Putting Batteries in Parallel Vs Series

Batteries can either be connected in series or in parallel, so when combining two or more batteries, the choice of which type of connection to make arises. If you have enough batteries to connect, you can even use both types of connections to meet specific requirements. When it comes to batteries in parallel vs series, choosing the right one could make or break a project. 

When batteries are connected in series, their voltages are added but their amp hours are not. When batteries are connected in parallel, their amp hours are added and their voltages are not. When connecting batteries in series, it's important for the amp hours to be the same. When connecting batteries in parallel, it's important for their voltages to be the same. All batteries connected in series experience the full load current. All batteries connected in parallel receive a portion of the current that is related to the resistance of that specific battery compared to the total resistance of all batteries in parallel. Considering the fact that batteries in parallel usually have matched electrical characteristics, this usually means that each battery in parallel experiences 1/Xth of the load current where X is the number of batteries connected in parallel.  

In this article, we will go over the electrical properties of batteries in parallel vs series. We will also explain several use cases where one or the other is better. 

Is It Better To Have Batteries In Series Or Parallel?

That depends on the application. Two equal batteries in parallel will share the load equally but don’t add their voltages. This means that if you have a battery that has a high enough voltage to run your load and you just want more capacity, then adding batteries in parallel is the best choice. If, on the other hand, a single battery does not have a high enough voltage, you will need to put the batteries in series which increases voltage and does not increase amp hours.

Is It Ok To Put Batteries Of Different Capacities In Parallel?

It’s not ideal. If a smaller capacity battery is connected to a higher capacity in parallel, all sorts of unexpected things can happen. It can seem fine and stable at first, and some setups like that can last a very long time, but there are many unknowns that are at play. 

For example, consider connecting a 36v 15ah battery and a 36V 5ah battery in parallel. If both batteries are at the same voltage when they are connected together, nothing bad will happen. There will be no spark, nothing will get hot, and everything will work just fine. In fact, whatever you add that battery setup to will indeed have a longer running time compared to a lone 36V 15ah battery. 

If the two battery packs have a similar resistance, they will even burn through their capacity evenly. The problem is that if both batteries are using the same amount of current but one battery is smaller, the smaller battery will run out of energy first. This doesn’t seem to happen because, behind the scenes, the larger battery is recharging the smaller battery. If you place a current meter between the two batteries, you would see current flowing from the larger battery to the smaller battery in order to produce the smaller battery's percentage of the given current. 

This places undue strain on both batteries, reducing efficiency. It also damages the batteries because it allows them to be charged and discharged in an unregulated manner that is inconsistent with the constant current charge method that is required for proper lithium-ion battery charging. As you would expect, this reduces the life of the batteries.

What Is The Advantage Of Batteries In Parallel?

When equal batteries are connected in parallel, their capacities are added and the load current is split equally between them. Because it does not affect their voltage, putting batteries in parallel is an easy way to upgrade a battery-powered device without having to change anything in the circuit other than the addition of batteries in parallel. 

Do Batteries In Parallel Drain Equally?

Ideally, yes. As long as the batteries have the same capacity and resistance, then they will drain at the same rates. If two batteries of unequal capacities are joined in parallel, they will seem to drain at the same rate, but in reality, the smaller one is draining quicker and the larger one is recharging at an unregulated rate to compensate. This can damage both batteries and lead to a loss of overall capacity. 

Does Putting Batteries In Series Add Capacity? 

The short answer is no because capacity is measured in amp hours and when batteries are put in series, their amp hours are not added. The long answer is yes because when batteries are put in series, their voltages are. Total power over time is measured in watt-hours which is amp hours multiplied by the voltage. So, if the voltage doubles and the amp hours are left the same, overall watt hours are doubled, doubling the capacity. 

The tricky part is that power is the fact that current multiplied by voltage is power. So this means that if the voltage is doubled, then the rate at which power can be used is also doubled. This means that if you use the extra power that you get by putting batteries in series to increase their voltage, then you will see no additional capacity in the operation of your equipment. 

If, on the other hand, you do not tap into the extra power afforded by the higher voltage, and use the same amount of power that you had before with the lower voltage, then you will experience a doubling of the capacity.

Do you need to balance batteries in parallel?

Not exactly, but there are still some things you need to know. Current flows when there is a voltage difference. Even two 52-volt batteries are unlikely to have the same voltage at any given point.

This is because the ‘52’ in a 52-volt battery is an average, or nominal, figure. In reality, a 52-volt NMC lithium-ion battery could be any voltage between around 36 and 58.8 volts. So, if two 52V batteries are connected together and they are not at the exact same state of charge, an unregulated amount of current will flow from the higher voltage battery to the lower voltage battery until they equalize. 

Depending on how much of a voltage difference there is, this could cause the connections between the batteries to get hot and it could damage both the sending and receiving battery. This is why it's important to ensure that both batteries are at the same voltage before connecting them in parallel. 

When To Put Batteries In Series

Batteries should be put in series when you want to achieve a higher power output or a single battery does not have a high enough voltage to operate in a given application. For example, if I needed to run a 24-volt inverter but all I had was 12V batteries, I would need to put 2 of those batteries in series to achieve the proper voltage to run the inverter. 

When batteries are in series, energy has no way of moving from one battery to another. Both batteries can contribute to the amount of energy put into the wires and therefore into the load, but the batteries cannot send energy back and forth between each other.

As long as those batteries have the same resistance when they are placed in series, whatever charge voltage you give them is divided equally between the batteries. This means that if you have two 3S batteries in series you can use a 6S charger to charge them while they are connected in series. 

When To Put Batteries In Parallel

Batteries should be put in parallel when you want your battery to be able to support a higher current or you want your battery-powered device to run longer. For example, if I needed to run a 12-volt inverter for 6 hours and I had three 12-volt batteries that could each run the inverter for 2 hours, I could put all three 12-volt batteries in parallel so that I would not have to keep changing the battery every 2 hours. 

When batteries are connected in parallel, the load current is shared between them equally so long as all batteries that are connected in parallel have the same resistance. This means that if you have 4 batteries connected in parallel, each one of them is only doing 25% of the work and only seeing 25% of the wear and tear that it would otherwise see if it was operating the device by itself. 

Conclusion

Whether you are building a DIY battery pack or a complex system that has sophisticated electrical requirements that can't be met with a single battery or you are just trying to double the run time of your DIY powerwall or battery-operated equipment, it's important to have an understanding of electrical differences between batteries in parallel vs series. If you know about how each configuration works and what the benefits are, then you will be able to design the perfect system for a given application. 

There are a few key aspects to remember when considering batteries in parallel vs series. When batteries are connected in parallel, their capacities get added together but their voltages do not. When batteries are connected in series, the voltages are summed but their capacities are not. If batteries are connected in a series configuration then it's ideal for each battery to have the same capacity (amp hours). In contrast, when batteries are put in a parallel configuration, it becomes essential to make sure their voltages are the same. When batteries are connected in series, each battery experiences the same full load current as all other batteries in series. In contrast, when batteries are connected in parallel, they receive only their share of the current, which is proportional to each battery’s resistance compared to the total resistance of the parallel array. 

We hope this article helped you learn everything you wanted to know about batteries in parallel vs series. Thanks for reading!!