Re: (ET) Fully charged pack voltage

[Christopher Zach <cz alembic crystel com>]

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