John Drake Services, Inc.
1427 E. 68th Street
Long Beach, CA 90805
Always wear eye protection when working around batteries.
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Battery State of Charge and Measurement
State of charge:
If you are looking for complex formulas and equations, this is the wrong
place to look.
I will present this as complicated as my simple mind can handle.
The state of charge is used to determine how much available power is
in your battery(s).
We will be concerned with deep cycle storage batteries, not automotive
Automotive batteries are usually rated at Cold Cranking Amps, the amount of amps
a new fully charged battery can deliver to the starter for a specific amount of time,
at a specific temperature.
These numbers mean nothing to us.
First off, storage batteries are rated in Amp Hour capacity.
It can be shown in a 5 hour, 10 hour, 20 hour or 100 hour discharge rate.
The slower you draw power from a battery, the more you will get out of it.
Usually they based on the 20 hour rate.
As an example, the Trojan T-105 six volt battery has the following factory
5 hour rate is 185 amp hours
10 hour rate is 207 amp hours
20 hour rate is 225 amp hours
and the 100 hour rate is 250 amp hours
Please consider that you will never get the full store of energy from a battery.
As the power level drops, so does the voltage and it can get to a point where
there is not enough pressure (voltage) to operate your loads - but there is
still energy left in the battery.
Use these amp hour capacity numbers as a guide.
Measuring state of charge:
There are basically three ways to determine how much power
is left in a battery.
The first is to read the specific gravity of the electrolyte (liquid) in
In a lead acid battery, the electrolyte is a solution of distilled water and sulphuric acid.
A fully charged battery in good condition, will have most. if not all of the acid dissolved
into the water.
By drawing a sample of the electrolyte into a hydrometer, a glass tube with a bulb on
The heavier (or denser) the electyrolyte, the more acid it has in it and the higher state of
charge in the battery.
This will push the hydrometer (float) higher in the glass tube.
Some hydrometers also have a thermometer and temperature adjustment scale inside.
The higher the electrolyte temperature, the less accurate it will be unless you use the
temperature adjustment scale.
This method is excellent for comparing the specific gravity of each individual cell to find a
soft (weak) cell.
But they have their limitations.
If the battery has just been charged there will be gas bubbles in the electrolyte which will
cause it to show a lower specific gravity.
If the battery has just been put under a load (discharged), some of the acid in the lead plates,
which would naturally migrate from the plates back into the electrolyte while resting, will not
bring up the density and show an incorrect specific gravity.
Always let the battery(s) rest (nothing in or out) for as long as possible to get the most accurate
state of charge based on the specific gravity.
The second method is to use a volt meter to read the voltage of the entire
battery as well as in each cell.
Reading the voltage of a battery is a field expedient method (quick and dirty) to determine the state
of charge of a battery.
It also suffers from the same pitfalls as when using a hydrometer.
You would want to let the battery rest before relying on the voltage to to check its state of charge.
It can also be used, like a hydrometer, to check for weak cells.