backchannel: isolator and dc-dc

from a comment on this YT video:

What if your RV already has an isolator? Should I install DC to DC after the isolator? > >

I think this is worth talking about.

in parallel

Most people wouldn’t need both an isolator and DC-DC charger. I can’t think of many good use cases for this setup. Maybe put a hard switch on the isolator and use it for self-jumpstarting? Put a HVD on the isolator and use it as a poor man’s smartpass?

upgrading/replacement

There are valid reasons for “upgrading” to dc-dc, although the isolator is typically replaced by the DC-DC in these scenarios:

1. you need to control current demand from the bank, thereby protecting the alternator. A 20A renogy DC-DC would limit draw to 20A instead of 50A or 100A or whatever thirsty AGM or Lithium want. Less of an issue with FLA due to higher internal resistance.
2. folks with lead banks and no fixed solar who drive a lot will benefit from the higher voltages DC-DC chargers make.
3. DC-DC charging eliminates backfeeding elevated bank voltages back to the chassis. I think it’s generally a non-issue, but included here for completeness

Related: I did some back-of-envelope math one day and found that when lead-chemistry bank voltage is low the break-even between isolator and 20A DC-DC charging is something like a half-hour. It’s a bit of a Tortoise & Hare scenario. For shorter drives the isolator (Hare) will shovel in more Amps (no 20A limit!) before Vbatt rises to alternator voltage and current falls off in pseudo-Absorption. For longer drives the DC-DC (Tortoise) will result in more overall current since 20A will still be flowing above the alternator’s voltage output right up to Vabs.

serial

People who already have an isolator (the scenario in question) might leverage the existing isolator install to feed the DC-DC. Perhaps this is what OP meant in their question up top.

Instead of the isolator feeding the bank it feeds the input of the DC-DC. Benefits:

1. don’t have to remove the isolator - can revert quickly
2. the isolator’s trigger now becomes the DC-DC’s trigger

The only drawback I can see is the constant 0.5A load of the electromagnet in the isolator when the circuit is energized. I don’t think that is meaningful against the deluge of alternator output.

thinking about my own situation

As of today I have 874 days on the GC2 FLA bank and still going strong. If/when I replace them with LiFePO4 I’d likely inline a 20A DC-DC like the renogy after the existing Battery Doctor voltage sensing relay.