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Owning a mobile or off-grid solar power system means obsessing about how much charge is left in your deep cycle battery bank.
A battery bank is a set of batteries that are interconnected that store electrical energy. Other than solar power systems, a battery bank is also used in cars, RVs, and boats. The remaining amount of charge that a battery bank has is known as the “stage of charge”. You can determine the state of charge by looking at the AGM battery voltage chart or a deep cycle battery voltage chart.
Most owners are concerned about the life span of their batteries, and some are worried if they have enough voltage that meet their needs. If you own a solar power system but are unsure whether you need to get a battery, then ask the trusted experts at Energy Matters to help you get started.
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Understanding battery voltage
Understanding the relationship between battery voltage and state of charge (SOC) is paramount for maximising battery lifespan and ensuring a reliable energy supply.
Battery voltage explained: What is deep cycle battery voltage?
The voltage of a deep-cycle battery is a key indicator of its state of charge. It is typically measured in volts (V). The voltage of a fully charged deep-cycle battery can vary depending on the type of battery and its specific characteristics.
- Battery voltage is a crucial indicator of a battery’s health and performance. It measures the electrical potential difference between the positive and negative terminals of the battery.
- The voltage of a battery is directly related to its state of charge (SOC). A fully charged battery will have a higher voltage than a partially charged or discharged battery.
While not totally accurate, the easiest way to determine this is with a multimeter if your solar regulator or charge controller doesn’t have a voltage readout.
The state of charge does vary a little between sealed lead acid, flooded, gel and AGM deep cycle battery types and also between brands. Even the weather can play a role.
Batteries perform better in colder weather and have a higher state of charge reading on an AGM voltage chart. In warmer conditions, the battery’s performance decreases, as well as the state of charge. To ensure that you only get the best battery state, it is crucial to keep these factors in mind.
What should a 12-volt battery read when fully charged?
A 12-volt deep cycle battery should ideally read between 12.4 and 12.7 volts when fully charged. If the voltage is lower than this range, it may indicate that the battery is not fully charged or has a problem.
12v lead acid battery
12-volt lead-acid batteries are one of the most common deep-cycle batteries. They are available in various sizes and capacities. The voltage of a 12-volt lead-acid battery should be monitored regularly to ensure that it is in good condition.
State of Charge (SOC)
A deep-cycle battery’s state of charge (SOC) refers to the amount of energy it can store. It is typically expressed as a percentage. A battery’s SOC can be determined by measuring its voltage or using a battery monitoring system.
Battery voltage state of charge chart
The battery voltage chart below shows the voltage and approximate state of charge for each type of battery, including AGM batteries, lead acid batteries, and car batteries.
Note: The figures in the AGM battery voltage chart, lead acid battery voltage chart, and car battery voltage chart are based on open circuit readings. That is when the deep cycle battery hasn’t been under load for a few hours. This scenario may not occur very often in a battery based system that’s in continual use.
So, the best time to take the reading is early in the morning before the sun hits your panels, in the evening as the sun is setting, or when it’s very overcast. If you take a reading while the battery is receiving charge, it could read anything up to 14.5 volts.
If you take the reading when the panels aren’t exposed to the sun, as there will likely be power being drawn at the time, you can assume that whatever the voltage reading, it’s a conservative estimate. Once all load is removed from a battery, the voltage can bounce back up substantially. Take a look at the chart below:
| State of Charge | Sealed or Flooded Lead Acid battery voltage | Gel battery voltage | AGM battery voltage |
| 100% | 12.70+ | 12.85+ | 12.80+ |
| 75% | 12.40 | 12.65 | 12.60 |
| 50% | 12.20 | 12.35 | 12.30 |
| 25% | 12.00 | 12.00 | 12.00 |
| 0% | 11.80 | 11.80 | 11.80 |
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Battery depth of discharge
The general rule of thumb: the less your deep cycle battery discharges before recharge, the longer it will last.
Here’s an example:
A Sonnenschein Solar Bloc 100 AH Gel Battery discharged to a depth of 70%, i.e. with only 30% or 30 Ah (amp hours) remaining, will have a lifespan of around 1200 cycles, which is quite impressive.
However, if it only discharges to 50%, the expected number of cycles skyrockets to around 1700! If a cycle is a day, that adds over 1.25 years to the life of the battery.
Depth of discharge, also known as DOD, shouldn’t be any more than 50% in most deep cycle batteries in order to get the best value for money. So if you have a 100 AH battery, consider the cut-off discharge depth being 50 AH.
Measuring your depth of discharge is a very important calculation you should make when choosing the size of a deep cycle battery.
Here’s another example:
If you want to power a laptop computer, check the amps rating on the adaptor. It’s likely to be somewhere between the 3 and 5 amp mark. This translates to probably around 2 – 4 amps an hour under normal usage as your laptop won’t be using the full amount at all times. So, based on the lower end:
100 AH battery = 50 AH available capacity/2 amp draw = 25 hours usage.
Four main types of conventional deep cycle and lead acid batteries
There are many lead acid batteries on the market. But there are four types that are commonly used – sealed lead acid, flooded lead acid, gel, and AGM.
