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PS810

DS21904C-page 14

4.4

EOD Voltage Look-up Table

4.4.1

NEAR EMPTY RESERVE

CAPACITY POINT

Available capacity in the battery varies with tempera-

ture and discharge rate. Since the remaining capacity

will vary with temperature and discharge rate, a

shutdown point, or End-Of-Discharge voltage, will also

vary with temperature and discharge rate in order to

leave enough reserve energy in the battery to allow a

system to perform a shutdown procedure under all

conditions.

The End-Of-Discharge voltage look-up table allows for

entry of 64 different voltages as a function of 8 different

temperature regions and 8 different discharge rate

regions. Linear interpolation is used to determine the

End-Of-Discharge voltage within the regions.

It is recommended to use the table in one of three

ways:

Constant Shutdown Voltage

If the system does not require any reserve

energy, then the same voltage can be used in all

table entries. This should be the lowest voltage

the battery can safely discharge to, or the lowest

voltage that will support system operation,

whichever is higher.

Constant Remaining Capacity

To give a warning of a certain amount of capacity

left, the voltage that represents this capacity for

a certain temperature and discharge rate is used

in each entry. Plotting voltage versus capacity

for different temperatures and discharge rates

will identify these points

Constant Reserve Energy for System Shutdown

If the system needs a certain amount of energy

for a system shutdown procedure that will

require a certain discharge rate and a certain

amount of time, then the discharge rate could

change suddenly as soon as the End-Of-

Discharge voltage point is hit. The voltage

entries should then represent a remaining

capacity that exists at the future discharge rate

of the shutdown procedure.

Plotting voltage versus capacity at various tempera-

tures and discharge rates, along with the voltage

versus capacity at the shutdown procedure discharge

rate, will identify the voltage point at which shutdown

should occur assuming a switch from the first discharge

rate to the shutdown discharge rate.

The residual capacity entries represent the difference

between the ideal capacity of the battery and the

capacity available at the specific temperature and

discharge rate. To compensate remaining capacity, the

residual capacity will be subtracted from remaining

capacity. The compensation adjustment will happen in

real time to avoid sudden drops or jumps. Every time

the temperature decreases by one degree, a new inter-

polated value will be subtracted from the remaining

capacity. Every time the temperature increases by one

degree, the remaining capacity will be held constant

until the discharged capacity equals the interpolated

value that should have been added to the remaining

capacity

(to

avoid

capacity

increases

during

discharge). This compensation will not begin until after

the fully charged status is reset, allowing the remaining

capacity to be 100% when the battery is full.

TABLE 4-1:

V_EOD LOOK-UP TABLE EXAMPLE

Table 4-1 is an example of the various voltage values

that will signal the shutdown points as a function of

temperature and discharge rate.

Table 4-2 shows the actual names of the values in the

memory. Table 4-3 shows the value definitions.

<-10°

<0°

<10°

<20°

<30°

<40°

<50°

>50°

< 0.2C

3200 mV

3100 mV

3050 mV

—

< 0.5C

3125 mV

—

< 0.8C

3075 mV

—

< 1.1C

—

< 1.4C

< 1.7C

< 2.0C

> 2.0C

Residual Capacity

20%

10%

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