This definitive and practical guide contains everything you need to know about solar batteries and their usage in solar power systems for your residential home, off-grid property, RV, van, camper or boat.
Here you’ll find the expert answer to the following essential questions:
- What is solar battery storage and how do solar batteries work?
- What is a deep cycle solar battery?
- Which solar battery is suitable for my solar project?
- What is a solar battery backup?
- What is a solar battery charge controller?
- How to charge a battery with a solar panel?
- Which basic battery parameters do you need to know as a solar power user?
- How to connect solar panels with batteries?
- What is the best battery for a solar system?
- What kinds of batteries are used in off-grid solar panel systems?
- What is the cost of solar batteries?
- What is most important when sizing a battery?
And much more…
Let’s get started!
What is solar battery storage and how do solar batteries work?
Batteries are devices build of electrochemical cells connected in series.
An electrochemical cell consists of two electrodes immersed in an electrolyte solution.
When a circuit is formed between the electrodes, a current flows.
Such a current is a result of chemical reactions occurring between the electrodes and the electrolyte.
Therefore, an electrochemical cell converts chemical energy into electrical energy.
Some battery types can only be used ones, others are rechargeable.
Solar power systems use rechargeable deep cycle solar batteries.
What is a deep cycle solar battery?
Batteries that can be frequently discharged down to such a value (at least 50% or even 80%) are called ‘deep-cycle’ batteries and are intended to use in solar power systems.
Car starting batteries are widely available at low cost.
However, they are not suitable for use in solar power systems due to the fact that solar batteries are frequently discharged down to 50% (or 80%). i.e. automotive batteries are not ‘deep-cycle’ ones.
Such frequent discharges are detrimental for the lifespan of any automotive battery because these batteries are designed to provide high current for a very short time.
That’s why they are called either ‘starting’ or ‘cranking’ or ‘shallow-cycle” batteries.
Our advice to you is to avoid saving money on the battery by purchasing a car starting one.
Batteries used in electric vehicles (fork-lift trucks) are much more suitable for solar power systems.
What is a solar battery backup?
Solar panels generate electricity only when the sun is shining.
Therefore, they generate electricity and do not store it.
Some electrical devices and appliances are used not only in daytime but also at night or when it’s cloudy, so you need to store electricity to use them.
The most popular solution to this problem is to use batteries.
Off-grid systems are typically provided with a battery backup to store the solar-generated electricity:
The load can be, for example, a TV set or a laptop.
Since such devices operate not only in the daytime, a battery is needed to ensure their operation during night hours.
The Schottky diode prevents the solar panels from the reverse current flowing from the battery to the solar panels during the night.
In such a system, the solar-generated electricity is used for charging the batteries, usually through a charge controller:
Here is how to connect a solar panel to battery and inverter:
What is a solar battery charge controller?
Charge controllers prevent solar batteries from overcharging and over-discharging.
Furthermore, the charging voltage of any battery must be strictly regulated.
If the charging voltage is too high, the battery might get damaged (by possible loss of electrolyte, excessive gassing and plate damage).
If the charging voltage is too high, the plates might get ‘sulphated’ and the available battery capacity is reduced.
If you connect a battery directly to a solar panel, after getting fully charged (on a bright sunny day), there is a risk that the battery might get overcharged and thus damaged.
A single overcharging is not dangerous for lead-acid batteries.
Regular overcharging, however, reduces the battery lifespan since it causes loss of electrolyte (by gassing) and damaged plates.
To prevent the battery from overcharging when it’s fully charged, you should connect a charge controller between the solar panel array and the battery.
A properly selected charge controller will enhance the life of your battery bank.
How to charge a battery with a solar panel?
Battery charging denotes the process of ‘putting’ electrical energy into the battery cells.
Battery discharging denotes the process of removing the stored energy from a battery.
The energy can be removed not only by connecting electrical loads to a battery, it always decreases over time.
This is called ‘self-discharge’.
The most popular rechargeable batteries available on the market today are lead-acid, lithium-ion and nickel-cadmium.
