Battery Testers

© Brooke Clarke 2007

There are a number of reasons why you might want to test the capacity of a battery.  For household type batteries it's good to know if it's the battery that has died or the device holding the battery.  When working with rechargeable batteries it's good to know the state of charge, i.e. what's the capacity.

I've found a number of cases lately where the Radio Shack battery tester shows in the middle of the green, yet the battery is not powering the device.

These testers estimate the remaining life of a battery.  For Carbon Zinc and to a lesser extent Alkaline this can be done by measuring the battery voltage while a light load is applied.  If the proper load is chosen for a given battery capacity the loaded battery voltage works well as an indicator of remaining life.  The idea is to determine the state of charge using as little of the batteries capacity as possible.

Aug 2007 - Added test jack table to TS-183 web page so that the Radio Shack load resistors could be compared to the TS-183 load resistors.  The Radio Shack tester showed a 9 volt battery as good yet the battery would not work in the Stamps.com postal scale.  The TS-183B showed the resistor as bad after abut 5 to 10 seconds.  There's a 100% difference in the load resistors for this voltage.  Need to check the others.

In addition to the test leads this battery tester also has metal contacts for a 9 volt battery on the front panel, the negative contact and be used on the bottom of AA, C, D battery and the red lead to make testing easier.  On the right side is a slot for button cells and part of the front panel can be depressed to make contact to them.  For most common batteries I have used this tester, but now am no longer as confident in it's abilities.
 

Since the above Radio Shack tester does not work very well I've been on the lookout for an improved tester.  ZTS makes a number of handheld battery testers and the Mini-MBT (Multiple Battery Tester) is what got.  They have a utility and a design patent.

6823274 Apparatus and method for testing remaining capacity of a battery, Nov 23, 2004, ZTS, 702/63 ; 320/125; 320/136; 702/64; 702/65; 702/79

uses voltage at end of resistor load pulse.  D515444 Battery Tester - packaging for the 6823274.

When a battery is connected it's immediately under load and after a few seconds the loaded voltage is read.  The capacity is based on the loaded voltage at the end of the test time.   The patent has data on the load resistance for 8 different battery types as well as capacity information in 20% steps for 3 & 6 Volt versions of the CR123 photo battery and for the 1.5 volt alkaline terminal.

The four battery types that the MBT will test are:

Cell Type	Details
Ni-MH	1.2 volt
Alkaline	AA, AAA, C, D, N
Lithium	CR123, CR2, CRV3
Alkaline	9V

I checked the same battery that the Radio Shack tester showed in the green and this tester either shows it as bad or on the second try it failed to start the test, meaning it's dead.
Also checked some CR123 cells that have been here for years.  They test at different levels.  I haven't made any controlled discharge tests to see how accurate this tester is, but at a first look I like it.
The Mini tester uses a socketed 18 pin micro controller without a seperate MUX IC and without an external crystal.  The circuit board is soldered to the positive and negative battery terminals and so can not easily be removed.  There's a 4 post 0.1" header for testing the unit, but it's between the PCB and back cover so is not accessible after the front cover is removed. 

Instead of using a switch like on the Radio Shack tester, this one uses 4 different positive battery terminals and a common negative test lead.  There's a groove around the tester to hold the lead and a pocket for the short probe body on the left side not shown in the photo.  The On button is on the right side.

There are two lower priced ($30) models this one (Mini-MBT) and the MINI-MBT9R.  The difference is that on this one the lower left terminal is for the CR123 Li-MnO2 primary battery and on the other one it's for a Rechargeable 9 volt battery.  I specifically wanted to try the CR123 battery test since I use these in some of my products (5BA, 90BA Battery Adapters).

The TS-183 works the same way.  You pick the test jack to match the battery instead of using switches.  The load is permanently associated with the jacks.  For example on the TS-183 jacks 1, 2, 3, & 4 are for single cell batteries and 1.40 V is the 80% indicator.   Jack 3 is for a "D" Zinc Carbon battery and uses a 6 Ohm load.  The ZTS tester is using a 9.1 Ohm for alkaline cells with a 80% voltage of 1.41 V.  ZTS could eliminate the whole relay circuit and thus lower the cost of the tester since one of the connections to the cell under test is by the probe.  That's to say whoever is testing the battery has to hold the probe on the cell   If there was a socket holding the cell then the relay has some merit, but in this case it's not likely someone will "forget" and keep holding the probe on the cell for an extra hour or more.

