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'[OT] Solar-Charging Comments'
2012\05\20@073930 by Bob Axtell

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The conversation seems to be over, but I have been out of touch about the solar-charging of
NiMH (or Lead-Acid) batteries, and I am trying to catch up..

I just completed a three-year testing period, mostly to address problems with solar battery charging> I'll pass along what we discovered:

1. Battery case temperature MUST be closely monitored. The ideal charging point is VERY temperature-dependent. You will need to reduce or suspend the charging current on hot days.

2. NiMH batteries are MUCH better than lead-acid systems in reliability and weight. But the NiMH packs are VERY expensive.and until the costs come down, you will probably specify lead-acid instead.

3. There are wonderful chips to charge NiMH packs. But these chips work when the supply is steady. Unfortunately, solar charging doesn't work that way: most of tne time the sun winks on and off behind clouds. This fact plays hell with those smart charging chips. We were able to get a reasonable, reliable charge by charging a large electrolytic then release a "charge burst" into the battery when the level was exactly correct You have a different current algorithm.for each type.

Everyone knows that the best battery is Lithium-Ion, but the problem with overcharging and overtemperature is so serious that I don't consider LiIon at all for solar applications.

I'll be glad to answer questions.

--Bob Axtell



.. . .

..

2012\05\20@090248 by RussellMc

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> Everyone knows that the best battery is Lithium-Ion, but the problem
> with overcharging and overtemperature is so serious that I don't
> consider LiIon at all for solar applications.

Mostly sounds somewhat familiar.
I would be extremely interested in knowing how you rate LiFePO4 as
opposed to standard LiIon.


         Russel

2012\05\20@163717 by Harold Hallikainen

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I've not been following this thread closely, but the mention of throwing
energy away when the battery is full just bothers me. the great thing
about grid-tied solar is that the grid will take whatever you can
generate. So, I assume solar powered battery charging is used only when
grid tie is not practical (extremely remote locations or self-contained
low power devices like highway emergency phones, solar powered
calculators, etc.).

Many years ago, I helped some students with a solar powered car. It used
lead acid batteries. We did a form of maximum power point tracking by
having each solar panel (there were lots) drive a boost converter where
the duty cycle was controlled by a PIC. The PIC dithered the duty cycle
while watching the output current to the battery, adjusting for maximum
current out of that individual converter. Later, I read an LTC application
note on MPPT that indicated that the MPP of a solar cell was close to the
same point as a fixed voltage out of the cell. So, they just used a
standard boost converter but tied the feedback to the input voltage
instead of the output voltage. Very simple! I was impressed.

With the very low price of solar cells right now, it seems practical to do
simple grid tied systems. I remember reading years ago about distributed
inverters where the inverter is attached to the panel. You buy panels as
you need (or can afford) them, and just plug them in. I think stuff like
this is now available on the market. If you put a low power radio in each
panel, you could also gather statistics on how each panel is doing. Each
panel could transmit its current status on a random basis every few
seconds or minutes. A central receiver would gather the data and summarize
it, probably on a web interface through wi-fi.

I just pulled my most recent electric bill. The rates are:

$0.12845 for first 154.7kWH
$0.14602 for next 46.41kWH
$0.29940 for next tier (up to about 300kWH)

So, right now, a grid tied solar that produced 16kWH per month would save
me about 30 cents per kWH. At 30 cents/kWH, it certainly seems practical.

Now, if I could just convince my condo association to let me put something
on the roof...

Harold


-- FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!
Not sent from an iPhone

2012\05\20@180146 by Bob Blick

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On Sun, May 20, 2012, at 01:37 PM, Harold Hallikainen wrote:

> simple grid tied systems. I remember reading years ago about distributed
> inverters where the inverter is attached to the panel. You buy panels as
> you need (or can afford) them, and just plug them in. I think stuff like
> this is now available on the market. If you put a low power radio in each
> panel, you could also gather statistics on how each panel is doing. Each
> panel could transmit its current status on a random basis every few
> seconds or minutes. A central receiver would gather the data and
> summarize
> it, probably on a web interface through wi-fi.

Enphase Energy, very popular around here, makes grid-tie microinverters
that talk to a data hub through the AC line.

Best regards,

Bob

-- http://www.fastmail.fm - Faster than the air-speed velocity of an
                         unladen european swallow

2012\05\20@183801 by Dave Tweed

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Harold Hallikainen wrote:
> I've not been following this thread closely, but the mention of throwing
> energy away when the battery is full just bothers me. the great thing about
> grid-tied solar is that the grid will take whatever you can generate. So,
> I assume solar powered battery charging is used only when grid tie is not
> practical (extremely remote locations or self-contained low power devices
> like highway emergency phones, solar powered calculators, etc.).

There's a big downside to grid-tied systems, too. Because of regulatory and
"common practice" issues rather than technical issues, a grid-tied system
has no local storage (batteries) at all, and when the grid goes down for an
extended period (an increasingly common problem here in the northeastern US),
your grid-tied solar energy installation does you no good whatsoever.

-- Dave Twee

2012\05\21@102203 by M.L.
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On Sun, May 20, 2012 at 9:02 AM, RussellMc <spam_OUTapptechnzTakeThisOuTspamgmail.com> wrote:
>> Everyone knows that the best battery is Lithium-Ion, but the problem
>> with overcharging and overtemperature is so serious that I don't
>> consider LiIon at all for solar applications.
>
> Mostly sounds somewhat familiar.
> I would be extremely interested in knowing how you rate LiFePO4 as
> opposed to standard LiIon.

I might have written firmware for a battery backup unit that might use
A123 cells.
The specified temperatures were very extreme, and it was promised by
various authorities that the cells would handle the heat very well.

With A123 being very expensive (for you and me) I don't see anyone
using their cells for bulk non-portable installations any time soon.

What I did remember is that the people were ridiculously secretive
about their technology. For a mass produced item that was extremely
cost sensitive. There was nothing state of the art in it.
-- M

2012\05\22@072900 by Bob Axtell

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On 5/20/2012 4:37 PM, Harold Hallikainen wrote:
{Quote hidden}

Moat of our clients pump into thew grid, but some, like those of rural Sonora, MX operate with battery arrays. however, my intent was to discuss simple battery matters, not storage methods.

I consider pumping into the grid a storage method.

--Bob

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