hi
We know that batteries deliver less amphours at low temperature and
at high currents. Volts drop quicker. That's my starting point. My
question that I still do not hear an answer to is this:
If the battery is a bank account and its harder to get the money out
in cold weather and when you want to get your hands on a lot at
once... Does this actually mean that some of the money gets lost?
What happens to it? Is it perhaps available later when the bank
warms up or the demand gets less hectic? Is there really less money
in there or does it just seem like less due to the conditions?
I notice that Ah capacity is actually defined as how much Amphours
you can get out before the battery reaches a certain terminal
voltage. I am wondering whether it is the ability to maintain
voltage that is the limiting factor whereas the chemicals in there
can still deliver amphours, given the right temperature and time
later. You can certainly see recovery take place when a battery
warms up and/or operates on lighter loads.
One last time what happens to the chemicals (lead and lead oxide)
that represent Amphours of charge in the battery plates? For me this
is a little bit like current of 10 amps entering one end of a piece
of wire and only 9 amps coming out the other end. I understand that
the volts go down due to voltage drop (in this analogy) but loss of
current is entirely a different matter.
Thanks for any help with this rather obscure question.
Hugh
A lead-acid battery is an electro-chemical processor (just like you
and other living things). When you and your battery are cold or hot,
performance changes because the chemical process is affected by
temperature. Cold equals sluggish chemical reaction, reduces the
capacity to perform work, and affects battery performance
linearly. Battery chemistry is well understood. When I get
some time, I'll google for temperature-based formulas and charts
unless someone else posts the links first.
----- Original Message -----
From: <mailto:h...@scoraigwind.co.uk>Hugh
To: <mailto:re-wrenches@lists.re-wrenches.org>RE-wrenches
Sent: Friday, January 15, 2010 12:02 AM
Subject: Re: [RE-wrenches] discharging Rolls batteries
Hi Jamie,
Remember, as batteries cool actual capacity is reduced, so if 200AH
is 50% @ 25C it is significantly more than 50% @ 5C. Thus, you are
discharging more deeply.
But earlier you put it this way:
Regarding temperature effects on capacity, earlier responses are
spot on as the lower capacity is totally as a result of slower
reaction times as a result of lower temperatures.
There is an issue here that I need to understand better. You state
that a battery has lower capacity in low temperatures. Suppose you
take a fully charged, 400 Ah battery and cool it down to -5 degrees
C where according to our numbers it will only have 80% of its
nominal capacity. You then remove 160 Ah (say 10 amps for 16
hours). It will then be 50% discharged. Now warm it up again to 20
degrees or whatever. My question is: will you only have 200
amphours left in it now? And if so, what happened to the other 40
amphours? Does low temperature operation actually lose amphours, or
is it just more sluggish? What is the chemical explanation for the
lost amphours?
I understand batteries as a chemical process of converting amphours
into chemical changes. I assume that a given amount of electrical
charge converts a given amount of lead into lead sulphate (and
likewise) back again. I understand that cooling will make this
process less efficient and thereby result in a rise in charging
voltage and a drop in discharging voltage. But does a low
temperature actually mean that a given amount of lead being
converted to sulphate actually give you less amphours electrically?
(I have similar questions in relation to Peukert's equation where
high discharge rates impact on the amphour capacity. The capacity
apparently 'recovers' when the discharge rate is reduced. To what
extent is the capacity actually lost by using high discharge rates
and to what extent is it just a voltage effect that impacts on the
terminal voltage, rather than the actual chemical state of the
battery?)
I hope you can follow my descriptions.
--
Hugh Piggott
Scoraig Wind Electric
Scotland
http://www.scoraigwind.co.uk
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--
Hugh Piggott
Scoraig Wind Electric
Scotland
http://www.scoraigwind.co.uk
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