Tuesday, September 30, 2014

15. WHAT'S THIS STEP CHARGING?                                 

I covered in the prior post about the ill consequences of charging a lithium-ion battery using constant current, constant voltage (or simply CCCV). The damage incurred within the battery during charging with CCCV is attributed to a series of undesirable side reactions that effectively reduce the effectiveness of the primary energy storage reaction. In other words, during charging, one desires that all the energy goes into the ideal reaction that stores energy inside the battery. In reality, CCCV charging promotes a number of bad reactions that effectively damage the internal structure of the battery, and reduces the battery's ability to store electrical energy.

I will not go into the details of these side reactions; they are fairly involved and can be quite complex for the average person. But they are reasonably understood by our scientists. For example, one of them is the formation of lithium metal deposits when lithium ions combine together. Others relate to the physical damage to the electrodes and the decomposition of the electrolyte solution.

A few of these undesirable side reactions, but certainly not all of them, have been shown to exhibit a dependence on voltage. Specifically, some of the damage accelerates when the voltage of the battery approaches 4.35 Volts....or in other words, when the battery is approaching 100% full charge. This is why a common tip is to charge the battery up to about 80% instead of the full 100%. 

So step charging, probably introduced several decades ago, was an early attempt to charge the battery very gently at the higher range of voltages, or when the battery is approaching full. It is simple: it means reducing, or stepping down the current, when the battery voltage reaches say 4.1 Volts, or say around 60% or 70% of its maximum charge. However, extensive tests and results over the past many years have shown that the damage reduction was at best minimal. There were indeed a few cases where step charging seemed to have helped, but these were few and far in between, and worse yet, there was not much consistency. In other words, step charging did not deliver a solution.

I will leave the discussion of better, more sophisticated, charging methodologies to another post, but let me address here why step charging fundamentally is flawed or at best, incomplete.

First, step charging is only attempting at alleviating the amount of charging when the battery is nearing its highest voltages. But the damage to the battery is not only due to high voltage. It is due to more complex reactions of which voltage is but only one parameter. Failing to recognize the relationships between all the damage elements makes step charging quite ineffective. This is particularly acute in more modern lithium-ion batteries with high energy densities (or higher capacities). 

Second, step charging, much like CCCV, is an open loop solution. In other words, it has no knowledge of the battery's inner reactions, inner health, inner status, and consequently  has no means to measure or assess the rate at which these undesirable damaging reactions at taking place. So let's say for the sake of example, we have two batteries from the same type and vendor, but with  manufacturing variations between them (which is very common). Let's further say that one battery is better, and that its damage seems to occur at an onset voltage of 4.1 Volts. Let's also say that the difference in manufacturing causes the damage in the second battery to occur at a lower voltage of say 4.0 Volts. So if step charging reduces the charging current at 4.1 Volts, then one battery will see an improvement but the other will not. And if one were to say let's drop the charging current at 4.0 Volts to be safe and cover both batteries, then there is a serious penalty to charging times -- charge times will balloon significantly.

So in a nutshell, if someone is promoting to you step charging as a solution, my advice is simple: RUN!

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