Tri-Metric program question [RE-wrenches]

Wed Oct 18 16:59:48 PDT 2006

Wrenches,
I posted this question a week or two ago, and was surprised that a) others
had faced the same issue, b) nobody had worked out a simple, effective,
proven solution. While I respect Mick's suggestion of a relay-based B+
control on the monitor as probably working just as he envisioned, the extra
cost and complexity make it not the first approach to try.

So the answer appears to be somewhere in a combination of these four
approaches:

1) Set the float voltage slightly high, in order to increase charge current
relative to discharge current during float/sell conditions. This will lessen
accumulated losses that cause the meter to slowly lose %-of-full indication
(thanks for this idea, Todd).

2) Set the charge efficiency factor at or close to 100%, so that routine
charge-discharge cycling has less of a tendency to accumulate as an
inaccurately low %-of-full, or takes longer to do so.

3) With an Outback system that has a Mate, the AC IN hot button will easily
access a "force bulk or float" command, which initiates an automatic bulk
charge cycle. I can instruct the homeowner to do this occasionally - say,
every couple of weeks, or when the monitor is reading too far out of whack.
This is an especially good step for a system with flooded batteries, as they
otherwise only vary between float/sell and resting voltages, and a periodic
bulk charge cycle would be good for them. It's probably only useful with
flooded batteries and involved homeowners (who are already caring for
flooded batteries).

4) It might work with sealed batteries to set the Tri-Metric charged voltage
parameter below the float/sell voltage, and the charged current parameter
just above the current necessary to maintain float. That way, the Tri-Met is
held at 100% and is constantly being reset as long as the grid is up.
Following or during an outage, the low charged current parameter keeps the
meter from resetting until charging current drops way down, even though the
voltage setting is artificially low. I wonder how these settings would
affect monitor accuracy during an outage - I suspect not much.

Feedback? Preferences among these options? What combinations would you try?

Thank you,

Allan at Positive Energy

Excerpts from previous posts about this issue:

We have been including a Tri-Metric monitor with our Outback
grid-tie-with-battery-backup systems, so that the homeowner has some sort of
state-of-charge indication during an outage. We have learned to be selective
about whether to include a Mate as well, as it's not as user-friendly for
our typical non-technical grid-tie customer.

The issue is that the Tri-Met is fundamentally designed for off-grid use,
and uses charged-voltage and charged-current parameters to reset the monitor
on a regular basis. As the PS1 keeps the batteries in float, the Tri-Met can
develop an accumulated error that is only reset after a grid outage and
recharge cycle, which could be months at a time. We have seen this happen,
where the % of full slowly drifts down over days or weeks.

I called Ralph Heise about this, and he had suggestions, but no clear
answers - in fact he is looking for ideas that have worked too. He suggested
the following two ideas, which make sense in theory:
1. Set the charged voltage parameter below the float voltage (far enough
below to accommodate the effect of hot-weather temperature compensation -
say 52.4V if float is at 53.6 - and set the charged current setpoint just
above the float current - 1 or 2A, I would guess. This keeps the monitor
reading 100% while in float, and the combination of voltage and current
settings would prevent premature resetting based on voltage-above-float
alone.
2. Set the charge efficiency factor unrealistically high - say at 100% - so
that this keeps the % reading at 100% while in float. This would make the %
reading slightly high during outage cycling, but it could be easily reset
each charge cycle.

Has any Wrench out there come up with a good solution? How would you set up
a TriMet for this application? Or otherwise, how do you address the bigger
issue of a monitor that is used only during an outage?

________________________

If you are having problems with the amp hour meter on a grid, tied,
battery based system, try raising the float voltage slightly on the
inverter/charge controller. Yes, it will slightly lower the efficiency
and cause the batteries to consume a bit more water, but the slightly
additional charge current will offset the discharge pulses and keep the
meter on track... also from what I have read, slightly overcharging wet
cell lead acid batteries does more good than harm. I have also found
that a few charge/discharge cycles will get the meter to recalculate
battery efficiency higher (the e-meter starts out a default of 90%) so
the +/- pulses are treated closer to being equal.

When a battery based grid tie system has the grid operating, the meter
is not necessary. When the grid goes down is when the customer needs the
meter and it is then that it is important for the meter to read
accurately, starting with a 100% charged reading.

Grid tie inverters with batteries constantly send small charge and
discharge currents into the batteries. Because batteries are not 100%
efficient, amp hour meters treat charge currents and discharge currents
differently. It takes more charge current to compensate for a given
amount of discharge current. This is why amp hour meters on battery
based systems tend to accrue a discharged SOC inaccuracy.

The only way I have found to resolve this issue, so when the grid goes
down the meter starts at 100% is by setting the float/sell voltage
slightly higher so the batteries constantly get way more charge then
discharge currents. Another thing which helps is to cycle the batteries
a few times so the amp hour (link-10) meter recalculates the battery
efficiency higher from the default of 90%.
Todd

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