Re: Charging Deep Cycle
We are playing with semantics over the word "charging". "Boosted" might be a better term. Maybe that's why jumper cables are called "booster" cables and not "charging" cables. When jumpers are used between vehicles, the running engine is indeed "charging" the discharged battery. If another battery is simply used to "jump" another, the term "boosted" is more applicable. Left for a period of time both batteries will equalize at some level, neither of which will be fully charged. If that were not the case, dual battery switches, isolators, and ACR's, Add-A-Battery systems would not be necessary. That's certainly not the only reason they are used but it is one reason.
I agree that when paralleling two otherwise independent batteries, the additional battery adds A "Boost" to the Starter in the sense that it assists the Primary Battery in starting the engine.
As the system voltage in this situation will never exceed 12.6 volts, I propose that it does Nothing to "Boost" the actual charge level in the Primary Battery, Just to assist in cranking the Starter.
No Lead Dioxide will be generated in the Discharged Battery and so no "Charging" can be claimed.
If the two batteries were to remain paralleled for a long enough time the "Booster" Battery would give up its capacity to the Discharged battery
(in the form of heat) until the the internal voltages became equal.
But the Discharged Battery would not gain ANY capacity in the process.
If the Engine in the "Booster" vehicle is running at the time, the system voltage will be above 14 volts and as such also above the "Float Voltage" necessary to bias the battery chemistry to convert the Lead Sulfate back into Lead Dioxide and and actual "Charging" of the battery could be claimed.
Battery Switches, Isolators, ACR's, and other Add-a-Battery devices all combine the Batteries when a charging device is also present.
They will/should separate the batteries when the system voltage fall below the Battery's Float Voltage.