Thermal Runaway in AGM Batteries
In Defense of Lithium Ion Batteries
Yes, it's true; thermal runaway can and does happen in Valve Regulated Lead Acid (VLRA) Absorbed Glass Mat (AGM) batteries, and is not a new phenomenon afflicting Lithium Ion batteries alone. A Google search of "Thermal Runaway in AGM Batteries" will reveal enough technical papers on the subject to keep even the most techno-savvy occupied for a long while, and will save me trying to explain the why's and wherefore's here. The simple fact is that, given the right conditions, an AGM battery can, and will, start a self-perpetuating internal heating process (thermal runaway) that can end in catastrophic failure with the possibility of fire and explosion. So why is there so much negative publicity and bad karma surrounding Lithium Ion batteries, when AGM's are prone to much the same maladies?
All Lithium Ion batteries are not created equal
The term "Lithium Ion" encompasses a wide range of very different chemistries that all fall under the Lithium Ion family banner. Some make-ups have a very high energy density, that is to say that they pack one heck of a punch, but with the downside that under certain conditions they are very susceptible to a thermal runaway situation that will usually have catastrophic consequences. One such common high energy-density Lithium Ion chemistry is lithium cobalt and its derivatives, and many of the well publicized failures, including the Boeing Dreamliner fires, have involved lithium cobalt batteries or a similar chemistry. Tesla cars, like the one involved in an accident that produced "burning debris exploding like fireworks" use nickel cobalt aluminum batteries. But there are other versions of Lithium Ion that have less energy density but are inherently very safe, safer in fact than lead-acid batteries. The most common Lithium Ion offering in the marine market these days is Lithium Iron Phosphate, whose chemical designation is LiFePo4. Unlike some other Lithium Ion chemistries, in particular lithium cobalt in its various forms, thermal runaway is not initiated in a LiFePo4 battery by physical damage like a puncture or crushing. The same cannot be said for AGM batteries when damaged physically or in a thermal runaway condition, where acid spill and the release of an explosive mix of hydrogen and oxygen is potentially extremely dangerous. In fact, in the highly unlikely event that thermal runaway is initiated somehow in a LiFePo4 cell for any reason, the emitting of vapors is the most noticeable result.
Heat is the enemy of all batteries
Apart from greatly reducing their life-span, storing and/or cycling batteries in elevated temperatures can greatly enhance the possibility of a thermal runaway situation in any type of battery: lead-acid or lithium ion. But for thermal runaway to begin, there must initially be a "thermal event" to take the battery up to the point at which the internal temperature takes off on its own and is unstoppable. If this elevated temperature were solely the result of a hot environment, then the temperatures required for thermal runaway to be initiated, in both LiFePo4 and AGM batteries, would be so high that the vessel would almost certainly have to be on fire. But then what if the cells buried within the battery were being heated internally by an over-charge or over-discharge condition?
The dangers of over-charging and/or over-discharging
This is where there is a very real possibility of inducing enough heat into an AGM or LiFePo4 battery to initiate thermal runaway. If a mains powered charger, or engine-driven alternator, or even a wind or solar generator develops a fault and charges a battery at too high a voltage, it could possibly push the internal temperature up to the point where the self-heating process takes over and thermal runaway begins. Charging at a rate of charge (amps) in excess of the manufacturers recommended limit will also raise the internal temperature significantly. Similarly, an excessive discharge will also cause the cells within a battery to heat up, and if the ambient temperature is high, then the critical temperature will be arrived at faster than in a cooler environment. To prevent this situation from arising, a properly engineered and designed Lithium Ion battery system will have many built-in safeguards that make thermal runaway almost impossible. AGM batteries have no such safety mechanisms.
Built-in safety mechanisms in Lithium Ion battery systems
One huge advantage of Lithium Ion battery systems is that they are constantly monitored and controlled right down to cell level. An electronic Battery Management System (BMS) is employed to keep a look-out for signs of overheating, over-voltage and under-voltage, excessive discharge, unbalanced cells, etc., and will disconnect loads and charge sources long before there is any real danger. In a top-of-the-line system like the ones that Coastal Climate Control offers, there are even warnings if a sensor should go faulty! This monitoring and safety system is one significant benefit that is not generally available in AGM battery systems, and as a bonus there is a whole host of logged data available from the BMS for analyzing battery performance and fine tuning the system.
Lithium Ion batteries are safer than Lead Acid
That indeed is the bottom line. First choose the right Lithium Ion chemistry, ensure that the control systems are designed specifically for the intended application (marine propulsion, marine house systems, etc.), and choose a respected supplier with in-depth knowledge of the product. Then enjoy all the benefits that a Lithium Ion battery system provides, while knowing that the system is safer than the lead-acid equivalent.