Aren’t All Lithium-Ion Battery Management Systems (BMS) the Same?

This is the fourth in a six-part series on lithium batteries:

  1. What’s Inside a Lithium-Ion Battery?
  2. How is a Lithium-Ion Battery Different than a Lead-Acid Battery?
  3. Which is a Better Lithium-Ion Battery, NMC or LFP?
  4. Aren’t All Battery Management Systems (BMS) the Same?
  5. What is the Future of Lithium Batteries?
  6. Beyond Lithium Ion – What’s the Future of Energy Storage and Renewable Energy Generation?

No battery manufacturer will tell you their batteries are unsafe. However, we at Green Yachts believe less than 1% of lithium batteries are safe enough to put on a marine vessel. The difference between safe and unsafe batteries is not the chemistry as our last blog discussed (read more), but due to the wide differences in how the BMS (Battery Management System) functions.

We experienced a thermal runaway event on a sailboat with a LiPO4 lithium battery that provided power to a bowthruster because the BMS did not properly regulate battery charging (fortunately we were at the dock). And yet that battery complies with all these safety standards:

  • 2006/66/EC on environmental EU compliance
  • 2004/108/EC on electromagnetic compatibility
  • IEC 62133, safety requirements for portable sealed secondary cells
  • IEC 62619, safety requirements for secondary lithium cells and batteries
  • 3, procedures, test methods and criteria relating to class 9 lithium-ion batteries
  • IEC 62281, safety of primary and secondary lithium cells and batteries during transport

Most battery safety standards such as ASTM F3353-19 or the American Bureau of Shipping Guide for Use of Lithium Batteries in the Marine and Offshore Industries were influenced by Corvus ( , the battery manufacturer whose batteries have been at the center of every major thermal runaway event on electric commercial vessels worldwide, such as this one in which they promised updates, which never happened (

If you think you’re getting a deal on lithium-ion batteries, be careful. Many cheaper lithium-ion batteries, such as what one can buy on Alibaba, don’t even have a BMS. They employ battery balancers that only optimize cell voltage and protect from over and under current while charging.

Needless to say, all of the above alarms us at Green Yachts. We actively tell commercial operators and recreational boaters alike that if they don’t use a battery that Green Yachts deems safe, they are putting themselves and their crew at risk.

Given the importance of the BMS for lithium-ion battery safety and the limited information that battery manufacturers provide about their BMS, how do boaters know which batteries are safe?

The answer is that most of the time, it is not possible. On occasion, one can see public information that gives a clue. Take the battery that had the thermal runaway event while we were on board. The manufacturer’s manual says that the BMS for their lithium batteries are a “drop-in replacement for lead acid batteries.” If you read our blog post about lead-acid vs lithium-ion batteries (link), you know that this should be a red-flag indicator. However, it is not reasonable for boaters to be able to read through a manual to know if a battery is safe.

At the end of this blog, we’ll list the batteries that Green Yachts believes has a robust enough BMS to provide the best available safety. First, we’ll go through the function of a BMS and what differentiates a good BMS from one that is not safe and/or causes unnecessarily rapid battery degradation.

The first task of a BMS is to balance the SOC among all the cells in battery string (a string being any group of batteries connected in series).

Imbalance happens when one cell’s SOC become higher or lower other cells. Causes of this can be cells with different Coulombic efficiency, which can happen if there is low quality control in cell production. Other causes of imbalance are excessively long periods of time without shore charging, cells having different net current, different discharge rates or different current leakage.

In other words, imbalance occurs and because of this, a BMS is programmed to balance the SOC between cells. There are two ways that a BMS can do this, passive and active balancing.

According to Dr. Gregory Plett (, passive balancing drains charge from cells with too much charge through discharge and/or energy dissipation in the form of heat. Active balancing moves charge from “high charge cells” to “low charge cells” in order to balance the state of charge between cells in a battery bank.



Passive balancing is less costly. The simplest passive balancing method is a fixed shunt resistor is placed in parallel with each cell and drains a cell with a high SOC. A slightly better version of this method includes a BMS-controlled switch that is closed when a cell has too much charge allowing them to drain. Passive balancing does nothing to increase the SOC in weak cells and since battery life is affected by the charge of the weakest cell, batteries with passive balancing do not provide as much energy since some is drained and they don’t last as long before degrading.

