East Penn Battery Charging Guidelines

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The maximum charge rate in amperes should be no more than 1/3 of the battery’s reserve capacity minute rating. If the terminal voltage exceeds 16.0 volts while charging, reduce the charge rate....

AGM 12-VOLT BATTERY CHARGING CONSIDERATIONS:
Not all chargers are really AGM compatible and can do significant damage to an AGM battery. Large “wheeled chargers,” found in many shops, which exceed 15.4 volts must be avoided. In the rare occurrence that an AGM battery needs to be charged outside of the vehicle’s charging system, charging voltage should be
13.8 –14.8 @ 77°F (25°C). Not to exceed 30 AMPS.

Charging a 50Ah 100 minute RC rated AGM battery at 14.8V and 30A sounds very aggressive to me.
 
It's not 14.8 volts AND 30 amps, it is OR. Lead acid batteries including AGM are charged at a constant voltage. Holding the voltage down controls the current. A smaller battery will inherently accept less current at 14.8 volts than a larger one.
 
An older battery guide (2007) from East Penn had different charging parameters than the 2020 guide linked in the post above:

Follow this tip for the longest life:
Use the highest initial charging current available (up to 30% of the 20-hour capacity) while staying within the proper temperature-compensated voltage range.

IMPORTANT: Always use an automatic temperature- sensing, voltage-regulated charger! Set charger at 14.4 to 14.6 volts at 68°F for AGM. Do not exceed 14.6 volts for AGM.

Our recommended float voltage is 2.25 to 2.3 volts per cell for gel and absorbed [AGM] models.
 
.....Lead acid batteries including AGM are charged at a constant voltage.
Many , if not most, battery chargers use constant current for the initial bulk phase, followed by constant voltage for absorption phase, followed by constant voltage for float.
 
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I think the 1/3 reserve minutes capacity rating, is a mistake.

Most AGM batteries say 1/3 the amp hour rating at the 20 hour rate., not 1/3 the reserve minutes rating

33 amps max, for a 100Ah battery.

Most AGMS, but Odyssey, Northstar or Lifeline, recommend no more than 1/3 the Ah capacity, as a max initial charging rate.

A 100Amp hour 12v battery is group 27 or larger, 12.5 wide 9.25 tall 6.5 wide thereabouts. Deplete this to the point it needs a jumpstart and the alternator will feed it well over 33 amps, for some variable amount of time, its not like lead acid batteries after jumping are exploding melting or failing after an unintentional discharge to the point a jump is required.

I largely ignore reserve minutes ratings, as the battery would need to have been healthy and fully charged when 25 amps was applied to it, and the minimum voltage the vehicle would still operate on, is so variable and largely unknown until it quits. Some vehicles might have no issues at running with a battery only able to maintain 11.1 volts, others would give up long before depletion to that voltage level.

Keep in mind manufacturer charge recommendations are kind of the best one size fits all to reduce warranty returns in expected usage for those it is marketed towards.

My small Asian AGMs, when deeply cycled, have gotten 10 times the maximum recommended charge amperage initially, briefly, and usually double to triple the max rate till they reach absorption voltage of 14.7v. I've doubled the max recommended charge amperage when well depleted, for upto an hour and 15 minutes. They do not get excessively hot and I do not worry and still behave well. The fear is excessive heating

When shallowly cycled their max amperage rate is not even able to be held for long, I just hook them upto a 100 amp power supply limited to 14.7v and let them feed on as much as they want. its when they are well depleted I worry about them as they can accept high amperage for long enough to heat up excessively. I also worry when they are older and am more timid with what voltage I allow, or more precisely the time at higher voltages when well depleted.

I do not do long term float. I charge at a temperature adjusted absorption voltage till amps taper to 0.5% of capacity ( 20 hour rate) then disconnect.
When deeply cycled I insure they get at least a c/3 charge rate if not every time at least once every 7 deep cycles.
Shallow cycle duty, I don't really worry about the rate as they cant accept high amperage for long anyway.
 
Just curious if any one here ever found specific charging guidelines from Clarios or Johnson Controls for their AGM or flooded batteries? I did a google search around a year ago and never found anything.

What puzzles me is that many car battery manufacturers do not print charging guidelines on their battery label. But I have seen charging guidelines printed on the label for an Exide AGM battery I had and for a X2 Power (Northstar) AGM battery I had. I think Odyssey also provides an informative label, plus they also provide detailed charging recommendations in their downloadable technical manual.
 
i just let my C-Tek 4.3 muse do the thinking after pressing button for the correct choice + ALL my batteries last longer than EVER!!
 
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