Difference Between Active And Passive Battery Balancers
Balancing lithium-ion batteries is crucial for ensuring the safe, efficient, and long-lasting operation of the battery pack. In a lithium-ion battery pack, individual cells are connected in series to increase the voltage and overall energy storage capacity. However, due to manufacturing variations and the inherent characteristics of lithium-ion cells, the cells in a pack can become imbalanced in terms of their state of charge.
This can lead to a number of problems, such as reduced overall performance and capacity, increased risk of thermal runaway, and reduced overall life of the battery pack. Balancing the battery pack ensures that each cell in a battery pack is at the same state of charge, which helps to mitigate these issues and ensure safe and efficient operation.
Active balancing redistributes charge among the cells in a battery pack to ensure that they all have the same state of charge with a dedicated circuit, which monitors the voltage of each cell and adjusts the charging and discharging current accordingly. Active balancing is more accurate and faster than passive balancing. On the other hand, passive balancing relies on Ohm’s Law and the natural cell and balance resistor characteristics to bring cells to the same state of charge. Passive balancing is generally less accurate and slower than active balancing and may take longer to achieve the desired result.
In this article, we will explain the difference between active and passive balancers, and we will elaborate on how lithium-ion batteries work, why lithium batteries need to be balanced, and the pros and cons of active or passive balancing.
What’s The Difference Between Active And Passive Balancers?
Active balancing refers to the process of actively redistributing charge among the cells in a lithium-ion battery pack to ensure that they all have the same state of charge. This is typically done using a dedicated circuit in addition to BMS (battery management system), which has the capability of transferring energy from one cell group to the other. Active balancing can be performed in real-time and is typically more accurate and faster than passive balancing.
Passive balancing, on the other hand, relies on the natural discharge rate of the cells to bring them to the same state of charge. This is achieved by simply placing a small load on the cell or cell group. Passive balancing is generally less accurate and slower than active balancing and may take longer to achieve the desired result.
Both methods have the advantage of maintaining the cells in a battery pack at an equal state of charge, but active balancing does that much more effectively. This extends the overall life of the battery pack. It also allows for a higher state of charge and discharge rate, which can be beneficial in applications where high power is required.
Passive balancing, on the other hand, is simpler and less expensive to implement, as it does not require any additional circuitry other than the always-required BMS.
The most important thing to remember is that active balancing is more accurate and efficient but requires a dedicated BMS, while passive balancing is simpler and less expensive but less accurate. Both methods have their pros and cons and the choice between them will depend on the specific application and requirements.
What Is Balance Current?
Balance current is the measure of how fast an active or passive balancer can balance. It is the current that is used by a battery management system (BMS) to redistribute charge among the cells in a battery pack, as part of the active balancing process. The balance current is typically a small fraction of the overall charging or discharging current of the battery pack, and is used to adjust the state of charge of cells that are out of balance with the other cells in the pack.
The balance current is determined by the BMS or external balancer based on the voltage and state of charge of each individual cell. The current is used to adjust the cell voltages in real-time to ensure that all the cells in the pack are at the same state of charge. Balance current can be positive or negative depending on whether the current is flowing into or out of the cell in question.
In summary, balance current is the current used to ensure that all the cells in a lithium-ion battery pack have the same state of charge.
Cell Balancing DIY Lithium Battery
We are so excited to be building our DIY lithium batteries for our truck camper. So excited that it is easy to overlook some of the critical and slightly tedious first steps. Balancing a battery hadn’t even occurred to me when we set out on this project. But balanced batteries are an essential first step in maximizing the capacity and extending the life of our LiFePO4 batteries.
What Does it Mean To “Balance a Battery Cell”?
Battery cell balancing is the process of equalizing the voltage and charge of cells across a battery. When constructing a battery, we are assembling a collection of cells to work together as a single power source. Ideally, these cells are brand new with the same potential and wear. Realistically, even new cells will have some degree of variability that will become more apparent over their lifespan. To have these cells work well together as a single battery and wear evenly they have to be balanced. A cell with a limited capacity will fill up faster when charged and limit further charging of the other, healthier batteries. Alternatively, that same cell with a limited capacity will also discharge more quickly than the other cells, limiting any further discharge. Overcharging or over-discharging can result in permanent damage to all of the cells. Therefore, the weakest cell will determine the capacity of the entire stack. To get the most out of our cells, we balance them to serve our purposes best.
