From: muratlanne@gmail.com   
      
   "Bob F"  wrote in message news:sok163$kfc$1@dont-email.me...   
      
   On 12/5/2021 9:23 AM, Jim Wilkins wrote:   
   >   
   > How did your project to salvage the Saft NiCds go?   
      
   I still haven't done much with them The one I played with did seem to   
   charge OK.   
   So many projects, so little time.   
      
   I am doing an experiment with a NiMh battery pack in an old panasonic   
   wireless phone designed for NiCd's. I bought some NiCd replacements that   
   turned out to be NiMh. Fried a couple of those in the phones. They'd   
   last about a year, then quickly and completely die from overcharging. I   
   put a AC timer on one phone supply set for an hour a day with the last   
   NiMh battery. It started about 2/3 charged, and has stayed there for 2-3   
   weeks so far. Maybe they will last a lot longer. I will adjust the   
   charge time if the charge level drops.   
      
   -----------------   
      
   Unlike Li and Lead, the voltage on a NiMH doesn't indicate state of charge   
   very accurately, because their discharge voltage curve is nearly flat.   
   Battery data sheets are very helpful to understand them. The rule of thumb   
   to run power tool batteries down before recharging prevents simple   
   timer-controlled fast chargers from overcharging nearly full battery packs.   
   It isn't inherently necessary even for NiCds, but it covers all common   
   battery types and chargers adequately without having to learn and remember   
   each one's particular needs.   
      
   Adjusting the charge timer is an easy solution as long as you can determine   
   the setting that just reaches full charge. I think 1.5 to 1.55V per cell is   
   a safe and conservative choice for a 'dumb' charger without feedback.   
      
   Once the cells reach full charge the internal chemical process changes and   
   any further current heats them, although their voltage may continue to rise.   
   The process change in fully charged 'wet' lead-acids is to electrolyze the   
   water into hydrogen and oxygen which escapes out the vent and doesn't damage   
   the cell until the level drops below the plate tops, other sealed battery   
   types are less tolerant of the change. That's why charging rates and times   
   are supposed to be limited.   
      
   It appears that prolonged charging at the C/10 rate or below is safe, C   
   being the Amp-hour capacity rating of the cell. You won't get far designing   
   a faster 'smart' charger unless you can monitor the cell voltage and   
   current. Although it doesn't guarantee a full charge, stopping the charge   
   when the voltage rises to a preset limit appears to work without much   
   complication. I've read and observed several choices for that limit, from   
   1.5V to 1.7V per cell. Another fairly simple way to end a rapid charge is to   
   watch for temperature rise by taping a temperature-sensing thermistor to the   
   pack. That appears to be how the multi-voltage NiMH charger for my   
   Dewalt-clone NiMH packs operates, via the third terminal. I opened the   
   charger and connected test point wires to the battery socket so I could   
   record a charge cycle.   
      
   My cheap homebrew datalogger is an old laptop plus TP4000ZC DVMs, which can   
   record voltage, current, resistance or temperature. It can also record the   
   discharge of a cell to measure its actual capacity. You import the file of   
   recorded values into a spreadsheet to do the math, such as summing the   
   individual Amp-seconds to get the Amp-hour total.   
      
   jsw   
      
   --- SoupGate-Win32 v1.05   
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