Reduced capacity in AGM batteries...cause?

Ralph Day · August 2010

Hi all
I was visiting a friend's off grid cottage and he remarked that his 6 year old AGM's were only operating at 70% or less of capacity. The MX60 charge/absorb settings were all fine for the Absolyte AGM's installed, but now he never gets any float time let alone any absorb time.

Here's my thinking. The RTS from the MX60 is on the outermost cells of the battery bank. The bank is in an unheated basement which is subject to freezing temperatures for at least 6 months of the year (when there's few visits to monitor things). Could the RTS be causing an overcharge/overvoltage condition on some of the cells in the middle of the bank if it's sensing ambient air temp and exterior battery cell temp? Would this drive the total bulk charge voltage to dangerous levels for AGM batteries?

If/when the bank is replaced should the RTS be installed inside the core of the bank if possible? Would this leave the outer cells in a potentially undercharged condition?

Or on the other hand, am I barking up the wrong tree? I live with Surrette FLA batteries so AGM's are a whole 'nother ball game.

Thanks

Ralph

dmiller · August 2010

Re: Reduced capacity in AGM batteries...cause?

70% after six years is pretty good, isn't it? If that's not enough capacity then that bank is done.

I'm not sure why someone would run AGM at home unless there was a reason to be concerned about freezing. The RTS can be insulated to better reflect internal temps. But outback isn't concerned about that in their installation instructions so I doubt it's a big issue.

Good AGMs can take a faster charge than FLA so it seems unlikely his setup is a problem. Sounds like a well designed system. He might see if Absolyte recommends equalizing. Obviously his solar CC can't do that, but maybe he has a backup system that can. A good question to ask Outback is how the charger should be setup if the batteries never reach absorption.

n3qik · August 2010

Re: Reduced capacity in AGM batteries...cause?

One should never equalizing AGM batteries. At the higher equalize voltage, you run the risk of venting the batteries. Because they are sealed, once vented, they are ruined.

At 6 years and at 70%, he and the system are doing very well.

I would add another panel or two to prolong the time till the battery bank needs replacement.

dmiller · August 2010

Re: Reduced capacity in AGM batteries...cause?

If the owner won't/doesn't want to do occasional battery maintenance then I guess AGM is a good choice. I'm always surprised in RV at the high percentage of FLA batteries. Many people try to save a few bucks buying FLA but damage the batteries due to poor maintenance. The experts who advise the purchase of FLA are more attuned, more careful and often don't take into account how their client may be different.
Consider also that AGM's have much better shock and freeze characteristics than FLA. Not that these characteristics matter in this instance.
Buying a new battery bank is the time to reevaluate system size, but it's likely he's pretty well dialed in.

niel · August 2010

Re: Reduced capacity in AGM batteries...cause?

for some agms 70% would mean time to replace them.
as to the temp sensor this is an interesting thing with a larger bank as the center batteries would be warmer than the outer batteries. i suppose it would be better to place it inside the bank rather than on the end, but not in the middle either where it would be the hottest and then it won't reflect the outermost batteries' temperature. i think you'd want to try for an average temp of them.
now what we don't know is how deep and how often he may have cycled his batteries. i suspect they are nearing the end of their useful life and new ones should be gotten. it is an opportunity to expand not only the battery bank, but also the pvs to charge them. i assume he has been giving the batteries a proper charge as that failure would strip away some of the lifespan too.
as to the possibility of outgassing due to differences between the battery temperatures, i would have say it may have been possible under the right circumstances. take all of the batteries from being interconnected with each other and check the at rest voltage about 3 or 4 hours later on each one of them. there may be a bad cell in one or 2 of the batteries dragging down the rest of them.

Ralph Day · August 2010

Re: Reduced capacity in AGM batteries...cause?

Thanks for the responses. I am guessing that 70% is the capacity now, just not sure.

Ken, could some outgassing have occurred when bulk-ing with temp sensor in a cold location...on outside of bank? That was my original thought. I know my MX60 will drive up the bulk/absorb setpoint by 2 volts when the basement temp is in the 60'sF.

It's always easier to spend someone else's money

.

Ralph

Solar Guppy · August 2010

Re: Reduced capacity in AGM batteries...cause?

Your trying to fixing something that isn't broken ... 70% at 6 years is spectacular performance, anything you touch or change couldn't possibly be an improvement so if your looking for something to fix, find something else to play with :roll:

boB · August 2010

Re: Reduced capacity in AGM batteries...cause?

6 years IS a pretty good like though, especially if they have had lots of Amp-hours in and out of the bank.

