I need some help in understanding basic battery charging. I read about the three common stages: Bulk Charging, Absorption Stage and Float Stage, but I can't understand exactly what the controller does in the first two.

Let me explain my problem:

Bulk Charging:

During the Bulk phase of the charge cycle, the voltage gradually rises to the Bulk level (usually 14.4v volts) while the batteries draw maximum current. When Bulk level voltage is reached the absorption stage begins"

  1. Does it mean that the controller voltage is gradually rising so it always will be above battery voltage, while the battery voltage is rising due to the current from difference on voltage (controller-battery)?

  2. Can the current to the battery can be calculated by Ohm's law: (Vcontroller-Vbattery)/Battery Resistance?

Absorption :

"During this phase the voltage is maintained at Bulk voltage level for a specified time (usually an hour) while the current gradually tapers off as the batteries charge up"

  1. The controller voltage is constant, or the battery voltage is kept constant? If it is the battery voltage, how can you keep constant voltage of a battery, if flow of current is into the battery, that means increasing voltage...

  2. Why does the current decrease with time if the Battery Voltage is being set constant? Does the resistance of the battery increase when it's more charged?

Sorry for the damn questions, but I have some holes in my education... Thanks in advance.

  • \$\begingroup\$ It's important to point out that these rules seem to be for lead/acid chemistries. If you try to apply those rules to Lithium-based batteries, you will likely burn your house down, EVEN IF you use approximately "correct voltage" batteries. \$\endgroup\$
    – Jon Watte
    May 14, 2014 at 4:04

1 Answer 1


During the bulk charging stage the charger works in a constant current mode. That is, the charger supplies a constant current to the battery while monitoring the battery voltage. As the battery charges, the voltage increases even though the current stays constant.

No, you can't use Ohm's Law to calculate the current into the battery. It's important that you understand that in the bulk charging stage the charger does not try to set a certain voltage...the charger supplies a fixed current instead. It's incorrect to think of the battery as having some kind of resistance value.

Once the battery voltage rises to a pre-determined voltage (14.4 V is common for a 12 V lead-acid battery) then the charger changes its mode of operation from constant current to constant voltage. The charger starts providing a constant 14.4 V output while monitoring the current. As the battery charges it will draw less and less current. Once the current falls below some pre-determined level the battery is fully charged.

If you supply a constant current to a battery then the voltage will rise as the battery charges. If you supply a constant current and allow the voltage to rise to high then the battery will be overcharged and possibly damaged. If you supply a constant voltage to a battery then the current will gradually fall as the battery becomes fully charged. It's important to pick the appropriate voltage for this phase of charging so that the battery becomes fully charged but is not overcharged. Ideally, the voltage used in the constant-voltage phase will also be adjusted for temperature.

You can think of the battery like a water-bearing container with a small leak. If you have a constant current ("bulk") of water flowing into it at a greater rate than the leak, then the level of the water in the container will rise. Think of the 14.4 V set-point as a level marked on the inside of the container. Once the water reaches that level, you have to taper off the flow of water coming in (down to about the leak rate) if you are to maintain at approximately that level.

  • \$\begingroup\$ you said: "It's incorrect to think of the battery as having some kind of resistance value" ,and that the charger suplying constant current in bulk mode, how the controller know how to change it's inner voltage , to get constant current without using Ohms law? you need some kind of model of the battery to do it(for the control algorithm),if its not Ohms law, what is it? you also mentioned that in constant voltage mode , the battery and controller voltage are the same,while current flows from the controlller to the battery, how it happens without diffrense in voltage between the2? \$\endgroup\$
    – Roman
    May 14, 2014 at 3:48
  • \$\begingroup\$ You simply use a current sense circuit of some sort. This may be a small high-power resistor with an opamp, or a fancier sensor like a Hall effect sensor, or even something that measures the voltage drop across the synchronous rectification MOSFETs in a DC DC converter. You don't need a detailed model of the battery itself, other than not setting the allowed charge current too high. \$\endgroup\$
    – Jon Watte
    May 14, 2014 at 4:06
  • \$\begingroup\$ As @JonWatte said, the charger measures the current directly and does not use Ohm's Law to calculate it. \$\endgroup\$
    – Joe Hass
    May 14, 2014 at 11:58

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