What is “four state” battery charging?

[Joel Gambino]

My solar charge controller is a “four state” charger. Lately I’ve been trying to understand what happens during the various states of charging. At this point I think I have a pretty good handle on what is going on and figured I share what I think I know and open it up for discussion and clarification. I know a lot of the forum readers use solar power and may be interested.

The information below applies to wet lead acid batteries since that’s what I use. It may or may not apply to other battery chemistries.

The four stages of battery charging are Bulk, Absorb, Float and Equalize. Equalize is really a battery maintenance operation and not really a charging mode. Below is a top level description of each mode. Most of the information below comes from the Bogart Engineering Technical Manual for their SC-2030 Solar Charge Controller, which is available online. Recommended reading if you want more detail.
Bulk:
When batteries are in a low state of charge (below 85%) they accept current (Amps) at a relatively high rate. In bulk mode, the voltage is typically below 14 volts and the current flow is high. Basically pump all the current that is available to the batteries.
Absorb:
The Absorb phase occurs when the battery reaches approximately 85% of full charge. At this point, the battery has become more resistant to accepting charge so the charge controller must increase the voltage to push in the amps required to reach full charge. The voltage during this phase is around 14.4 volts. (Cold batteries need higher voltage for optimum charging, so if temperature compensation is available, the charge controller will adjust the absorb voltage with temperature). Optimizing the absorb cycle is important because there is limited solar illumination during the day and we are trying to achieve a full charge as often as possible. In addition to temperature compensation, some of the higher end charge controllers will also allow you to configure the nominal absorb voltage based on data from the battery manufacturer. The trick here is to charge the batteries as fast as safely possible. You could charge at a lower voltage, but that would take longer. You could charge at a higher voltage but you risk boiling off electrolyte and damaging the battery. Finding the proper voltage is the magic here. Consult your battery manufacturer’s documentation to get these parameters for your batteries.
Note that the definition of full charge can vary depending on the type of charge controller. Some battery manufacturers recommend replacing the amp-hours removed plus some fixed percentage. This requires a way to measure the current flowing in/out of the battery. For example the manufacturer of my Crown CR 235 batteries recommends replacing 115%-118% of the amp hours consumed. Charging this way requires a system that is capable of tracking the amp-hours in and out. Otherwise your charge controller may time the absorb cycle or use other means (I’m not clear on what the other controllers do in absorb). Consult your charge controller manual to determine its capabilities.
Float:
Once the battery reaches 100% charge, the charge controller goes into Float mode. The float mode maintains the battery at full charge. Float voltage is about 13.2 volts. Too high a float voltage will cause the batteries to “boil off” electrolyte.
Equalize:
Equalization is a process that overcharges the battery to remove sulfation and equalize the state of charge across the cells. This helps improve the longevity of the battery. Overcharging can cause the battery to loose electrolyte, so whenever you equalize, you should check the electrolyte level before and after the equalization.
General thoughts:
Read the manual for your charge controller and understand how it actually charges the batteries. Then look online for the specification sheet for your batteries and see how well the charge parameters of you charge controller match up with the battery manufacturer’s recommendations.

If you supplement solar with a generator, use the generator early in the day. As described above, batteries charge faster when they are in a low state of charge (Bulk charge state). Run the generator early in the day to supply lots of amps when the batteries readily accept charge, then use the solar to deliver the last 15-20% over the rest of the day. This will minimize the generator run time and allow the solar to bring the batteries to full charge over time.

Make sure ALL of your loads pass through the shunt. You should have one cable on the negative terminal of the battery that cable goes to the shunt. All of the loads are attached to the other side of the shunt. That way all current in and out of the battery is accounted for.

Please share your thoughts. I’m still learning, so I’m open to feedback/clarifications.

Joel

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