There are some  aspects that could be improved. 

• Using a post for the battery positive terminal seems to be upside down.  Either use a socket for the positive terminal or make the post for a negative terminal.  It's not as bad as balancing a pin on the point of another pin, but not that far different.
  
• The display is at the top so when you are holding a battery upside-down on one of the posts with one hand and holding the probe with your other hand the display is somewhere behind your hands.  Better to place the display at the bottom.  If you turn the unit upside down then the display reads backwards and the labels are all upside down, and you need to be left handed, but it still a better way to use the stock tester.
• The On button seems to get activated whenever the unit is handled.  Maybe put it on the top narrow face where you don't normally hold the tester.
• The word Pulse" does not seem to have anything to do with how it works.  The load resistor is always connected to the test terminal so the battery sees the load as soon as the circuit is compleated.  When powered up the tester scans all the terminals and when it finds on with a voltage starts a timer.  At the end of the time (always 3 seconds?) it reads the loaded battery voltage.  So there's no pulse involved.  Then, after making the measurement the relay opens the load resistor to keep from draining the battery.  Maybe this is what they mean by pulse? 
  

These are nit picks.  This is be far the best battery tester I have.

They make more expensive testers that can test a longer list of batteries.

Army dry battery tester.  Works by measuring loaded voltage.

The set can be carried on a belt and tests radio batteries, Vietnam era.  Uses high current load to activate Magnesium batteries.

Tester for LiSO4 primary batteries BA-5590/U, BA-5598/U and BA-5588. 

4725784  Method and apparatus for determining the state-of-charge of batteries particularly Lithium batteries, Feb 16, 1988, 324/427 ; 320/149; 320/150; 320/162; 340/636.15

Uses pulse loading and voltages measured at various times in relation to the load pulse as well as temperature correction.  Capacity depends on manufacturer as well as model.

The idea here is to drain the battery using a known load until it's voltage is down to the end voltage and record the delivered Amp hours, or better delivered Watt hours.  Note that when the battery starts the test it's voltage is highere than at the end where the voltage is the pre assigned ending voltage.  So the early amp hours are at a higher voltage than the ending amp hours.  The early Watt hours are higher than the ending Watt hours.  So reporting the run time using a load resistor or constant current source is not as accurate as reporting the delivered Watt hours.

The results for a given battery will be different as a function of the load. 

Resistor

A fixed resistor is the load.  The load current decreases as the battery voltage goes down. 

Filament Flashlight Bulb

The resistance of the bulb changes in a non linear way with the battery voltage.  The bulb gets dimmer as the battery voltage gets lower.  How dim does the bulb need to get to say the battery is dead?  Kind of a fuzzy feely thing maybe not the best capacity test method.

Constant Current

When a constant current load is used the delivered Watt hours are directly proportional to run time.  This is the most constant load type. 

Constant Power

Switching Mode Power Supplies, like used in most modern battery powered devices consume what's pretty close to a constant power in Watts.  This means at higher battery voltages they draw smaller currents than when fed lower battery voltages.  This is the oppsite of the resistor load.

This is an electronic load that can be set to look like a constant resistance or constant voltage load.  By using a computer to control the load and read back the battery voltage and current almost any test sequence can be accomplished.  There is a low voltage limitation of  2.5 Volts.

Watt's Up

Maha 777Plus

Maha MH-C9000

Triton2

New for me Oct. 2007 are the "Ready to Use" rechargeable batteries.  Sanyo eneloop brand in AA Ni-MH format.  Capacity of 1.9 to 2.0 AH.  Testing capacity can be typically done in less than 24 hours, but shelf life may take months or years.  See the eneloop Ready To Use battery web page for the test plan.

self-discharge (Wiki) causes most rechargable batteries to loose charge just sitting.

Chemistry	% / month
Ni-MH	30
Ni-Cad	15 to 20
Li-xx	2 to 3
Ready to Use Ni-MH	1.5

This is the current a battery can source when a load of 10 milli Ohms is applied for 200 milli seconds.  So far the only mechanical relay I've found that can handle 40 Amps (needed for "D", "F" and No. 6 dry cells) with a contact resistance will below 10 milli Ohms  is the one used in car starter motor circuits typically rated for 400 Amps.  Solid state devices have way too much resistance.  A method of getting the required load is to connect just under two feet of 14 AWG copper wire across the terminals.  The problem is only connecting it for 200 milliseconds and then disconnecting.  If left connected longer serious problems with the battery or load may occur.