Early Battery Failure

papab · October 2021

I've had a Renogy 200 Ah AGM battery for a little over 2 years, but it has been used very little. It have kept it charged up, always charged it up pretty quickly after use and kept it on a float charge at 13.3 - 13.4 V. I haven't really used it much until recently and I've found that it doesn't have any capacity. I have a Trimetric battery monitor. I just did a discharge test and with a 16 amp load it only had 27 Ah capacity down to 11.0 V (when the inverter trips off). After the discharge test, VOC after 2 hours was 12.7V ?.

I charge the battery using a 3 stage charge controller, a Victron solar charge controller or a Xantrex charger (when I have grid power).

Initially I didn't have it on the batt mon, I was using another battery as my primary battery, and had this one disconnected. When I did use it I monitored the finish amps pretty carefully using the victron app and had it switch to float at around 2 or 3 amps. Most trips in the van were a few days or a week at most before I was on grid power and had it on float.

On my most recent trip, it was at around 85-90% SOC by morning. I tried to charge an ebike that only needed about 100 Wh and it couldn't do it. When charging it takes a long time (5 hrs), to get to 106% returned percent of charge and around 0.5% end amps.

I'm really picky about taking care of this so I'm stumped as to what could have gone wrong. The only mistake I can think of was one time I left it charging overnight at 14.2 V. Another time it was in a Phoenix garage for 5 or 6 days with temperatures peaking at around 105 or 110F.

Any ideas as to why this has failed?

350 W rooftop Solar, parallel

Victron MPPT 100/30, temperature compensated

Xantrex Truecharge2 60, temperature compensated

Renogy 12V/200Ah AGM

papab · October 2021

Bump. I'd really like to understand what caused this to fail. Any ideas?

Photowhit · October 2021

I take it the amps are being measured at 12 volts and not a 16 amps load at 110 volts AC?
Did you exceed the discharge capacity?
Most batteries will have a recommended discharge rate. Most batteries rate rated at a discharge of 1/20th of their capacity, 17 amps would be close to 1/12th.

With the batteries returning to 12.7 volts at rest, I suspect the inverter shutting down was due to voltage drop due to too small of wiring, rather than complete discharge of the battery.

Marc Kurth · October 2021

All AGM batteries are not the same. Sadly, I have a lot of customers who have experienced very short lifespans with Renogy AGM batteries. Renogy optimistically publishes only 600 cycles at 50% DOD under ideal conditions. That should give you a clue about their design.

If I recall correctly, Renogy specifies 13.6v to 13.8v for float - but check me on that.

papab · October 2021

Photowhit said:

I take it the amps are being measured at 12 volts and not a 16 amps load at 110 volts AC?
Did you exceed the discharge capacity?
Most batteries will have a recommended discharge rate. Most batteries rate rated at a discharge of 1/20th of their capacity, 17 amps would be close to 1/12th.

With the batteries returning to 12.7 volts at rest, I suspect the inverter shutting down was due to voltage drop due to too small of wiring, rather than complete discharge of the battery.

Yes, 16 amps at 12 Volts. It wasn't actually a constant load. I was charging up an ebike battery and it starts at about 175W and tapers down. I'm going to run another test with a constant, smaller load. I don't expect much better though. I can't think of anything I have that would be a 10 amp, 12 V load, so it will have to be something through the inverter again.

No, it isn't small wiring, I have a short run of 1/0 cable. The battery monitor with a voltage sense wire right on the battery was showing 11.0 volts also.

papab · October 2021

Marc Kurth said:

All AGM batteries are not the same. Sadly, I have a lot of customers who have experienced very short lifespans with Renogy AGM batteries. Renogy optimistically publishes only 600 cycles at 50% DOD under ideal conditions. That should give you a clue about their design.
If I recall correctly, Renogy specifies 13.6v to 13.8v for float - but check me on that.