Take a voltmeter and measure individual batteries in the string, if they are indeed in a string. OR, while charging, take your hand and feel if any of the batteries are hotter than others. Sometimes it can be a simple im-balance problem.

And, some AGM batteries actually have recommendations for equalizing. Don't know about these Surrettes though.

boB

Dapdan · August 2010

Re: Reduced capacity in AGM batteries...cause?

Ralph,

Indeed you can equalise the Absolyte IIP by GNB. I have been doing this for the past year and half with success in every bank. This is my initial thread when I was first starting out.

http://forum.solar-electric.com/showthread.php?t=4624&highlight=low+rest+voltage+gnb

I am now in the process of recovering 5# 1000Ah @ 24V bank (Absolyte IIP 100A-19).

Cheers...
Damani

Dave Angelini · September 2010

Re: Reduced capacity in AGM batteries...cause?

Ralph Day wrote: »

Hi all
I was visiting a friend's off grid cottage and he remarked that his 6 year old AGM's were only operating at 70% or less of capacity. The MX60 charge/absorb settings were all fine for the Absolyte AGM's installed, but now he never gets any float time let alone any absorb time.

Here's my thinking. The RTS from the MX60 is on the outermost cells of the battery bank. The bank is in an unheated basement which is subject to freezing temperatures for at least 6 months of the year (when there's few visits to monitor things). Could the RTS be causing an overcharge/overvoltage condition on some of the cells in the middle of the bank if it's sensing ambient air temp and exterior battery cell temp? Would this drive the total bulk charge voltage to dangerous levels for AGM batteries?

If/when the bank is replaced should the RTS be installed inside the core of the bank if possible? Would this leave the outer cells in a potentially undercharged condition?

Or on the other hand, am I barking up the wrong tree? I live with Surrette FLA batteries so AGM's are a whole 'nother ball game.

Thanks

Ralph

There is something wrong here! When a battery system loses capacity it does not cause a system to not float and go thru absorption!!! In fact, it is the opposite! A no brainer sign that a system has lost capacity is that it goes thru these stages faster! The system is either being asked to do more now, or there is a flaw not related to capacity in my opinion!

Ralph one of the nice things about your other battery the "mean one" is you will never have this kind of question. In space systems these and other sealed batteries have an independent charge controller or crowbar that stops a system from overvoltaging a sealed battery. The Outback CC's are notorious here BTW. Until they do this on earth, I stay away! It is not worth it to give up an SG measurement!

Ralph Day · September 2010

Re: Reduced capacity in AGM batteries...cause?

Dave,
I've noticed when the mx60 on my system searches for a new MPP it will sometimes ratchet up past the absorb/bulk setpoint. Would a quick overvoltage cause an outgassing in an AGM?

I'm also one who appreciates SG measurements. Once per month, every cell SG and water top up. Keeps you on top of how the bank is behaving.

Ralph

Dave Angelini · September 2010

Re: Reduced capacity in AGM batteries...cause?

It should not if it is quick, but like I said, in space some of the drawings I have seen from satelites where my day jobs product goes have a redundant a crowbar circuit or charge controller. This limits voltage on sealed batteries. I would want that also. Last thing, the xwcc and the morningstarsdo not have the Outback problem that I have seen with overvoltage.

Photowhit · September 2010

Re: Reduced capacity in AGM batteries...cause?

A basement that is subject to freezing 6 months out of the year!!! That's pretty far north, or a walk out with no insulation... maybe no door and poor insulated cabin, but still bet you can't get there in the lower 48 states (yes I know this is the WWW) Maybe not shy of artic circle unless the above.

6 years sounds pretty good to me, though I agree the not reaching float, might indicate bad cells, if multiple strings it might be worth checking individual batteries to see if you have a single bad battery bringing the rest down. Likely have to completely disconnect and wait a few hours. If a single string, don't bother as you'd likely not want to replace a single battery.

BB. · September 2010

Re: Reduced capacity in AGM batteries...cause?

Ralph,

What is the Battery Bank Voltage?

From the Outback user manual (PDF)

12V system -30mV/degree C
24V system -60mV/degree C
36V system -90mV/degree C
48V system -120mV/degree C
60V system -150mV/degree C

So, for a 2 volt increase on cold batteries for a 48 volt bank, that would indicate a RBTS temperature of:

2 volts / -0.120 v/C = -16.7 C change
25C - 16.7C = 8.3C
8.3C * 1.8 F/C + 32F = 46.9F Battery Temperature

And the Absorb Voltage for AGM from Outback:

[FONT=Fixedsys]
SEALED LEAD ACID or AGM / GEL  12V    24V    48V
ABSORB voltage set point      14.4V  28.8V  57.6V[/FONT]

I guess I am with the others... Not reaching Float would seem to indicate that it is staying in Absorb "all day"? Would seem to indicate that the load >> charge supplied by the solar panels

What is the resting voltage of the bank?

