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 minimal.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 draw).
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).
Mike