I see that they do spec 13.6-13.8 for float. That's unusually high isn't it? When I bought the battery 2 years ago they didn't have a manual or specs so I went with the normal specs for an AGM. At least they have a good datasheet now, even if the batteries are lousy.

On their troubleshooting guide they suggest 14.2 for 2 - 4 hours for equalization. I'm trying that today.

RCinFLA · October 2021

AGM's have limited electrolyte so long float charging can cause water loss. It is more common to have this failure in AGM and Gel cells.

AGM's don't suffer from positive plate grid corrosion but can build up lead oxide layer thickness on positive plates that increases cell resistance. About a 10-20% discharge should burn off excessive oxide build up.

As with other lead-acid batteries they can get sulfated. Negative plate needs higher voltage to fully charge, 13.8v or greater, but don't leave high for float due to electrolyte loss.

If failure is caused by electrolyte loss there is nothing to fix it. If sulfated you can discharge some then try short periods of high current recharge. Repeat several times if you see improvement in capacity.

BB. · October 2021

When the AGM battery was disconnected--How many months did it go without charging... More or less, 6 months between chargers is typical specifications (before sulfation sets in)--And at what temperature--Very roughly if stored at ~95F (for example), the time between charging falls to 3 months maximum (at higher temperatures, self discharge rate increases).

-Bill

papab · October 2021

Is low capacity and normal open cell voltage consistent with sulfation?

papab · October 2021

BB. said:

When the AGM battery was disconnected--How many months did it go without charging... More or less, 6 months between chargers is typical specifications (before sulfation sets in)--And at what temperature--Very roughly if stored at ~95F (for example), the time between charging falls to 3 months maximum (at higher temperatures, self discharge rate increases).

-Bill

I don't think I let it go more than a few days without putting it on a float charge. I had a switch so I could connect/disconnect easily. Temperature between high 40's and 75 in my Colorado garage. Less than a week in an AZ garage.

BB. · October 2021

Yes, sulfation causes low resting voltage and higher cell resistance and low capacity (sulfur from electrolyte "locked up" into hard sulfate crystals, lower electrolyte SG means lower voltage and capacity, hard sulfate crystals "insulate" some surface are on plates...

Cycling can improve capacity. Controlled EQ can help AGM (holding 14.2-14.4 volts for hours/days--Too high of voltage can cause battery to overheat and vent--Venting is never good).

What was your typical worst case current draw? AGM can supply more peak/surge current than FLA batteries (even up to C/1 or faster discharge--But keeping your average peak current to C/5 or C/8 is probably going to be healthier for for your batteries).

Short term of 'running hot' probably did not "kill" the batteries--Typically for every 10C/18F over room temperature (25C/77F), the batteries "age" 2x faster... I.e., at 95F a "5 year" aging life battery will die in 2.5 years (assuming it was held at 95F for those 2.5 hours) (the 10C increase in temperature for 1/2 life or 10C decrease in temperature for a 2x longer life--A standard engineering rule of thumb for many temperature related effects).

-Bill

papab · October 2021

RCinFLA said:
AGM's don't suffer from positive plate grid corrosion but can build up lead oxide layer thickness on positive plates that increases cell resistance. About a 10-20% discharge should burn off excessive oxide build up.

Wouldn't the discharge test I did (& am repeating) satisfy the 10-20% discharge? Or should it be a higher rate?

papab · October 2021

@""BB."

I think the highest current draw might have been C/7 (2 ebike charging at once), but if any, that would have been only once or twice.

Typically only fridge at ~2.2 amps, 50% duty cycle and LED lights. Espar heater pulls about 8 amps when it starts up, but then settles down to a couple. A coffee creamer heater pulls a lot, but it's only a couple of minutes.

Since the OCV is good it doesn't sound like sulfation. I forgot to write it down but I think it was 12.8 or 12.9 after full charge & over night rest. I did a 14.3V conditioning charge this morning for 3 hrs, on top of a full charge, now running a 9 amp discharge.

