Aargh!  I had to cancel the order that I had placed with
http://www.batterystation.com because they couldn't put tabs on those cells.
Seems that they were a special lot somewhere in a warehouse in the far west
that didn't have a welder to attach the tabs.  So, I went to the site that
Brooke suggested (batteryspace.com) and ordered two of the 7.2V 1600 maH
2/3A NiMh packs for $12.95 each.  Higher maH rating, plus already assembled
into a pack.
As an afterthought; I'm wondering if I could install a "smart charger"
inside the unit to properly charge the battery pack instead of leaving it on
trickle charge all the time.  From the schematic, I'm guessing that the unit
trickle charges the battery pack at around 10ma.  Normally, NiCd and NiMh
batteries like to be trickled at 0.1C, which would be 160ma for the battery
pack I've ordered.  I doubt that the little power supply in the 731B could
handle such a drain.  That's why the thought about installing a smart
charger.  Are "smart chargers" quiet or do they emit any EMI or RFI that
might conduct to the output of the unit?

Thanks for all the info and advice.
David
dgminala at mediacombb dot net

Hi David:

Ni-Cad chemistry is very different than Ni-MH.  It's OK to trickle
charge Ni-Cad but it's a no-no to trickle charge Ni-MH.
You might ask Battery Space if it's OK to use their "smart charger" when
it's always connected to the battery pack.  Or, if they have them, get
Ni-Cad packs.

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.End2PartyGovernment.com/

Dave M wrote:

0.1C is not trickle charge, it is the normal charge rate for 14 hours recharge time.
Most NiCad and NiMH batteries will not like that too much as a trickle charge rate and will get and stay warm, at least those that I have had experience with. The batteries in the HP 8116 function generator were trickle charged at that rate and cause extensive damage when they get tired of it. I have had better luck

Trickle charge would be 1/10th or 1/20th of that.

Didier KO4BB

Sent from my BlackBerry Wireless thingy while I do other things...

-----Original Message-----
From: "Dave M" dgminala@mediacombb.net
Sender: volt-nuts-bounces@febo.com
Date: Mon, 22 Aug 2011 10:16:18
To: volt-nuts@febo.com
Reply-To: Discussion of precise voltage measurement volt-nuts@febo.com
Subject: Re: [volt-nuts] volt-nuts Digest, Vol 24, Issue 19

Aargh!  I had to cancel the order that I had placed with
http://www.batterystation.com because they couldn't put tabs on those cells.
Seems that they were a special lot somewhere in a warehouse in the far west
that didn't have a welder to attach the tabs.  So, I went to the site that
Brooke suggested (batteryspace.com) and ordered two of the 7.2V 1600 maH
2/3A NiMh packs for $12.95 each.  Higher maH rating, plus already assembled
into a pack.
As an afterthought; I'm wondering if I could install a "smart charger"
inside the unit to properly charge the battery pack instead of leaving it on
trickle charge all the time.  From the schematic, I'm guessing that the unit
trickle charges the battery pack at around 10ma.  Normally, NiCd and NiMh
batteries like to be trickled at 0.1C, which would be 160ma for the battery
pack I've ordered.  I doubt that the little power supply in the 731B could
handle such a drain.  That's why the thought about installing a smart
charger.  Are "smart chargers" quiet or do they emit any EMI or RFI that
might conduct to the output of the unit?

Thanks for all the info and advice.
David
dgminala at mediacombb dot net

volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

On the contrary, the manufacturers of NiCd cells consider 0.1C to be the highest
rate that can be applied indefinitely to a NiCd cell.  It is by definition the
trickle charge rate.  At the 0.1C rate, the cell will be fully charged in 14 to 16
hours.... The extra 50% allows for losses in the process.

If you charge at a C/40 rate, it will take 64 hours to fully charge your pack,
which is too long for most applications.

C/250 to C/300, which are the maximum indefinite term charge rates that modern
NiMH cells can take are ridiculously long.  If you want to "trickle" charge at
that rate, you will need a charger that can boost the cell up to full charge
at a higher rate, and then drop down to the C/250 to C/300 rate as a maintenance
charge rate.

-Chuck Harris

shalimr9@gmail.com wrote:

Hi Chuck:

There's a fundamental difference in the chemistry of these two
batteries.  The Ni-Cad is what's called endothermic so the temperature
remains relatively constant during charge, but the Ni-MH is exothermic,
meaning that the battery temperature is rising along with the state of
charge.  So you can leave a Ni-Cad on charge at a 0.1C rate and it will
be charged in a half to full day and the temperature will be warm but
not hot,. BUT, if you put a Ni-Cad on 0.1C charge it will get very hot
and degrade the battery and might even catch fire or explode.  Hence the
constant rate recommendation of C/250 (or lower).  It's not to charge
the battery but to keep it from discharging.  Some other charge
mechanism needs to be used in addition to the constant rate for charging.

Ref Handbook of Batteries, third edition, Linden & Reddy, pg. 29.22.

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.End2PartyGovernment.com/

Chuck Harris wrote:

Hi Brooke,

Yes, I know.  I remember explaining it to you 5 or 6 years ago.

-Chuck Harris

Brooke Clarke wrote:

..........................^NiMH^