backchannel: How do you size a VSR?

> > How do you size up the ampage for a VSR? > >

Reminder: a VSR is a Voltage-Sensing Relay is a type of isolator that uses sensed voltage to decide when to open/close.   The benefits are:

  1.  no need for a trigger wire powered when the vehicle key is turned from the OFF position

  2. the VSR doesn’t connect until sufficient voltage is sensed, meaning the battery is being charged


from this RVwiki article

> > If an isolator is **oversized** it will cost more for no benefit and will self-consume somewhat more energy to hold the combining circuit closed.[15)  ]( If an isolator is **undersized** (less common) it will not be able to carry enough current, resulting in overheating and/or [sudden shutdown]( > > > > Most AGM {discharged to 50% DoD} will pull about C/3 (33A for a 100Ah bank) but premium brands may do more. Flooded lead-acid batteries tend to pull less current (C/5, 20A per 100Ah of bank). If your flooded back will only pull ~40A, or your AGM bank 70A then there is little reason to spend more money on a 150-200A isolator {over a 100A version} > >

DC-DC chargers convert power as needed and so can maintain a given output precisely.  They are much more expensive so they are typically sized to the minimum charging current required by the bank:

  • AGM min = C/5 (20A per 100Ah of capacity)

  •  FLA min = C/10 (10A per 100Ah of capacity)

  • there is no min for Lithium;  charge as slow as you like