Properly wiring a battery bank is as important as determining the size of the battery that you need. However, this connection process is not always as clear as we would like, so in this article, we will approach all the knowledge that you need to wire your battery bank at home.
Basically, batteries can be wired in two ways: series or parallel. Let’s examine what each of these connections mean.
Batteries In Series
What happens when you connect batteries in series?
Each battery has specific parameters such as the nominal capacity, the maximum depth of discharge, efficiency, lifespan, and nominal voltage. This last parameter is very important as it determines the charging voltage that the batteries will need to get charged.
The basic structure of the battery is composed of a battery cell which is a plate that has approximately a 2V output. Connecting multiple battery cells in series allows obtaining battery units of 4V, 6V, 8V, 10V, and 12V.
Now, this principle inside the battery unit also applies when you wire the battery bank, in other words, when connecting the batteries in series you will increase the nominal voltage output of your resulting battery bank. However, the electrical current that flows through them will remain the same.
Thus, if a battery unit has 12V and has a 5Ah output, then, connecting the same battery in series will increase the resulting nominal voltage to 24V and will maintain the same 5Ah.
Advantages and disadvantages of series connection
The first and maybe most important advantage is that the charging and discharging electrical current of the battery bank is lower. This is positive because it reduces ohmic losses in the form of heat and reduces costs because lower gauge sizes are needed for the cable that connects batteries and the charge controller. Also, this lower electrical current means that there will be fewer hot spots in terminals of connections, which drastically reduces the chances of fire or ignition. Another good thing about a series connection is that the same charging current will flow through all the batteries, meaning that they will all be charging/discharging at the same rate.
On the other hand, the main disadvantage of a series connection is that it has very low reliability because if one of the connections comes loose or breaks, then all the power is lost.
Batteries in Parallel
What happens when you connect batteries in parallel?
Meanwhile, when connecting the batteries in parallel, the voltage will remain the same and the electrical current will increase.
Thus, if a battery unit has 12V and has a 5Ah output, then connecting the same battery in parallel will increase the output to 12V and 10Ah.
Advantages and disadvantages of parallel connection
Alternatively, a parallel connection offers the best reliability since even when one of the strings of batteries suffers some malfunction, the rest of the batteries will continue working and will not interrupt the power supply. Of course, the power output of the battery bank will be lower, but it will not leave the load completely out of service. Moreover, by making a parallel connection, you will increase the capacity of your system.
However, a parallel connection also means that the charging current that is used to charge all the bank will be divided between every string of batteries. Although theoretically, it should be “equal”, the truth is that due to manufacturing differences or low-quality connections, the charging and discharging current that will flow throughout the strings can be different. This creates a unbalance that is undesirable for the long-term performance of the battery bank.
Can I put different voltage batteries in parallel or series?
You might find yourself in a situation where you may need to replace a battery within the battery bank and that you do not find the one that you purchased before. Thus, you may ask yourself if it is possible to connect two batteries voltages either in series or parallel connections. The short answer is no, but let’s see why that is.
Let’s assume that we have two batteries, where B1 has a higher voltage than B2 and that you want to connect them in series. Now, the first issue that you will have is finding two batteries with different voltages that have the same capacity, (Ah) which is not possible. Even if you find models with similar capacity (Ah), the problem will be that the resulting capacity of the bank will be the lowest one between the two of them.
The capacity is expressed as ampere-hour capacity, meaning it is highly dependent on the electrical current that flows in or out of the battery. Since in a series connection the electrical current has to be the same (due to physics laws) then, the highest current that can exist in a circuit with two different current ratings is the lowest one between the two of them.
However, this issue should only mean that you will be underusing the highest capacity one. That will not kill you, but that is not all.
As we discussed before, if you choose to connect them in series, the resulting battery bank voltage will be the sum of the two battery voltages. Thus, if B1 is 12V and B2 is 6V, then you should be having around 18V, so, what is the problem?
The issue is that charge controllers or battery-based inverters use the voltage of the batteries as a reference of the state of charge of each battery. Therefore, if you use two batteries with different voltages the charging source will not be able to accurately predict what is the state of charge of each battery. Then, it will set a charging current that will not be the required one, neither for the lower voltage one nor for the higher voltage one.
