Battery capacity is rated at start of one combustion enegine for 6 starts-up taking 5 seconds each. In case when one battery is a source for more than one starter, the total capacity should be calculated as a sum of the required capacity of the greatest starter and a half of the required capacity for each of the remaining starters.

Calculations were carried out based on the Polish standard PN-W-89509:1997 Merchant ships, batteries, selection, in accordance with the following equations:
, where

Q_r – required of capacity of the starting battery,

C – capacity related to the starter power (curves are available in the standard),

a – temperature coefficient (curves are available in the standard) taking into account reduction of the capacity due to electrolyte temperature,

0,75 – undercharge coefficient,
, where

S₁ – minimum cross-section of the feeding cable due to heating [mm²]

I_zw – short circuit current in the starter during start-up (curves available in the standard),

j – permissible current density shall not exceed 20 [A/mm²]

, where

S₂ – minimal cross-section of the cable due to voltage drop related to short-circuit current I_zw, [mm²]

l – lenght of the cable, [m],

U_n – rated voltage of starter, [V].

Author: Daniel Czarkowski