The Tri-Met Dance: was battery shoot-off [RE-wrenches]

[Allan Sindelar]

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----- Original Message ----- 
From: "Kurt Nelson" -----Original Message-----
From: John Blittersdorf

<I have my battery capacity on my 6 year old L16G's set at about 68% of
rated ah capacity and it goes up and down from 50% to 100% fairly
accurately.

Greetings John -- I have never understood why trimetric recommended
setting battery capacity at something less than what the pack will
deliver.  Is this essentially another way from keeping folks from
over-discharging their battery or are you saying that the true capacity
isn't what they are rated at (and that's at C/20 and we typically don't
hit them that hard).>

Kurt,
It's both, in my humble opinion. I ALWAYS set the TM capacity a bit low, for
a variety of (sometimes contradictory) reasons.
1) Most people watch %, not voltage/current. Most people can remember only
about three numbers at once: full at least once/week; occasionally down to
50%; never below 20%. So I want that occasional 20% to still be operating,
or I'm trying to explain why their inverter shut off at 38%, when it could
be any of a number of reasons.
2) Battery efficiency varies with age, but few users will reduce their
programmed efficiency as the batteries get old.
3) Load profiles affect readings in multiple ways, such as how really full
the batteries get up to (as opposed to full enough to reset the TM
regularly, as you point out); a large array and hot charge setpoints will
give more effective battery capacity.
4) Varies with BOS equipment: a PWM or MPPT (which is also PWM) controller
is set up differently as to the charged algorithm than an older on-off
controller (which some folks refuse to upgrade).
5) As others have pointed out, many batteries don't give claimed output in
real-world applications.
6) C/ rates are all over the map, so Peukert's ghost is always lurking, and
plays with TM numbers.

I will generally tell one person (the one who most understands and cares for
the system) that I have set capacity more conservatively than the battery
mfgr.'s numbers, and suggest that it stay a secret.

<You also write: I have noticed lately that I am hitting 24 .1VDC at 60%
indicated on the e-meter so I need to push my rating down again.  I'll
try 55.>

<Isn't this just an indication that you have lost capacity and
efficiency?  I would have thought changing the battery efficiency from
94% to something like 89 would be a better way?  That said, doesn't the
Emeter track battery charge efficiency?>

You need to change both, as both capacity and efficiency reduce as batteries
age. Efficiency determines recharge accuracy on the TM, but if programmed
capacity stays too high, the voltage will drop out before the % says it
should, and the system could shut down.

In my own system, with a fairly high array/battery ratio and pretty regular
complete daily fills, I set the TM to 97-98% efficiency; it stays pretty
accurate that way when it doesn't reset for a week or so.

<My Emeter and my Trimetric both drift if they don't get reset to 100%
fairly regularly.  I have a customer who has both a Link (early E) and a
Trimetric and they were tracking very close until he recently had
generator problems and hasn't hit 100% for a few weeks.  Now they are
reading way off, both from each other and from what is.>

This drift comes up a lot in winter with some systems, especially ones with
too little array. It is the result of all of the above issues. A 1%
inaccuracy in programmed efficiency can mean a 20% error in 3 weeks if it
doesn't get reset. It's related to 3) above: if the array is small relative
to either battery capacity or load profile, the TM has to be set up to reset
(reach 100%) loosely, or it never resets in winter, and accuracy goes down
the tubes.

<Thanks for your thoughts on this John and Kurt.>

Welcome to the real world! It's very different from the world of manuals and
theoretical setpoints...

Allan at PosNRG

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