Lead-acid batteries are often the preferred choice for residential solar power systems as they are widely available, cost-effective and reliable.
However, they need regular maintenance and are not suitable for small and/or mobile solar panel systems.
Lithium-ion and nickel-cadmium batteries are more expensive than lead-acid ones but they are the ideal match for small solar-powered devices and small solar panel systems.
A single lead-acid battery cell has a voltage of 2.1 V when fully charged.
A lead-acid battery bank comprising 6 cells connected in series has a voltage of 12.6 V in the idle state (when not connected to a device).
This voltage increases in the process of battery charging, when you connect a solar panel to the battery and decreases when you connect a load to the battery and discharge it by draining the stored electricity.
The lead-acid battery is suitable for a variety of stationary solar power applications.
Each cell of a lead-acid battery has a nominal voltage of 2 V.
Therefore, a 12 V solar battery consists of 6 battery cells connected in series.
Lead-acid batteries are usually available as 2 Volt cells or 6 V or 12 V battery banks.
A standard car battery is an example of a 12 V battery bank.
Both alkaline (nickel-cadmium and nickel-iron) and lithium batteries are quite dissimilar to lead-acid ones in terms of being sealed (their electrolyte is dry), portable and maintenance-free.
These batteries are always more expensive than lead-acid ones but are selected for solar panel systems due to the following obvious benefits compared to their lead-acid counterparts:
- Lighter and smaller in size
- Fully maintenance-free
- Longer lifespan
- Can be completely discharged without damaging the cells
- Can be left in a state of low charge for a long period.
Basic battery parameters
A battery ‘cycle’ means a single sequence of battery charging and discharging. Lead-acid batteries can be divided into two types – ‘deep-cycle’ and ‘shallow-cycle’.
Deep-cycle batteries are designed for deep discharges and are used in solar power systems, while shallow-cycle batteries are not designed for deep discharge and are intended for use in cars and vehicles.
The advantage of a shallow-cycle battery is its ability to provide a high starting current which is not a primary requirement in a solar panel system.
Instead, for solar panel system much more important is the battery’s ability to be discharged at least down to 50% and be recharged many times over a couple of years.
Therefore, you could use an automotive battery for your solar system but you should be careful not to discharge it deeply, which presents an obvious inconvenience.
‘Capacity’ is the amount of energy a battery can store.
Capacity is measured in amp-hours (Ah). For example, a battery of 200Ah can provide a current of 1A for 200 hours or 5A for 40 hours or 10A for 20 hours or 50A for 4 hours.
It should be noted that the rate at which a battery is discharged affects its capacity.
For example, a battery of 200 Ah discharged at 5A might provide this current for 40 hours.
The same battery discharged at 10A, however, might only be able to deliver this current within 15 hours, rather than for expected 20 hours.
For this reason, in battery datasheets and specifications you can often see the letter “C” next to the number indicating the battery capacity.
For example, C40 refers to the capacity upon the battery discharged over 40 hours.
As a rule, when a battery is cold, its capacity decreases.
‘Charging current’ is the current supplied by the solar panel to the battery and stored in it.
Lower charging currents (about 5% of the capacity) are better for a battery.
A rule of a thumb is that the maximum current during battery charging should not exceed one-tenth of its capacity.
However, some solar batteries, i.e. Li-Ion solar batteries, can withstand charging currents higher than one-tenth of their capacity without the substantial decrease in their lifespan.
Therefore, a 200Ah battery is recommended to be charged at a maximum current of 20A.
Should this value be exceeded, the battery cells might get damaged.
Another important battery parameter is ‘state of charge’.
The state of charge measures the energy remaining in the battery.
A fully charged battery means a 100% state of charge, while a half-charged battery means a 50% state of charge.
Alternatively, this can be denoted by ‘depth of discharge’ (DoD) indicating how much a battery is discharged before it is charged again. Therefore, a state of charge of 80% and a depth of discharge of 20% refer to the same battery state.