Should I try a couple full discharge/charge cycles? What discharge rate?

Or should I try 14.3V overnight? It's 15C on the battery right now.

Vibration damaged a cell? It's sitting on the Van floor.

Marc Kurth · October 2021

I'm big on following manufacturers' instructions because they tend to know their products.

In my world of Concorde batteries, factory-recommended Conditioning is hitting the battery with 15.5v (at 77F) for up to 8 hours while watching for venting and temperature rise. Using temperature compensated charging/conditioning is imperative, as is avoiding excess venting. We have gone as far as 18v at very low current for long periods to bring life back into heavily sulfated cells. Yes, this is right out of their Tech Manual.

I am also heavily involved with Fullriver, and MY (not theirs) approach on recovering capacity on the L16 DC Series is slightly more gentle and much a shorter duration. No, I am not an expert in the field. Yes, I have a long list of success stories to go with my methods.

Marc Kurth · October 2021

One more comment: In my experience, as Bill said: Sulfated batteries will normally exhibit a low resting voltage. I will add that a healthy-looking resting voltage on a battery that falls flat on its face under load, is what we see on a battery that has been cycled to death or is dried out. I know that you said that isn't your case - just explaining what I see a lot of.

papab · October 2021

Marc, yep that makes sense from the symptoms. It make me wonder if I'll find a hole in it when I pull it out and look closely at it.

RCinFLA · October 2021

papab said:

Is low capacity and normal open cell voltage consistent with sulfation?

Sulfation term is usually used to denote hardened, un-rechargable lead-sulfate crystals. It should have an open circuit votlage similar to a corresponding percent of state of charge OCV, (lower voltage)

For a flooded battery, this results in lower SG reading as sulphur is locked up on sulfated plates so it cannot recharge back to electrolyte. This depletes sulfuric acid concentration in electrolyte resulting in lower SG. Can't measure this on AGM cell however.

So sulfation is a partially discharged battery, permanently stuck at the partially discharged state due to the locked up lead sulfate on plates. Sulfation is a permanent capacity reduction. It starts out with normal discharge voltage slump but after some time it drops to greater levels like a normal discharged battery, just happens sooner then it should because battery has less capacity.

papab said:
Wouldn't the discharge test I did (& am repeating) satisfy the 10-20% discharge? Or should it be a higher rate?

Yes, too much lead oxide coating just causes more terminal voltage drop with load current. As it is burned off the terminal voltage drop improves, A normal cell will show this within the first minute or two of discharge after been on float for some time. The voltage will initially dip down at a greater rate within first minute then start to build back up after a couple of minutes of constant load current.

My guess for your case, AGM electrolyte has dried out. It can not support ion flow due to high electrolyte resistance. This would give normal open circuit terminal voltage but a lot of voltage slump with current load. This occurs rather quickly after start of load current.

Marc Kurth · October 2021

RCinFLA said:

(Snipped from post)
.............................Yes, too much lead oxide coating just causes more terminal voltage drop with load current. As it is burned off the terminal voltage drop improves, A normal cell will show this within the first minute or two of discharge after been on float for some time. The voltage will initially dip down at a greater rate within first minute then start to build back up after a couple of minutes of constant load current...........................

This is a valuable piece of information that is most often not even noticed, or ignored by most people. The delayed voltage rise can be pretty dramatic with large loads relative to the battery bank size. Good post!

papab · October 2021

For grins I plotted up the Voltage vs DOD when I did the last discharge test. First I was running a 9 amp (@12V) load with the inverter. You can see that the voltage dropped like a rock when the DOD got to about 27 Ah. Then, right away, no recharge, to see how much better it would be with a lower load, I reduced the load to 3.6 amps, then 2.5 (12V load, no inverter for the last one). All the data came from a Trimetric battery monitor. I've crosschecked the voltage with the solar charge controller. The load was checked with a Kill-a-watt meter when it was a 120V load.

Early Battery Failure

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