In vehicle applications, the use of a second battery is very common. Most users choose one of the widely available lead-acid battery technologies. These include starter-type wet batteries, sealed-valve regulated, Gel and AGM types. Each type has different characteristics, but they are all essentially energy storage devices.
The last two articles have covered two topics: first, how to store energy in a battery by charging with an AC-DC charger or MPPT solar controller, and second, how to make up for energy losses that happen in a lead-acid battery when charging takes place.
How do you monitor the state of charge of a battery? There are three main ways.
Measuring the Specific Gravity (SG): In a ‘Wet’ battery (with removable cell caps) a Hydrometer can be used to measure the Specific Gravity. This reading (say, a number such as 1.26) is directly related to the concentration of the acid/electrolyte, and is a very accurate indicator of a battery’s state of charge. This method is confined to wet batteries, requires a specialised tool and cannot easily be carried out when in the field.
Battery Voltage: This method can be used as a rough indication of the battery’s state of charge. Note that the battery should be at rest or undisturbed for several hours to obtain the best result. While a battery is being used, energy is flowing in and out, affecting the voltage and potentially giving an inaccurate reading.
Amp Hour meter: This device is specifically designed to monitor state of charge and is the most accurate method. The monitor keeps track of the current flowing in and out of the battery, and uses an algorithm (which can be adjusted) to continuously calculate Amp Hours being charged or discharged.
For example, assume a discharge current of 5A for 12 hours (equals 60 A/H). In real time, the discharge changes continuously, which the monitor then takes into account. The Current is measured by means of a shunt which is placed in the negative circuit of the installation, preferably as close to the battery as possible. (A shunt is a precision resistor able to measure voltage drop caused by AC or DC current passing between its poles.)
Energy efficiency of a battery
When a battery is charged or discharged, losses occur. The battery will absorb about 25% more energy during charging, in comparison to when it is discharged. High charge currents and high discharge currents will also affect this efficiency. The greatest losses occur because the voltage is higher during charging than during discharging.
As the battery voltage rises during charging, the temperature also rises, which causes gassing. Certain batteries are designed not to gas excessively in comparison to others – this is an advantage of Sealed, AGM and Gel batteries.
Charge efficiency of a battery
Amps going into a battery (the charge current) are transformed into chemical energy which is stored in the plates of the battery. But, due to gassing and heat, not all the energy is stored in the battery. Amps charged over time are lost in this process (here Amps x Hours = A/H).
The charge efficiency can range from 70% to 95%. A battery monitor must take charge efficiency into account, otherwise the reading will be in-accurate. Efficiency must be set manually on the monitor; the recommended setting is 85% for a new, fully-charged lead acid battery.
Over time, the battery will become more inefficient; and this value must be adjusted to ensure accurate readings.
Useful features of a battery monitor
Using a simple voltmeter with an audible alarm does not really provide a full view of your battery’s usage patterns.
Rather get a smarter monitor that shows more information. The Victron Energy BMV-700 range provides a digital display of real-time Voltage, charge or discharge current, and power values. It also has an A/H counter and a Time to go (TTG) value.
The TTG is a time-value, linked to the algorithm monitoring the real-time current values, which is displayed during discharging.
Linked to the TTG is a discharge floor setting, which has a default setting of 50%. The TTG therefore will indicate how long the battery can sustain the existing discharge until it reaches, for instance, 50%. A relay is built into the monitor which can be activated via the settings menu. Various alarms for Voltage and SOC are also available.
Advanced information is found in the Historical Data menu, where event counting and data logging information can be seen. By using the Bluetooth-driven Victron Connect App, the user can view this history menu on his or her smartphone. This information is also available directly from the monitor, using its menu function.