Whatever charging electrical current is selected by the charging source, it is likely to be higher than the specified for the lower voltage one and lower for the higher voltage one. Thus, the lower voltage battery will charge faster, to an overcharge point, while the higher voltage one will never be fully charged.
On the other hand, during the discharge process, the weakest battery (lower capacity) will drain first. Even after reaching the point where the weakest battery should start recharging, the circuit will still be providing electrical current to the load as the strongest battery will still have capacity available and will not have reached the point of recharge from a charge controller perspective. By the time the strongest battery has fairly discharged, the weakest battery will be extremely discharged.
Repeating this cycle multiple times leads to two main problems in the best-case scenario. The first is that the lifespan of both batteries will become deeply affected, reducing to maybe half or more than what it should be. The second one is sulfation, a phenomenon that occurs when batteries are overcharged or when batteries are never fully charged. Sulfation leads to the formation of lead sulfate crystals that drastically decrease the efficiency of the battery system. In the worst-case scenario, results can be leakage or ignition.
Now if you think that the problem is making the connection in series, the truth is that making a connection in parallel with batteries with different voltage would not only be worst but would be a disaster. In addition to all the problems stated before, another one comes up.
Since in parallel both batteries must have the same voltage, the higher voltage battery will always try to charge the lower voltage battery, the problem is that unlike a controlled charging source, the higher voltage battery will simply provide all the current possible to charge the lower one. This will take it to overcharge and probably heat the lower voltage one rapidly. If there is a high difference between voltages, then the chances of ignition or gas expulsion are very high.
Can I put different amp hour batteries in parallel or series?
The problem that we exposed under a series connection in the previous section will be the same problem that you will have in this case, even if they are both 6V. The lower capacity one will be overcharged during charging periods and will deeply undercharge during discharging periods, while the higher capacity one will never reach a full charge. This will lead to sulfation, shorter lifespan, and in the worst cases leakage or ignition.
Meanwhile, putting batteries with different Ah but similar voltages in parallel (assuming both are around 12V for instance), is maybe the lesser evil of all. However, you must know that even when connecting batteries from the same technology, brand, and model, there can be differences in voltages. The chances increase when you select two batteries from different brands or models, even if they both say 6V. Despite that differences should be minor, they will still impact the performance of the system, maybe leading to sulfation and reduction in efficiency.
Can I put batteries in series and parallel at the same time?
Yes, it is possible.
Actually, battery banks are designed by making series/parallel connections of the same battery. The reason why you want to do combinations is that you will increase the reliability of your system and you will also have a higher capacity. Moreover, by adding batteries in series, you increase the voltage of the battery bank, which at the same time, reduces the amount of current that you need to supply specific power or energy demands. This becomes very important for applications with higher power demands.
Problems with batteries in parallel
As we have mentioned in the advantages/disadvantages section, there are some issues when connecting batteries in parallel. The main issue occurs when multiple strings are connected in parallel, banks with over 4 strings of batteries will generally have unbalances during the discharge and charging process. This is because no matter if batteries are the same model, there will be inherent differences between one battery and the other, so, voltages will never be exactly the same, especially as time passes.
Another consequence of this phenomenon is that batteries with the higher voltage will try to charge the batteries with the lower voltages to equalize the system, these are known as circulating currents and they are undesirable. This is why it is important to place fuses in every string to protect them from circulating currents and also using diodes is sometimes recommended as these electronic devices only allow the flow of current in one direction.
This effect becomes harmful when there are more than 4 strings, but as long as you size a system with 2-4 strings, there should not be a problem.
Also, keep in mind that when increasing the number of strings, the short-circuit current also increases, therefore, as more parallel connections are involved, protection devices for each string become more important.
Lithium batteries in parallel
If you are wondering whether or not it is possible to apply all of this knowledge not only to lead-acid batteries but also to lithium batteries, the answer is yes. No matter what battery technology you use, the concepts explained here apply as well.
What is better? Series or parallel?
There is no better or worst choice among series and parallel connections. Both of them are needed in the design of battery banks. Although parallel connections generally have more issues associated with charging/discharging processes, learning how to use the properties of each one of them is needed to ensure a good design.