AGM Mystery Capacity Crash Below 75% SOC UPDATE

[BFL13]

Update 28 Sep.

I had an issue with one of my AGMs last Spring where it seemed to act well until all of a sudden it crashed under load when below 75% approx. Now they are all doing it.

https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/29883938.cfm

I decided to wait till Fall to do anything about it all. Well, the other day while camping with the whole bank of 450AH worth (as rated--not actual amount now) everything went well at first--high amp draws on MW, kettle, and toaster, no problem-- until:

Morning voltage 12.7, indicating approx 75% with these batts according to spec sheet, and ran the kettle (90 amps) three times within an hour or so (coffee), no problem with voltage drop or anything, and then toaster 70 amps, and poof! Voltage drop crashed to big amount, inverter quit. Yipes. BUT-- voltage then bounced back to 12.7 and we still had "12v" for all the usual stuff in the RV no problem.

That day, got the batts nearly full via generator and solar and then all the high draw things worked again no problem. Until next morning, same thing again.

A few kettle runs (voltage drop 0.7v each time to 12.2--as low a drop as it ever was when all new, so perfect as the baseline) and then poof! Voltage drop crashed next time, and that was that. Except bounce right back to 12.7 (supposedly about 75% SOC with these batts--and AH count confirmed by -90AH from whatever the capacity really is of the 450 rated.)

And again after that we still had 12v ok at normal low RV voltages for all other things rest of the day--just no high draws allowed.

Ran some tests at home today, and all three AGMs are doing this. Act perfectly with low voltage drops until down to about 75% and then they can't handle high amp draws, but still can do low amp draws.

The change-over is not gradual, but is a drop off a cliff kind of thing for the high amp draws. Ok at low amp draws.

Back when I had old and tired Wet batts, at some point they got so low in capacity (SOC), they could not do the high amps at lower SOC (say 65% instead of down to 50% as before they got tired) without hitting the inverter low voltage. BUT this is way different. There is this sudden crash.

What does it all mean? What has happened inside these AGM batts that makes them do that? (I am not asking what to do, but just want to know what the heck is going on)

Thanks.

PS--I will not attempt the Lifeline recovery thing Mex mentioned in the linked earlier thread. Too scary! And these are not Lifelines that are stronger built than most other AGMs either. So I will toss them if I need to before trying that, even if it would work maybe.

[MEXICOWANDERER]

Perfectly golden toast, mango preservatives, and Finca Renaldo toasted coffee beans. Yep, a toaster is important 😛

Is this conversation about wet?
Regular AGM
Or telecom

Batteries?


nothing wrong with your memory BFL. Telecomm batteries charge slower than regular AGM batteries. They are at their best using the hundred hour rate.

[BFL13]

Mr Wiz, it seems there is a good reason to be careful like that with this kind of battery.

Actually I have no proof it is because they are AGMs. It is just that this never happened doing the same thing with Wets.

I suppose someone going from T-105s doing what I do, would be in for a nasty surprise if he went to the T-105AGM thinking it is the same thing. I don't know.

I don't mind using the stove top instead of the kettle, but I refuse to give up the electric toaster! 🙂

[pianotuna]

Hi BFL13,

Yes they care about gassing voltage, even though they are recombinate.

That said, so long as they don't vent voltage can be above the gassing number.

[MEXICOWANDERER]

There's an alternate way.
Add 300 watts of solar panels. Start gen in AM. Use for heavy loads and let panels charge from around 9:00 onward.

People who insist on 100% solar remind me of cruising sailboat skippers who won't move an inch without a breeze.

[MrWizard]

A side note
I try to keep my batteries above 70%
At 50% I am not going to be running big inverter loads like MW or kettle or toaster

I'll run the generator or do without
Just because the standard is to not go below 50%
Doesn't mean doesn't mean go ahead and do 50% every camping night and do back to back heavy loads every them every morning

Even with my less intense loading and higher SOC, I charge them to full everyday
Unless there is something that prevents me from doing so

[BFL13]

PT, do AGMs care about the gassing voltage? I have no idea. Was something about pressure for squeezing stuff into the mats instead? I have a good memory, but it is short!
---------
I am still confused, so please clarify if the idea is to charge the 100AH battery at constant 5 amps until battery voltage reaches 16.2, then stop?

So Vabs would be set to 16.2 and you get constant 5 amps up to some high SOC, but not to 100% full yet--since there has been no absorption stage with tapering amps.

I have one charger that goes to 15.8 with a 4.1 amp setting, but none that does 5amps. I can put two 100s in parallel and set the charger to 10 amps. But the plan was to do one battery at a time.

EDIT--but the two 100s were at 90% of original Capacity on their last test for that in April. So can I say they are now 90AH so 5% is 4.5 so that 4.1 is "close enough"?

At 15.8 I would have to swap to my 45 amper to get the rest of the way to 16.2. No idea what amps that would be, but might be 10 or less at that stage.

Would that do any good? What if I start with them already full like they are now and just jack up the voltage to 16.2? Like you do when seeing if you "hit the wall" ?

Or do I have to discharge them some way down so the 5 amp thing will run for a while before they get to 16.2?

[pianotuna]

Just a note,

I had occasion to visit my RV today (for an oil change). The 556 amp-hours of battery bank were charging at 14.2 volts and 0.9 amps. Ambient temperature was 8 C (46 f). That's approximately 0.6 volts below the gassing voltage.

[MEXICOWANDERER]

Five percent amps as calculated against tOtAl amp hours of the battery being treated 100 = 5

220 is 11 constant amperage. Higher than specified amperage royally screws up the Peukert factor.

