Introduction
The bq25505 was designed with the flexibility to support a variety of energy storage elements. The
availability of the sources from which harvesters extract their energy can often be sporadic or time-
varying. Systems will typically need some type of energy storage element, such as a re-chargeable
battery, super capacitor, or conventional capacitor. The storage element will make certain constant power
is available when needed for the systems. In general, the storage element also allows the system to
handle any peak currents that can not directly come from the input source. It is important to remember
that batteries and super capacitors can have significant leakage currents that need to be included with
determining the loading on VSTOR.
To prevent damage to a customer’s storage element, both maximum and minimum voltages are monitored
against the internally programmed undervoltage (VBAT_UV) and user programmed overvoltage
(VBAT_OV) levels.
To further assist users in the strict management of their energy budgets, the bq25505 toggles a user
programmable battery good flag (VBAT_OK), checked every 64 ms, to signal the microprocessor when
the voltage on an energy storage element or capacitor has risen above (OK_HYST threshold) or dropped
below (OK_PROG threshold) a pre-set critical level. To prevent the system from entering an undervoltage
condition or if starting up into a depleted storage element, it is recommended to isolate the system load
from VSTOR by using an NFET to invert the BAT_OK signal so that it drives the gate of PFET, which
isolates the system load from VSTOR.
For details, see bq25505 data sheet (
).
1.3
Design and Evaluation Considerations
This user's guide is not a replacement for the data sheet. Reading the data sheet first will help in
understanding the operations and features of this IC. In this document, “secondary rechargeable battery”
or "VBAT_SEC" will be used but one could substitute any appropriate storage element.
System Design Tips
Compared to designing systems powered from an AC/DC converter or large battery (for example, low
impedance sources), designing systems powered by Hi-Z sources requires that the system load-per-unit
time (for example, per day for solar panel) be compared to the expected loading per the same time unit.
Often there is not enough real time input harvested power (for example, at night for a solar panel) to run
the system in full operation. Therefore, the energy harvesting circuit collects more energy than being
drawn by the system when ambient conditions allow and stores that energy in a storage element for later
use to power the system. See
for an example spreadsheet on how to design a real solar-panel-
powered system in three easy steps:
1. Referring the system rail power back to VSTOR
2. Referring the required VSTOR power back to bq255xx input power
3. Computing the minimum solar panel area from the input power requirement
As demonstrated in the spreadsheet, for any boost converter, you must perform a power balance:
P
OUT
/ P
IN
= (V
STOR
× I
STOR
) / (V
IN
× I
IN
) =
η
(1)
where
η
is the estimated efficiency for the same or similar configuration in order to determine the minimum
input power needed to supply the desired output power.
This IC is a highly efficient charger for a storage element such as a battery or super capacitor. The main
difference between a battery and a super capacitor is the capacity curve. The battery typically has little or
no capacity below a certain voltage, whereas the capacitor does have capacity at lower voltages. Both can
have significant leakage currents that will appear as a DC load on VSTOR/VBAT_SEC.
3
SLUUAA8 – September 2013
User's Guide for bq25505 Battery Charger Evaluation Module for Energy
Harvesting
Copyright © 2013, Texas Instruments Incorporated
