Date: Thu, 15 Feb 2007 08:32:38 -0500
Reply-To: Dennis Haynes <dhaynes@OPTONLINE.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <dhaynes@OPTONLINE.NET>
Subject: Re: Batteries don't die, they get murdered
In-Reply-To: <45D3F915.8070808@colorado.edu>
Content-type: text/plain; charset=us-ascii
You're right, my bad! I wrote before thinking all the way through. Open
circuit voltage, at rest is the key. However, as soon as any load is placed
including internal leakage, the high resistance of a cold battery will drop
the voltage quickly. As for why, as the electrolyte cools it becomes denser.
Has the same effect at the plates as increasing the specific gravity.
However, the chemical reaction speed drops so the current delivery goes
down.
Dennis
-----Original Message-----
From: Richard A Jones [mailto:jones@colorado.edu]
Sent: Thursday, February 15, 2007 1:09 AM
To: Dennis Haynes
Cc: vanagon@GERRY.VANAGON.COM
Subject: Re: Batteries don't die, they get murdered
Dennis wrote:
> The open circuit voltage of a battery goes up as temperature goes up. The
> required charging voltage goes down as temperature goes up.
>
> Dennis
This is not true. Why do you state this? From what source?
It is true that battery "capacity" goes down with temperature.
That's why sizing guides suggest 50% capacity at freezing
compared to 80 degrees. But resting voltage goes UP and so
does required charging voltage.
One of the links I included in my original post states:
http://www.trojan-battery.com/Tech-Support/BatteryMaintenance/Charging.aspx
"6. Correct the charging voltage to compensate for temperatures above
and below 80o F. (Add .028 volt per cell for every 10o below 80o F and
subtract .028 volt per cell for every 10o above 80o F)"
And from:
http://www.batteryuniversity.com/partone-13.htm
"A cold battery would show slightly higher voltages and a hot battery
would be lower."
And from:
http://www.solarnavigator.net/lead_acid_batteries.htm
"Battery charging voltage also changes with temperature. It will vary
from about 2.74 volts per cell (16.4 volts) at -40 C to 2.3 volts per
cell (13.8 volts) at 50 C. This is why you should have temperature
compensation on your charger or charge control if your batteries are
outside and/or subject to wide temperature variations."
And:
http://www.uuhome.de/william.darden//carfaq9.htm#stages
see the chart cited from Bosch showing declining charging voltage
with increasing temperatures--after section 9.3.1.
I admit this seems counter-intuitive. It took me a while to catch
on. I maintain the photovoltaic system at a mtn hut at 10,500'
where we had several years of problems with low voltage cutoff of
the lights. I finally figured out that 14.4 charging voltage when
the batteries are 18 degrees F meant batteries that were less than
40% charged. Trojan told me the charging voltage should be 15.8!
Since setting the charge controller, we have had no problems.
The compensation factors for charging voltage and for measuring
resting voltage are the same--.028V per cell per 10 degrees F.
Call Trojan at 800-423-6569--I did.
Here are battery state-of-charge numbers at 30 degrees F:
100% 13.57
90 13.46
80 13.34
70 13.21
60 13.08
50 12.94
I have measured the resting voltage with the batteries below 20
degrees and seen voltages well above 13V. I can imagine why the
charging voltage needs to go up with lower temperature--voltage =
pressure and a colder solution--but I can't imagine why the resting
voltage goes up. That's why I say counter-intuitive. Maybe some
chemical engineer can explain. No battery site I have found tries
to explain it. Also, battery sites tend to point out the relationship
between temperature and charging voltage, but neglect it with resting
voltage--although including it with specific gravity tests. E.g.:
http://www.trojan-battery.com/Tech-Support/BatteryMaintenance/Testing.aspx
Anyway, lower temps mean higher battery resting voltage, higher
charging voltage requirements--but lower battery capacity.
I am not making this up.
Richard
Boulder
