Charging Rates for AGM Batteries

[MN_Traveler]

As a follow-on to my earlier question re. uneven charge/discharge character on my house batteries, and concern that it is being caused by corrosion-induced degradation of the battery connections - I have been looking over past discussions on AGM batteries. A lot of good discussion, lots of pros-and cons. In the interest of NOT initiating another long discussion on "which is better", I am asking for feedback on the following question (that is still unanswered for me), and ask that the discussion be limited to only that question .... :-)

QUESTION: some "informational" data on the web indicates that AGM's are NOT able to take as fast a charge-rate as flooded cells. Other information indicates that AGM's actually CAN take a faster charge-rate than flooded cells. My manual for my Magnum Energy charger/inverter indicates that the Bulk charge setting for AGM's is 14.5 VDC, versus 14.6 VDC for flooded cells (which implies only a slightly slower charge rate for AGM's. What is the experience of the AGM users out there: do AGM's take a much longer time to charge than wet cells (from an equavalent state of discharge), or is the the difference small in practice?????

Thank you, as always, for sharing your experience !!!

[GaryKD]

Hi MN_Traveler,
As best I can tell think of this as the other way around. A charging system will present X amount of charge capability to a battery. The amount of charge received depends on the battery. A battery needing quite a bit of charge will accept more than a battery almost at 100% of full charge. The important thing to remember is have a charger capable of presenting more than enough charge capability to the battery(s).

AGM fully charged 12 VDC battery is 12.8 VDC. A fully charge flooded deep cycle battery is 12.6 VDC. This might not seem like much difference. However, depending on the charger, one would not want the charger to "turn off" at 12.6 VDC if one has AGM batteries.

For me the most important thing to remember is for the charging source to stay charging until 12.8 VDC is reached (I have AGM batteries). Next is to have a charger that provides more than enough charging voltage for the battery to accept whatever amount of charge it needs.

[beenthere]

remember also that volts going in is not always how many amps the charger is pushing which would be like you describe about being able to charge faster...

chargers have limits on haw many amps they push for charging and the volts do not always coincide with that...say your battery is mostly dead and the volts going in will coincide with that but the amps the charger is pushing for bulk will be the max it can deliver..until it goes to absorbstion then amps go down edct...jeff

[MN_Traveler]

Ummm - I probably simplified things too far for my question. When I asked about Rate ... I meant RATE (not total time). All else being equal (depth of battery discharge, same internal resistance for the battery bank (which may not be accurate for wet cell versus AGM), and the same charger delivering my 14.5 versus 14.6 VDC bulk charge (my charger does FLOAT at 13.5 VDC for AGM and 13.4 VDC for flooded). sorry: I guess to get at this question I need to be either really specific, or very, very general (I am an engineer, so we can get specific if you folks want to). I suspect though that for now a really simplified approach would be more productive.

So lets try reducing the question to the following: for those of you who are using AGM's, do you notice a significant difference in time to recharge (or at lease finish the Bulk step) for AGM's versus wet cells, assuming the banks having started from a similar depth of discharge?

[beenthere]

like i said it comes down to amps in not voltage...until the end for float

[macnut]

One of the major advantages to an AGM battery is that it can accept a faster charge rate than fooled cell batteries. Gel batteries are sometimes confused with AGM, and that may be why you are getting differing information. You should get consistent info from the battery makers....Lifetime, Trogan, etc.

[TQ60]

Amps is the flow of current to replace the charge in the Battery.

Voltage determine the amount of amps during charge.

The VRLA or AGM batteries FLOAT at 2.25 volts per cell, or 13.5 VDC.

The float current should never excede 0.1% of capacity, so a 100 AHR Battery should never be charged higher than 0.1 Amp after it is fully charged.

They have a lower internal resistance so they can be charged faster..
BUT AT A PRICE... they have a chamber to retain the gasses and allow them to recombine and stay inside the Battery.

If charge rate is high enough to cause excess gassing the relief valve will "gass off" or release the excess gass, and there is no way to replace it, so capacity is reduced.

Seek out the manufacturer data sheet for the specific Battery.

It will specify the charge currents, voltage for charge as well as the actual capacity based on the loading from the installation

The rating of the Battery is usually at the 8, 10, or 20 hour rate, it puts out a certian amout of amps for that time to get total.

Say it is listed as "100 AHr @10 HR.

That means it will do 10 amps for 10 hours.

Place a 50 amp load and it will NOT run 2 hours.

Plus pay attention to end voltage, 1.75, 1.80 or 1.85 VPC, sets the end of cycle.

Plenty more information if you search for it.

[GaryKD]

Hi MN_Traveler,
Being a retired engineer, I'll answer your revised question as simply as possible. I notice no difference in the time to charge between flooded cell or AGM batteries.

[dons2346]

"So lets try reducing the question to the following: for those of you who are using AGM's, do you notice a significant difference in time to recharge (or at lease finish the Bulk step) for AGM's versus wet cells, assuming the banks having started from a similar depth of discharge? "


I see a significant reduction in charge time. I have never timed it down to the minute but it appears that charge time is reduced about 30%

[macnut]

Here's a link to the technical manual for lifeline batteries. Pages 10-12 provide a table comparing AGM to other battery types.

http://www.lifelinebatteries.com/manual.pdf

Hope this helps!
Al