Heat soak starter coil, or weak battery?

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A bit outside our budget but these are the batteries G.O. probably means, they stand in banks on the floor of a telecom exchange To maintain 52 volts ( negative with respect to ground ), are on constant float charge in a ventilated room 100% on-line and have a design life of 20 years, the plates however are 8 mm. thick and the enclosure is glass, very heavy and expensive .
They need topping up with de-mineralised water and are tested on a regular schedule;
https://www.exponentialpower.com/stt2v700.html