Factors change the behavior of a battery. I've had to let vigorously bubble for as long as 15 hours to maximize the specific gravity.

[BFL13]

I do have current limiting by having chargers with that, so they do constant amps until the battery acceptance rate gets down to that limit when amps then taper with rising battery voltage.

Just how long it stays in constant amps is simply a matter of the charging rate--meaning the percentage of the charger amps to the AH capacity of the battery bank. A higher rate will get the battery up to the charger's voltage sooner, and at a lower SOC. Using a lower rate will make the bulk stage go on for longer and absorption will start at a higher SOC

At the 20% rate for AGMs, bulk will end at about 75% SOC and amps will then taper.

If Mex says to hit the 100AH battery with 15 amps, that is a 15% charging rate. The 15 amps will be constant till about 80% SOC and then taper. (unless you start raising the voltage) is that what he really meant? Do that until you have voltage up to 16.2 and then quit?

My lowest amp charger with adjustable voltage is a 45 amper. That would be 15 amps each for a bank of 300AH worth. I could use the 45 on the 8D 250 AH and be over a bit. Using it on both 100s is way over 15 each. Mex says do them one at a time at 15 amps each. (He forgot that one is a 250 and the two others are 100s I think)

So now we get to the current limiting at below charger current limit using the light bulb? Get some lower constant amps but still have the adjustable voltage?

What is the point of doing that for 15% that tapers at 80% SOC? Battery acceptance above 15v with the battery nearly full is under 4 amps and tapering. By 16.2 it will be 1 amp maybe?

Beats me. Anyway, I would rather just go try it out and see whether it will work for now the way it is.

[Sam_Spade]

BFL13 wrote:
I don't understand this. I don't want current limiting, I want to maintain the current higher at some constant DC amps instead of it tapering as battery voltage rises.

Hard to do (NOT a feature of any power supply intended to be a battery charger) because the battery behaves like a variable resistance.

When the battery is low, the resistance is also low and the charger supplies a high current. As the charge goes up so does the resistance and less current flows.....normally.
To maintain a constant high current, the voltage has to also go UP.

BUT that is NOT a good thing to do.
A working voltage higher than 15 volts can be bad for some devices elsewhere in the system.
Forced high currents tend to "boil" (electrolyze) the water faster.
Then what parameter are you going to design the system to "watch" so it knows when the charge is done ??

A really LOT of the theory that is often discussed in battery threads has no practical use for most RV owners. Some can actually be dangerous.

[BFL13]

I don't understand this. I don't want current limiting, I want to maintain the current higher at some constant DC amps instead of it tapering as battery voltage rises.

Right now, each battery is at full where it accepts about that 0.5a/100AH at 14.7v Floating now at 13.6v. If I put my charger on set to 16.2, amps will jump to not very many, and then battery voltage will hit 16.2 almost right away.

Amps will taper if I leave it running at 16.2. But Mex said to quit when you get to 16.2.

I cannot see howinheck I can shove 15 amps in when that is way above the acceptance rate at those voltages.

BTW I just finished adding the two (full) Wet 27DCs to the (full) AGM bank so that got done. I cranked the charger to 14.7 and amps went to around 3 amps and then quickly fell to 0.3 amps for the whole shebang. So I cranked it down to 13.6 again.

Now I need to try it out as though camping (or go camping).

[MEXICOWANDERER]

• Current limiting is essential and voltage control is too. I would not wander too far above 16.0 volts as heating rises exponentially
• NAPA AUTO PARTS carries 25, 50, 75. and 100 watt TWELVE VOLT SCREW IN LIGHT BULBS
• Here's another eBay way of doing it.

https://i.ebayimg.com/images/g/twkAAOSwxXJcwqyS/s-l1600.jpg

I choose the 100 watt size to limit the heat expended and the resistor is physically larger and tougher.

2.0 ohms = 6.0 amps @ 12volts
0.5 ohms = 12.0 amps @ 6 volts.Why so many amps? Golf car batteries are higher capacity at half the voltage.

If you have an unregulated battery charger put ANOTHER resistor in SERIES with the first one. An assortment of primary resistors noted.

The formula of roughly 5% or 10% current can be daunting to newbies. Simply inquire by asking "I am equalizing a pair of (BCI group size) batteries with THIS charger, so what power resistor do I use? It's an all or nothing deal when choosing a resistor. So all of the info is needed. Be prepared. There can be no guesswork at deriving at the correct resistor for the job.

These resistors used-to-be expensive. Now they are dirt cheap, small, and tough).

I much recommend equalizing batteries one-at-a-time

[pianotuna]

Yes.

BFL13 wrote:
Is there any point in doing it to 16.2v with tapering amps?

Current limiting using a light bulb.

[BFL13]

MEXICOWANDERER wrote:
Socket-too em, BFL!

Charge Steady 15 amps EACH until voltage rises to 16.2 volts then quit. Way way way (Enough 'way's'?) below venting pressure to forget about it...

I can bring each one to 16.2v with my PowerMax (will do up to 16.5 constant voltage), but amps will taper of course.

I just can't figure out how to implement your light bulb trick with the equipment I have in order to achieve constant amps. You have explained it several times, so regard me as hopeless.

Is there any point in doing it to 16.2v with tapering amps?

[MEXICOWANDERER]

Socket-too em, BFL!

Charge Steady 15 amps EACH until voltage rises to 16.2 volts then quit. Way way way (Enough 'way's'?) below venting pressure to forget about it...