Otherwise, if absorb happens very quickly and the controller sits in Float most of the day--then that would seem to indicate that the charge controller is supplying more than enough power or battery capacity is falling.

Have you checked the programming on the Outback? Voltage points, timers, current levels, RBTS compensation value?

If the batteries are nearly fully charged and you still have good amounts of current being supplied--the Battery Caps should start getting warm (recombining of H2+O2 into water).

If the caps are hot--then maybe over charging. If some caps are hot and other cold--perhaps the catalyst has failed (corroded). If the caps are not hot, then either the battery is not yet fully charged or the catalyst has failed and the batteries may start venting.

-Bill

Ralph Day · September 2010

Re: Reduced capacity in AGM batteries...cause?

Thanks for the replies. The bank is 24 volt. Not sure about all the numbers, but the mx seemed to be at defaults for AGM batteries. I'll send the thread to the owner for his perusal.

Ralph

n3qik · September 2010

Re: Reduced capacity in AGM batteries...cause?

May not applie to AGM. Most of the Gel cells from the UPS at work go to low voltage/short cell(s).

Just today, overcharged a Gel 12V 17AH battery. Got very hot and now has a resting voltage of 4.5VDC. OOOPPPPSSSS!!!!

Like others, bet a shorted cell/battery.

Ralph Day · September 2010

Re: Reduced capacity in AGM batteries...cause?

The latest information from the owner of the battery bank:

Hi Ralph:

The insulated basement is cool all year. In the summer it's about 18 degrees C and in the winter about -5 degrees. The batteries never feel hot. I think the capacity is more like 50% rather than 70%. The rest voltage is 25.2 V. I don't think it is going into Absorb mode. During the day it reaches 28 volts when charging and the cells are all equal voltage. There are two banks running in parallel for a total of 25000 watt hours capacity at 24 volts at the outset. The heat conduction within the package is pretty good. I doubt that the temperature sensor is off more more that a couple of degrees from the core temperature of the pile.

Thanks,

Ralph

mike95490 · September 2010

Re: Reduced capacity in AGM batteries...cause?

the core temperature of the pile

Get your dosimeters and lead briks !!

Dave Angelini · September 2010

Re: Reduced capacity in AGM batteries...cause?

There still is not any info on what the load is and if it has increased over the years. Another missing piece of info and mistake made by folks is the capacity rating? i.e. is this number he gave a 20 hour rate? Lastly, if he is going down to 24V every day, then the internal resistance of the cells is suspect. Still too many variables to go into failure analysis!

Dapdan · September 2010

Re: Reduced capacity in AGM batteries...cause?

Bill,

Allow me to correct something you typed:

If the batteries are nearly fully charged and you still have good amounts of current being supplied--the Battery Caps should start getting warm (recombining of H2+O2 into water).

If the caps are hot--then maybe over charging. If some caps are hot and other cold--perhaps the catalyst has failed (corroded). If the caps are not hot, then either the battery is not yet fully charged or the catalyst has failed and the batteries may start venting.

Normally the Absolyte AGM do not come with an installed catalyst cap. The cap is usually value that regulate the internal pressure and keeps it a 5psi. The catalyst caps are usually bought "after market" and are installed after removing the factory cap. These caps are usually blue and manufactured by Philadelphia scientific.

Therefore in his case (assuming no catalyst caps installed) the valve or factory caps will not get hot or warm. However if they are really gassing one may here a hissing sound as the gas escapes.

Cheers...
Damani

BB. · September 2010

Re: Reduced capacity in AGM batteries...cause?

Damani,

I did not know that... Always learning something new.

Thank you,
-Bill

niel · September 2010

Re: Reduced capacity in AGM batteries...cause?

hey mike add some boronated water to that too.:roll::p

i have to ask if ralph is confusing the actual ah capacity left in the batteries or dod figures as these are 2 different things? i think he may not be the only one confusing this as concorde states for their sunxtenders that below 80% the battery needs replaced. capacity means that a battery that comes from the factory has an amp hour rating and if say a 100ah battery is now at 70% capacity that means that it can only deliver 70ah of its original 100ah.

Reduced capacity in AGM batteries...cause?

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