Uneven charge across string

 

We have 12 2volt AGMs which are manufactured in 6volt blocks each
containing 3x2v cells. Each cell is (supposedly) completely isolated
and has its own external terminal posts.

These are all wired in series and connected to a Studer charger.

charger
\
Pos - 6v-6v
Neg - 6v-6v

The two blocks at the front, connected to pos and neg respectively,
seem to be absorbing more of the voltage.,

Voltage across each cell in the forward blocks during charge is around
2.4-2.5v, whereas at the two blocks to the rear it's around 2.2-2.3
volts across each cell.

It's as if the charge current isn't filtering downstream properly,
which doesn't make a lot of sense. Any ideas what could be causing
this? I'm going to change the configuration around tomorrow.

 

wrote:


Looks like you need equalization, consult the manufacturer.  Cell readings
of  2.01-2.08 volts under load before charge is a major difference.

Is there any chance that the two greatly different cells have a different
thermal invironment such as sunshine on them and not the others?

It is near impossible to manufacture battery cells such that all have the same
capacity.  The lower capacity cells will charge faster and have higher voltage
than the others.  Higher capacity cells will have lower voltages.  Equalization,
done properly, will apply a controlled overcharge to the lower capacity cells to
allow the higher capacity cells to charge fully.


Bill Kaszeta
Photovoltaic Resources Int'l
Tempe  Arizona  USA
bill@pvri-removethis.biz

 

Along with the other suggestions, I'd ask exactly how you're measuring the
voltage.  If you are tapping into the inter-cell connector cable, then you
need to consider the bolted connections.  It is best if you can measure cell
voltages right on the terminal post, not on the bolts, nuts, or cable
terminations.  If you have a tiny variation in the inter-cell resistance
from one cell to another, and you measure the voltages from the cable
terminations or bolts instead of the terminal post directly, you will see
higher then average voltage on charge and lower than average when
discharging heavily.

Do these cells have something else that is different about them?
Warmer/cooler?  Use more water than the others?  I assume they are all the
same age and were placed in service at the same time?

daestrom

 Several years ago I visited a company near Palm Beach, Fla. that made kits
to convert gas cars to electric.  Several of the employees had electric cars
of there own design. One fellow designed a battery charger that charged each
2 volt cell separately without disconnecting any wires from the 72 volt
string. His was a flyback topology with separate windings for each 2 volt
cell. Each winding had one schottky diode and one cap in the output. to each
2 volt cell. He regulated one of the 2 volt cells to 2.4 volts max. He said
he had to cheat and parallel the primary winding of 6 transformers with 6
secondaries each. He used three flyback transformers from an old tv set to
get enough of a window for all of the secondaries. He used one transistor
for all three transformers. He had some critics at first but after three
years of every day use and no battery failures more and more folks started
to take notice. The flyback does not start charging all of the out puts
exactly as expected but as the cells reach the voltage regulated mode they
even out which was not expected.

Ray