Bulk rate is maximum amps the charging source can supply until the absorption voltage is reached, at that point the amps required to hold the ABSV will taper. The longer the battery is held at ABSV, the more the amps required to hold ABSV will taper. At some point, either time, or the amps required to hold ABSV fall below a threshold and triggers float mode.
This is how pretty much all 3 stage automatic chargers work, but they all have differences in how high the ABSV is set, and some of them cannot hold their bulk max amp rate on the way to ABSV.
And ultimately it is up to the battery how many amps are required to reach and hold ABSV. AGMs have much less internal resistance and can really suck up huge charging currents and tax the charging source.
Some AGM's state to feed them all the amps you possibly can, Some other AGMs say not to exceed a 30% rate, or 30 amps on a hundred amp hour battery. Most important thing one can do with any particular AGM is to find the manufacturer recommended ABSV, and find a charging source which allows one to set this figure, or one that comes the closest.
All charging sources are basically compromises, and with AGMs since there is no way to check Specific gravity readings, which is the Battery polygraph, one determines state of charge by when the battery can no longer accept a certain amount of amps at the recommended ABSV.
It really depends on the battery manufacturer specs on how much current the battery can accept, and the ABSV stated by the manufacturer, corrected for temperature.
I know a lot of info states to limit the charging amperage as it is safer and 'better' for the battery, but it really depends on when the next discharge cycle begins and hohw much time one has to get the battery as near to 100% as possible before that next discharge cycle begins.
Somebody who has days to allow a charging source to top off a battery, can go low and slow, but the guy on the generator who has limited hours when he can run the generator should force as much amperage into the batteries as possible, and not exceed the manufacturer bulk amp rating, if one is listed.
All the top quality AGM's like Lifeline, Odyssey, and Northstar basically state there is no easily attainable limit on Bulk current, where as lesser AGMs will state the 30% maximum. Lifeline says their 100 amp hour battery can accept 500 amps no issues, until 14.4 is reached and the battery will in fact enjoy this much current, but supplying that much current is nearly impossible with charging sources commonly available.
If your battery bank was still taking 28 amps at 13.2 volts after requiring 48 amps to hold at 14.8v, they needed much more time at 14.8. Float mode should only require a small percentage of amp hour capacity to hold the float voltage. Premature efloatulation is the demise of many a battery, and is the fault of the charging source reverting to float voltage way too early.
The charging source which lets one dictate how long ABSV is held is a rare creature. All the algorithms by the converter/charger makers are compromises which basically say we know what is best for your batteries and do not question us, when each battery has slightly different requirements so a one size fits all charging source is more like a one size fits none.
How widely the charging source misses the mark is directly proportional to battery lifespan and performance during that lifespan.
When any givern converter cannot hold ABSV long enough the battery is not being fully charged. Finding a charging source which can take over when the converter ends puts one into the power supply area.
a 30 amp power supply with adjustable voltage should have no problems taking over when a converter decides to drop to float.
I have been contemplating getting one of these to replace my Schumacher 2/12/25 amp "smart" charger which misses the mark each and every time.
http://www.amazon.com/MegaWatt-S-35...Supply/dp/B00JZBE97U/ref=cm_cr_pr_product_top
They offer larger amperage models. The voltage is adjustable to the mid 15's so it should be able to do a EQ cycle on flooded batteries, but these need to be monitored as they are not automatic.
I reprogram my solar controller to do 16v every 2 weeks or so. I found mid 15's doubled and tripled the time required to max out the specific gravity, so now I don't screw around, just push it to 16v after a normal charge cycle and hold it at 16v until the SG stops rising or reaches my max expected baseline.
AGMs in general, should not really be fed voltages above 15, with a few exceptions, but again the most important thing is the recommendations of the battery manufacturer, and few if any commonly available charging sources can fully charge a battery in the minimum possible time required to do so, and getting as close to 100% as possible daily, when cycling nightly, as is the common pattern in Van dwelling, is most important aspect of recharging to yield the best battery life.
But when the efforts required to reach 100% as often as possible cost more than a replacement battery, the battery depleter needs to draw a line of what they are willing to do to get every possible cycle out of the battery. When setting up a system it is highly desirable to have charging source come as close as possible to battery manufacturer specs. Those who already have batteries or charging sources should find one which meets the other, as close as possible.
Often when a charging source such as a converter is not behaving as expected, it is the cabling between the charging source and battery which is too long and too thin and the voltage at the converter output terminals is very different than the voltage read at the battery terminals. Always check for the difference and do not trust voltage readings taken in the middle of a circuit, or at just one end. The most accurate readings are taken right at the battery terminals, especially at higher charge rates, or discharge rates. Since most charging supplies do not have separate voltage sense wires, they are not accounting for voltage drop in the circuit and just assume battery terminal voltage is the same at converter output voltage, when this is basically an impossibility with a hungry battery, a high amp charging source, and anything less than a short length of 4/0 cable from one to the other.
So the short list, always set the ABSV limit according to battery manufacturer specs, and if possible hold it until specific gravity no longer rises.
With AGM's hold ABSV until amps required to hold ABSV drop to 0.5 amps for a hundred amp hour battery, dropping to float voltages.
Any time float voltage is triggered before this 0.5 amp per 100 A/h threshold with AGMs is reached, tha battery is less than fully charged and float voltages might take another 2 or 3 days to indeed fully charge the unused battery.
Check out this video about how voltage and amps work in relation to each other on a fully charged battery: