The 1315 only parallels the batteries when the battery connected to the
charging system reaches 13.2 volts.

When you first start the engine and the alternator light goes out, the
aux still isn't connected (and won't be) until the primary battery is
fully charged.  It'll also drop off the aux battery (even though the
alternator is still charging) should the primary battery drop to 12.8
volts ... hence the "Smart" relay designation.

Mike

Michael Elliott wrote:
> Thanks for that, Mike.
>
> Using the van's ALT light as a trigger to close the bridging relay
> between the starter and aux relay adds the same "smartness" to the relay
> system as the 1315 possesses. Even then, if a hosed aux battery drags
> down the charging system voltage to the point where the ALT light comes
> on again, the relay would disconnect the aux battery (which would cause
> the light to go out, bridging the batteries again . . . but the 1315 has
> a time delay feature which prevents this from happening over and over
> and over again rapidly. Might just make it happen more slowly.
>
> So to me, the 1315 looks like a fine solution, and it's a neater package
> than my relay + a bunch of wires and things, but it doesn't appear to
> have any significant advantage over the relay system.
>
> This is an interesting thread to me. If anyone can come up with a likely
> situation where the 1315 trumps my relay rig (below), then I'd like to
> read about it.
>