Battery Charging Stage (Bulk, Absorbtion & float)

[Olegkuskhov]

Hi all, i found in the internet the picture that i attach below. The picture is explaining about battery voltage which proportional to the percentage of battery. I have 12 V 20Ah sealed lead acid baterry. Let's assume that after continuous usage, the battery voltage is become 11.81 V which is only 20% of it's full capacity. At this time we want to employ bulk charging which is maintaining a constant current charge (which the voltage is varying) until the battery reach 12.78 or 90 % of it's full capacity.

Then for absorbtion, we set the charging voltage to be 12,89 and wait until the charging current aproaching nearly 0 A. This time we can conclude that the baterry is about 100% full.

Please correct my statement if I'm wrong, because this is what i can understand from the internet. My question right now is, how do we employ float charging?. Some article said that we can do pulse charging for keeping the battery at it's voltage level and not self discharging. Or there's another way?. I ask this because right now i make solar charge controller and i need this knowledge to implement the code for charging process? Thank you in advance

[radiolistener]

Your table shows EMF voltage. In order to measure EMF voltage properly you're needs to keep your battery disconnected from charger or load for about 1-2 days.

Just after charge or just after discharge the voltage on battery terminal will be different, because there is some chemical processes in the battery. It needs to wait for about 1-2 days to stabilize it before measurements.

Regarding to the battery charge modes. You're just needs to limit max voltage 14.4V and current to 0.1*C (where C is battery capacity). There is no needs to change voltage or current. Just wait until current drops down to about 0.01*C or current just stops to drops for at least one hour (what happens first). After that you're needs to disconnect charger from the battery. This mode is named "Cycle charge".

There is also "Standby charge mode". In this mode you're needs to use 13.8V and 0.1*C max limits. In this mode you can keep the battery connected to the charger forever. But this mode is not good for battery lifetime.

[Olegkuskhov]

Thank you so much man for the answer. I appreciate it a lot. I have seen your idea is implemented by people and i dont have any idea about this chargingw way. Your response seems strenghten my understanding.

[RoGeorge]

Beware the voltage varies with temperature.  These are some generic specs I've found while looking for thumb rules:

Code: [Select]

Lead Acid generic specs
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At a comfortable temperature of 20°C (68°F), gassing starts at charge voltage of 2.415V/cell. When going to -–20°C (0°F), the gassing threshold rises to 2.97V/cell.

Battery status -40°C (-40°F) -20°C (-4°F) 0°C (32°F) 25°C (77°F) 40°C (104°F)
Voltage limit on recharge 17.1 16.2 15.3 14.7 14.1
Float voltage at full charge 15.3 14.7 14.1 13.8 13.5
or lower or lower or lower or lower or lower

gassing at 20*C for V >= 14.4V
nominal, no load = 12.6V
floating charge = 13.5...13.8V

Note that the typical 14.4V is the stop charging voltage, not the floating voltage.  Float charging (when a voltage source is kept on the battery at all times) is to be done at lower voltage, typical is 13.8V, but read the markings/label printed on the battery's body.

Best thing to do to identify proper voltages is to read the battery label, and if possible, find the battery specs/datasheet for your particular brand and model.  Lead acid batteries are similar but they are not the same.  Follow the battery manufacturer's instructions.

[Olegkuskhov]

Thank you George for reminding me about the temperature. I actually made my own charger as this is my final year project. I made synchronous  buck converter with MOSFET driver and microncontroller to do all the things (sensor reading, pwm generator, user interface and many other things). Most of the time as well after i see the replies from you guys, i realize that i need to implement enable and disable of the MOSFET driver so that i can enable or disabe charger.

My initial design is to use MOSFET driver with enable pin which is IR 2104. I bought like 10 of those from "china" and none is working. Later by the time i just currious why none out of 10 is working. I started to look all over the initernet for answer and someone is actually telling that the ir2104 series that not working according to it's logic, might be a rebranding ir2103. I was curious to see if this was the case. turns out 9 out of 10 is actually works as ir2103 and i ended it up using them in my design which is already finish. Now, i need to implement the algorithm where i need to enable and disable the ic to turn on or turn off the charger.

Below is the pinout comparisson of those 2 ic. The mosfet driver section that i ended it up use also being shown in second picture. I propose that adding transistor in pull down network on VCC pin as shown on the third picture may working as enable pin replacement. People said that this is not a good way of implement enable and disable because there maybe some capacitor that still charge and will make bad things happen. I also attach the internal ir 2103 block diagram on the 4th picture. Thanks in advance.

[radiolistener]

I forgot to say that charge parameters depends on temperature, as RoGeorge said.

It is important to heat the battery to about 20-25 °C before charge.
Don't charge it when it cold, if you get battery from winter weather, just put it at warm room for 8-12 hours before charge...