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Re: (ET) new batteries
On Mon, 18 May 2009, Christopher Zach wrote:
Michael S Briggs wrote:
Under-load voltages are largely meaningless. THe voltage drop across the
internal resistance depends on the current being pulled from the battery
pack. So, you can get a pretty low "under-load" voltage from a fully
charged battery if the current is high enough. And likewise, if you are
only pulling a very small current from the batteries, a battery that is
fairly dead can still have a decent voltage across it if the current is
I beg to differ: Under load voltages are the only ones that really matter
since that's what we use our tractors (and cars in my case) for.
The floor voltage is 10.5 volts per battery on a 12 volt battery, or 5.25v on
a 6 volt battery. Draw it lower than that under load and your battery will be
in danger of going bad. Some batteries can go to 10v (5v), but you don't want
to draw it much lower than that.
Yes, you don't want the loaded voltage to drop below a threshold value -
but that is the extent of the usefulness of loaded voltage. A loaded
voltage of 11 Volts does not tell you diddly about the state of the
battery, if you don't know how much current is being pulled out of it.
On my car (50 batteries, 2 strings) I am happy if the pack is above 11.5 vpb
when driving around at a 60a draw and 11.0v while accelerating (120-150a
That's the key though - you specified a current draw. As I said, if you
don't know the current draw, a loaded voltage is meaningless (since the
voltage drop over the internal resistance is dependant on the current
coming out of the pack).