Yep- And that is the problem. I'm trying to convey this in text and it may not come across but I'll try to cut thru the market'eering since I am stuck at the airport lounge - so I'll try to get this down
I dont know your use case so lets assume normal starting battery. Other applicaitons could vary but the concept is based on physics/chemistry so it doesn't really change outside of the temporal timeframes.
first principle / axiom - volt per volt AGMs recharge faster than standard flooded cells (even if the charge voltage is lower - it is an ohm's law thing)
The listed "cycle service" you show is like a total loss cycle (discharge to like 50% then recharge off a big charger bank - think a backup UPS inverter type service that has an effectively "infinite power" charger system for when grid power comes back on and you want to recharge the battery as fast as technically possible for efficiency reasons). In this application you'd deep discharge and actively monitor the state of charge with a shunt and know exact battery charge status at all times. With this you can do a controlled charge back to near 100% and transition from a very rapid charge rate with 14.8VDC then roll off to the tail/trickle charge at the lower 13.5VDC to prevent boil off for the last ~15% of charge cycle (bulk charge at high amps enabled by the lower cell resistance of AGM). That aint how 98.9% of cars work.
A standard alternator is a constant voltage system (drive the field to the set point voltage +/- the temperature correction and power limit but basically constant voltage). If you bump alternator voltage by cheating the sense signal with a diode you can certainly drive the buss voltage up by 0.7VDC. In a "normal automotive application" the battery is there for the start cycle and then is recharged in tens of minutes (~15% depth of discharge or less). So you are already living in the float charge voltage region of SOC there is no down side to keeping the voltage lower at the OEM setpoint even if you go deeper into the battery state of charge with parasitic loads. The battery will do its thing and recharge faster than the OEM FLA/SLA battery would.
The AGMs are lower resistance so if you don't have a higher voltage bulk charge voltage and just charge at the lower "float voltage" (AKA the stock alternator setpoint) it will recharge faster than an OEM flooded cell would but not as fast as it is capable of recharging (basically inefficient for what the battery could handle but they don't care). If you had a "smart" charger/alternator (a closed loop charge controller) on the car you could ramp up charge voltage and recharge noticeably faster. This higher voltage would be usefull if you did super short trips and never actually drive long enough to let the battery charge but then you'd have a way worse issue if you were on a normal FLA battery in the same use case so that is a bogus use case since either way you'd have dead batteries all the time no mater the battery type. But most cars don't have CL voltage controls so leave the voltage set point lower so you don't boil the battery by over charging.
And there is the problem condition- if you set the target voltage higher for a higher charge rate, AKA diode on the sense line and drive up alternator voltage, and then drive long enough to charge the battery you will damage/boil the AGM cells by not rolling back to a lower "float" voltage at the higher state of charge.
The "cycle service" voltage they list is a kinda goofy/contrived spec that really comes back to "maximum charge amps" under some condition that they don't list (I can't find a northstar tech sheet that isn't marketing wonk, so another AGM G27 sheet is below - note that the bulk charge limit is listed in amperage not voltage).
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Throw a shunt on your battery and see what it is doing for charge rate and what the load actually needs.
Street creds- Designed/serviced/operated properly high draw systems for mobile applications, did chassis/suspension and harness design work for 2 major OEMs, and lived/live off grid on solar/battery inverters (230A split phase service on 22kW of panels plus three generations of battery technology and building out a new system currently)
And I'm out
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