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Inverter dumps the battery at 10.6V?

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Posted by Bedraggled on October 18, 2004, 12:28 am
Ok.  I'm looking confused right about now.
I am reading that the lead acid battery is considered
totally discharged at a specific gravity of around 1.13...
which several tables indicate will occur at around 11.8V.

? I have two small inverters (400W and 150W)
that I use for emergency lighting...
both disconnect the battery at well under 11.8 V.
(one of them disconnects at 10.5 or sok the 400W inverter
drops the battery at 10.0V ) [??!]

Overdischarging of the battery is damaging to its plates.
I would expect that the battery disconnect voltage would
be at or above 11.8V...not well below that point.
Are these inverters badly designed?...not taking proper care
of the battery...? ..some piece of the puzzle I am not seeing..

or am I even more confused than I think I am?

*stares at data sheet....(staring is not helping...sigh)

*puts message into bottle, casts the bottle into the cyber sea.

*plays with the crabs on the beach.

Posted by Ron Rosenfeld on October 18, 2004, 1:20 am
On Mon, 18 Oct 2004 00:28:00 GMT, "Bedraggled"

Probably that those voltages are measured with the battery at rest -- i.e.
no charging or discharging for at least three hours.

Under load, there can be a considerable drop in battery voltage.

Having said that, those cutoff voltages seem a bit on the low side to me.
I would probably use 11.0V for a 12V system, but I might change that
depending on the particulars of the system and my experience with the


Posted by Bedraggled on October 18, 2004, 2:11 am

Just ran an experiment and ran the inverter hard for a few hours.
The "heads up, I'm about to disconnect the battery" alarm went off
at 10.8V.
I turned all the loads off...let the battery sit for about 20 minutes..
...battery voltage now at 11.8 volts.  Right at the "advertised"
"battery is discharged " voltage.

It does seem that the battery disconnect voltage is trying to
account for the fact that the load is pulling the battery voltage down.

at any rate...the majic seems to be working.

[I did not allow the test to continue to the cutoff voltage [10.0 V]
No sense in slapping the SLI battery (not a "deep cycle" type) that hard
if I don't have to.  Will consider the disconnect as more of a "fuse" than
a normal operating mode.

thanks for the input..
A single clue can solve a puzzle!

Posted by Ron Rosenfeld on October 18, 2004, 2:30 am
 On Mon, 18 Oct 2004 02:11:03 GMT, "Bedraggled"

Glad to provide it.

If it's not a deep-cycle type of battery, you should take care not to
discharge it very deeply.  Usually, with thin plates, a few deep discharges
can really do them in.


Posted by Bill Kaszeta / Photovoltaic Re on October 18, 2004, 4:11 am
 Most lead-acid batteries are rated for capacity at a specific rate
and final voltage, usually a discharge over 8 hours to a final
voltage of 1.75 volts per cell (10.5 volts for a 12-volt battery).

While the batteries can be discharged to this voltage, it is
only for a limited number of times.  Battery life is greatly
extended if the depth of discharge is limited.  For a typical
golf cart traction battery:
300 cycles to 100% discharge (10.5 volts)
600 cycles to 80%  (about 11.6 volts)
1000 cycles to 50%  (do not have the data at home)

A lot depends on the quality of the battery, but the relative
cycle life follows the above.

Bill Kaszeta
Photovoltaic Resources Int'l
Tempe  Arizona  USA

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