Larry wrote:
. . .
Nice charger, but you'll only see 40A charging them for a few minutes
before the charge quickly tapers off to a safe charging level below 20A.

Hmm. Assuming 2 6v Trojan T-105s or equivalent (or an 8D), 220ah
capacity, discharged 50%, a 40 amp charge rate is only 20% of the
bank's capacity, quite safe. The bank probably won't even get warm.
If you're in a hurry, charging at a 40% rate, 80 amps (typical
alternator output), would shorten the life of the bank slightly, and
warm things up a bit, but still would be a "safe" charging rate.

A 40 amp charger should put out 40 amps for two hours or so, before the
rising voltage causes the charger to start tapering off (70%-80%
charged) down ultimately to 5 to 10 amps or so ( 2% to 5% of bank
capacity, depending on age and battery design), where it should stay
for a half hour or so (14.4 volts, Trojan says 14.8!), to gas the
batteries for a while to mix the electrolyte thoroughly and bring the
bank up those last few amp-hours to full charge. This is *not* an
equalization charge. Then you can either turn the charger off, or, if
it's smart, it'll drop back to a float charge, 13.2 volts or so, so you
can run stuff without discharging the bank.

OTOH, if you're hooked up to shore power and have all night to charge,
there's nothing wrong with charging at 10% of the bank's capacity (20
amps or so), which will prolong the bank's service life slightly. But
there's nothing *unsafe* about an initial 40 amp (20% of capacity)
charge rate on a discharged bank.