www.alcad.com

Battery Sizing

Scope

•
•
•
•

Battery selection criteria
Electrochemistry
Comparison of Lead Acid vs Nickel Cadmium
Battery Sizing – Understanding load profiles
Where and how you can save money

The Basics – Build a Load Profile
•
•

•

•

WHERE TO BEGIN
When will the battery discharge
LOADS TO CONSIDER
Continuous
Non-Continuou ... See more

Momentary
WHAT HAPPENS FIRST
Followed by?
For how long?
MARGINS
Design
Aging
Effects of temperature
Fluff

NORMAL OPERATION
AC Available
No Faults

2 amps for
indicating lights,
relays etc.

~
6 AMP
CHARGER

4.160KV
Switchgear

30 mA to float
charge battery

12 AMP TRIP
CURRENT

Basics

Charger goes to
current limit, puts
out 6 amps MAX.

~
6 AMP
CHARGER

4.160KV
Switchgear
0 mA to float
charge battery

Remaining 6
amps comes
from battery.

Load requirements exceed maximum charger output

12 AMP TRIP
CURRENT

ABNORMAL CONDITION
Charger Output Limited
Breaker Operating

1 amps max
coming from
charger.

4.160KV
Switchgear

~
6 AMP
CHARGER

0 mA going to
the battery

Remaining 11
amps comes
from battery.

Charger output is limited in some way.

12 AMP TRIP
CURRENT

ABNORMAL CONDITION
AC Failure
Charger Output @ Zero

No current
available from
charger

4.160KV
Switchgear

~
6 AMP
CHARGER

Full load comes
from battery.

Charger output is lost completely, due to rectifier or
AC failure.

12 AMP TRIP
CURRENT

Continuous Loads

• Loads that are energized for the duration
of the duty cycle
• Are normally supplied by the charger
• Can have a big effect on battery capacity
• Examples
Continuously operating motors / pumps
Relay coils
Indicating lights

Non-Continuous Loads

• Energized for only a portion of the duty
cycle
• Can be turned on or off automatically or
by operator action
• Special considerations:
If inception is known, but end is not, run to end of duty cycle
If end is known, and inception is not, consider the load from the
beginning of the duty cycle.

• Examples
Emergency lighting
Lube oil pumps
Communication

Momentary Loads
• Very short in duration, can be fraction of a
second
• Lead Acid - IEEE 485
Even though the load may last for only a few cycles, you must
treat it as lasting one full minute

• Nickel Cadmium - IEEE 1115
Even though the load may last for only a few cycles, you must
treat it as lasting one full second

• Examples
Switchgear operation
Engine starting
Field flashing

Dealing with Multiple
Momentary Loads
• Discrete Sequence Known
Load is maximum current at any instant

• Sequence Unknown
Load is sum of all loads in that period
Usually a greater demand on battery

Dealing with Multiple
Momentary Loads
IEEE States:

• If a discrete sequence can be established,
the load for the period shall be assumed to
be the maximum current at any instant
Example:
1 Trip - 3 breakers 45 amps
2 Trip - 5 breakers 75 amps
3 Trip - 4 breakers 60 amps

Duration
0.5 sec
1 sec
1 sec

Since we can determine the sequence, the load for the period
would be 75 amps for one minute (for lead acid)
Nicad can be broken into 3 distinct loads, or, 75A for 3 seconds

Dealing with Multiple
Momentary Loads
•

IEEE States:
If a discrete sequence cannot be established, the load for the period
shall be assumed to be the sum of all the loads occurring within that
period

Example:
Trip
Trip
Trip

- 3 breakers 45 amps
- 5 breakers 75 amps
- 4 breakers 60 amps

Duration
0.5 sec
1 sec
1 sec

Since we cannot determine the sequence, you must treat
the load as occurring all at once
180A for 1 minute - Lead
180A for 1 second - Ni-Cd

The Duty Cycle

M o m e n ta r y lo a d
1 m in - L e a d A c id
1 s e c - N ic k e l C a d m iu m
N o n C o n tin u o u s L o a d

C o n t in u o u s L o a d

Sizing Margins or
Making Batteries Bigger
• Aging Factor
25% Recommended

•
•
•
•

Applicable to:
All types of flooded lead acid
VRLA
Nickel Cadmium

• Exception is Plante`

Sizing Margins or
Making Batteries Bigger

• Low Temperature Correction
For operation below rated temperature

• High Temperature
Improves performance slightly
Not normally used in sizing calc’s.
Design margin for maximum life

Sizing Margins or
Making Batteries Bigger

• Design Margin
Normally considered for future equipment or load growth
Allows for operation at lower than expected temperature
Can cover for less than adequate maintenance

• Almost every sizing has one!

Sizing Margins or
Making Batteries Bigger

• If the calculation requires a 220 Ah
battery, and the next cell size up is 250 Ah
-• The 30 Ah difference is a 13% margin,
“designed” in
• An additional margin of 10% might not be
required

Knowledge is not
only Power

It’s Money
• Develop load profile using worst case
• Try to determine sequences
Not knowing requires conservatism
Conservatism can increase required capacity
Increased capacity - More $

• Closely review various sizing factors
Low temperature increases battery size
Aging factors are good ideas - ensure long reliable life
Evaluate design margins, especially after sizing a battery

Have we lost anyone

So far we’ve covered...

