DPRG mailing list archives: [DPRG] Batteries in parallel

Tue Mar 27 10:15:34 CDT 2007

John,

The batteries are primary cells, not rechargeable.
I understand that 9v batteries have a high internal resistance and will
balance out, but my customers will inevitably replace only one battery
and leave an old low voltage battery installed. This will drain the new
battery into the old one and shorten the life of the unit. The end
result will be that the customer will blame the machine.

I think that either the PNP or P channel MOSFET solution will work
pretty well. I'll have to prototype it and try it out.

Rick

________________________________

From: jdrumm9015 at aol.com [mailto:jdrumm9015 at aol.com] 
Sent: Monday, March 26, 2007 3:08 PM
To: Rick Bickle
Cc: dprglist at dprg.org
Subject: Re: [DPRG] Batteries in parallel

Rick,
  So?  Just call it "charging".  Are the 9V batteries you plan on using
rechargable or primary cells?
If they are primary cells (i. e. non-rechargeable) just live with the
slight differences in cell voltage and they will all balance out in a
very short amount of time and the batteries would enter the operational
mode of life. Eventually the first one runs out of chemical activity and
starts to drag the pack down.  A low voltage alarm would sound (like a
smoke detector) and you would replace all the batteries, since they are
all nearing death.

  However if you plan on re-charging the 9V batteries insitu and not
replacing them you will need isolation devices as you are  planning on
right now.

Regards,
John Drummond

-----Original Message-----
From: rbickle at intconsys.com
To: dprglist at dprg.org
Sent: Mon, 26 Mar 2007 10:45 AM
Subject: [DPRG] (no subject)

Hello all, 
I've got an application where I want to put a number of 9v batteries in
parallel to increase the current capacity. I need to prevent the higher
voltage cells from discharging into lower voltage ones, but I don't want
to have the 0.7 volt diode drop. (Germanium diodes don't have enough
current capacity.) Doe anyone have a circuit to do the same job as diode
isolation without the voltage drop?

Rick 
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