Calculation of recharge rate for RAP power system

 Hello,

I currently have a stand alone remote power system for my house.

It consists of 14 x 185W Sharp Panels (5 amps each), and 24 Hoppecke
OPZS batteries in a 48 volt configuration.  The batteries are the 1200
series, and have a C10 of 1210.  I use a PSA RAP 5 inverter.

Generally speaking, we bring in anything from 9-15kwh of power a day
(on average), but we tend to only use around 6 or 9 kwh of power a day.

My question is this.  Although we bring in more PV power than the power
used each day, it seems to take a long time to charge from a specific
gravity of 1.23 to the max of 1.24.  After a few days of cloudy
weather, sometimes we go backwards a little bit with our power, and the
SG drops from the usual max level to 1.23 (which I understand is around
90%).

The regulator is a plasmatronics PL60, and is currently going into
float each day, however the SG is still at 1.23, but it now crawling up
over 1.23 (but only after a few days).  So far this week, I've brought
in around 12kwh of power more than what I've used.

Generally, it takes about a week or so to move to the full 1.24, but I
was wondering if there's a formula or calculation tool I can use to
calculate the amount of recharge I can get each day?  It would be good
to be able to know this, as its tempting at the moment to just throw
the generator on for a few hours and max it up if its going to take an
excessive period to charge the remainder with the panels.

regards,

  It is a good exercise to watch SG change during charge and discharge of lead
acid batteries. What you will find is that SG lags the battery state of
charge by many hours. This is because the concentrated acid deep within the
plates takes a long time to diffuse out to the surrounding solution. If
batteries are trickle charged or 'floated' for about a week after a full
charge you can be sure of getting an accurate 'full charge' SG reading.

Use of an amp-hour meter is the best way to keep track of your batteries on
a daily basis. Just make sure you put back in more amp-hours than you
discharge each night. You may need to set your float voltage a bit higher
for this to happen. You can 'sulphate' and permanently damage your batteries
if you undercharge them on a daily basis.



NM

 Thanks for your reply.

Given that the batteries are 1500AH each, I suppose it will take some
time to charge them fully.  Looking at it from a different viewpoint,
my surplus power for the week is equivalent to what I'd get if I ran
the generator for 2 hours, so all things considered, it might take a
week or two or charge it fully.  I think the main thing that concerns
me is the point you raised, and thats about the batteries taking an
excessive period to return to 100%.  The system runs an equalisation
phase once a month, but I'd certainly prefer that it returned to a full
charge SG within a few days of dropping 10% than taking a week or more.
 I'll have to double check the float setting as you've suggested, as it
could be the problem i.e. its floating the battery too early to prevent
a full charge.

regards,

 I have given up on expecting to see a full SG reading except under special
conditions. If you cycle your batteries daily like I do there simply aren't
enough sunlit hours in a day to do this. I discharge about 20% of the
battery capacity each evening and the solar recharges them about 22.5%
during the day to make up for losses in the battery system. I usually end up
with about a 1.225 SG at the end of the day.

Once a month I do a long equalization using the generator to supply my
evening needs for one night so the batteries can stay topped up for at least
24 hours. After this I carefully check the SG and record the numbers for
each cell to make sure no cell is going south on me. If a cell is low I
continue the C/100 equalization charge for another day.

NM

 
I'm not sure that means what you think it means.  AFAICT equalizing
requires sufficient overcharge to get every cell gassing enough that
the bubbles will stir up the electrolyte and eliminate stratification.
I doubt C/100 will do it...

Another thought, since your panel charge rate is only C/17, why not
just bypass your controller for a few days instead of equalizing?  As
long as you get sufficient bubbling to stir up the electrolyte, and
get all the cells into overcharge, you've pretty much covered the
requirement for equalizing, yes?