[time-nuts] HP 105B: Modern replacement for NiCad battery pack?

Van Horn, David david.vanhorn at backcountryaccess.com
Fri Jan 5 14:00:40 EST 2018

In a former job I designed battery charging systems for NIMH cells.
Some cells will TOLERATE long term trickle, some spec ZERO trickle current.  Get a data sheet and read it.  

Violating that spec, or low quality cells can get you a battery pack that will arbitrarily, and without even being connected to anything, suddenly overhead and spew boiling lye out the end. One prototype did this with the pack literally sitting disconnected on the desk while we were out for dinner.  It took a few layers off the PCB that was sitting nearby.

I had specified cells from a Japanese company, but the accountants insisted I use cells from China at half the price.   Cost us a 100% recall of battery packs.

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Mark Sims
Sent: Friday, January 5, 2018 10:54 AM
To: time-nuts at febo.com
Subject: [time-nuts] HP 105B: Modern replacement for NiCad battery pack?

Since NiMH cells typically have over twice the capacity of NiCad cells, a C/10 charger will charge them at less than C/20.    It's best to trickle charge NiMH cells in the C/30 to C/40 range, but depending on the cell C/20 might be OK.   Measure your charge current on a discharged pack and calculate the C rate.  Tweak the charger supply for a C/30 to C/40 rate.


NIMH is not necessarily a drop in. While it will work fine in the short term, crude chargers that implement constant trickle like C/10 can be (emphasis) tolerated (/emphasis) by some NIMH cells, and totally out of spec for others.
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