Battery State of Charge and Measurement

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

batteries.

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

capacities:

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

each cell.

  

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

one end.

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. 

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