Active balancing causes cells with a high SOC to send charge to cells with lower SOC bringing the cells into uniform balance without energy drainage. It also extends battery life by preventing a weak cell from remaining weak. Often it is the same cell in a module that becomes weak rather than a random rotation and if that one weak cell allowed to consistently have a low SOC as a passive balancing system allows, that cell can degrade and force early battery retirement. Active balancing is achieved by using switched capacitors, transformer/inductor designs and active employment of DC/DC converter techniques. A BMS can also use multiple strategies in concert if necessary, but most of the time, a BMS will make lots of small adjustments constantly in balance rather than need to make large energy transfers between cells.

This brings us to the question of how a BMS knows what the energy characteristics of the cells are and how a BMS knows cell information is the second big differentiator between a high and low quality BMS.  The BMS monitors the following variables:

  • Total voltage OR voltage of each individual cell
  • Average temperature OR temperature of individual cells
  • Charge (Ah) being discharged or charged

All BMSs calculate the SOC and adjust the SOC based on the amperage discharged or discharged. There are many ways this is done, but most BMSs use Coulomb counting to track SOC and changes to SOC. SOC is important to know one’s range, but it is not critical to battery safety and thus using an algorithm to calculate SOC is perfectly fine and safe.

However, voltage and temperature are critical to battery safety. If you open up a good BMS, you will see that there are individual wires going to each of the cells. These wires directly monitor temperature and voltage of individual cells. A less safe strategy employed by most battery manufacturers is to use an algorithm to calculate individual cell voltage because they don’t run a wire to each cell and can’t know the voltage of each individual cell.

The difference between monitoring and calculating is important. If one calculates, the accuracy is lower. In order to not have too many false positive readings outside voltage operating parameters, the parameters need to be relaxed. Therefore, when calculating voltage, one has a less accurate methodology and lower tolerances of variation before a BMS responds. In contrast, when there is a wire running to each individual cell, a BMS has direct and accurate information for each cell. A BMS can have tight tolerances for voltage imbalances. For example, Valence batteries depower output from a battery when there is more than a .3V imbalance between cells in a battery bank. An EPTechnologies BMS shuts down a string at .3V and also shuts down a battery string if the BMS does not receive individual cell voltage information for a few seconds. It is hard to imagine a thermal runaway event occurring when such direct monitoring and aggressive safety protocols are in place and this is exactly why monitoring individual cell data rather than calculating it off of data from a module bus bar is supremely important for lithium battery safety.

One of the questions we often get at Green Yachts is whether one can stick a Tesla battery in a boat and our answer is no. Tesla batteries, both the 18650 and the newer 4680 cells are small and cylindrical. A Tesla battery pack has thousands of these cylindrical cells. It would be impossible to run an individual wire to thousands of cylindrical cells. All battery manufacturers who make truly safe batteries use prismatic cells, which are bigger and in the shape of a rectangle. A battery module might have 2 to 48 cells depending on the size of the battery and it is much easier to run a few dozen wires than a few thousand. Thus, one way boaters can tell a battery is not safe is if it has cylindrical cells and this is why we at Green Yachts say most Tesla batteries (some LFP Tesla batteries are made with prismatic cells) are not safe.

If you have made it through this blog, pat yourself on the back. You now are equipped with an understanding of what makes a lithium battery safe that exceeds 99% of boaters and unfortunately, many battery manufacturing companies. Please share this post with other boaters. Please use this knowledge to make smart choices about batteries.

And speaking of smart choices, here are the battery companies that Green Yachts believes makes a safe battery with active balancing and direct individual cell monitoring:

Cleantron – recreational 48V NMC batteries

EPTechnologies – commercial and recreational NMC batteries (and their batteries have an extra safety feature of a temperature activated fire retardant system within each battery module)

MG Batteries

Spear Power Systems


Of note, the batteries from these five companies are not as inexpensive as other offerings as one can find on the market. It’s not that these companies are simply asking a higher price. These manufacturers spend more to build a safer battery and this costs more. But, these batteries often last longer due to active balancing, which saves money and of course, one cannot put a price on safety.

Privacy Settings
We use cookies to enhance your experience while using our website. If you are using our Services via a browser you can restrict, block or remove cookies through your web browser settings. We also use content and scripts from third parties that may use tracking technologies. You can selectively provide your consent below to allow such third party embeds. For complete information about the cookies we use, data we collect and how we process them, please check our Privacy Policy
Consent to display content from - Youtube
Consent to display content from - Vimeo
Google Maps
Consent to display content from - Google