Tri-Metric program question [RE-wrenches]

Sat Oct 21 09:53:24 PDT 2006

We are dealing with the same issue.  The customer reported that the meter drifted down at about 5% every other day. I am curious as to how much drift others are seeing.

The parameters have been adjusted to those suggested by Alan and Ralph (of Bogart Engineering).  I'm sure we will hear if the problem persists.  Can anyone report success (or otherwise) from the reprogramming?

Thanks,

Drake 

----------------------------------------

From: Allan Sindelar <allan at positiveenergysolar.com>

Sent: Wednesday, October 18, 2006 5:16 PM

To: RE-wrenches at topica.com

Subject: Re: Tri-Metric program question [RE-wrenches] 

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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