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.

How to Decide if You Need Batteries in Series Vs Parallel or Any Combination?

Deciding between connecting batteries in series, parallel, or a combination requires understanding the voltage and current requirements of the device that you are powering and the batteries you plan on powering it with.

If you connect batteries in series, it will increase the voltage. If you increase the voltage, you can get more power out of the battery without increasing the amps. In contrast, connecting them in parallel will divide the load current between batteries in the parallel array, putting each battery under far less stress than it otherwise would have been if it were powering the load on its own. 

Scenario 1: Extend The Overall Life of The System

If you have a large enough battery and that battery has the proper discharge voltage range for your application, then you technically don't need to worry about putting batteries in series or parallel.

You could, however, still benefit from putting your batteries in parallel. This is because regardless of how well a battery-powered system is running, running the same system with 2 batteries in parallel will always increase the life of the system. 

This is because batteries will all eventually wear out. So, if you have 2 batteries running any load, then those batteries will take twice as long- and sometimes even longer- to wear out because when batteries are put in parallel, the load current is distributed among all batteries in the parallel array. 

Scenario 2: Low Range or Low Runtime

If you are not happy with the current (pun intended) range or run time of your battery-powered system, but you are totally happy with its performance, then you would greatly benefit from putting your batteries in parallel. 

Whatever machine or equipment you are running will last twice or more as long as it did before. You may be asking, how could it run for more than twice as long if I am only doubling the battery capacity? Well, that’s a great question, and here is the answer: 

Let's use an Ebike for this example. When an Ebike is not able to go as far on a single charge, you are using the battery toward the end of its discharge curve to get you the rest of the way.

The more and more a battery's voltage drops below nominal, the less efficient it is. This is because, towards the end of the discharge curve, a battery’s internal resistance exponentially increases. This is why, after all, batteries die in the first place. The chemical reaction inside is no longer able to provide the voltage it used to because its internal resistance has become too high. The only way to lower it is to charge the battery or add dual batteries in parallel to your ebike!

So, if you double the amount of capacity you have, that means that your batteries will spend far more time at optimal, much more efficient voltage ranges. This is why you can sometimes get 2.5x more range or run time by simply doubling the size of the battery. 

Scenario 3: Low Performance or Speed

So, you've got a motor-based application, and it's not performing as well as you'd like? One thing you can do is increase the voltage. By putting batteries in series, you can boost the voltage, like connecting two 12-volt batteries to get 24 volts or connecting two 24-volt batteries to get 48 volts.

Voltage, resistance, and current are related to each other. The power delivered to a load depends on its resistance and voltage. For example, if you have a 2-ohm motor connected to a 24-volt power supply, it'll draw 288 watts of power, with 12 amps of current. 

If you need your motor to spin faster, you can increase the voltage. If you give the same motor 48 volts, it will draw 24 amps of current which comes out to 1152 watts of power. So, as you would expect, the motor would be a LOT faster. 

But keep in mind, most systems use speed controllers to regulate motor speed. Also, in the case of a brushless motor, a speed controller is required. This means that when you increase the voltage in a motor-based system, you get access to more power without having to use it all. So, if you're almost happy with your motor's performance at 24 volts and need just a bit more speed, you can double the voltage and adjust the speed controller way down to find that sweet spot.

The best part is that higher voltage lets you run the motor, wires, speed controller, and battery- really every part of the system- more efficiently and at lower currents. And, contrary to popular belief, putting your batteries in series does give you more capacity in terms of watt-hours, which can be really useful in practical applications.

So, putting batteries in series to increase the voltage in a motor-based system can lead to better performance, improved efficiency, and even greater capacity. Just make sure that your controller can handle the increased voltage.

Scenario 3: Battery Voltage Too Low For Application

Let's say you have an inverter that requires a 12-volt input but all you have are a bunch of 6V batteries. In this example, let's assume we have eight 6-volt 10Ah batteries. If you put all of those batteries in parallel, you would end up with a 6-volt 80Ah battery. That is 480-watt hours, which is quite a bit of power, but it’s still not a high enough voltage to run the inverter. 

On the other hand, if you put all of those batteries in series, you would have a 48-volt 10Ah battery, which is still 480-watt hours but now the voltage is way, way too high to run the 12-volt inverter. So, what can you do? 

You can put the batteries in a series-parallel configuration to get a compatible voltage while using the capacity in all of the batteries.

Take two of the batteries and put them in series. This will make a 12-volt 10Ah battery. Now you have 6 more batteries left, so do the same thing with those batteries and form 3 more series battery pairs. Now that you have four 12-volt 10 Ah battery pairs, I’m sure by now you know what to do with them. Put those four 12-volt batteries in parallel!

This will form a 12-volt 40Ah battery that is, you guessed it, also 480-watt hours. Unlike the other 480-watt-hour configurations, however, this one can run a 12-volt inverter. Make sure your inverter is properly sized for whatever voltage you intend to put through it.

Wiring Series VS Parallel Battery Connections

How To Wire Batteries In Series

To wire batteries in series, simply connect the positive terminal of one battery to the negative terminal of the next. Keep connecting them in this manner until you reach your desired voltage. It’s really that simple. Regardless of how many batteries you connect this way, you will be left with one free positive end and one free negative end. So, if you have two 12-volt batteries, connecting them in series will give you a total of 24 volts. 

The positive connection on one battery that is not connected to anything will be the main positive connection for the series group. Then, as you would expect, the negative connection on the other battery that isn't connected to anything else is the main negative connection for the series group. 

How To Wire Batteries In Parallel

To wire batteries in parallel, it’s super simple. All you have to do is connect all the positive terminals together and all the negative terminals together. So, if you have two 12-volt batteries with 50 amp-hours each, connecting them in parallel will still give you 12 volts, but the capacity will double to 100 amp-hours.

As far as where to make the main positive and negative connections, you can tap into the battery at any positive and negative point, but it's best to choose points that are equidistant from each terminal. So basically, you want to make your main positive and negative connections as close to the middle between your two batteries as possible. Or, at the very least, make sure the distance from your connection point is the same to each battery.  

Remember, before you start wiring your batteries, make sure they're the same voltage, capacity, and type (e.g., lead-acid, lithium-ion, etc.). Mismatched batteries can cause problems and even be dangerous.

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.

This is one of the more less-obviously-damaging but more-commonly-made battery errors. It is very common, in fact, for people to add mismatched, smaller batteries in parallel to larger e-bike battery packs. On the surface, it seems fine. It seems like as long as the batteries are in the same voltage range and are at the same exact voltage when they are connected that it works just fine. 

While it's true that doing this probably won’t cause any fires and will indeed provide more range overall, if you place a current meter between the batteries you would see that there is a lot of totally unregulated charging and discharging going on between the packs. 

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 more quickly 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. 

And really, if you are running very high currents and/or operating the batteries at or near their absolute maximum limits, then everything becomes important to make them even, neat, and symmetrical. Remember, energy will always take the path of least resistance. Even if your connections are the same quality and your batteries are 100% balanced, if the length of your power cables is substantially different, it will cause one side to take on more than its fair share of the load. 

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 balance. 

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 batteries. This is why it's important to ensure that both batteries are at the same voltage before connecting them in parallel. 

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.

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!!