Preventing Thermal Runaway

Keep your boat's electrical system in check by paying attention to the details.

For the boater, the old adage “You reap what you sow” usually rings true. Having peace of mind while out on the water is the ideal experience, and something all boaters desire. But it is important to pay close attention to detail, especially when it comes to the boat’s electrical systems. Some issues can be easily prevented by a thorough inspection of your electrical systems, and one of the most important of these issues is thermal runaway.


What is Thermal Runaway?

Thermal runaway occurs when chemical reactions generate heat, and this increase in heat causes the original reaction to accelerate, thereby generating more heat, and so on. When a boat’s batteries are charging, they heat up—this is completely normal. However, as the charge rate increases, so does the amount of resistance to the charging process. At a certain point, if the heat induced with charging is not dissipated enough, the battery begins a process called thermal runaway.

When a battery reaches this level of heat, even the previously used charge rate causes the battery to overheat. As this cycle of overheating continues, the process escalates. If the charging device does not sense that the battery or batteries are overheating, they could catch ­ fire or even explode.

Thermal runaway is common not just on boats, but also in other applications. A few years ago, laptop manufacturers had to recall thousands of laptops because of this issue.

What Causes Thermal Runaway?

While performing electrical audits, we often see instances of preventable thermal runaway. Therefore, we strongly recommend constantly checking and questioning the integrity of a boat’s electrical systems. In many cases, by looking at the causes of thermal runaway, it can be prevented before the process even starts. Here are two common causes:


  1. A faster battery charging time makes boating a lot more convenient. One way to achieve this is by increasing the size of the alternator and/or charger. This increases the size ratio of the alternator/charger to the batteries, which is acceptable within a certain range. However, if this balance is thrown off too much, the alternator/charger will charge the battery more quickly than it can handle, thus causing thermal runaway.
  2. Utilizing a temperature sensor is crucial to preventing thermal runaway, but the wrong setup can cause problems. The temperature sensor hooks into the charger and/or smart regulator and scales its charging amperage based on the battery temperature. But having the wrong temperature sensor or placing it incorrectly on a battery bank will give an inaccurate temperature reading. This in turn will prevent the charging device from detecting thermal runaway.

What Can I Do To Prevent It?

Taking care of your boat’s batteries is usually simple, but some issues are hard to detect. The following tips are easy to implement for most boaters. However, the safety and reliability of your electrical systems should not be taken for granted, so if you are unsure or need guidance, contact a professional for assistance.

  1. When looking at your remote panel, make sure the reading on the temperature sensor is at the ambient temperature of the batteries’ location. Look at this reading only when the batteries have not been charged for 24 hours.
  2. The placing of the temperature sensor is critical. It must be placed on the battery and not on the charger. Speci­fically, it must be placed on the warmest battery in a group of batteries, typically the one in the middle. You can use a digital thermometre or your hand to feel the temperature. Wrong placement can lead to a situation similar to Scenario #1.
  3. The charging current should be limited to appropriate battery capacity. For instance, a flooded battery should never have a charge rate of more than 25 percent of the battery’s capacity. You can also consider getting a smart charger or regulator with a temperature sensor, which will recognize the signs of thermal runaway and shut down the charger automatically.
  4. Since the temperature sensor is unfused, it should be located on the negative terminal of the battery. If a short occurs, there will be no consequences apart from the sensor ceasing to work. However, if placed on a positive terminal, an electrical short could cause a fire.
  5. Each temperature sensor is intended to be paired with a specfic charger, even though they all use phone jacks as the physical connection to the inverter or charger. Therefore, temperature sensors should not be interchanged between brands or models. Replacing the temperature sensor and pairing it with the old charger could lead to a situation similar to that in Scenario #2.

What Can Happen If Thermal Runaway Is Not Prevented?

To better demonstrate the causes and effects of thermal runaway, here are some scenarios we have seen over the years:

Scenario #1: Approximate Cost - $5,500

A boat owner had a three-by-three battery setup (nine batteries in one battery bank) on his boat. To measure the temperature of a group of batteries accurately, the temperature sensor must be mounted on the warmest battery, in this case the one in the middle. Unfortunately, the temperature sensor was mounted incorrectly on one of the four corner batteries. Under high charging and thermal runaway, the middle battery swelled so much that it practically warped into a balloon shape. Five of the nine batteries were completely ruined, and the four remaining corner batteries were left in mediocre condition. It cost the owner $5,500 just to replace the batteries, teaching him a curt lesson in the importance of positioning the temperature sensor in the correct place.

Scenario #2: Approximate Cost - $2,400

Having just installed a new battery charger, the old temperature sensor was refitted with the new charger. This setup seemed to work fine. However, the temperature reading showed approximately 303 degrees F when it should have read 72 degrees F. This resulted in the battery being constantly undercharged. Eventually the battery bank was damaged and had to be replaced, costing approximately $2,400.

In Closing

Thermal runaway can be easily prevented if the necessary steps are taken early enough. It’s easy to relax and enjoy the sights and sounds of the open waters worry-free, after having paid attention to the small details—especially when you know your battery system is healthy.

About the author: Jeff Cote is a systems design engineer and owner of Pacific Yacht Systems, a full service shop delivering marine electrical and navigation solutions for recreational boats. Visit their website and blog for info and articles on marine electrical systems, projects and more:

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