Re: Doable or dumb?
Yes, you could also charge the Varta offline then add it to the string. The "hole" in the string will be filled in by the bypass unit anyway to keep the rest happy whilst charging, though you won't be able to take much power! Does your bypass have a reverse current protection diode?
13.8V charge is really just good for floating a fully charged battery indefinitely, to get a full charge at that voltage from empty will take a loooooooong time for the last bit. That's likely why Powerline suggest a regular "topping" charge since you're not usually just floating a standby battery in a solar PV application, you're running in a part cyclic mode. Lead acids are not happy for any significant time in anything other than a fully charged state.
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