[RE-wrenches] discharging Rolls batteries

[Kent Osterberg]

Hugh,

I'll take a try at explaining what is going on with battery temperature 
and high discharge rate issues.  I hope Jamie will clarify, if I've got 
it wrong. 

A battery's capacity is the number of amphours that it will provide from 
full charge down to a terminal voltage of 1.75 vpc.  It isn't literally 
dead at that time -- it isn't chemically exhausted -- chemical reactions 
just aren't happening fast enough to keep the terminal voltage up to 
1.75 vpc.  The rate that chemical reactions happen depends on 
temperature, electrolyte concentration, plate area, and more; so a 
battery's capacity depends on the temperature and rate of discharge.

If a battery is cold, chemical reactions happen slower so the voltage 
drops to 1.75 volts sooner than it would if it were at 25°C.  With a 
rapid discharge, the voltage drops to 1.75 volts with fewer amphours 
removed because the rate of chemical reactions isn't fast enough to keep 
the terminal voltage at 1.75 vpc.

If a battery is discharged at C/20 and 25°C the specific gravity will be 
(about) 1.12 when the terminal voltage gets to 1.75 vpc.  If the battery 
is colder, or if the discharge rate is higher, the specific gravity 
(corrected to 25°C) will be higher at the end of discharge to 1.75 vpc.  
That's because not as many chemical reactions took place before the 
battery couldn't sustain 1.75 vpc.  Since there are still chemical 
reactions that can take place, the battery can sustain a 1.75 vpc 
terminal voltage if it is warmed or the discharge rate is reduced.

Using the Rolls S530 as an example.  The capacity at 25°C is 400 Ah at 
C/20 or 296 Ah at C/6.  For FLA batteries, the capacity changes by 
0.8%/°C (I think that is only true at C/20), so at 0°C and C/20 the 
capacity of the S530 is 320 Ah.  If we discharge it to 1.75 vpc by 
removing 320 Ah at C/20 and 0°C and then warm it back up to 25°C, we 
could remove another 80 Ah before the terminal voltage again drops to 
1.75 vpc.  You could also think of this process in reverse order: remove 
320 Ah at C/20 and 25°C, the voltage will be 1.82 vpc and 80% DOD,  now  
cool the battery to 0°C and the voltage will be 1.75 vpc and 100% DOD.  
Same idea with rapid discharge.  Discharge 296 Ah at 25°C and C/6 rate 
and the terminal voltage will be 1.75 vpc.  Reduce the current to C/20 
and you'll be able to continue discharging the battery to 400 Ah before 
the terminal voltage is 1.75 vpc.

Starting the generator when the battery voltage is 1.96 vpc is about 40% 
DOD (-160 Ah from a 400 Ah S 530 or similar battery) at C/20 and 25°C is 
a good strategy to get good life out of the battery.  If the battery is 
cold or the discharge rate is high, fewer amphours will be removed 
before the generator starts.  At 0°C a 1.96 vpc value should be reached 
at (about) -128 Ah, which is 40% DOD based on the capacity at 0°C.  So a 
voltage based strategy for starting the generator gives (about) the same 
% DOD after temperature correction of the battery capacity. 

Outback's Flexnet DC is the only device that I'm familiar with that can 
start the generator based on the amphours removed from the battery.  It 
is supposed to display SOC and it is programmed with "SOC" values, but 
the battery capacity isn't compensated for temperature or discharge 
rate, so the SOC display is only accurate if the battery is actually at 
25°C with a C/20 discharge.  If the Flexnet DC is programed to start the 
generator at 60% SOC it'll discharge 160 Ah from a 400 Ah battery 
regardless of the temperature or discharge rate.  Just using its SOC 
programming, you wouldn't notice the fact that a battery has less 
capacity when it is cold or discharge rate is high.  In cold conditions 
or with high discharge rates, it'll let the voltage go lower before the 
generator starts.  If it was cold enough, the battery voltage could go 
below 1.75 volts and not start the generator.  Fortunately the Flexnet 
DC can also watch the voltage.

Kent Osterberg
Blue Mountain Solar, Inc.