BQ24650 Datasheet, PDF (12/33 Pages) Texas Instruments – Synchronous Switch-Mode Battery Charge Controller for Solar Power With Maximum Power Point Tracking

bq24650
SLUSA75 – JULY 2010
Regulation Voltage
V_RECH
Regulation Current
Precharge
Current
Regulation
Phase
Charge
Current
DETAILED DESCRIPTION
Fastcharge Current
Regulation Phase
Fastcharge Voltage
Regulation Phase
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Termination
V_LOWV
I_PRECH & I_TERM
Charge
Voltage
Figure 16. Typical Charging Profile
BATTERY VOLTAGE REGULATION
The bq24650 uses a high accuracy voltage regulator for the charging voltage. The charge voltage is
programmed via a resistor divider from the battery to ground, with the midpoint tied to the VFB pin. The voltage
at the VFB pin is regulated to 2.1V, giving the following equation for the regulation voltage:
VBAT
= 2.1 V ´
é
êë¹⁺
R2 ù
R1 ^úû
(1)
where R2 is connected from VFB to the battery and R1 is connected from VFB to GND.
Li-Ion, LiFePO4, and sealed lead acid are widely used battery chemistries. Most commercial Li-ion cells can now
be charged to 4.2V/cell. A LiFePO4 battery allows a much higher charge and discharge rate, but the energy
density is lower. The typical cell voltage is 3.6V. The charge profile of both Li-Ion and LiFePO4 is
preconditioning, constant current, and constant voltage. For maximum cycle life, the end-of-charge voltage
threshold could be lowered to 4.1V/cell.
Although it's energy density is much lower than Li-based chemistry, lead acid is still popular due to its low
manufacturing cost and high discharge rates. The typical voltage limit is from 2.3V to 2.45V. After the battery has
been fully charged, a float charge is required to compensate for the self-discharge. The float charge limit is
100mV-200mV below the constant voltage limit.
INPUT VOLTAGE REGULATION
A solar panel has a unique point on the V-I or V-P curve, called the Maximum Power Point (MPP), at which the
entire photovoltaic (PV) system operates with maximum efficiency and produces its maximum output power. The
constant voltage algorithm is the simplest Maximum Power Point Tracking (MPPT) method. The bq24650
automatically reduces charge current so the maximum power point is maintained for maximum efficiency.
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