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:
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no need for a trigger wire powered when the vehicle key is turned from the OFF position
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the VSR doesn’t connect until sufficient voltage is sensed, meaning the battery is being charged
sizing
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) ](http://rvwiki.mousetrap.net/doku.php?id=electrical:12v:alternator#fn__15) If an isolator is **undersized** (less common) it will not be able to carry enough current, resulting in overheating and/or [sudden shutdown](http://rvwiki.mousetrap.net/doku.php?id=electrical:12v:alternator#sudden_disconnection). > > > > 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:
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AGM min = C/5 (20A per 100Ah of capacity)
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FLA min = C/10 (10A per 100Ah of capacity)
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there is no min for Lithium; charge as slow as you like