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Bulk Absorb Float Equalize: What Does It Mean

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  • Bulk Absorb Float Equalize: What Does It Mean

    Time for another Sticky Thread about battery charging to discuss what Bulk, Absorb. Float, and Equalize mean and how It works. There will be a test at the end so pay attention. First we need to get some math out of the way. Don’t worry, it is 5th grade math, and I wil be doing the math for you, but you need to understand the mechanics of it. Just a few simple formulas.

    Power (watts) = Voltage x Current. Example 10 volts x 10 amps = 100 watts. Your take away here is if either voltage, current, or both go up, Power has to go up. Vice Versa if either voltage or current go down, Power must go down. See simple.

    Voltage = Current x Resistance. Just like Power above if the variables of current or resistance go up or down, so does voltage. In this discussion Resistance will be a fixed value of .010 Ohm’s. That makes it real easy to understand because Voltage will follow whatever the Current does as as it varies. Simple again huh?

    Amp Hours. Takes 16 years of higher education to learn and figure out and is an extremely complicated formula that will make your head spin Amp Hours = Amps x Hours. Example if you draw 10 amps from a battery for 10 hours, you used 10 Amps x 10 Hours = 100 Amp Hours. Told you it was hard.
    Test Question. What is Watt Hours.

    C-Rate is also really simple and is Expressed as C/x. Where C is the value of the battery in AMP Hour Capacity, and x is a number specified in hours. Anytime you see a battery capacity it has to be qualified at some Hour charge/discharge rate called C-Rate. Most consumer batteries are rated at the 20 hour C-Rate. So for example if you have a 100 AH battery means if you either charged or discharged the battery at 20 hours is 5 amps, or C/20. Use the 100 AH example. 100 AH / 20 H = 5 Amps. So if I tell you to charge a 100 AH battery at the C/10 rate all I am telling you is to charge it at 100 AH / 10 Hours = 10 Amps. Ok that was the hard one but is 5th grade Algebra and just an extraction of the Amp Hour formula. All I am doing is factoring out either Amps or Hours from Amp Hours. I can say Amps = AH/H is the same thing as C-Rate. I can also say Hours = Amp Hours / Amps.

    OK enough math. If you understand that then you can understand the rest. I am going to discuss how a AC powered battery charger works first. Modern Smart Charges use a 3 or 4-stage charging algorithm called Bulk, Absorb, Float, and the 4th is Equalize. It boils down to just two algorithms. 1 is Constant Current (CC), and 2 is Constant Voltage (CV) Bulk is CC, and Absorb, Float, and EQ is CV. Nothing more, nothing less, it is that simple. Just fancy names to confuse Americans. Russians and Chi-Coms already know this stuff and learned it in grade school. In the USA it takes 16 years of school.

    So we go to CHARGERS/BATTERIES-R-US store and we buy a 10 amp charger to charge our 12 volt 100 AH battery. The perfect C-Rate to charge a battery is C/10, and we have a 10 amp charger and 100 amp hour battery. Perfect match. Our charger is a Fancy 3+1 Stage Charger which means it has Bulk, Absorb, Float, and EQ we get to play with. Our new shiny battery has an Internal Resistance (Ri) of .01 Ohm’s we will call Ri. Our new battery has sat on he shelf for a year and has discharged to 12 volts Open Circuit Voltage (OCV) All OCV means is the battery is disconnected and is Open Circuit and the resting voltage is some value. Ours right now before we charge it is 12 volts. We want 12.6 volts on a rested battery with a OCV of 12.6 volts.

    OK our charger can only charge at a maximum rate of 10 amps. It cannot supply any more than 10 amps. We look in our battery manual at it says to set Bulk/Absorb to 14.4 volts. We connect our 10 amp charge and see it is taking 10 amps of charge current which means we are in Bulk charge mode of 10 amps Constant Current. But we look at the battery voltage and it only reads 12.1 volts, but we have the voltage set to 14.4 volts. WTF is going on? Answer is simple we only have a maximum of 10 amps of charge current going into a battery with an OCV of 12 volts and .01 Ohms of resistance. Have you figured it out yet? I will give you a minute.Stop and think, and then see if you got it right.