Shallow-cycle (automotive) batteries should never be discharged below DoD of 20% (80% state of charge).
Deep cycle batteries are much more tolerant to discharging but nonetheless, DoD below 60% (40% state of charge) is not recommended.
‘Self-discharge’ is a process where batteries lose charge as a result of being left uncharged for a long period.
Such a process depends on the temperature but also on the type, condition, and age of the battery.
Although the typical self-discharge rate is about 5% per month, it could be higher with dirt and dust accumulated on the top a battery and in case of high ambient temperatures.
Therefore, you should avoid storing a battery for long periods without using it.
For lead-acid batteries, the self-discharge process is additionally aggravated by the process of sulphation – an irreversible chemical change occurring with the battery plates.
You should also mind that if a lead-acid battery is left unused and uncharged for a period of one month, it might not be able to regain its rated capacity even after charging it.
Automotive (car) batteries, also known as SLI (‘Starting, Lighting and Ignition’) batteries are lead-acid shallow-cycle batteries designed for use in vehicles.
They are not a good storage option for solar panel system as they are damaged when regularly discharged by more than 20% DoD (i.e. 80% SoC).
However, they are both low-cost and locally available in various brands and capacity.
Selecting a SLI battery for your solar panel system only seems reasonable if you are on a very limited budget and your daily consumption is relatively low.
In such a case, a SLI battery of capacity between 60 and 100 Ah will probably be enough as long as you don’t let the battery get discharged below 80% state of charge unless you wish your battery to withstand just a couple of cycles.
We recommend you selecting truck batteries that have a larger capacity than car ones – 100-120 Ah.
Li-Ion batteries are gaining popularity mostly for use in mobile solar panel systems where low weight and limited space are more important than cost.
The main advantages of Li-Ion batteries: they are lightweight (30% of the size of a lead-acid battery), compact (50% of room needed for a lead-acid battery), the maximum DoD is 70-80% (compared to 50% for lead-acid batteries), they have twice longer battery lifespan as compared to lead-acid batteries, provide more capacity in low-temperature conditions and are fully maintenance-free.
The essential drawbacks of Li-Ion batteries are: their higher price (compared to any other battery type), the need for a dedicated battery management system, as well as the risk that the battery voltage operating window might be incompatible with the input operating window of the inverter.
Here are some batteries you could also encounter on the market as labeled ‘solar’:
- Lead-antimony batteries – they have capacity of 80-200 Ah and low self-discharge rate. These batteries offer a greater number of cycles (between 1,000 and 2,000) and can be operated between 3 and 5 years as long as they are not exposed to discharges below 75% of the SoC. For this reason they are better than typical SLI batteries and are sometimes referred to as ‘modified SLI’. Another drawback is the frequent need for electrolyte refill. These batteries are used in fork-lifts or golf-carts but are not the best option for solar panel systems.
- Lead-calcium batteries – although not completely sealed, these batteries are enclosed and are considered maintenance-free since no electrolyte refill is needed. The big disadvantage is their low tolerance for deep discharges which means that they should be avoided for solar power systems.
- Valve-regulated lead-acid (VRLA) batteries – a lead-acid battery that is sealed and therefore can be called ‘maintenance-free’. To prevent gas collection inside the batteries, these batteries have safety valves that play an important role during charging. VRLA batteries are much more expensive than traditional lead-acid ones. Also compared to lead-acid counterparts, these batteries are less tolerant to deep discharges. Another drawback is that they require some special settings on charge controllers.
RV/Marine batteries are a compromise between vehicle batteries (also known as ‘cranking’ batteries) and deep-cycle batteries.
RV/Marine batteries are used in small photovoltaic systems where size is often as important as performance.
What is a solar power battery bank?
A solar power battery bank is a set of batteries connected together to increase the total voltage, the total current or both.
Below you can find some common examples how to wire a solar battery bank where batteries are connected in series, in parallel or mixed.
The total voltage and current of batteries connected in series are as follows:
If you, however, connect batteries of different capacity in series, the total capacity obtained is equal to the lowest capacity in the string:
Therefore, you should avoid connecting in series a set of batteries of different capacity.
