Early Battery Failure
papab
Registered Users Posts: 66 ✭✭
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
Comments
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.
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
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.
-Bill
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
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.
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.