    Wait for it. Figure it out yet?



    Go back to the math of Voltage = Current x Resistance. We have 10 amps charging into .01 Ohms. What is the voltage? 10 amps x .01 Ohms = .1 volts. Our battery OCV is 12 volts. Is the light turning on yet?

    Wait for it.

    12 + 0.1 = what?

    More math using what you already know. A charging Battery Voltage = Battery OCV + (Charge Current x Ri).

    Put it all together and we have 12 volts + (10 Amps x .010 Ohms) = 12.1 volts you see on the battery terminal. The output voltage of the charger has rolled back to 12.1 volts because it is current limited to 10 amps. It cannot do any more than 10 amps. To take the battery from 12 to the 14.4 volts instantly would require the charge current to be 240 amps. If you hit a 100 AH battery with 240 amps it goes BOOM and you have a really bad hair day full of battery acid and fire.

    So we go happily along at 10 amps CC. As the battery charges the OCV begins to rise to equal the charger voltage of the 14.4 volt set point we set the charger too. Once the battery OCV reaches 14.3 volt magic happens. We automatically go into Absorb mode. Nothing really happens, nothing switches or does anything. Simple Ohms Law is still at play. As soon as the battery OCV reaches 14.31 volts charge current starts to taper off. At 14.31 volts Charge Current = Charger Voltage – Battery OCV) / Ri. So 14.4 – 14.31) .01 Ohms = 9 amps. As the battery saturates and reaches 14.4 volts all current stops because the Charge Voltage and Battery OCV are EQUAL.

    Remember Absorb is a CV mode of a fixed voltage, not a current. Contrary to what your solar charge controller says Absorb is not a Timed Event. It is a C-Rate Event. When Charge Current tapers to C/33 ends the Absorb phase or in our example 3 amps, the battery is fully charged, and now the charger actually does something. It switches to Float Mode and we sat that voltage to 13.6 volts. The charger lowers the voltage to 13.6 volts and no current flows. After an hour or two, the surface charge on the plates self-discharge, and then the Charger will have a little current flowing to overcome the battery Self Discharge rate. It will be very small current. Float is to keep the battery 100% charged and can be left on for eternity. Now if you turn something on of 10 amps or less, the charge will supply the power, not the battery. If you turn the charger off, turn on a load, then the battery supplies the current and you are discharging.

    OK EQ or equalize. Is nothing more than a CV mode. Its voltage is just higher than all the other three modes. Say 16 volts. Take the same battery at 12 volts on the same charger, and it will charge CC at 10 amp limit of the charger until the battery voltage reaches 16 volts. But do you let it do that? NO. You will cook your battery. You manually stop it when your Hydrometer says to stop it when all the cells are equal Specific Gravity. It is not a TIMED EVENT although all chargers have a time limit in case you fall asleep with your hydrometer. It can take up to 24 hours.

    OK now you know how chargers work. Let’s talk about Solar a minute. A commercial AC charger is a stiff source with unlimited energy and time. Your solar is a very soft source of unknown power and only a few hours to get the job done. It does not have time to go through all stages properly in most case. This is especially true in Winter months during short days. Bulk mode in a solar charge is not CC. It is Constant Power. What Power?. Who knows, depends on how strong the sun is. In our AC charger example it was up to 160 watts to supply 10 amps @ 16 volts. So when we set it to 14.4 volts and in Bulk it was a constant 144 watts @ 10 amps. So say you have a 160 watt panel and a 10 amp controller. In Bulk at a few brief seconds around noon it was 10 amps for a second. What is it 30 minute after noon?. Something less than 10 amps. If it is hazy maybe 5 amps. All your controller can do is transfer the maximum available power at the time. It cannot do any more than that. So that makes it Constant Power, not CC. This is what makes a 3 Stage Solar Charger useless in the practical sense. You have to change strategy to Maximum Smoke 1 stage charger. You set the voltage to Bulk = Absorb = Float. Throw away your battery Owners Manual and it voltage set points. They are useless. Instead you use your Battery Hydrometer to set the voltage. To learn how read this thread.
    Last edited by Sunking; 01-21-2016, 09:01 PM.
    MSEE, PE