BPD1002 datasheet(9/15 Pages) BEL

9 / 15 page

BCD.00617 Rev AB,16-Nov-2015

Page 9 of 15

MELCHER

The Power Partners.

Accessories Data Sheet

Backplanes BPD, BPF for T Series

300

0.6

0.2

0.4

1.0

100

factor y

(Irms bat /Iload)

Rsys [m ]

Number of LT units:

06037

Battery specification (at 20 °C):

• 24 cells, 2.23 V/cell, float charge voltage 53.5 V

• Permissible ripple current I bat: 0.05 C (5% of battery

capacity)

• Temperature coefficient –3 mV/°C per cell

• Max. discharge current 20 A

• Capacity at –10 °C and 0.2 C discharge: 70%

Calculation of battery size:

Load 20 A, back-up 3 h, ageing loss 20% therefore ageing

factor 0.8, derating factor for temperature and discharge

current 0.7

Qbat [Ah] = I load • t back-up /(fageing • f TAIdischarge)

= 20 [A] • 3 [h] /(0.8 • 0.7) = 107 [Ah]

next closest battery configuration::120 Ah or 95 Ah

Decision: Battery with 95 Ah (Ri: 28 m

Ω)

Calculation of the ripple current:

The ripple current to the battery can either be measured or

calculated. The diagrams allow a good estimation of the rela-

tionship between the ripple current to the battery Irms bat and the

load current I Load of a given system resistance RSys.

The system resistance is the sum of the resistance of the

supply and the AC resistance of the battery.

The AC resistance is defined as the relationship between the

AC voltage and the AC current to the battery measured with a

capacitively coupled 100 Hz AC source.

In the example the system resistance is:

RSys

= Rsupply rms + Ri AC bat

= 16 m

Ω + 16 mΩ = 32 mΩ

Permissible ripple current:

Irms bat = 0.05 • 95 Ah = 4.75 Arms

Calculated ripple current:

With 3 LT 1740 without capacitance on the back plane

Irms bat = Iload • factor y = 19 A • 0.198 = 3.76 Arms = 0.04 C

With 3 LT 1740 and 50 mF on the back plane

Irms bat = 19 A • 0.16 = 3.04 Arms = 0.032 C

With 3 LT 1740 and 90 mF on the back plane

Irms bat = 19 A • 0.13 = 2.47 Arms = 0.026 C

Fig. 11

Irms bat/Iload versus system resistance (Rsys). Up to 6 T units in

parallel, without additional output capacitors

Fig. 12

Irms bat/Iload versus system resistance (Rsys). 2 and 3 T units in

parallel, with additional output capacitors

2 units + 50 mF

2 units + 90 mF

3 units + 50 mF

3 units + 90 mF

300

0.6

0.2

0.4

1.0

100

Rsys [m ]

0.16

factor y

(Irms bat /Iload)

Number of LT units plus additional output capacitance

06038

All three examples give a value below the permissible ripple

current.

Additional capacitance on the back plane not only reduces the

AC ripple on the battery, but stabilizes the output voltage during

fast load changes.

Where the ripple current should be reduced even more, the

system resistance RSys could be increased with a special

choke set into the battery supply line. For further information

please contact the Company.

Rback plane 1

Rtot supply lines

16 m

Ω

–

Ri AC bat

LT 1

LT 2

LT 3

–

1.5 m

06039

1 The resistance on the back plane (Rback plane) is already

taken into account in the following two graphs.

Fig. 10

System set-up

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