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Crank Amp Ratings for Lithium Batteries

Why are XS Power Lithium Powerpacks for automotive use not rated in cranking amps? The simple answer is because you cannot safely extract energy that fast from any lithium-ion battery on the market today (even if some competitors claim a crank amp rating.)  Sustained high rate discharges from lithium-ion batteries will cause dangerous electrolyte venting and explosive fires. After such an uncontrolled high rate discharge permanent damage is done to the individual cells. This is a characteristic of the Lithium-ion cell’s chemistry and has been confirmed with our in-house Firing Circuits battery tester.

Well, will it start my car? Yes! XS Power Lithium Powerpacks can discharge high currents for short bursts and can easily handle a modern cranking sequence. For instance, XS Power Lr48 batteries have started large V8 engines with high compression rates and sizes up to 932 cu. in. In fact each XS Power Lithium battery goes through a complete test regimen that includes high rate discharging, charging, and sustained loads for several cycles before it is shipped.

If you really want to know… Cranking amp ratings were established by the BCI for lead acid batteries and are defined in the SAE J537 specification. The procedure was established in 1914 and has been periodically revised since as needed. A BCI cranking characterization test is a 30 second test that discharges the battery with a fixed amp load until the battery falls to a cutoff voltage, usually 7.2V.

Each cranking amp number specifies a temperature and the temperature is important.  The temperature is not usually listed but can be determined from the suffix. Typically “CCA” is @ 0°F (although it can be at -20°F) and “CA” (or sometimes “MCA”) is @ 32°F. Additional ratings have come into play known as “HCA” (@ 80°F) and “PHCA” (@ 80°F but limited to only 5 seconds.)

The test is to apply a fixed amperage load to the battery and record the voltage drop. When the voltage declines to a predetermined cut-off point the test stops, typically 7.2V. Therefore if a battery is rated at 500 MCA it should be able to deliver 500A at 32°F for more than 30 seconds before dropping to 7.2V. If it runs more than 30 seconds, it passes the test. If it fails to go 30 seconds, it fails.

As you can imagine this test stresses everything in the battery, including the internal buss bars, the grids, the plate chemistry, and the electrolyte. Commonly the battery will be at 75% Depth-of-Discharge or more at the end of this test. Why is it not at 100% DOD?

Capacity vs. Discharge rate

To fully deplete all the energy from a battery requires a much slower rate of discharge. The energy must be pulled out over a long period to time typically hours. This rating is the amp-hours rating and is the fundamental capacity of the battery. The reserve minutes is another method of expressing the capacity although it is a shorter test and yields a lower capacity number than the amp-hour rating. Battery chemistries have unique rates of charge and discharge. Attempting to charge or discharge a battery outside of these rates has detrimental effects some being very serious.

For example, lead acid AGM batteries can stand the high rate discharge of the crank amp test but the battery’s life is reduced.  On the other hand, high rate esp. high voltage charging of lead acid AGM batteries is very harmful. Lead acid AGM batteries like most others will convert the excess amperage into heat and it is this heat that boils electrolyte, warps plates, and vents acid.

Lithium-ion batteries have different characteristics. They more forgiving on charge rate and can take a higher rate of charge to a point. The optimum rate of charge for a lead acid battery is 0.3C (or 0.3 times the amp-hour capacity.) In contrast lithium-ion batteries can be charged at 1C or even higher. XS Power Lithium Powerpacks are commonly charged at 1.5C.

To put this in perspective, a 45Ah XS Power AGM battery should be charged at 15A (0.3 x 45Ah = 15A.) If the same size battery were made from lithium-ion cells you could safely charge it at 45A or even 67.5A (1.5 x 45Ah = 67.5A!) But what about discharge?

Lithium-ion batteries are less forgiving on high rate discharges than lead-acid. Maximum discharge rates vary from manufacturer to manufacturer but all are currently under 60C. For instance given a 60C rate, an 8Ah Lithium-ion pack should be discharged at less than 480A for ten seconds at room temperature. Thirty second rates are commonly half this or less. Rates above 60C for thirty seconds or more cause serious problems.

Today, only the best high rate lithium-ion cells can survive delivering all their energy in about 90 seconds. None can do the 30 seconds required for the BCI test cranking characterization test and therefore a BCI “CCA” or “CA” rating is not possible.

Attempting a BCI test designed for lead acid batteries on any lithium-ion battery will destroy it in about 15 seconds. Discharging any lithium-ion battery too aggressively will cause dangerously high temperatures. Extreme internal temperature can travel through the cell and heat the electrolyte. At some point the electrolyte will boil, build pressure, and vent. When it vents the damage is permanent and the electrolyte is flammable. It is common for the electrolyte to flame once vented especially if the buss bars connecting the cells are hot.

It is for these reasons that XS Power Lithium Powerpacks have a computer built into every pack. This computer constantly monitors voltage and temperature and can avert disaster in milliseconds. Don’t take the step to Lithium-ion in your high performance car with an inferior and dangerous battery. Choose XS Power for years of safe, trouble-free service!

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