When the battery set gets low [RE-wrenches]

[John Raynes]

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I wonder how much of that loss in efficiency is due to the fact that a lot 
of the re-charging is happening at lower voltages, hence the cable losses 
will increase as a percentage of total charging power.

Anybody have sufficient long term experience in observing the charge 
efficiency calculations of an E-meter or Trimetric, that they can quantify 
the loss in efficiency on a typical system, based on maximum depth of 
discharge?  I'm always looking for more good reasons to give to customers 
as to why they need to keep their battery state of charge higher than 
they're inclined to.

Had to give the bad news to a new client yesterday on a small system that 
was being perpetually drained.  Original batteries failed (They were deep 
cycle Exides) so he replaced them with 18 6-volt automotive batteries, 9 
strings of 2 batteries.  Has a freezer sucking 125% of his total solar 
capacity.  Wonders why the new batteries aren't helping.

9 parallel strings at 12 volts.  That's a new record in my book.

John Raynes
RE Solar
Torrey, UT


At 01:53 PM 7/31/2005 +0100, you wrote:
>Yes, I have definitely noticed that.  When my huge great battery gets low 
>it is very ungrateful for any charge that comes its way and it is very 
>unrewarding to jam a load of amps into it for short periods. For a few 
>years I had the habit of running my engine driven welder to charge at 
>about 2kW, while using only a couple of hundred watts in the house.  A 
>gallon of fuel later the batteries would be no happier. Nowadays I run a 
>tiny little generator and all the power gets used directly (saving my 
>batteries from discharging much) and a gallon of fuel would last me about 
>15 hours.  Makes more sense to me.  I wait for cheap and abundant wind 
>energy to persuade my batteries back to life later.
>--
>Hugh

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