Trojan AGM issue

[Jemplayer]

Hi All,

6 months ago we put in a set of the new Trojan SAGM 06 375 AGM's. X2 banks of 8 units paralleled to copper bus bars w/ equal take off cable lengths etc.

In the first 3 month, batteries reached full charge (i.e. C/200 tail current) in around 4-4.5 hours of absorb.

Daily DOD is around 15- 20% per night.

The last few months, the battery bank is taking much much longer to reach the full charge tail current of C/200, and even after 6.5 hours in absorb, I can only reach about a C/100 tail current. There is no longer anyway I can get to the C/200 tail current in the 8 or so hours of recharge time in the day.

The system is paneled to have the batteries bulk charged by 10:30-11am or so, and we have sun till 5 pm. So despite being paneled correctly for the DOD and having 5+ hours of absorb time daily, I cannot fully charge the bank any longer.

Discharge voltages are also lower and voltage in the morning just before the sun come up, was about 50.2v, but is now about 49,4, so much lower after same DOD, indicating they are not being fully charged.

I am quite experience with batteries and been off grid for 7 years. Bulk and absorb voltages are per Trojan spec sheet and temp compensated etc. I check the bank voltages and each battery voltage during charging as well as under discharge and at high amp loads to see if there is a failing unit/s. I cannot find a problem battery, they are all equal voltage pretty much under load both big and small.

any ideas?

[PNjunction]

The bad news about AGM: (which I use) - it's the "last 1-percent issue".

For the sake of conversation, let's switch to a stable charge source - solar will have to be modified, but know this first...

In the lab, C/200 for a tail-current when in the CV/absorb stage is the textbook value. But in the real world, as the battery ages and the internal resistance starts to rise, you may never reach that value, and over-charge. Basically, you should stop CV when there is no change in the tail current for 1 hour - regardless of the value. Nothing is happening but recombination and no charging that last 1 percent left.

Trying to force an agm to reach the idyllic C/200 current value in CV, rather than watching for a "stall" in current for no more than an hour, is how cases get broken. As the system ages, this stall current will get a bit higher and higher so the good engineer HAS to become involved checking every few months.

But we aren't done yet! No agm is truly fully charged unless it has at least 8 to 16 hours of float *following* the finish of the CV / absorb stage. For a single 12v agm, that would be floating at about 13.6v for 8-16 hours. If your agm allows for higher float, say 13.8v, then you can shorten that time period.

In the real world, even with cyclic use with AC chargers, one may not even have that amount of time during shift-changes and so forth, unless it is an overnight deal.

What I'm about to say is a bummer - if you can afford the downtime, once in awhile, set your system to allow the AGM's to just float all day, ensuring that they have reached that "stall" current in CV mode first. I know, it can make one crazy to do that.

The reality is that unless you do this, you just walk down the capacity of an agm, little by little as that last 1 percent hard sulfates over time. This is why agm's need a bit more of a specialized ops plan than a flooded you can just EQ and add a bit of water from time to time.

Here's the crazy thing - one would never think of using a genny to float agm batts for 8-16 hours once in awhile - too costly. So years back I thought about agm's being totally off grid ... and came up with having a smaller agm (or other batt) sitting around nicely charged to do just this float operation on the main bank. Getting pretty whacky now right? But I suppose it could be done.

[Mike90250]

I'll quote myself from a few minutes ago:

The other way to complete a full charge cycle, is early morning, 5am, start your genset and get thru BULK with the generator, and then let the solar handle absorb and float all day. In BULK, you are using your genset most efficiently .