The total voltage and current of batteries connected in parallel are as follows:
If you need a higher capacity, you should connect maximum 2 or 3 strings in parallel.
Otherwise, the overall battery bank capacity will be reduced in case of a faulty battery in any of the strings.
Furthermore, a string comprised of many low-voltage high-capacity batteries connected in series is a better option for getting the desired capacity than a string composed of one or two high-voltage batteries connected in parallel.
Here is how you can increase both overall voltage and capacity by mixed connection of batteries:
How to connect solar panels with batteries?
Below you can find a picture of how to charge a 12V battery with solar panels.
The battery bank has a total voltage of 12V and capacity of 700Ah and is to be charged by a solar array comprising 4 solar panels.
To prevent the battery bank from overcharging and overdischarging, a charge controller is needed.
What is the best battery for a solar system?
Selecting a battery for your solar panel system should not be underestimated.
Battery performance affects the overall system performance while battery life affects the system cost. Before all, you should investigate what batteries are currently available on the market.
The worst option is to choose an automotive battery (SLI) which has the shortest possible lifespan due to its intolerance to frequent discharging. If using vehicle batteries seems inevitable to you, you’d better choose truck batteries rather than car ones due to their higher capacity (over 120Ah compared to 50-60Ah at car batteries).
A poorly operating battery can not only reduce the performance of your solar system but can also damage your household devices and appliances.
To choose the right battery means not only to find the battery matching technically to the rest of the system components but also to make the right decision to invest more money on a costlier battery that will be worth your investment over time.
Before deciding what is the best battery for solar storage, you should consider the following:
- Price. Although there are very good batteries at high prices, it is not a good idea to spend a fortune on the battery in case of a small system. Furthermore, if your consumption is limited to a couple of low-power devices (obviously a small panel system), using a truck battery might be the cost-effective solution.
- Capacity. It can be estimated by using the battery sizing methodology provided in our book “Off Grid And Mobile Solar Power For Everyone: Your Smart Solar Guide” available on Amazon. Furthermore, considering the battery aging, you should choose a battery of slightly larger capacity than calculated. Also, remember than colder temperatures make capacity decrease.
- Lifespan. The better batteries have a longer cycle life (greater number of charge and discharge cycles). A great disadvantage of lead-acid batteries is their low tolerance to deep discharges. Operation at high temperatures reduces the battery lifespan. Therefore, capacity and lifespan are differently affected by temperature and you have to prioritize. When designing a solar panel system, to take account of environmental temperature, you should include a battery temperature multiplier in the calculations – see the section on battery sizing below.
- Maintenance. If batteries appear hard to maintain, you should select maintenance-free batteries which are always costlier.
- Mobility. Lead-acid batteries are totally unsuitable if you are going to build a mobile solar power system and install it on a caravan, camper, motorhome or RV, since they are not tolerant to vibration and tilting.
- Size. After calculating how much capacity your system will need, be sure to refer such estimated capacity to the room space you need to install it.
- Replacement. Be sure to choose a battery that can easily be replaced when needed. This means ordering a battery available at the local market rather than ordering it from abroad, which is both costly and time-consuming.
You should select the battery not just as one of the system components.
Choosing the best battery for your solar system is vital since the battery affects both the cost and the overall system performance.
You should start with making a quick research about what is available on the market.
Only in case of no other option available, you could use an SLI (automotive) battery for a small off-grid system (10Wp-1kWp).
However, we recommend you to use a truck SLI battery (100-120Ah), rather than a car SLI battery (50-60Ah).
Of course, these are general guidelines and the battery capacity should match the solar power system size.
The higher the solar power generated the higher the solar bank capacity.
If you are not building a residential solar panel system, you should consider using sealed batteries – alkaline (Nickel-Cadmium or Nickel-Iron) or Lithium-Ion (Li-Ion).
