I've been meaning to get back to this question for some time now, but I wanted
to have all my information straight first, since it can all be a bit
confusing.
Olin Kane (okane@cscns.com) wrote:
> Does anyone have data of a similar nature that indicates how much power
> a fully charged battery loses as a function of decreasing temperature? This
> is kind of the other side of the coin (ie a warm battery starts a car better
> than a cold battery).
While it's true that a warm battery starts a car better than a cold battery,
it's not because a cold battery has less energy content than a warm one. A
warm battery is just more efficient in delivering this energy. I'll
illustrate why this is in a minute.
First, let me explain typical car (starting) battery specs. You'll usually
see two ratings on car batteries: the "Cold Cranking Rating" and "Reserve
Capacity." The cold cranking rating is the current a battery can deliver for
30 seconds at low temperature, usually 0 deg. F, and maintain a terminal
voltage of at least 7.2 volts. The reserve capacity rating is the length of
time a battery can deliver 25 amps and maintain a minimum terminal voltage of
10.5 volts at 80 deg. F. The latter is really the energy rating of the
battery: a 60 minute reserve capacity rating would be a 25 amp-hour battery.
But since car batteries are not typically used to deliver 25 amps over
extended periods of time, it's typical to see only the cold cranking rating on
batteries today.
So why does a cold battery not start a car as well as a warm one? Well,
obviously, it's usually not just the battery that's cold, the engine is too.
And an engine is harder to start when it's cold, and not just because the
crankcase oil resembles marmalade more than lubricant at very low
temperatures. Gasoline is also less volatile at lower tempatures, and tends
to condense out onto the (cold) manifold, so it's harder to get a combustable
fuel-air mixture to start the engine. This accounts for a good part of the
slow cranking and hard starting in cold weather that people tend to overlook,
but there is also an electrical (actually electro-chemical) reason.
The reason is the terminal voltage of a battery drops with temperature, due to
a decrease in battery chemical reactivity with decreasing temperature. The
reactivity drops by about half for every 20 deg. F drop in temperature. The
drop in reactivity also causes the internal resistance of the battery to
increase, exacerbating the terminal voltage drop under load. Here's a table
of what the terminal voltage of a car battery would be versus temperature when
loaded with 1/2 the cold cranking rating for 15 seconds:
Min.
Voltage Temperature 'F
9.6 70
9.4 50
8.9 20
8.5 0
Let's see how this effects starting. The impedance of a starter motor, and
the resistance of the connections and contacts in the starting circuit, don't
change much with temperature. So if a starter draws 100 amps from a battery
with a 200 amp cold crank rating, the battery will be delivering 9.6 KWatts of
power at 70 'F. At 0 'F, if the rest of the conditions were the same, the
battery could only deliver 8.5 KW, which is an 11% drop in power. But the
conditions won't be the same, as mentioned above. The congealed crankcase oil
will make the engine harder to crank over, making the starter draw more
current than at 70 'F. Because of the higher battery internal resistance, the
terminal voltage will be even less. The overall effect is that the starter
cranks the engine slower, making it even harder to get it to fire off.
To answer your question, Olin, a battery doesn't lose power at low
temperature. What it loses is some of it's ability to deliver power at lower
temperatures, as shown in the above table.
Pat Vilbrandt Fluke Corporation Everett, Washington USA
pwv@tc.fluke.COM or: { uunet, uw-beaver, sun, microsoft }!fluke!pwv
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