The 14.8/.9v likely only lasts for a short while after engine starting. Once warmed up it likely drops to under 14, as a continuous 14.9v always, would overcharge the original engine battery.
Most people's solenoids come on with the key to on, and off during actual engine cranking, then back on after starting. This is not any significant issue for the batteries, but the solenoid develops 2x as many connection disconnection cycles and will wear out faster. Also the solenoid allowing house battery to contribute current to starter motor will wear out contacts faster.
Ideally the solenoid would only be triggered after the engine started, but it is not always easy to find a circuit which becomes live only after engine starting to use to trigger the solenoid. Enter the illuminated manual switch on the trigger circuit so the driver has more control over when the batteries are paralleled.
Usually when Solenoids fail, the contacts fuse together leading to no battery isolation with engine off. This is not so obvious to the driver who does not have a individual voltmeter on each battery. With a failed solenoid whose contacts have fused together, both batteries voltage will always read the same or very close, even when the house battery with a properly functioning solenoid is well depleted and should read lower when measured at battery terminals.
A failed solenoid usually means the engine battery gets inadvertently cycled with house loads and the problem only becomes obvious once the starter battery AND the house battery together, fail to start the vehicle, and this is usually at the end of the life of both batteries. This can take quite a while after the initial solenoid failure, and usually both engine and house batteries, in addition to solenoid, need to be replaced.
As the starter and house batteries will have different resistance and resting voltages, one battery will always feed the other when no charging sources are present, leading to the premature demise of both.
The solenoid might make the same exact noises when the contacts fuse together, or similar enough thae one's ears might not detect a difference in operation, so the wise user occassionally ensures there is actually isolation with engine off. House loads should not affect engine battery voltage with engine off. A properly isolated Engine battery voltage should always be 12.6v or higher, while it is expected that a 50% depleted house battery would be around 12.2v, so a voltmeter on both batteries regularly observed should easily indicate solenoid failure and prevent solenoid failure from taking out engine starting battery from unnecessary cycling of it.
Solenoid failure is not something to freak out about, just something to be aware of, and it is why i recommend people spend the bucks for the higher amp rated solenoiids with the silver tungsten contacts as the initial higher expense will likely be cheaper in the long run, compared to a 15$ 90 amp continuous duty budget solenoid whose contacts fuse together and eventually takes out both batteries and likely requires a jumpstart at some point before replacing both sets of batteries, and the solenoid.
. That's sort of where my mind was going; Potential problems being rust and the shorting.
