ISL78610 Datasheet, PDF (48/98 Pages) Intersil Corporation – Multi-Cell Li-Ion Battery Manager

ISL78610
Auto Balance mode performs balancing autonomously and in an
intelligent manner. Thermal issues are accommodated by the
provision of auto balance sequencing (see “Auto Balance
Sequencing” on page 48), a multiple instance Balance Status
register and a balance wait time.
During Auto Balance mode the ISL78610 cycles through each
Balance Status register instance, which turns on the balancing
outputs corresponding to the bits set in each Balance Status
register instance. While each cell is being balanced, the amount
of charge withdrawn is calculated. Balancing stops for a cell
when the specified amount of charge has been removed. See
“Auto Balance SOC Adjustment value” on page 48.
When Auto Balancing is complete, the End Of Balance (EOB) bit
in the Device Setup register is set and BEN bit is reset.
To set up an auto balance operation:
• Set the Balance Mode bits to ‘11’ for Auto
• Set the Balance Status Pointer to ‘1’
• Set bits in the Balance Status register to program the cells to
be balanced in the first cycle (e.g., to balance odd cells, set bits
1, 3, 5, 7, 9 and 11)
• Set the Balance Status Pointer to ‘2’
• Set bits in the Balance Status register to program the cells to
be balanced in the second cycle (e.g., to balance even cells, set
bits 2, 4, 6, 8, 10 and 12)
• Set the Balance Status Pointer to ‘3’
• Set bits in the Balance Status register at this location to zero to
terminate the sequence. The next cycle will go back to balance
at status pointer = 1.
• Write the B values into the Balance Value Registers for each
cell to be balanced.
• Enable balancing, either by setting the BEN bit in the Balance
Setup register or by sending a Balance Enable command.
Once enabled, the ISL78610 cycles through each instance of
the Balance Status register for the duration given by the
balance timeout. Between each Balance Status register
instance, the device does a Scan All operation and inserts a
delay equal to the balance wait time. The process continues
with the balance status pointer wrapping back to 1, until all
the Balance Value registers equal zero. If one cell Balance
Value register reaches zero before the others, balancing for
that cell stops, but the others continue.
• Disable balancing either by resetting the BEN bit or by sending
a Balance Inhibit command. Resetting BEN, either directly or
by using the Balance Inhibit command, stops the balancing
functions but maintains the current Balance Value register
contents. Auto Balancing continues from Balance Status
register location 1 when BEN is reasserted.
AUTO BALANCE SEQUENCING
The first cycle of the auto balance operation begins with the
balance status pointer at location 1, specifying the first Balance
Status register instance. For the next auto balance cycle, the
balance status pointer increments to location 2. For each
subsequent cycle, the pointer increments to the next Balance
Status register instance, until a zero value instance is
encountered. At this point the sequence repeats from the
Balance Status register instance at balance status pointer
location 1.
For example, using two Balance Status registers, the ISL78610
can balance odd numbered cells during the first cycle and even
numbered cells on the second cycle.
Between each cycle, there is a delay time. This delay is set by the
balance wait time bits (see Table 16 on page 46)
Cells are balanced with periodic measurements being performed
during the balance time interval (see Table 18). These
measurements are used to calculate the reduction in State of
Charge (SOC) with each balancing cycle.
As individual cells reach their programmed SOC adjustment, that
cell balance terminates, but the balance operation continues
cycling through all instances until all cells have met their SOC
adjustment value.
AUTO BALANCE SOC ADJUSTMENT VALUE
The balance value (delta SOC) is the difference between the
present charge in a cell and the desired charge for that cell.
The method for calculating the state of charge for a cell is left to
the system designer. Typically, determining the state of charge is
dependent on the chosen cell type and manufacturer, is
dependent on cell voltage, charge and discharge rates,
temperature, age of the cell, number of cycles and other factors.
Tables for determining SOC are often available from the battery
cell manufacturer.
The balance value itself is a function of the current SOC, required
SOC, balancing leg impedance and sample interval. This value is
calculated by the host microcontroller for each cell. The
balancing leg impedance is made up of the external balance FET
and balancing resistor. The sample interval is equal to the
balance cycle on-time period (e.g., each cell voltage is sampled
at the end of the balance on-time).
The balancing value B for each cell is calculated using the
formula shown in Equation 1 (see also “Balance Value
Calculation Example” on page 80):
B = ⁸----¹--₅-⁹----¹-  CurrentSOC – T argetSOC  _d-^Z---_t
(EQ. 1)
Where:
B = The balance register value
CurrentSOC = The present SOC of the cell (Coulombs)
TargetSOC = The required SOC value (Coulombs)
Z = The balancing leg impedance (ohms)
dt = The sampling time interval (Balance cycle on time in seconds)
8191/5 = A voltage to Hex conversion value
The balancing leg impedance is normally the sum of the balance
FET r_DS(ON) and the balance resistor.
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FN8830.1
June 16, 2016