backchannel: Does a DC-DC charger link both batteries banks when not charging?

from this thread on Will’s DIY Solar forum.

the question

A question from a new user:

Does a dc to dc charger link both batteries banks when not charging?

the simple answer

A consistently-clueful poster responded:

No

That is straightforward and correct enough, particularly given the user was new to the forum and could be struggling with the basics.

a longer answer

OP’s language is ambiguous (probably from unfamiliarity with the topic) and exceptions can hide in the nuance. We can discuss the details here without muddying the waters for OP.

“link”

It doesn’t link them in the combining/paralleling sense even while charging. In normal operation the charge goes one way: alternator -> DC-DC -> house bank

See below for exceptions.

“charging”

Possible exceptions, depending on what we mean by charging. In this context charging often is shorthand for charging by alternator but other scenarios are possible:

• even when the engine is stopped the starter battery voltage may be high enough to keep a voltage-triggered DC-DC active for a short while
• some DC-DC have starter battery maintenance features. These will send power back to the starter battery when the house battery is “full” or “full enough”. See respective manuals for details.
• some DC-DC setups allow connection of the two for self-jumpstarting. Ctek does this through the Smartpass. A relay can be paralleled with the DC-DC as Redarc describes here, etc. The actual “link” is not done with the DC-DC in this scenario.
• If an external starter battery charger is placed on the vehicle and the DC-DC is triggered by voltage this can cause the DC-DC to start charging the house battery. This can lead to cycling weirdness if the DC-DC draw is greater than the starter battery charger can provide.

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