Which 36 volt trolling motor battery bank should you buy?

Not amp hour for amp hour, no, and a straight swap needs checking. Three 12 volt batteries wired in series add their amp hour rating together at 36 volts, while a single lithium pack is rated on its own. Lithium delivers a much larger share of that rating at a hard discharge than lead acid does, so a 50 Ah lithium pack often covers a day that would need noticeably more lead acid capacity to match.

No. A single bank charger is built for one 12 volt battery, and a 36 volt system, whether it is one lithium pack or three lead acid batteries in series, needs a charger designed for that voltage or a bank set up per battery. A three bank 12 volt charger works on a series lead acid bank because it charges each battery individually before they are wired together as 36 volts for the motor.

Yes, without exception. The voltage is different but the chemistry inside is the same LiFePO4, and a lead acid profile causes the same problems at 36 volts as it does at 12: either it never reaches the bulk voltage the pack needs, or it holds the pack at a float voltage lithium cells should not sit at. Buying a pack bundled with a matching charger, like the LiTime models that ship with one, removes this risk entirely.

Motor draw and hours on the water decide it, not the thrust rating alone. A 101 or 112 lb thrust motor run hard for a full tournament day pulls enough current that a 50 Ah pack may not last, while the same motor used for a few hours of casual fishing rarely comes close to emptying it. Running the actual draw and planned hours through a runtime calculator before buying the larger pack avoids paying for capacity that sits unused.

Using published figures, a LiTime 36V 50Ah pack weighs 33.14 lb, while a single VMAX MR137-120 Group 31 AGM weighs 75 lb, and a series bank needs three of those to reach 36 volts. The comparison is not a perfectly equal amp hour swap, since three 120 Ah batteries total more rated capacity than the 50 Ah lithium pack, but it puts a real number on the weight difference anglers ask about most.

The series interconnect cables between the three batteries come out entirely, along with two of the three boxes or trays, since the single pack replaces all of it in one case. The main feed to the motor, the fuse near the battery post and the on-off switch stay in the circuit and generally do not need to change size, though it is worth confirming the fuse rating against the motor's published draw either way.