Chris,Thanks for all the info - I should have clarified that I was talking about the "at rest" (unloaded) voltage. Because of the significant internal resistance of these batteries (and how the internal resistance changes with state of charge due to the conductivity of the electrolyte being dependant on the concentration of sulfate and H+ ions), loaded voltages aren't particularly telling. I think that's where much of the confusion was coming from - the posts I was finding that say to charge the pack until it is 42 or so volts are apparently talking about the loaded voltage, but I was assuming they were talking about the at rest voltage. Your info on what the OEM charger does is very helpful - I was wondering if it did any current tapering.
Thanks, Mike On Thu, 23 Apr 2009, Christopher Zach wrote:
Michael S Briggs wrote:I just got a nice new pack of Trojan T-605s (slightly lower capacity than T-105s, with a proportionately lower price). I'm trying to figure out what the voltage of a fully charged new pack *should* be, and I have seen very different things as skimming through the archives. My understanding is that the chemical reactions taking place in a lead-acid cell produce a potential difference of 2.13 Volts, so a fully charged lead-acid battery measured with a very high impedance voltmeter should measure that 2.13 Volts per cell, or 6.39V for a 6V battery, and 38.34 V for a 36V pack. But, I have seen statements in the archives saying that a fully charged pack should have a voltage of 42V or so (various posts saying anywhere from about 40V up to 44V).Voltage varies with the load on the battery. For example when the battery is at rest, a 12 volt battery that is fully charged should be about 12.9 volts.When under charge, things change. For example on my AGM batteries, a "full" battery will be at about 14.4 volts (for a 12v battery). On other types, it depends on the documentation from the manufacturer. Also as you charge you want to reduce current as the batteries get full to keep them from over-charging. The charger on the Elec-trak does an excellent job of this: It will put in 20+a up to about 43 volts, then taper back to 1a by 45 volts.When under load (you're driving) things change again. A battery is considered "done" when it's voltage is 12 volts *at rest*. At rest means sitting for 15 minutes with no load on it.Under load, you can go down as far as 10.66 volts per 12 volt battery before you run the risk of damage. The more you load the battery, the lower the voltage goes. The Elec-trak will usually cut itself off at about 30 volts (5 volts per 6v battery) due to the contactors dropping out.Note also that these numbers are per *battery* not the whole pack. It's totally possible to have 6 batteries, with 5 of them at 6 volts under load and the sixth at 5 volts. If you continue to use it, the 5v battery will start to take damage. This is how batteries die: One goes low, but it's not enough to shut down the tractor because the others are still good. It gets hammered more and more each time till it fails. This is why you can't rely on a low-voltage cut-out on the tractor to protect the batteries.With NiCD batteries, everything above is totally different :-) Chris