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December 12, 2016  

Can AGM VRLA lead acid batteries be fast charged? (YES!)

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Question: Why do sealed lead acid manufacturers specify that you can't charge at a rate higher than C/3? And why are they wrong?

First let's define the C/3 term. If the capacity of a battery is C amp-hours and you want to charge in 3 hours, you would be charging at C/3 amps (C amp-hours/3 hours = C/3 amps). If you want to charge in half an hour you would charge at C/0.5hours, or 2C. A 15AH battery would have a C/3 charge rate of 15AH/3H = 5 amps.

As we discuss below, it has been shown that the dominant sealed lead acid technology, VRLA AGM, can be charged at high rates without damage. But the battery manufacturers often say to charge below C/3.

The reason for this is found in Linden's Battery Handbook. He says "The VRLA battery is capable of recombining the oxygen produced on overcharge up to the C/3 rate of constant current charge. At higher rates the recombination reaction is exceeded by the rate of gas generation." [my italics]

To fully understand this statement you also have to realize that no oxygen is generated during the charging of the battery. Oxygen is only generated when the battery is being overcharged. So what the battery manufacturers are thinking is that if you don't charge a battery at higher than C/3, you also won't overcharge the battery at higher than C/3. So even if you use the dumbest charger on the planet it won't cause the battery to vent.

This completely discounts the fact that a quality battery charger will never overcharge the battery at all. In fact to overcharge the battery at C/3 you have to be at a much higher voltage than is normally available from a 12 volt charger. To force a current of only C/5 through a fully charged battery requires 16.2 volts. (I couldn't find the voltage necessary for C/3, but it will be much higher).

Typically a multi-stage charger will run at 14.4 volts during the fast charging phase and 13.8 volts during the float charging phase. At 13.8 volts it is impossible to generate oxygen even for a fully charged battery, and even at 14.4 volts the oxygen generation rate for overcharge is about C/100.

Linden also discusses quick-charging in which he sets a constant voltage and lets the battery accept what charge it will. His conclusion is "These data show that the thin-plate VRLA battery can be fast-charged to 100% of the rated capacity in less than 1 hour."

So the bottom line is that it is OK to charge a sealed lead acid battery fast as long as you don't overcharge it. Modern CC-CV (constant current-constant-voltage) and multi-stage chargers can perform this function very well. They ensure that when the battery is full the applied voltage is less than the gassing voltage, and no damage can occur. Fast charging is important to the electric vehicle industry, and long battery life is essential, so a lot of the quick-charging research has been done for this application. Here is a link to a fast-charge algorithm that achieves both http://www.sciencedirect.com . Using this algorithm for sealed lead acid batteries they showed that they can safely restore 50% of the charge in 20 minutes, or an average rate at 1.5C, or at a 4.5x rate over the rate that the battery companies suggest.

In the book Electric Vehicle Battery Systems By Sandeep Dhameja he states that "Fast charging does not exhibit detrimental effects on battery cycle life." He is talking about a charge rate of 8C to 9C, though he does not charge at this rate beyond 80% state of charge.

In the book Valve-regulated Lead-acid Batteries by David Anthony James Rand, P. T. Moseley, J. Garche, C. D. Park it says:

" . . . it is now abundantly clear that thin-plate VRLA batteries can be fast charged with excellent results. Contrary to previous beliefs, for a given VRLA product, the imposition of aggressive charging algorithms that minimize the effects of the oxygen cycle and finish the charge relatively quickly can result in superior cycle lives."

In other words, fast charging will give you more cycles than staying below C/3!

How does this relate to the PowerStream DC UPS products?

These are constant-voltage, limited current chargers. They take the car's charging voltage and subtract about 0.5 volts to use to charge the auxiliary battery. They also limit the current available for battery charging. Since the vehicle's alternator charging voltage is regulated between 13.5 and 14.5 volts, this has the effect of applying 13.0 to 14.0 volts to the auxiliary battery with a current limit. This is the perfect range for charging a sealed lead acid battery without fear of overcharge.

At a float voltage of 14.0 volts the overcharge rate of a fully charged battery is C/1000, and at 13.0V the overcharge rate is less than C/5000.

This type of charge does not force current through the battery. It offers the battery a voltage. If the battery can absorb current at that voltage then current flows and the battery charges. If the battery wants too much current at the available voltage then our current limit kicks in, cutting back the voltage to limit the current to the maximum allowed by the charging circuit.

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