# Batteries in Series vs Parallel

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.

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.

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.

## Battery FAQ

I have made a few articles describing various battery systems with diagrams. View the diagrams here:

### 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.

#### References

1. Connecting batteries in parallel. Battery guy.
2. Connecting batteries in series. Battery guy.
3. Batteries series-parallel vs series for longevity. Solar-electric Forum
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## 17 thoughts on “Batteries in Series vs Parallel”

1. Great information

2. I ordered 4 deep cycle gel cell batteries, but not all in the same order and when I got them, I found that one of the batteries was slightly different. The batteries were physically the same size and all 4 were 12V 100 AH batteries. I was planning to make a 24V storage pack by putting them is series and parallel to make a 200 AH, 24V pack, but I fear that the string with the odd battery will charge differently. Should I be worried?

Long answer: can you recondition the battery? Do you know what happened to the faulty one? Is there a visible difference between them? I think it’s best to try to recondition the battery so that the voltages are matching. Otherwise, the faulty one will ‘drag down’ the other 3 batteries. Can you ask for a replacement where you got them?

3. What are your thought on this idea. I was thinking of wiring 3 12 volt lead acid batteries in series to be charged by some 36 volt solar panels wired in parrallel. I was going to use a direct connection to the batteries with no controller other than a possible solenoid to completely disconnect them when the reading on one battery reaches 12.4 volts or so.

Then I would draw power from one battery at 12 volts leaving the other batterries and the solar panels to always keep the one Iw as drawing from at 12 volts.

basically using the 3 batteries themsleves as a step down resistor series to get 12 volts.

This is because I have lots of 37 volt solar panels.

Thoughts?

• Hello George, I answered your email, but since you are posting here I think you didn’t receive it.

What you are going to do is dangerous. I would never recommend wiring the solar panels directly to the batteries because the voltage is not a good indication of the state of charge of a battery. When you are charging a battery, it will have a higher voltage, which gives you a false reading. Also, it will take the voltage down (voltage drop on the terminals) and not give you an accurate reading if you attach loads.

I recommend getting an MPPT charge controller at 100VDC input voltage and then wiring them to your batteries. Most likely, 2 in series and then parallel these.
Never draw power from one battery. Always use a DC to DC converter across the whole battery bank. They are cheap. Ensure the charge controller can charge 36 volts because these are quite rare unless you put in a custom charge profile. Most of them are 12, 24, or 48 volts.

4. I had two 110mAh 12v batteries in my 24v system, and now I added another two 110mAh 12v. The problem is the real capacity of the older ones are lower than the new ones. So I put the older ones in series between them (since that was the way they were conected alone before), and add the two new ones in series between them and in paralel with the older ones. Is that conection OK or I should switch them? The thing is getting more complicated as I want to add a third pair latter.

• Connect the two old batteries in series and connect the two new batteries in series. Then connect those two 24V batteries in parallel to the charge controller. You will now have a 2S2P battery pack. I’m pretty sure you mean Ah instead of mAh.

5. Hi Nick,
I see many single LifePo4 battery packs coming out on the market that are 24v, 36v, 48v and even 72v. Is it better to use these or to wire up say 3 or more 12v LifePo4 batteries in series that would achieve the same voltage and ah capacity as one battery pack?

Thank you,

• Hello Dean, those higher voltage battery packs are perfectly fine. A 48V battery is just 4 12 batteries placed in series. If it’s a 100Ah 48V battery, then they placed 16 3.2V 100Ah cells in series. They will have their own BMS. I recommend the server rack batteries. Make sure you can carry them because they can be heavy.

• IF I HAVE 16 3.2V 280AH BATTERIES IN SERIES TO MAKE THE 48V SYSTEM BUT NEED MORE WH CAN I GET ADDITIONAL BATTERIES OF THE SAME CHEMISTRY AND PUT THOSE IN PARALLEL, I WAS THINKING OF GETTING 4 MORE 3.2V BATTERIES 280AH ( BECAUSE I HAVE 16 OF THOSE ALREADY AND RUN THE ADDITIONAL 4 IN PARALLEL ON THOSE TO GET MORE POWER IS THAT OK OR SAFE??

• 4pcs 3.2V will give you a 12V battery, so connecting it is unsafe because it’s a different voltage.

6. Just a few comments on Nicks comments. As an electrician dealing with solar installations, I think my experience should count here.
Re George, July 2021: What he plans is not really dangerous, but definately unwise, and the inclusion of a quality MPPT charge controller would be the best move.

Re Dean May 2022: Be very careful linking Lithium batteries in series.
*Read the manufacturers spec sheets very carefully*, and only buy quality name brands, not those from, lets say, “less reliable sources”. This is because not all battery monitoring systems are made equally, and some, cannot handle more that the battery itself due to limitations of the electronic components inside them. Also, Lithium batteries are not actually that heavy. After all Lithium is the third element on the periodic table below Hydrogen and Helium. I’m not a young man any more, and I can still easily carry 2 12V, 100Ah batteries, one in each hand. Lithium batteries are, however a little more challenging to maintain, as it is seldom known to many that the inbuilt battery monitoring system ( BMS) will stop the batteries from charging should they reach zero degrees celsius, and will not allow then to start charging again until the *internal* battery temperature reaches 5 degrees celsius. My suggestion here, and I’m sure Nick will agree, is buy a charger suited to Lithium batteries, or a battery to battery charger which is the method I have opted to use.
Nick, hope you don’t mind my comments here. Like you, I’m always happy to help others and provide my experience to everyone.

• Awesome, thanks for your contribution Steve!

7. Hello
I have two 100 watt panels charging two 12v, 101 amp hr batteries
I using a 6k watt inverter And a charge controller

Batteries are connected in parallel. I have two 80watt trough heaters running and nothing else

Problem is, they only run for approximately 5-6 hours and the batteries are dead

Any thoughts?
Thank you
John

• 12V*202Ah=2424Wh
160W*6h=960Wh
60W*24h=1140Wh
960Wh+1140Wh=2400Wh
Conclusion: Swap your power-hungry inverter for a smaller, more efficient one. Use a maximum of 1000W inverter for a 12V system. The one who sold you a 6kw inverter for a 12V system should go to solar jail 🙂