# Relationship between battery voltage and state of charge

### Battery voltage and state of charge - Energy Matters

to measure your battery's voltage. relationship of the curves is similar for all deep cycle lead-acid If voltage is to be related to battery state of charge, then. Voltage vs State of charge, you will find a chart for your chosen battery chemistry. Temperature, will probably have a simple relationship, also in the datasheet. DEEP CYCLE BATTERY VOLTAGE & STATE OF CHARGE. If you're the To learn more about the difference between them, view our deep cycle battery guide .

It is interesting to note that a charged 12 volt lead acid battery at rest not powering loads and unused for at least 3 hours will read about Hook up a load and the voltage will drop to about Another way of looking at this is to use an analogy of a water pump a battery is an electric pump.

The pressure in PSI a pump delivers is like a battery's voltage. Let's look at a 12 PSI pump with no loads the pump is running but the overflow valve is turned off. The pump will run and the internal pressure of the pump will build to some point higher than 12 PSI.

### Voltage Readings & State of Charge : Technical Support Desk

Once the valve is opened and the water is free to flow into the loads, the pressure will drop to the rated output pressure of 12 PSI, but only if the load is not too big. If the load is then reduced or removed, the pump will catch up and return to it's rated 12 PSI pressure.

If the pump has an infinite source of water such as a lake or the water utility this is like the grid, no battery the pump will never run out of pressure, and as long as the pump is operated at or below it's 15 GPM level it will hold 12 PSI. However, a pump that is connected to a water tank with a finite capacity, will start to lose the ability to hold pressure as the level of water in the tank drops.

Thinking of siphoning water from a bucket, as the level of the water drops the volume of water exiting the siphon slows down.

## DEEP CYCLE BATTERY VOLTAGE & STATE OF CHARGE

When the tank is full it is capable of feeding more "pressure" to the pump inlet due to gravity, and the pump always has enough water available to maintain its rated pressure and volume.

However, if the water tank gets low the pump will not have enough water volume coming in to maintain 12 PSI at 15 GPM. If the loads are taken away from the pump by closing the valve on the outflow, even with the low pressure in the tank the pump will eventually pump up to 12 PSI. It will just take it longer to get there. Then when the valve is opened the pump will sustain 12 PSI for a brief while, but since the tank is no longer feeding the pump as fast as needed the pressure will eventually drop.

This analogy can be restated by replacing the pump with a battery, pressure with voltage, volume with amps, outflow valve with a switch, water with electricity, and the water tank with the battery electrolyte. Then the current will be the normal charging current of So the first point to be drawn in fig.

This is starting point A. As charging continues, initially the current will remain the normal charging current of Meanwhile the battery becomes more and more charged so the lines representing higher states of charge are crossed. This means that from starting point A in fig. To find where this vertical line stops, one has to calculate when the voltage regulator will start reducing the field current.

Between the point where the regulator is connected and the battery poles, there is a voltage drop of So if the regulator starts reducing the field current when it senses a voltage Vr of This is point B in fig.

Now charging continues and battery voltage will rise further, but the current decreases. To find out how the current decreases, one has to look at the voltage drop mentioned above, which represents the difference between the voltage Vr sensed by the regulator and battery voltage Vb.

Voltage Vr will remain constant at This means that the equation representing the relationship between I and Vb is: This is the end point D of the charging process. Even charging a battery for a week would not make the current drop further and voltage Vb reach By now, the energy that is still pumped into the battery is not used for the chemical reaction that goes with battery charging see annex C: Electrolysis of water, meaning that water is transformed into hydrogen gas and oxigen gas the explosive gas mixture that is formed during battery charging.

The water that is lost this way, has to be replenished by topping up with destilled water once in a while. Corrosion of the grid of the positive plate, see annex C: These undesirable reactions go only slowly because the current is so low. Still they go faster than in a car because the voltage regulator has been readjusted to a slightly higher voltage. Therefor a battery must be disconnected once point D is reached. The charging characteristic in fig.

When battery voltage reaches Getting just this little into the corner of the danger zone won't harm the battery at all. But it shows how important it is that the charger complies with the data that were used in calculating the charging characteristic. If the regulator would be adjusted slightly higher and the resistance between battery poles and regulator would turn out to be a bit lower, the charging characteristic could end up well into the danger zone.

Another interesting point is point C where the battery is just fully charged and charging can be stopped. Then all the electricity that has been taken from the battery, has just been charged back into it. Generally, they will not measure battery voltage Vb but use the indicator on the switchboard, thus measuring voltage Vi.

A battery can be considered fully charged when: The current has dropped below 4 A see note below ; and: The voltage Vi as measured with the indicator has risen above So then the point at which the battery is fully charged, will determined by the criterium that the current should drop below 4 A. Still voltage Vi must be checked to make sure that the current has dropped to less than 4 A because the battery is charged, since there could be other reasons why the current is so low: The charger runs short of water so its power output is just enough for a few amperes charging current.

The nozzle is partially blocked, with the same effect on the power output. There is something wrong with the field current, maybe too many or too few lamps are switched on, the connections are poor or the brushes are wet. The current indicator is wrong and underestimates the actual current. The most likely cause for this is corrosion of the surfaces in the indicator switch operate the switch a few times to see if it goes back to a normal value.

In this case, the voltage measurement part probably still functions. So to prevent that batteries that are not yet charged, are disconnected because the current is below 4 A for one of those reasons, also voltage Vi should be checked. The graph of fig.

## Technical Support Desk

For smaller batteries, the state of charge lines would end up closer towards the Y-axis and such batteries might not be fully charged when the current has decreased to 4 A. Therefor small car batteries can be considered charged when the current has dropped to 2 to 3 A instead of 4 A. So for those batteries, point C should lie at a current of 2 to 3 A. For small batteries, the other points on the charging characteristic remain virtually the same.

Still the choice of having point C at a charging current of 4 A seems wise considering: At a higher temperature, a battery accepts charging current more easily, so when reaching point C, it will be charged a bit more than fig.

Suppose the operator will check the battery only once every hour.

This means that on average, batteries will be disconnected half an hour after they have passed point C and then they will be charged considerably more see next paragraph. So point C is the minimum at which a battery can be considered charged enough.

### batteries - Ratio Between Voltage and Charge - Electrical Engineering Stack Exchange

On average, batteries will be charged quite a bit more because the operator will only check once in a while. Allowing batteries to be disconnected a bit early means that total charging time see next paragraph will be shorter, which is important because: It means more convenience for users.

They can bring their battery late and still have it recharged the same day. It also means that the charger can serve more users without having to charge during the night.

Allowing that batteries are disconnected a bit early reduces the risc that they will be seriously overcharged, which would damage them just like undercharging does. It is important that operators have a simple and clear criterium for when a battery is charged well enough and point C provides this.

Another way of analysing the charging process is by of looking how charging current, battery voltage and some other variables vary over time. This comes down to calculating how much time it takes to go from one state of charge line to the next. As long as the current is constant at After passing point B, the current is no longer constant so that for each traject between two state of charge lines, a mean current must be calculated. For this, the current at the beginning and at the end of the traject has been read from fig.

The results of this calculation are presented in fig. In this figure, also the voltage Vi at the indicator is printed because this is the voltage that is measured on the switchboard. After 3 hours and 50 minutes, the charging process passes point B and the voltage regulator starts reducing the field current. After 5 hours and 30 minutes, the charging process passes point C and the battery can be considered just charged. After 7 hours, the charging process reaches point D and the battery is charged completely.

Charging any further means overcharging and will only damage the battery. The charging process as a function of time. Things will be quite different in case: