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Author Topic: Fast charging VRLA batteries!  (Read 1815 times)

Doca

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Fast charging VRLA batteries!
« on: April 28, 2013, 09:13:00 PM »


In many places on the WEB you will see that it is strictly recommended charging the SLA (Sealed Lead Acid) battery with current NOT greater than C/3. As soon as someone comes on a forum and ask how to quickly charge SLA battery, other rush in with advice to keep strictly to the manufacturer's recommendation, and that battery must be filled slowly for several hours. Recent research, however, shows that this is not so and it is possible to quite successfully, and without damage to the battery, charge this type of battery much faster and with much larger currents.


Let us first say what it really mean charging current that is C/3.  Battery capacity is usually marked with a "C" and in this type of battery is always in ampere-hours or "Ah". So if the battery is 12V/7Ah and if you follow the manufacturer's instructions, charging current should by no means be greater than C/3, and that means 7Ah/3 = 2.3 Amp. For standard charging the manufacturers still recommend even less current like C/5 to C/10.

Recent research in the automotive and defense industries are carried out daily by a number of researchers and all those exploring the opportunities that these batteries are charging as fast as possible. David Linden in his book "Battery Handbook" states: "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. " (VRLA = Valve Regulated Lead Acid batteries are SLA batteries which have a safety valve that regulates the pressure inside it, and that is the case with most of today's SLA battery).
This actualy mean that free gaseous oxygen is NOT generated inside the battery during NORMAL charging process, but its creation begins when we start overcharging. If this overcharge occurs with currents that are less than C/3, the battery itself is (without any damage) structurally capable to absorb all generated oxygen even if the overcharge continues. The problem and the damage of the battery will happen if overcharging is performed with current greater than C/3 because then the quantity of generated oxygen will be higher than the battery's capability to absorb it.

Elsewhere in the book Linden discusses the rapid charging and says that he actualy set a fixed voltage from the charger and let the batteries "pull" as much as it needs without limiting the current. After that, he concludes: "These data show that the thin-plate VRLA battery can be fast-charged to 100% of the rated capacity in less than 1 hour."  Some other researchers have also been working on this problem so Sandeep Dhameja in the book "Electric Vehicle Battery Systems" states: "Fast charging does not exhibit detrimental effects on battery life cycle.".  He was using the charging current from 8C to 9C, meaning 24 to 27 times more than what the manufacturers recomend, but he is not charging beyond 80% battery capacity. Also, David Anthony, James Rand, P. T. Moseley, J. Garche and C. D. Park in the book "Valve-Regulated Lead-Acid Batteries," they say, "It is now abundantly clear that thin-plate VRLA batteries can be fast charged with excellent results. Contrary to previous beliefs, VRLA for a given 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 allow even greater number of charge cycles than if you always use currents below C/3

I also have a need for fast charging two 6V/4.5Ah SLA battery for my bait-boat for carp fishing and I also performed some research on my own. So far I have tested 14 different SLA batteries with different voltage and capacity of the various manufacturers. Some of them are periodically charged with currents of 3C and 4C and I was unable to notice any negative effects. The way I solved my charger is given  in the accompanying diagram but I have to highlight that it is just MY way to do that and that it could be done in several other ways.

Specificity of my case is that the charger has to work with the input voltage of 12V DC. Power source is car battery because it all has to work somewhere outdoor where there is no wall outlet and the mains voltage. My quick charger uses a factory-ready module so-called BUCK converter based on IC LM2596. BUCK converter is actually a switching voltage regulator that convert higher input voltage into a lower output voltage and with minimal losses. The one I used has the ability to control and continuously adjust output voltage and output current. Since no one want its bait-boat to be out of service for too long while waiting for the battery to be charged, the charging current is set at 2-2.5 amps. What this  factory converter doesn't have in it, which is important for quick charging, is precise control of voltage to which the battery is charged up and the ability to stop charging in a right moment , thus avoiding overcharging of the battery which is dangerous. I solved that with a small additional circuit containing a precision voltage source with TL431A, a comparator circuit with LM311 and Power MOSFET IRFZ44 with very low internal resistance when it is in a state of conduction. This whole additional circuit is powered directly from the input voltage, ie. with 12V from the car battery.

As long as the battery being charged is less than the reference voltage which sets the TL431, output from the comparator (pin 7) will be on the full potential of the positive supply voltage of +12 V. LED diode will not light and the gate MOSFETs is so driven that IRFZ44 will be completely open between its drain and source and will exibit very little resistance like several milliohms - almost like there is a short circuit between drain and source. The battery is then rapidly charged and when the voltage on it reaches 7.4V, output of the comparator will fall sharply to almost zero, which will immediately close the MOSFET and between its drain and source will not flow any more current - fast charging stops. LED diode will lite on marking the end of the process. If the battery voltage reached 7.4 V (2.46 V per cell) that is a sign that it is fully charged and that any further continuation
of charging will be overcharging, hence, the process must be stopped at that moment. There is a 22 ohm resistor connected in parallel MOSFET. I called it "Float resistor" because at the end of the process of fast charge, MOSFET will exibit very high resistance between Drain and Source thus will not pass any current and the only other path will be through that 22 ohms resistor. Its value is such that the current will be about C/100 which is 45-50mA in my case. That resistor will perform float charging of the battery.

I haven't designed PCB for this circuit because the schematic is simple enough to be easily done on the protoboard by someone with average skils.
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