Rechargeable Battery management guidelines

Dave Marlow, CBET University of Michigan Health System

• All rechargeable batteries should have some type of an initial date, a start of warranty date (by the supplier), a charged/tested date, or an installed date.

• Lead (SLA, Acid/Calcium) batteries need to be maintained in storage with the open circuit voltage above 2.11V/cell. The date of each recharge/ test should be noted on the battery.

•Batteries in critical devices should be tested and dated with in 6 months prior to installation (MFR. supplied batteries for these usually have been cycle tested, third party suppliers typically only check terminal voltage).

Rechargeable batteries should be maintained or replaced according to the device MFR’s. recommendation, unless we have documented that a different plan provides equal or better performance assurance and may be more cost effective.

Maximum battery life is expected to be no more than the following:

NiMH batteries should be replaced within 4 years of initial date on the battery.

Lead (SLA, Acid/Calcium) batteries should be replaced within 5 years after the initial date.

NiCd batteries should be replaced within 5 years of being installed (NiCd batteries can sit discharged for many years without significant degradation).

Li-ion batteries should be replaced within 10 years after the initial date.

It is possible for any rechargeable to fail in less that a year, if it is:

Exposed to excessive temperatures for excessive time.

Over charged.

Charged or discharged at excessive rates.

Exhaustively cycled. Batteries other than NiCd, may fail from being left completely discharged.

Additional Considerations for Smart Batteries: (Batteries with management circuits within the battery pack)  Smart Batteries usually have a battery gas gauge as part of them (you push a button and a bar of LEDs light up momentarily to indicate the level of charge in the pack as a percentage of existing capacity).A problem with this is, if the existing capacity is low, the display will be deceptive (100% of zero is still zero).

If the Smart Battery is using a SM Buss standard interface (most do), the information that you can read from it includes the following: Date of MFR Number of cycles on the battery Original spec. capacity Last measured capacity and the % error.

The device that uses or charges the battery may be able to display some or all of this info, if not a generic SM Buss reader usually can, providing the battery connector can be adapted to it.

With Smart Batteries and the ability to read them, you should not have to test them.

Semi-automatic calibrations may be required and they are normally done in the device or the charger designed to work with them.

(check the devices operators manual)

How to use the Smart battery info:

The date of MFR can be used as the initial date.

The number of cycles can help in predicting when the battery may need replacing based on experience with similar batteries.

The last measured capacity as compared to the original spec. capacity tells you how good the battery is. The % error is the confidence factor,1% error - a very good indication, 20% or more error - not good - calibration is needed. 

Smart Batteries are constantly using a little of the battery power to keep their logic awake, if not charged every few months the battery can run down and the logic will go to sleep, trying to stop a complete discharge, which can ruin other than NiCd batteries.

If the battery has not been asleep too long it can be awoke with normal charging, a longer sleep may require a boost charge to be applied manually with another power source and the battery may have suffered some loss of capacity. If it won’t wake up with a boost charge - it has died.

Another problem that a Smart Battery can have is a failure of its logic, causing:

Corrupted information.

Failure to communicate.

Will not charge or discharge.

Repair of these problems is normally not possible. .

Batteries can be tested in or out of a device (it depends on the device design and testing equipment availability as to which method is best).

So called quick or partial run tests are rarely an adequate test.  Testing the batteries in the device involves following the MFRs instructions, usually noting the time to low battery alarm and the final stop time. On devices that you have a large number of, this can be tedious and not a very productive use of your time.

Testing batteries out of the device involves connecting them to a battery analyzer and having it programed to give an appropriate test for the battery in question.

Then documenting the test, this can be as simple as noting the results on the battery it self, if there is a automatic record of the test, also note the test record number on the battery.  

For our Cadex Battery analyzers you will at least need to know the type of battery, the battery voltage and the mAH rating of the battery. With that info the battery analyzer can test most batteries with its other settings at their default standard settings.

One exception is the small button cell batteries they usually need to be set for 10 hour charge and discharge.

When in doubt as to the battery specs. check with the battery supplier or the internet.

More information is available at Cadex.com

Why test?

•To Assure Proper Performance in Critical Applications

•To Collect Data - to make decisions regarding Suppliers and How Often Replacement is Needed

•Failure Analysis

•Manage Replacement Costs

Mixed bag of challenges

•Different Battery Technologies

•Different Connections and Capacities

•Different Current, Voltage limits

•How Fast/ How Accurate?

•Medical Facilities with Different Capabilities

•User Training and Experience Differences

Minimal tools