Determining the Right ESS: AGM Batteries vs. Ultracapacitors for Micro Hybrid Start/Stop

We’re already familiar with the basics for how ultracapacitors function, but what about AGM (absorbed glass mat) batteries? What are the differences, and why are they so frequently confused?

AGM batteries differ from flooded lead acid batteries in that the electrolyte is suspended in the glass mats, as opposed to freely flooding the plates. In theory, this enhances both the discharge and recharge efficiency. When left unattended or not used daily, an AGM battery will hold its charge better than other types and are often found in deep-cycle applications, solar energy storage and vehicle starting.

There are many factors to consider when determining which energy storage system (ESS) to implement for start/stop. An ESS uses either one or two energy storage devices as a means of integration with the application it serves, so the choice rests on a couple of factors. As with most things, cost is important. AGM systems cost 4-6 times that of traditional starting batteries and do not achieve the required life cycle needed for automotive requirements. Ultracapacitors are better at meeting the life cycle necessary for this high performance application and their costs have steadily decreased – primarily because of decreased material costs and increased levels of automation in both the cell and module manufacturing processes. The predicted cost of an ultracapacitor-based system is similar to the cost of the current best solution, double AGM batteries.

AGM systems don’t respond well to large spikes in current draw and are not reliable at colder temperatures. The biggest problem with AGM batteries (or all other battery options) is the poor DC charge acceptance. This means that a battery has to be charged slowly, over a longer period of time (up to 10 minutes) vs. an ultracapacitor (15 seconds). Ultracapacitors can complete hundreds of thousands more charge and discharge cycles than batteries.

Micro and mild-hybrid vehicles are a great example to show the difference. The current ESS of choice for these vehicles employs AGM batteries or a pairing of ultracapacitors and an AGM. Because the hybrid system requires multifaceted optimization, there may be several considerations in the vehicle design that limit the ESS used for a particular size or capacity. As a general rule, the electrical loads in vehicles are ever-increasing and much of that increase is intended to reduce fuel consumption, done through moving mechanical or hydraulic systems onto the electrical bus. This typically lessens the average load on the engine because this design enables systems to only run when they are required to, rather than constantly. Many OEMs (original equipment manufacturers) have already moved several features standard to cars including cooling fans, power steering, brakes and fuel pumps, to the electrical bus. OEMs must take into account the level of power required by these systems, as well as the alternator’s ability to support them, when determining which ESS to use.

Specific to start-stop for micro-hybrid applications, low ESR ultracapacitors can go in parallel with existing batteries to provide robust support. Although the ultracapacitor unit would increase the cost of the system by roughly $100, there are other savings inherent in not having to run an AGM over a larger voltage cycle. Ultracapacitors can also provide acceleration for a mild-hybrid power application, with more efficiency and a smaller volume of required space than other ESS. To top it off, the ultracapacitor is approximately 40 pounds lighter than the AGM battery, thus saving weight and increasing fuel efficiency.

Also, due to poor DC charge acceptance, AGM batteries limit the number of stop events in the daily average commute to just three (average 30 minute commute), because they cannot charge fast enough. If the temperature drops between +3 and +7 degrees Celsius, the AGM based system turns off the start/stop function, because batteries do not charge well at cold temperatures. With an ultracapacitor based system, you could have 60 stop events in your 30 minute commute, even if the temperature is -40 degrees Celsius!

Many lone AGM systems currently in use are not long-term solutions for such applications, as they either do not last the length of the warranty or do not perform well enough to achieve a long life cycle. A hybrid solution – a combination of an ultracapacitor with an AGM or lithium-ion battery – is the right answer for the needs of both OEMs and consumers.

- Any questions or comments? Feel free to send me an email ( or reach me on Twitter at the@Ioxus handle.

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