AN1467 datasheet(9/16 Pages) MICROCHIP

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 2012 Microchip Technology Inc.

DS01467A-page 9

AN1467

- FLOAT – works in Constant Voltage mode

(typically 2.25V/cell). It is necessary for

lead-acid batteries to maintain full charge

(counter battery self-discharge). This state

has a time out, which switches to DONE

when it expires. After a defined battery relax-

ation period, it checks the battery current

and, if it is below the battery detection thresh-

old, it switches to IDLE. The “relaxation”

period is necessary after switching from the

charging voltage to the lower, floating volt-

age. Until the battery voltage relaxes, there

will be no current flowing to the battery and

the state machine might incorrectly detect

that the battery has been removed. The big-

ger the battery (capacity), the slower the

relaxation.

- DONE – is the final state for fully charged

batteries. The converter is stopped, but, if a

special STANDBY_MODE is enabled, then this

state will monitor the OC voltage of the bat-

tery and jump back to CHARGE, when it

drops too much. This way all types of batter-

ies can be maintained near full charge for

extended periods of time. This even works for

Li-Ion, if the voltage threshold is chosen care-

fully. If STANDBY_MODE is not enabled, this

state will switch automatically to IDLE.

- IDLE – stops the converter and does not do

anything, except wait for user input. A button

press will re-initialize the state machine and

start the charging process. To automate the

process, it is simple enough to add output

voltage monitoring that will start charging

when a battery is connected.

- FAULT – stops the converter and waits for

the user input. A button press will change the

state to IDLE.

One LED is used to signal the current charger state to

the user. The LED on/off states and blinking rates are

defined in Hardware.h (blinking rate depends on the

timer tick period). Each charger state sets the LED

behavior:

- LED off – IDLE state

- LED blinking 0.5Hz – PRECHARGE and

CHARGE states

- LED on – FLOAT and DONE states

- LED blinking 2 Hz – FAULT state

Charger Variables and Functions

The charger needs a few variables to keep track of the

battery parameters. Most of the variables are internal,

but some of them are also available to the main

program loop.

- battery_state – holds the current charg-

ing state of the battery charger. Since the

charger accepts only one battery, there is no

reason to differentiate between the state of

the charger and the state of the battery. This

variable is available to the main program

loop.

enum charge_states { IDLE = 0, FAULT

= 1, DONE = 2, PRECHARGE = 3, CHARGE

= 4, FLOAT = 5 };

- state_counter – is used as a time-out

counter for certain states:

• PRECHARGE will switch to FAULT on time

out

• CHARGE will switch to FAULT on time out

• FLOAT will switch to DONE on time out

- imin – is the minimum current value

recorded during the constant voltage phase

of the CHARGE state. When the value of this

variable falls below the minimum current

threshold (calculated as a fraction of the

battery capacity), the topping charge is done.

- imin_db – is the minimum current debounc-

ing counter. When the value of ISENSE is

smaller than imin for IMIN_UPDATE times in

a row, imin is updated with the value of

ISENSE

- iflat_db – is the flat current debouncing

counter. This counter is reset every time

imin is updated. When it reaches zero, it

triggers an end of charge condition.

The charger library has two functions:

Init_State_Machine() will initialize the

state

machine

debouncing

and

time-out

counters, set the charging voltage and current

limits and start the converter. This function

should be called when starting to charge from

IDLE (basically, a new battery is inserted).

Battery_State_Machine() contains the

code for each of the charger/battery state

machine and will handle state transitions based

on the measured current and voltage values.

This function expects to be called every second,

otherwise the time-out counters will measure a

different interval.

THE BATTERY CHEMISTRY

DEFINITION FILES

Lead-acid.h, LiCo.h, LiFePO.h and NiZn.h contain

example definitions for charging these chemistries.

As mentioned before, the values in the definition files

depend on the charger hardware implementation and

the number of ADC samples taken on each measure-

ment. In this case, we have a 10-bit ADC with a 5V

reference, and 4 samples are taken for every measure-

ment. The output current shunt is 5 mOhms, amplified

101 times. The output voltage divider is ¼.

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