The Absorption stage of lead-chemistry battery charging is characterized by constant voltage (Vabs) and decreasing current. The battery takes less and less current as it approaches full charge, and finally plateaus at a low rate. Current is the red line in this graphic:
[caption id=”” align=”aligncenter” width=”257”] https://chargetek.com/chargingbasics.html[/caption]
That leveled off terminal rate in the right third is called endAmps, finishing amps, tail current, etc. It is typically C/200 to C/100, or 0.5 to 1A per 100Ah of battery capacity. Fresh batteries tend to have lower endAmps reached more quickly, and tired / old / abused batteries tend to have higher endAmps reached more slowly (longer Absorption phase required).
> I have repeated this test with AGM batteries [in] worse shape that exceeded 7 hours to attain 100% SOC, likely due to sulfation.
If that is so, one might be able to record endAmps monthly as a metric for battery state of health. It might not tell you when a particular battery is toast, but increasing endAmps would be an indication of design, all other things being equal.
MainSail suggests endAmps greater than the 0.75%C (C/133) indicates the battery is no longer running at full capacity:
Over many years of capacity testing hundreds and hundreds of batteries, flooded, GEL and AGM I have yet to see a lead acid battery that could [deliver] its full storage potential at anything less than 0.75% in tail current at absorption voltage.
The wording is a bit odd there, and IMO means “worse (higher) tail current than 0.75% rather than “ < 0.75%”.
increasing Vabs when Absorption duration is limited
Charge controllers have different strategies for knowing when to end Absorption
high end controllers may hold Absorption until the specificed endAmps is achieved as long as it takes (or as long as sunlight is available).
mid-range controllers may hold Absorption for a duration specified by the user
low-end controllers usually hold Absorption for a set duration like 2 hours.
In each of these cases, batteries in poor health may require longer than the available time to drop to endAmps. One workaround for maxxed Absorption duration is to increase Vabs. I propose increasing Vabs in 0.1v - 0.2v steps until endAmps can be achieved within the available duration.
Caution: as Vabs creeps up you will either hit the maximum allowable setpoint (AGM, Gel) or need to water more often (flooded).
longshot: decreasing charge rate when Absorption duration is limited
Charge time remains relatively constant whether charging at max or min rates for the battery. Again quoting Mainsail:
charging AGM at C/2.5 (max rate) resulted in the battery being 63.3% “full”[3)](http://rvwiki.mousetrap.net/doku.php?id=opinion:frater_secessus:charging_faster#fn__3) at Vabs and being fully charged in **5hrs, 30 minutes** (20min Bulk, 5hr 10mins Absorption) charging AGM at C/5 (min rate) resulted in the battery being 77.4% “full” at Vabs and being fully charged in **5hrs, 42 minutes** (76min Bulk, 4hr 26mins Absorption)
It is conceivable that charging at a minimum rate (particularly for flooded that can be successfully charged as low as C/10) could also help absorption finish faster. Overall time would be about the same but the charging process would spend more time in Bulk which by definition is not time-limited.
My own set of flooded golf cart batts (2x 6v in series) is Absorped at 14.6v @ 70F and finishes at 1.79A, or 0.81% (C/123) in about 2.5 hours. This suggests I have lost some capacity in 589 cycles at 50% DoD (installed 9/24/2018). I wasn’t measuring it before so I can’t compare to new, and the loss of capacity is not noticeable in practical experience.
When my bank can no longer reach endAmps within the three hour max the controller allows I will try increasing Vabs.