Here’s a neat thing: battery cells will naturally balance themselves if they have the opportunity. I say “neat,” but this can be very dangerous. If two cells are at different voltage states, the cell with more voltage will discharge into the cell with less voltage. Connecting two lithium cells with a significant difference in charge could quickly send a large amount of current into the cell with the lowest voltage cell and cause damage to both. Our battery cells have been sitting in a corner for a while, allowing a lot of time to discharge. So, when we first hook up these cells, we need to be careful.
Our first foray into battery balancing begins with a simple prototype of our battery. Before diving into the official build, we want to make sure the entire system works and get a sense of how much space we need. Each of our batteries will be composed of 8 cells connected in series. It turns out, however, that series connections are far less tolerant of unbalanced cells. So we need to balance the cells in parallel first.
Four battery cells arranged in series.
Four battery cells arranged in parallel.
We use a multimeter to test, label, and sort all the battery cells in order of the voltage. Our batteries span 2.632 to 2.949-volts. That’s quite a variation. So, before we arrange them in series, we will balance them to bring the cells within a lower voltage variation.
Checking the voltage of a battery cell.
Marking voltage on battery cell.
Sorting battery cells by voltage.
Balancing the Cells
There are many ways we can bring them to closer charge states. Honestly, it’s a fascinating rabbit hole to dive down. But we want to keep this reasonably simple. The most intuitive solution is to charge each cell to the same voltage and then hook them up. But when our batteries first arrive, we don’t have a charger that supports an individual 3.2V battery. Instead, we use a slower, low tech solution. We connect sets of all the battery cells in parallel to get them all closer to the same charge. Over time, the cells will naturally balance as higher voltage battery cells slowly discharge into lower-voltage cells. To avoid any potential damage to the cells, we split the stack into two sets of eight to balance the extreme outliers before moving on to the full set of sixteen. This method of balancing takes a long time, so we set the cells aside and check the voltage of each battery throughout the week.
16p battery cell arrangement Connecting to the BMS with an iPhone App
Cell Balancing DIY Lithium Battery
We are so excited to be building our DIY lithium batteries for our truck camper. So excited that it is easy to overlook some of the critical and slightly tedious first steps. Balancing a battery hadn’t even occurred to me when we set out on this project. But balanced batteries are an essential first step in maximizing the capacity and extending the life of our LiFePO4 batteries.
What Does it Mean To “Balance a Battery Cell”?
Battery cell balancing is the process of equalizing the voltage and charge of cells across a battery. When constructing a battery, we are assembling a collection of cells to work together as a single power source. Ideally, these cells are brand new with the same potential and wear. Realistically, even new cells will have some degree of variability that will become more apparent over their lifespan. To have these cells work well together as a single battery and wear evenly they have to be balanced. A cell with a limited capacity will fill up faster when charged and limit further charging of the other, healthier batteries. Alternatively, that same cell with a limited capacity will also discharge more quickly than the other cells, limiting any further discharge. Overcharging or over-discharging can result in permanent damage to all of the cells. Therefore, the weakest cell will determine the capacity of the entire stack. To get the most out of our cells, we balance them to serve our purposes best.
Here’s a neat thing: battery cells will naturally balance themselves if they have the opportunity. I say “neat,” but this can be very dangerous. If two cells are at different voltage states, the cell with more voltage will discharge into the cell with less voltage. Connecting two lithium cells with a significant difference in charge could quickly send a large amount of current into the cell with the lowest voltage cell and cause damage to both. Our battery cells have been sitting in a corner for a while, allowing a lot of time to discharge. So, when we first hook up these cells, we need to be careful.
Our first foray into battery balancing begins with a simple prototype of our battery. Before diving into the official build, we want to make sure the entire system works and get a sense of how much space we need. Each of our batteries will be composed of 8 cells connected in series. It turns out, however, that series connections are far less tolerant of unbalanced cells. So we need to balance the cells in parallel first.
Four battery cells arranged in series.
Four battery cells arranged in parallel.
We use a multimeter to test, label, and sort all the battery cells in order of the voltage. Our batteries span 2.632 to 2.949-volts. That’s quite a variation. So, before we arrange them in series, we will balance them to bring the cells within a lower voltage variation.
Checking the voltage of a battery cell.
Marking voltage on battery cell.
Sorting battery cells by voltage.
