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Understanding Batteries and Charging Systems

To first understand battery charging a few definitions are in order to explain the charging phases that are used



My interest in this subject came from the desire to build my own emergency power back up system for use with my amateur radio setup. This article is a summary of the information learned from the reference sources below and was written to share with other interested folks in my local amateur radio club. This information has also helped me better understand the performance and limitations of the battery and charging system in my vehicle.


To understand battery charging systems, a few definitions are in order to explain the charging phases that are used. Not all battery charging systems use all the modes explained below.


Modern lead acid batteries will last longer and charge faster if they are charged in a particular sequence known as a three step charge. A fourth sequence known as equalization charge which can triggered by the user exists only in more sophisticated charging systems.


Bulk Charge

The first step, known as bulk charge, delivers a constant current rate (at or near the rated capacity of the charger) until the battery voltage approaches it gassing voltage typically around 14.4 volts. Depending on the ratio of the charging current to the ampere hour capacity of the battery, the battery will be charged anywhere from 75 - 95% of the battery's capacity. (See paragraph below on charging rate for more detail.)


Absorption Charge

The second step, known as the absorption charge, holds the voltage constant, typically at the above mentioned 14.4V, with the current dropping on a slowly decaying exponential curve.  Most of the relatively inexpensive chargers that have the absorption charge mode hold the voltage at this state until the charging current has dropped to approximately 10% of the rated capacity of the charger. This charge mode is needed to assure the last 15 - 20% of the battery capacity is fully charged.


More sophisticated charging systems use a microprocessor to monitor the history of the rate of current drop to determine when to exit this phase. In addition they will have temperature compensating circuit that alters the charging voltage during this phase.  High performance charging systems will also contain remote sensing of the battery temperatures. These features are necessary to avoid a thermal runaway condition, which under the mildest of failure modes cause loss of electrolyte out of the battery and generally shorting the life of the battery due to internal heating.  Rare but worse case failure modes include battery fires.  More discussion of this topic follows later.


Float Charge

The third step, known as float charge, holds the voltage approximately at 13.2 - 13.5 volts, enough to maintain the batteries charge without losing electrolyte through gassing. At this voltage the charge can be left connected to the battery indefinitely without risk of overcharging.  It is interesting to note that most automotive charging systems are set to have a maximum charging voltage of about 14.2 volts tapering down to about 13.5 volts. These systems also need to have temperature compensation; otherwise, either under charging or overcharging will occur depending upon the temperature of the battery. 


Equalization Charge

The fourth step, found only on the most sophisticated systems is known as the equalization charge. An equalization charge is needed generally only on flooded or wet cell batteries (not gel cells) as often as recommended by the battery manufacturer.  The purpose of this charge is to equalize the capacity of the individual cells of the battery, reverse electrolyte stratification (the separation of the liquid electrolyte into layers of different acid concentrations) and break up any residual sulfate which may remain after normal charging.  The charge cycle is in essence a controlled overcharge that tends to restore the battery back to its original charge capacity. Typically the charge mode involves taking the charge voltage to at minimum about 15.3V and in some cases as high as 16.2V. Needless to say this is not something to be done with out close monitoring or a charging system designed for this purpose, given the thermal runaway condition that could develop. Also this is not something to be done with sensitive electronics attached, whether it be your amateur radio equipment or with the battery installed in a vehicle, given the numerous computer chips found in today's cars.


Commercially available, large chargers usually found at full service stations can apply this high a voltage, not something to be done with in the battery in the car for two reasons; the risk of damage to the a fore mentioned computers and electronic devices and, battery "slobbering" (gassing at a high rate that deposits acid droplets) on top of the battery and adjacent metal, paint and other components near your battery.

What kind of charger do you have?

There are 1-Mode, 2-Mode and 3-Mode chargers.



This charger operates only in float mode. This charger can charge a battery to the 100% level, but it can take days due the lower voltage, and subsequent low charge current used).



A 2-mode charges makes uses of the previously discussed bulk mode and float mode.



This type of charger is designed to recharge a battery to 100% more quickly than a 2-mode type. It uses bulk, absorption and float modes.


Charging Rates, Drawing power from the battery while charging

What if you want to draw power from the battery while it is being charged?


It can be done, but you need to know the modes used by your charger and some problems to be avoided.


If the charger is a 2-mode, the charger should be large enough to supply the leakage current of the battery plus any draw from the external circuit (assuming it is a long term sustained loaded as opposed to an intermittent load. This is to avoid having the external load from over discharging the battery.


If the charger is a 3-mode, you need to read the reference material below to avoid getting into a situation where the charger locks up in absorption mode and either overcharges the battery or initiates a thermal run away condition previously discussed.


Charging Rate


The fundamental question here is how big a charger do you need for your particular size (ampere rating) of your battery.


The maximum battery charging rate is often specified by manufactures for example as C/5, where C refers to the ampere hour rating of the battery. C/5 for a 100ah battery would be and optimum charge current (in bulk mode) of 20 amps.  The size of an off the shelf charger should be generally chosen so that the C/5 rate is not exceeded.



This summary was compiled from a number of sources on the world wide WEB. Thanks and techncial credits go to these sources for enlightening us all.  Further reading is recommended to understand in greater detail the topics covered.

The http://www.ibexbatterysystems.com/ web site was one of the most helpful in understanding battery charging systems. Be sure to check out the IBEX Application Notes.

The http://www.amplepower.com web site have some very good reading also.

The Power Equation - A short primer on Battery Systems

Testing Batteries

Battery Equalization

PS Reports - Volume 1 - Issue 2 August 1997 - a detailed analysis of charging modes including current / voltage plots

Parallel Batteries - A discussion of why, safety questions, etc

Battery Temperature Compensation - more details about why thermal runaway can occur

Exploding Batteries and Boats - more discussion about failures you want to avoid

Excellent Article about Inverters, means of regulation, modified sinewaves and their effects on the efficiency of the devices they power

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