These batteries are very different to lead-acid batteries, since they are sealed, portable, maintenance-free, and are used to power relatively small devices. Also, compared with lead-acid batteries, they are much smaller, lighter and have a longer lifespan.
Nickel-Cadmium or Nickel-Iron batteries can be left fully discharged for extended periods.
Li-Ion batteries, however, can be damaged by deep discharges.
Nickel batteries can operate in a broader temperature range, while Li-Ion batteries perform well in colder temperatures and can get damaged by heat.
What is the cost of solar batteries?
The cost of batteries alone can reach between 25% and 50% of the total cost of an off-grid solar panel system.
Batteries of a longer lifespan cost more but are less expensive to maintain.
How to size your battery or battery bank
Battery sizing goes hand in hand with selecting the right battery type for your solar system.
The primary goal of battery sizing is to evaluate the minimum battery capacity required to meet the daily consumption of electricity:
Q = (Eday * DoA * BTM) / (DoD * CE * SV)
- Q is the Minimum battery capacity required (in Ah),
- Eday is the Average daily energy target (in Wh) estimated by the load analysis of the devices used daily. A load analysis is assessing the average daily energy consumption based on the power rating of each device and for how long it is used.
- DoA is the Days of Autonomy selected according to how important for you is to power the available appliances with no outages. The commonly assumed values are between 3 and 5 (days).
- BTM is the Battery Temperature Multiplier considering the negative effect of the temperature on the battery capacity.
- DoD is the permissible depth of battery discharge. As a rule, a lead-acid battery, unlike Ni-Cad and Li-Ion ones, should never be fully discharged. DoD = 80% is a commonly used value for lead-acid batteries, however, DoD = 50% will result in a longer battery life.
- CE (Cable Efficiency) denotes the losses in the cables between the battery and the charge controller, and between the battery and the loads.
- SV is the System Voltage that is typically 6V or 12V for small systems, 24V for medium systems, and 48V for large systems.
Having determined the minimum capacity required, here is what comes next:
- Select a particular battery type available on your market;
- Calculate how many batteries of such type you should connect in series and/or in parallel to achieve the above-estimated minimum capacity required;
- Size the overcurrent protection devices that are to be installed between the battery and the charge controller and between the battery and the inverter;
- Wire together all the components.
You can find more info on battery selection, sizing, and wiring along with good examples in our book “Off Grid And Mobile Solar Power For Everyone: Your Smart Solar Guide”.
Solar batteries in a summary:
In solar power systems, batteries store the electricity produced by the photovoltaic array.
- Batteries are the most expensive components of the solar panel systems and often, also the least durable ones.
- A poor functioning battery can reduce the overall solar system performance.
- The optimal battery types for residential and mobile solar panel systems are different. While lead-acid batteries are considered the optimal choice for home solar power systems, alkaline (Ni-Cad and Ni-Iron) and Li-Ion batteries are the best choice for caravans, campers, RV, motorhomes, etc.
- Batteries should get a full charge by the solar array regularly.
- Batteries used in solar electric systems for homes (typically lead-acid ones) need maintenance. Batteries used in mobile solar power systems do not need maintenance but are more expensive.
- You should only connect batteries of the same type, manufacturer, and age. Mixing different batteries is not recommended since this might reduce the system performance but also reduce the lifespan of each battery.
If you are more visual you can check out our video dedicated to solar batteries as well.
By watching the video below you will discover:
- basics of solar batteries and their application in solar power systems
- the most important battery features,
- what kinds of batteries are used in solar power systems
- how to connect them and what to avoid to have an efficient and healthy solar battery bank
1. Hankins, Mark. 2010. Stand-Alone Solar Electric Systems: The Earthscan Expert Handbook for Planning, Design and Installation, Earthscan.
2. Mayfield, Ryan. 2010. Photovoltaic Design and Installation for Dummies, Wiley Publishing Inc.
3. Pop MSE, Lacho, Dimi Avram MSE (2018-02-28), Off Grid And Mobile Solar Power For Everyone: Your Smart Solar Guide (Kindle Edition). Digital Publishing Ltd.
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