Battery advanced topics
In this knowledge article we will cover some more advanced battery topics including
- Selecting system voltage
- C rating
- Battery properties (Open circuit voltage, Cell voltage, A/h ratings)
- Detrimental battery effects (DOD, high rated charge controllers)
1. Selecting your System Voltage
In some cases it is more convenient to run a system at 12 volts. For example, in marine applications where off the shelf appliances are designed to run at 12 volts. However if the power requirements become excessive or the current flow in the cables start to cause voltage drop it may pay to increase the system voltage. This can be done by placing identical batteries in series. For example, to create a 24 volt system place two identical 12 volt batteries in series. You can now calculate your Wh storage using the same formula as in the basic battery knowledge article. Ah = (Total power consumption / (DoD x Vsystem). You will see when you do the maths that the power storage has now doubled. Lux solar controllers are designed to be operated on both 12 and 24 volt systems and automatically detect which one you are using.
2. C rating
The C rate is a measure of the rate of charge or discharge of the battery in relation to its capacity. That is, A 1C discharge rate for a 100 Ah battery correlates to a discharge of 100 Amps for 1 hour. Doubling the C rate to 2C would mean drawing 200 Amps for half the time. Similarly a charge rate of 0.2C would mean a rate of 20 amps for the same battery. Many battery manufactures provide specifications in relation to the c-rate. For example they may provide a maximum recommended charge rate in terms of C. Charging the battery above these rates will lead to an increase in the temperature of the battery and a shortened lifespan. Another example they may provide in terms of the c rate is the battery capacity. For example the battery may have a capacity of 100 Ah when discharged over a 10 hour period (0.1 C). As the rate of discharge increases the battery capacity will decrease.
3. Battery properties
In this portion we will discuss a few properties that are inherent to every battery.
Open circuit voltage
The open circuit voltage is the voltage of the battery under no load. As the load or the current flow out of the battery increases, the voltage will drop. This is due to the internal resistance of the battery.
Every battery has an internal resistance. In technical terms a battery looks like a voltage source in series with a resistor. When current is drawn from the battery this "internal resistor" drops voltage. So using ohms law, the battery voltage will now be the battery voltage minus the internal resistor size times the current drawn. This implies when we draw large currents the voltage will drop lower. This is exactly what happens when you start a car or a boat. If you measure the battery voltage during starting it will drop to approximately 10 volts. If you measure the open circuit voltage of the battery it might look reasonable, however as soon as any current is drawn, the voltage will collapse, this is due to the increased internal resistance. This normally means the battery is nearing end of life.
Amp hour ratings
Amp hour (Ah) ratings are in theory quite simple. It is the amount of amps you can draw from your battery for 1 hour. However the rating can alter with multiple factors. The amp hour rating is specified under certain operating conditions. Most manufacturers will specify the amp hour rating at a specific temperature under fixed discharge conditions. If the conditions vary from these, the capacity will change. For example if the temperature increases the battery capacity will increase and if it decreases it will decrease. Also if the battery is discharged at faster rates than specified the capacity will be reduced.
The cell voltage of each cell in a battery is determined by the battery chemistry. For example a lead acid will have a nominal voltage of around 2.1 Volts per cell and a lithium ion around 3.7 Volts per cell. If we add a cell in series we double the voltage. If we add a cell in parallel we increase the current capabilities of the pack.
A 12 volt boat battery is simply 6 individual lead acid cells wired in series.
4. Detrimental Battery Effects
A lead acid battery will discharge itself with nothing connected if left over a period of time. This is normally specified on the battery data sheet. For example if a battery is left over the winter period of 6 months. The battery could discharge to around 75 percent of its capacity. The self discharge rate mainly varies with temperature. The hotter it is the faster the chemical reactions take place inside the battery and the faster the self discharge rate. Having a Lux Solar system installed will mitigate this problem for your house battery.
The main issue with self discharge in the marine environment is on the start battery. Leaving the start battery over the winter uncharged is very detrimental to battery life. This will also cause an issue with starting. Lux solar are currently developing a system to mitigate this problem.
Temperature of operation has an impact on multiple facets of the battery. An increased operating temperature will:
- Decrease the life span of the battery
- Increase the rate of self discharge
- Increase the amp hour rating of the battery.