@brassmonkey001, to begin with I always recommend the Smart BatteryProtect over the BatteryProtect because, although it's a little more expensive, it makes setting the device so much easier through the app that it's well worth the additional expense; furthermore the Smart version adds a significantly greater degree of functionality in that you can custom-set your high/low voltage cutoff/restart thresholds to 1/100th of a volt, instead of being limited to the factory presets as is the case with the standard-version BatteryProtect.
So there's that consideration.
Otherwise, you simply need to calculate the loads that you're going to have controlled by the device. If you have <50A of anticipated loads, get the 65A model, etc. You want the version that is equivalent to your anticipated loads x 1.25... so if you expect to have right around 65A loads that you want controlled by this device, you should go the next size up to the 100A version.
A couple of critical points: This device is unidirectional in operation only, so you cannot connect it in such a manner that reverse current is ever, ever applied to it. Also, you cannot use this device between your batteries and an inverter; the exception is in the case that you have a Victron inverter (not an inverter/charger) that has a remote trigger, in which case you can use this device as the remote trigger in accordance with the new instructions posted on 10/29 regarding the proper installation/wiring instructions for these devices; this device is not to be used to interrupt an inverter's main connection to the battery, however. See here: https://www.victronenergy.com/support-and-downloads/manuals#item=manuals-batteryprotect
These new instruction supersede all previous manuals issued in print, including the manuals included with most units on the shelves today.
That simply depends on what power you need.
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