Instruction Manual

34
6.C.3: System “battery efficiency cycle” logged data:
Program modes affecting data: P14, P15, P32, P33, P47.
Data always on: This type of data is “always on”--it does not need to be programmed on. . It is
available either from the “display unit” using display modes AD29 through AD40 or as a downloaded
file using the computer interface. The downloaded file can then be accessed on your computer using
Excel, or other spreadsheet program which shows a history of previous charge cycle efficiency.
Purpose of data: This data is designed to determine how much charge (or amp hours) the battery
system loses during each charge/discharge cycle. An ideal system would lose 0 charge: you would be
able to remove the same number of amp hours while discharging as went into the batteries during
charge portion. However a real system will lose some energy each cycle due to two causes: (1) the
self discharge current of the batteries and (2) The extra charging that occurs after the batteries are
fairly well charged, which typically results in some gassing of the batteries.
When batteries are new the self discharge should be low. As they age the self discharge will
eventually increase.
Although it wastes some energy, the extra charging at the end of the charge cycle is useful to maintain
high capacity of lead acid batteries. Tracking the amount of extra charge required per cycle is useful
to see that the system is being charged sufficiently, and that the batteries’ self discharge is not
becoming excessive. Some experts recommend that lead acid batteries in solar energy systems
should be recharged with at least 110% of the amp hours that were discharged. This would represent
an “amp hour efficiency factor” of 100/110 = 91%.
Exactly how is data measured? First a brief description of how this measurement will be given--then
a more detailed description will follow. Starting from a “full” battery the PentaMetric measures the total
discharge (amp hours) leaving during its normal discharge period, and then separately measures the
charge required to recharge to “full” again for each discharge/charge cycle. As said before, a perfect
battery would result in equal amounts of “discharging” amp hours compared to “charging” amp-hours.
With a real battery however, the amount of discharge “amp hours” will be slightly less than the amount
of amp hours required to charge it originally, because of the battery’s “self discharge” current, and
also, as said, because extra beneficial charging is done which results in some charge being lost by
“gassing” of the batteries. There are two objectives of recording this data: one is to determine how
much energy is lost in one cycle, and thus determine if the “self discharge” of the batteries is becoming
excessive, due to aging or other problem. The other is to determine if sufficient additional charge is
being delivered to optimally maintain the batteries.
Here is a more precise description of how this is measured: First the PentaMetric determines when the
battery is “charged” by noting that the battery voltage has exceeded the “charge setpoint” voltage, and
the battery charging current is less than a “charge setpoint” amp value. (These setpoints are entered
by Program Modes P32 and P33.) Once this has been attained, the official beginning of the
“discharge cycle” may
occur after this, the first time the battery starts to discharge: that is when the
“amps” value goes from “charging” to “discharge”. This time, and amp hour values at this time are
recorded as possible
beginning data The reason for sounding somewhat uncertain, is that it is
possible that after only a tiny discharge has taken place, the battery may be re charged to “full” again,
(perhaps because a cloud obstructing the solar array has passed.) So if this occurs the old beginning
values are discarded, and new ones are taken when the battery again begins discharging. These
values are only kept and permanently recorded as the beginning data when the %full value drops
below 90% charge. Then the “begin” data is officially recorded, and two registers are zeroed with
reference to this already recorded data: one of which will accumulate all “charging” amp-hours, (i.e.
when “amps” value is positive) and the other records all “discharging” amp hours (i.e when the
“amps” are negative). Finally, when the battery again reaches “charged”, this is NOT taken yet as the
end of the cycle. The end is taken only when the next beginning occurs, as described above. So it
records data for a completely closed cycle. And, since we still don’t know if that is the real
beginning
time, the cycle results are not recorded, or shown in the display until the battery charge declines below
90%, into the next cycle. At this point two numbers are calculated and recorded. One is the ratio
between discharge amp hours to charge amp hours (the “charge efficiency factor” in %). The other is
the “average self discharge” amps, which is (amp hours charged) minus (amp hours discharged), all
divided by the number of hours between the cycle. This number, “average self discharge amps”, is
recorded between 0.00 amps and 9.99 amps.