The four main types have their own advantages and disadvantages. It is important to determine which type meets your budget, as well as your preferences. Let us take a closer look at deep cycle batteries and lead acid battery voltage, including AGM battery voltage.
Sealed lead acid battery
Sealed lead acid batteries are the most common type of deep cycle battery used at home. They are favored by residential and business properties as they are maintenance-free and have a long lifespan. However, they are also the heaviest type of battery.
Flooded lead acid battery
Flooded lead acid batteries require periodic maintenance and adding water to the cells, but they are less expensive than other types of batteries.
Gel batteries
Gel batteries are similar to sealed lead acid batteries in that they are maintenance-free, but they are more expensive and may not last as long. Gel battery voltage is typically around 2 volts per cell, which means a 12-volt battery delivers six cells.
AGM batteries
AGM batteries are the most expensive type of battery, but they provide the best performance and have the longest lifespan. AGM battery voltage may depend on the specific model and the manufacturer. AGM battery’s minimum voltage is 12 volts, meaning it can produce 12 volts when fully charged.
Solar charge controller
Other than lead acid batteries, you must also consider a solar charge controller. A solar charge controller is an important part of any solar power system. They regulate the flow of power from your panels to their batteries, thus, preventing them from overcharging.
One main factor when choosing a solar charger controller is the type of panel you own, battery size, and the amount of energy source you need to generate.
Factors affecting battery voltage and SOC
Several factors can affect the voltage and SOC of a deep-cycle battery, including:
- Battery type: Different types of deep-cycle batteries have different voltage characteristics.
- Age: As a battery ages, its voltage and SOC can decline.
- Temperature: Extreme temperatures can affect the performance of a battery.
- Charging and discharging: Frequent deep discharges and improper charging can shorten the lifespan of a battery.
Monitoring battery voltage and SOC
To ensure a deep-cycle battery is in good condition, it is important to regularly monitor its voltage and SOC using a voltmeter or a battery monitoring system.
Maintaining battery voltage and SOC
To maintain the voltage and SOC of a deep-cycle battery, it is important to:
- Charge the battery properly: Use a suitable charger and follow the manufacturer’s instructions.
- Avoid deep discharges: Deep discharges can shorten the lifespan of a battery.
- Store the battery properly: When not in use, store the battery in a cool, dry place.
- Have the battery serviced regularly: Regular maintenance can help prolong a battery’s lifespan.
- Temperature control: Store and operate your deep-cycle battery in a cool, dry environment as much as possible. Extreme temperatures can reduce battery life.
Why deep cycle batteries are ideal for Australia's energy needs
Australia’s increasing reliance on solar and renewable energy makes deep-cycle batteries crucial to modern energy systems. Their ability to endure frequent discharge cycles makes them ideal for off-grid homes, caravans, and solar-powered businesses.
The unpredictable weather conditions nationwide further highlight the need for reliable energy storage. By understanding battery voltage discharge and the impact of battery discharge rate, you can optimise your energy storage system for efficiency and durability.
Choosing the right battery voltage for your needs
Battery manufacturers aim to produce the best products in the solar industry, so it’s vital that you choose a reliable and reputable brand. With a number of factors to consider, it is no surprise that batteries can be quite confusing.
Completing the off-grid ecosystem: Solar panels & EV chargers
Deep-cycle batteries are often used in conjunction with solar panels for off-grid power generation. Exploring commercial solar panel installation costs can be a great first step if you’re considering a solar power system for your home or business. Integrating solar panels with your deep-cycle battery system allows you to harness renewable energy and achieve true energy independence.
Furthermore, electric vehicles (EVs) are becoming increasingly popular. Deep cycle batteries can be used for stationary energy storage in conjunction with home EV chargers to maximise the utilisation of your solar energy and potentially reduce reliance on the grid.
Energy Matters for your solar power needs
Deep-cycle batteries are the backbone of sustainable solar battery energy storage, especially in Australia. You can enhance your system’s reliability and longevity by mastering concepts like voltage vs. discharge rate and understanding how to manage battery discharge cycles.
Australian consumers can optimise the performance and longevity of their deep-cycle batteries and recreational adventures by understanding the intricate relationship between battery voltage, discharge rate, and SOC. View our deep cycle battery guide to learn more about their differences.
Energy Matters recommends consulting with a qualified solar and battery installer to assess your specific energy needs and select the most suitable battery system for your home or business, such as the Tesla Powerwall.
Energy Matters has been recognised for our continued excellence in the Australian solar industry. We provide our customers with high-quality resources, insight, and access to reputable solar quotes. Our team of solar experts can help you get up to 3 FREE solar quotes from pre-qualified and vetted solar firms in your area.