Balancing the Cells
There are many ways we can bring them to closer charge states. Honestly, it’s a fascinating rabbit hole to dive down. But we want to keep this reasonably simple. The most intuitive solution is to charge each cell to the same voltage and then hook them up. But when our batteries first arrive, we don’t have a charger that supports an individual 3.2V battery. Instead, we use a slower, low tech solution. We connect sets of all the battery cells in parallel to get them all closer to the same charge. Over time, the cells will naturally balance as higher voltage battery cells slowly discharge into lower-voltage cells. To avoid any potential damage to the cells, we split the stack into two sets of eight to balance the extreme outliers before moving on to the full set of sixteen. This method of balancing takes a long time, so we set the cells aside and check the voltage of each battery throughout the week.
16p battery cell arrangement Connecting to the BMS with an iPhone App
Intro to Cell Balancing
Note: this transcript was done by an AI with minimal human oversight in the interest of time. Please let us know if anything is unclear and we’ll work on fixing it.
Would you help describe what cell balancing is?
Luke: Each cell has a capacity and a state of charge and, in a balanced situation, you would hope that they all are sharing a similar state of charge with each other.
Bryan: And maybe even to start farther back. This is assuming that you have a battery pack where all the cells are identical. If you made a battery pack where cells were twice the capacity of their neighbors, it would look like it’s balanced but then they are imbalanced later on. So from step one, you have to actually make sure that your cells are matched in balance, or state of charge, from the factory. If one has 1 Amp-hour, it’s paired with others that have 1 Amp-hour so that your active balancing system has a chance of leveling out the pack.
A note on Active vs. Passive Balancing Systems
Active Balancing
The active cell balancing technique uses inductive charge shuttling or capacitive charge shuttling to transfer the charge between the cells. This technique is proven to be an efficient approach as it transfers energy to where the energy is needed instead of wasting it. However, this demands additional components to be added to the system, which in turn translates to increased cost.
Passive Cell Balancing
The passive cell balancing technique uses the idea of discharging the cells through a bypass route that is mostly dissipative in nature. It is simple and easier to implement than active balancing techniques as the bypass can either be external or be integrated — keeping the system more cost-effective either way. However, since all the excess energy is dissipated as heat, battery run time is adversely impacted and is less likely to be used during discharge.
Luke: If each parallel cell group has identical capacity, or as close to identical capacity as you can get them, each state of charge will represent an equivalent unit of capacity stored between each of the series groups.
Bryan: And to make the most effective use of the battery pack safely, at some point all of the cells need to stay matched together [at the same state of charge]. And so this is where cell balancing comes into play. Because if the battery pack gets too high and low, high voltage or low voltage on a single cell, that will destroy the pack. In the end, your pack is only as strong as your weakest cell. And in this case, balancing tries to make the other cells or the weak cell keep up with them.
Luke: Or drain the over performers to match the underperformer…
Bryan: Which, you bring up a point, that there are two different ways that you could balance this out, either by charging or discharging; the simple way is discharging. It’s just as simple as tying a resistor on and bleeding [or letting the battery discharge slowly through a small current]. Like popping up in a hole and a tank that’s overfilled and draining some of the excess out and then stopping it. And then there are more advanced ways to charge single cells and strings to balance them.
Luke: And whether or not that more advanced way has benefit for your needs is based on the amount of imbalance that that you may be expecting and in healthy cells. The rate of imbalance you should see approaches the difference in self-discharge rate that you’re seeing between subgroups [of cells]. Packs with an active [balance] system may not be able to do this well; [driving a current to balance defective cells can damage the pack and lead to pack failure]. The ability to charge the low cell may not be as valuable as it seems, in light of what’s causing the low cell to be…
Bryan: Right. To expand upon that, if you have a cell that’s self-discharged due to a defect, no [active] balancing system is really going to help you [maintain that cell’s operation in a functional battery pack] at that point.
Luke: [The active balancing system] doesn’t correct your defect or repair any damage done to your cell causing it to be defective. It does mask the effect of being able to have clear visibility that you had a defect though. [It can also funnel so much current into the defective cell that it makes the battery inoperable or fail at worse.]
The Importance of Cell Balancing and Pack Safety
There are many different concepts in cell balancing you mentioned: having a pack with all the same cells, having a pack with a bunch of recycled cells, or trying to repurpose cells that are out there. And so, even now that we understand what cell balancing is, why is it important or needed? Are there use cases where you definitely should not do it?And so, even now that we understand what cell balancing is, why is it important or needed? Are there use cases where you definitely should not do it?