Battery Charging

You could have given the cell type - the numbers suggest this is a 90 cell stationary vented [ it has screw-in caps to release gas & allow topping up with water] Nickel - Cadmium type Ni-Cd.

Float Charge - this is the continuous voltage which will keep the battery fully charged automatically, exceeding self-discharge, provided any service discharges are small & rare. The charger voltage regulation requirement is probably +/- 1%. The actual voltage depends on cell type and service e.g. engine starter batteries are slightly higher than long discharge standby types, each start removes some Ah.

Auto Charge [commonly called boost or equalise, second term is more accurate] - this is probably a manually initiated "gassing" charge (pushbutton) with automatic timed duration ~ 3 hours, which ensures the battery is fully charged on all cells, despite any small discharges N.B. Lead Acid batteries can have state of charge assessed by electrolyte specific gravity [or even voltage after open-circuit for 24 hours - O/C voltage = S.G. + 0.84 e.g. 1.210 S.G. has 2.05V open-circuit/cell] but there is no such measure for NiCd - one just has to overcharge periodically & conduct a discharge annually & log results to be sure of actual capacity (as with any type standby battery). "Gassing" will use a noticeable amount of cell level. Used every 6 months, say...

Manual Charge - this is a manually supervised charge to ensure full recharge after an emergency discharge - even at low ambient temperatures. Usually applied at commissioning & after a discharge with observation of individual cell voltages to ensure all cells are fully charged before reverting to float. Usually selectable only by an internal link rather than normally accessible control.

N.B. the previous voltages apply at about 20 Celsius & have to be increased for lower ambients to give sufficient charge current or maybe reduced at high ambient to avoid frequent topping up with distilled water. The float charge current electrolyses water at about 1/3 cc per amp hour per cell, if I remember right, & voltage/current may be chosen to give a long unattended time between top-ups. Float current is about 1/1000 C in amperes e.g. 0.1 amp for 100 amp-hour @ 25'C

Minimum voltage - if the cells are discharged flat, some will discharge first & become reverse potential, turning into electrolysis cells, bubbling fiercely. If discharge current is high, this can damage cell by overheating & mechanical effect of bubbles/overpressure. If the over-discharge is only at C/10 or less and rare, then a minimum volt cut-off may not be necessary - unreliability/cost of Min. volt detection & discharge cut-off contactor may not be justified. The practical minimum voltage is greatly influenced by discharge rate, 1 volt/cell or even 0.8V/cell may apply for engine starting with different requirements for cold break-away and crank duration.