Re: RV Inverter / Charger Opinions?
I agree that there are several Concorde / Lifeline statements that suggest their AGM batteries have virtually no charge current limitations:
Originally Posted by crewzer
The 100 A charger may well be overkill -- in both charge current and in cost -- for your proposed battery bank size.
AGM batteries like the (Concorde) Lifeline can handle very high charge current. However, charge current of ~30% of bank capacity may be the practical upper limit for AGM batteries.
Checking Lifeline's online FAQ, they claim "no input current limitations" - which seems to imply that a 100A charging current would be fine. I haven't found any documentation suggesting otherwise. Hmmmm....
But, there’s more to the story than just pushing Amps. The first notion to dispense with is that battery charging is a linear operation. Specifically, you can’t take 100 Ah out of a 220 Ah battery, hook up a 100 A charger, and recharge the battery in one hour (assuming 100% recharge efficiency).
Concorde / lifeline also says to “Charge 1 with voltage-regulated (constant potential) charger.” See: http://www.lifelinebatteries.com/manual2.php
This type of charge delivers full current in “bulk” charge mode until the battery voltage reaches the target absorption voltage, at which point the battery is ~80% to 90% fully recharged, depending on charge current. Ironically, while a high current charger will achieve the absorption target voltage rather quickly, the battery is less full (i.e., closer to 80% full than to 90% full) due to the high surface charge.
The charger then switches to absorb mode, which hold the battery voltage constant by slowly reducing charge current. The absorption stage for AGM batteries can last anywhere from 2-1/2 to 4 hours or more.
Here’s a somewhat simplified comparison of using a 100 A charger vs. a 40 A charger:
Starting at 220 Ah – 100 Ah = 120 Ah remaining, a 100 A charger would take about an hour to recharge the 220 Ah battery bank to 80% SOC (176 Ah). Assuming another 2-1/2 hours for the absorption stage, the 100 A charger would take ~3-1/2 hours to put 100 Ah back into the battery bank.
Starting at 220 Ah – 100 Ah = 120 Ah remaining, a 40 A charger would take about two hours to recharge the 220 Ah battery bank to 90% SOC (198 Ah). Assuming another 2-1/2 hours for the absorption stage, the 40 A charger would take ~4-1/2 hours to put 100 Ah back into the battery bank.
So, is the extra 60 A charge current -- and its attendant size and cost (don't forget BIG cables and a BIG DC breaker) -- worth it?
The 30% charge current “rule” for AGM batteries is used by several manufacturers. MK (East Penn) says to “Maintain Current <= 30 A per 100 Ah C20”. See: http://www.mkbattery.com/images/AGMBatteryCharging.pdf
Exide suggests something in the 30% range for their Marathon AGM batteries (i.e., 37 A recommended max for their ~105 Ah battery). See: http://industrialenergy.exide.com/ex...%202005-09.pdf
But, going the other way, Concorde, Lifeline’s corporate owner, recommends a “conditioning charge constant current charge rate” of 11 A for their Sun-Xtended 220 Ah AGM battery. 11 A is just 5% of capacity. See: http://www.sunxtender.com/xtenderservice.php
I hope this additional information is useful to you.
Jim / crewzer
Last edited by crewzer; May 29th, 2008 at 10:53 PDT.
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