheet, load the battery with the heavier load, and pick the battery that has the highest voltage in that condition. :slight_smile:

Caveat: Don’t use this process for 2-battery configurations – those should always be replaced in pairs with new batteries. Single-battery configurations only.


NO LOAD (Voltmeter Only) (this test did not find the bad batteries) :frowning:
‘Bad’ indicates a battery that wouldn’t operate a sensor
Test goal = 3V

Bat# Condition No Load
1 Used/Good 3.101
2 Used/Good 3.108
3 Used/Bad 3.119
4 Used/Good 3.118
5 Used/Bad 3.109
6 Used/Bad 3.114
7 Used/Good 3.116
8 Used/Good 3.118
9 Used/Good 3.117
10 New/Good 3.185
11 New/Good 3.233

7500-ohm ‘Constant’ Load (this test did not find the bad batteries) :frowning:
‘Bad’ indicates a battery that wouldn’t operate a sensor
Test goal = 3V

Bat# Condition 7500 ohms
1 Used/Good 3.066
2 Used/Good 3.097
3 Used/Bad 3.087
4 Used/Good 3.094
5 Used/Bad 3.049
6 Used/Bad 3.091
7 Used/Good 3.065
8 Used/Good 3.093
9 Used/Good 3.103
10 New/Good 3.180
11 New/Good 3.226

330-ohm ‘Pulse’ Load (this test DID find the bad batteries) :grin:
‘Bad’ indicates a battery that wouldn’t operate a sensor
Test goal = 2.7V

Bat# Condition 330 ohms
1 Used/Good 2.82
2 Used/Good 2.94
3 Used/Bad 2.62
4 Used/Good 2.87
5 Used/Bad 2.2
6 Used/Bad 2.61
7 Used/Good 2.80
8 Used/Good 2.85
9 Used/Good 2.91
10 New/Good 3.08
11 New/Good 3.10

So the longest lasting batteries will be in the order of:

11, 10, 2, 9, 4, 8, 1, 7 – with 3, 5, and 6 going to the trash. :slight_smile:


Good stuff. My brain hurts after reading it. Now I just need the right tools :thinking:


I was trying to think of a TLDR, but all I could come up with is “don’t test a battery unloaded”, lol.

It looks like one of your posts otherwise! :wink:


:rofl: indeed! I like it. I want to become more knowledgeable about electrical stuff one of these days. I still feel like a big n00b.


Nice writeup sir!


Nice, very interesting stuff, though some of it was a bit much for me.

I’m sure it’s will come in handy for many, and a bonus that the forum threads show up in google so more general questions could be answered by this as well.