• Various discharge scenarios
Pick the worst case

• Various load types
Continuous
Non-Continuous
Momentary

• Sizing margins / factors
Temperature correction
Aging factor
Design margins

Sizing - What’s needed
• Load profile
Include all prudent margins

• Voltage
Maximum
Minimum

• Manufacturers data
Yes, you do need us

• Capacity rating - Kt factors
Amps per positive plate - Rt factors

• Battery type
Flooded lead acid
Nickel cadmium
VRLA

The load profile
15 BREAKERS
TRIP

- 10A, 5 CYCLES

Sim./Brkr

CLOSE

- 7A, 5 CYCLES

Seq./Brkr

SP. CHG - 4A, 6 SECONDS

TWO OPERATIONS,
Beginning and end of 8 hr duty cycle

2A CONTINUOUS LOAD

EMERGENCY LIGHTING LOAD
1200 Watts - 90 minutes
Starts at outage

Seq./Brkr

The load profile

Trip 15 Breaker
1 min. - Lead Acid
1 sec. - Ni-Cd

150 A

Trip Breakers at
End Of Cycle

Close & Spring
Charge Loads

Trip Load

1 min. - Lead Acid
92 sec. - Ni-Cd

?
7A

10A

Lighting Load

2A
Continuous - 8 HR
90 minutes

Load profile defined

•

Nickel Cadmium
162A
19A
12A
2A
152A

1 second
92 seconds
88.45 minutes
390 minutes
1 second

•

Lead Acid
162A
19A
12A
2A
152A

1 minute
1 minutes
88 minutes
389 minutes
1 minute

Voltage window

• Maximum and Minimum values
Determined by DC powered equipment

• Allow widest possible range
Uses maximum number of cells

• More cells = lower end of discharge
voltage
• More efficient capacity utilization
• Least expensive battery

Impact of voltage window

• IEEE 485 Example:
140V - 105V window
60 cells, to 1.75 VPC

- 1,010.4 Ah req'd

• Wider voltage window
62 cells, to 1.69 VPC
- 944 Ah req'd
3% increase cell qty, 7% capacity reduction

• Narrower voltage window
58 cells, to 1.81 VPC - 1,186 Ah req'd
3% decrease cell qty, 17% increase in capacity

• 100Ah High rate ni-cd cell
One minute rate To 1.14 VPC
One minute to 1.05 VPC

243 amps
406 amps

No. of cells calculation
Max. Volts - Determines number of cells that can be adequately
charged.
Equalize value is normally used as determining cell voltage

Ex

140V max
2.33VPC
or
140V max
1.46VPC

= 60 cells (lead acid)

= 96 cells (nickel cadmium)

End of discharge calculation
Min. Voltage - Lowest value system designed to operate at
Min. Volts
# of cells = End of discharge voltage / cell
Ex. 105 VDC
60 cells
= 1.75 VPC Lead Acid
Ex. 105 VDC
96 cells

= 1.09 VPC Nickel Cadmium

Sizing factors

• Kt factors
Based on performance per rated ampere hour
Kt factor = Rated ampere hours
Amps available for time t

• Rt factors
Based on performance per positive plate
Used primarily with lead acid cells
Rt factor = Amps available for time t per positive plate

Capacity rating factors

• Kt factors
Determined from tabular data

• Examples
160 Ah rated cell
8 hr discharge rate - 20 amperes
Kt = 160 Ah / 20 amps
8 Hr rate Kt = 8

• One minute discharge rate - 320 amperes
Kt =
160 Ah / 320 amps
One minute Kt = 0.5

• Kt factors are multipliers in IEEE
worksheets

Capacity rating factors

• Rt Factors
Found in plate performance curves
Not all manufacturers publish them
When not available, use Kt

• Rt factors are divisors in IEEE worksheets

Did you know this?

• Most lead acid model numbers indicate
number of plates per cell
3CC7 = Seven plates per cell
4JC11= Eleven plates per cell

• One more negative plate than positive
Seven plates = 4 Neg - 3 Pos
Eleven plates = 6 Neg - 5 Pos

• 50Ah / positive plate
150Ah cell from above example (3 pos x 50Ah)
250Ah cell for eleven plate cell (5 pos. x 50Ah)

• Positive plates are same for the range

More things you should know

• Ampere hour nomenclatures
Most nickel cadmium
Some VRLA

• Pos. plates are not identical thru range
• Wider or taller plate = higher capacity
• More plates per cell = higher capacity

Real life
• Create a duty cycle
• Periods
Amps__________ Duration_________
Amps__________ Duration_________
Amps__________ Duration_________

• Voltage window
Max__________

MIn__________

• Calculated No. of cells
Lead acid___

Nickel cadmium___

• Environment
High Temp_____

Lowest Temp_______

• Aging factor _______ (not for Plante')
• Design margins
_______

Summing up

•

Battery Sizing is a science

•

Building the load profile is an
art
Electro-chemistries vary
greatly

•

•

You have more control over
your battery selection than you
think

www.alcad.com

Thank you for
your attention

Alcad: presentation title

37

... See less