AN1467

DS01467A-page 6

 2012 Microchip Technology Inc.

THE CC/CV CHARGER LIBRARY

Except for Ni-MH (and Ni-Cd), all the popular battery

chemistries on the market today use a form of constant

current, followed by a constant voltage charging (or

constant voltage with current limit) algorithm. Since the

hardware presented in this application note is capable

of regulating output voltage and current, using it for this

purpose comes only naturally. Also, one of the biggest

problems in synchronous chargers, the battery reversal

current, is solved by the driver diode emulation feature.

The converter’s maximum current is 8A, so it is some-

what impractical to use it for very small batteries (cur-

rent shunt amplifier output for 50 mA or less is close to

the noise floor of the ADC). Also, because this imple-

mentation has no burst mode at very low duty cycles,

the output ripple increases. It is best used with

lead-acid and lithium type batteries with capacities over

4 Ah (also probably not useful on batteries bigger than

80 Ah).

MULTI-STEP CHARGING

Properly charging the batteries requires multiple steps

and specific mechanisms for each chemistry.

Lead-acid batteries have generally a more sluggish

behavior when charging, and the process will take

longer. It is not recommended to charge using currents

over C/5, or the topping charge portion may not be

complete when the current threshold triggers the end of

the charge. Also, lead-acid batteries may be placed in

floating charge for extended periods of time.

Li-Ion chemistries have different charging require-

ments, in the sense that they will not absorb the over-

charge, so the current flow must be cut as soon as the

battery is full. The optimum charging current is C/2, for

a good balance between the charge time and the

battery life cycle. Deeply depleted cells have to be

trickle-charged until the cut-off threshold is reached.

Some Li-Ion chemistries are more resilient and

forgiving to abuse than others. LiFePO4 is one such

example.

Ni-Zn cells are similar to Li-Ion. No float charge should

be applied, and deeply depleted cells need to be

trickled back to life.

All these particularities need to be implemented in the

charger state machine and a set of charging parameters

written for each chemistry type.

TABLE 1:

BATTERY CHEMISTRIES

Chemistry

Pre-charge

Charge

Float

Lead-acid

No, if under 1.75V per cell for extended periods replace it. 2.4V/cell to C/40 current

Yes, 2.25V/cell

Li-Co

Yes. C/10 to 3.0V

4.2V/cell to C/33 (3%)

No

LiFePO4

Yes. C/10 to 2.7V (2.5V for some variants)

3.65V/cell to C/33 (3%)

No

Ni-Zn

Yes. C/10 to 1.3V

1.9V/cell to C/33 (3%)

No