[Elecraft] NiMH batteries How do I test?

[Mark via Elecraft]

Re:  The float voltage is 1.25 so the numbers are 10 (for 8 cells), 11.25 (for 9 cells), and 12.5 (for 10 cells)…






The voltages Fred lists are correct for a float charger where you are charging at a rate of less than 0.05Cma for an indefinite period of time.  The KXBC3 isn’t really designed to be used as a float charger, so it uses a somewhat higher rate of 0.1Cma (200ma) and is designed for 2000mah or larger cells.  That 0.1Cma is an industry standard constant current rate, and it is meant to be used for a limited amount of time, about 16hr. is what the manufacturers recommend


The datasheets available from most manufacturers don’t always list the voltages you can expect to see when using a 0.1Cma constant current charger, but Ansmann is the exception.  Datasheets for both their 2100mah and 2500mah cells show a sharp voltage increase when reaching the fully charged condition increasing from about 1.35v at hour 10 (assuming you start with a discharged cell), up to about 1.45v at hour 11 where the voltage levels off.  Their charge voltage curves continue nearly level (1.45v) from hour 11 out to hour 16.  That flat curve represents a fully charged cell that cannot accept any more energy for chemical conversion.  The 1.45v is 11.6v for 8 cells.  


As Fred mentioned, heat is the biggest enemy of any battery, so it would be best to stop charging once the voltage reaches about 11.5v, or even 0.3 or 0.4 volts sooner to avoid adding too much heat to the cells.  Hopefully, my cells will last quite a while that way, but I will not hesitate to properly dispose of them and buy new ones if they give me any trouble.  They can become very unreliable (read that as “frustrating”) once they start to deteriorate.


The KXBC3 should protect your cells in most cases, but it is probably best to avoid being too aggressive.  Remember that it limits cell temperature to 40 deg. C which is their specification upper limit, but like most electronic components, they will last longer if the temperature is kept down a bit.


Before retirement, I worked with several battery manufacturers on NiMH technology (and lithium technology as well), and their recommendations were quite similar to the above.  Please heed Fred’s warning, these numbers do NOT have any relation to lead acid (SLA these days) batteries, or lithium cells for that matter!


Mark

KE6BB





From: Fred Townsend
Sent: ‎Sunday‎, ‎August‎ ‎24‎, ‎2014 ‎07‎:‎30‎ ‎PM
To: Mark,KE6BB, elecraft at mailman.qth.net





Jim I agree with Mark but would like to add one thing. Heat is the enemy of the battery. As Mark says the heat goes up sharply at end of charge. This is how the smart charger knows the battery is fully charged. If you attempt another charge when the battery is hot you may damage the cells. This is why the charger is programed to protect the batteries. The nominal cell voltage is 1.2vdc so for 8 cells it is 9.6vdc, for 9 cells 10.8vdc, or 12.0vdc for 10 cells. The float voltage is 1.25 so the numbers are 10, 11.25, and 12.5 respective. Do not confuse NiMH with SLA which have a nominal voltage of 12.6 and float of 13.85vdc.
73
Fred, AE6QL