# Lead-acid battery: Voltage, State of Charge, and S.G.

Discussion in 'Electrical Engineering' started by Percival P. Cassidy, Aug 22, 2005.

1. ### Percival P. CassidyGuest

As I think about Lead-acid batteries and the voltage and s.g. as
measures of the state of charge, I am getting confused.

If I charge a battery fully, both the s.g. and the voltage will be
reasonable indicators of the state of charge, right?

Now what happens if I realize that the electrolyte level is low and add
water and let the electrolyte stabilize? The s.g. will be reduced, I
assume, but what happens to the voltage? Does that drop because of the
dilution of the electrolyte? If not, the voltage still indicates full
charge, but the s.g. indicates less than full charge.

Perce

Percival P. Cassidy, Aug 22, 2005

2. ### Bill Kaszeta / Photovoltaic ResourcesGuest

On , "Percival P. Cassidy" <> wrote:

>
>NNTP-Posting-Date: Sun, 21 Aug 2005 16:46:52 MST
>Date: Sun, 21 Aug 2005 19:46:52 -0400
>
>As I think about Lead-acid batteries and the voltage and s.g. as
>measures of the state of charge, I am getting confused.
>
>If I charge a battery fully, both the s.g. and the voltage will be
>reasonable indicators of the state of charge, right?
>
>Now what happens if I realize that the electrolyte level is low and add
>water and let the electrolyte stabilize? The s.g. will be reduced, I
>assume, but what happens to the voltage? Does that drop because of the
>dilution of the electrolyte? If not, the voltage still indicates full
>charge, but the s.g. indicates less than full charge.
>
>
>Perce

For single cells of lead-acid batteries:
Specific Gravity = Cell Open Circuit Voltage - 0.845

I think this is for 78F/25C temperature, for cells that have rested
(no charge/discharge) for at least an hour.

Bill Kaszeta
Photovoltaic Resources Int'l
Tempe Arizona USA

Bill Kaszeta / Photovoltaic Resources, Aug 22, 2005

3. ### ehsjrGuest

Percival P. Cassidy wrote:
> As I think about Lead-acid batteries and the voltage and s.g. as
> measures of the state of charge, I am getting confused.
>
> If I charge a battery fully, both the s.g. and the voltage will be
> reasonable indicators of the state of charge, right?
>
> Now what happens if I realize that the electrolyte level is low and add
> water and let the electrolyte stabilize? The s.g. will be reduced, I

> assume, but what happens to the voltage? Does that drop because of the
> dilution of the electrolyte? If not, the voltage still indicates full
> charge, but the s.g. indicates less than full charge.
>
>
> Perce

Here's, a very simplified description, that nonetheless
tells you what is happening. During discharge, heavy
"stuff" that is in solution is deposited on the plates
of the battery, thereby making the liquid lighter, so the
SG decreases. The opposite happens when charging - the
heavy "stuff" that is on the plates is driven back into
the liquid solution, making the SG increase.

When the electrolyte level is low, it is because the
water, not the heavy "stuff" has evaporated. Adding
water raises the level of the electrolyte and dilutes
it, just as you said. But the heavy "stuff" is still
in the battery, deposited on the plates. Charging the
battery forces that stuff back into solution, and the
SG returns to where it should be.

SG is, as you said, an indicator of the state of charge of
the water evaporated and then the level was brought back up
by adding water, both the SG and the voltage would indicate
that the battery was not fully charged. Charging it would
raise both the voltage and the SG to their fully charged
levels.

Ed

ehsjr, Aug 23, 2005
4. ### daestromGuest

"Percival P. Cassidy" <> wrote in message
news:Mn8Oe.5450\$...
> As I think about Lead-acid batteries and the voltage and s.g. as measures
> of the state of charge, I am getting confused.
>
> If I charge a battery fully, both the s.g. and the voltage will be
> reasonable indicators of the state of charge, right?
>
> Now what happens if I realize that the electrolyte level is low and add
> water and let the electrolyte stabilize? The s.g. will be reduced, I
> assume, but what happens to the voltage? Does that drop because of the
> dilution of the electrolyte? If not, the voltage still indicates full
> charge, but the s.g. indicates less than full charge.
>
>

The SG for a fully charged battery *assumes* normal electrolyte level.

For large cells, there is usually a 'level correction' that is applied to
the SG reading, much like the temperature correction. So for every xx below
normal, you subtract .001 from the SG reading to get the corrected reading.

It's best to add water just before charging. But it is important to not
over-water as the charging will create bubbles that can cause the
electrolyte to overflow. But the charging will help to mix the electrolyte.
If you add water to a battery and then leave it open circuit, the water
(being lighter than the acid) will not mix evenly and this can be bad for a
cell. It can also screw up tomorrow's SG reading if it hasn't mixed in yet.

daestrom

daestrom, Aug 23, 2005