StevieC.
I cannot find in the Odyssey AGM tech manual where they say in excess of 15 volts will pop the vents. The only reference I see to 15v is this statement:
Quote
The message to be taken from this graph is clear - in deep
cycling applications it is important to have the charge
voltage set at 14.4 - 15.0V. A nominal setting of 14.7V
is a good choice, as shown by the test results.
http://www.odysseybattery.com/documents/US-ODY-TM.pdf
In their 'reconditioning procedure' they say not to exceed 15 volts, but the reconditioning procedure consists of a relatively quick 100% discharge quickly followed by a high amp recharge, upto 14.7v, not to exceed 15v.
http://www.odysseybattery.com/documents/ODYSSEY_Battery_Reconditioning_Charge_Procedure.pdf
This is FAR different from recharging an 80% charged battery beyond 15v, as an 80% charged battery will not accept huge aperages for a long time and heat up significantly just from charging or build high pressures.
I've been deepcycling a Northstar AGM since Late November 2013, and using it to start my engine and deep cycle 240 times+ per year since July 2015.
It hass seen plently of time above 14.7v, and Northstar lists 14.46v as their recommended absorption voltage limit at 77f.
My point is that exceeding the recommended voltages is not an instant killer of AGMs. It is not as if they are one time vents, pop once and forever after useless. It is high rate recharging of a well depleted battery to and above 14.7v at a battery temperature of 77f in which the likely hood of the vents allowing some pressure to escape increases.
If exceeding the recommendations were instant death, my battery would have been recycled long ago.
Here in the Lifeline tech manual, Lifeline being a very well respected AGM deep cycle battery, their conditioning procedure is to take the battery to 15.5v for 8 hours, after a normal 'full' charge has been applied.
scroll to page 21
http://lifelinebatteries.com/wp-content/uploads/2015/12/6-0101-Rev-E-Lifeline-Technical-Manual.pdf
The deep discharge recovery procedure outline on page 22 requires special equipment and close monitoring with a constant current charging source, but they say voltages as high as 3.0 per cell can be expected. That is 18volts!
Granted Lifeline/Concorde are the ONLY AGM manufacturer to say voltages over 15 are OK. They are also Mil-$pec.
Note that the lesser AGM$, will say to limit charging amperage to 30% of capacity at 14.5 to 14.9v.
Here is a 67LB 100AH group 27 battery
https://www.wholesalesolar.com/cms/upg-ub-121000-agm-battery-specs-753736796.pdf
In 'cyclic' duty, is says the voltage range is 14.5 to 14.9v.
I recently got a Ub12180 as a portable 12vDC source, which is an 18Ah AGM, same as this.
https://www.sunwize.com/wp-content/uploads/2018/02/Data_Sheet-1-D5745.pdf
It says No more than 5.4 amps and the same 14.5 to 14.9v.
I discharged it well below 50%, removing 12 Ah of its 18 AH total, at an average 2.6 amp rate. It earned its 18 Ah rating providing 0.9 amps for 20 hours until voltage hits 10.5v so at 2.6 amps Peukert steps in and further reduces available capacity.
When I recharged it, I set my 40 amp adjustable voltage power supply to 14.7v and hooked it to the battery.
The initial inrush of current was 38.2 amps, which settled to about 32 amps within 10 seconds. Basically Instant absorption voltage, saturating charging.
5 minutes later it was still accepting 25 amps. The sides of the case were not bulging, the battery was not heating excessively, there was no abnormal noises coming from the battery. 5 minutes after that it was still accepting 22 amps, and I had things to do, and could not monitor the battery closely anymore, so I lowered voltage until only 10 amps were flowing, and then came back in stages to raise voltage slowly back to the 14.7v range keeping amperage in the sub 10 amp range the first two times..
I exceeded the recommended initial amperage 'limit' by a factor of 6. No popped vents, no thermal runaway. This battery 1 week off the charger reads 13.17v.
My point is that AGMs are not going to instantly be destroyed by exceeding the manufacturer recommended voltages. Like all lead acid batteries, they are extremely tolerant of voltages outside of the manufacture 'recommeded' ideal which is really more of what they figure will lead to the least possible amount of warranty returns in how they figure the majority of consumers will employ the battery.
An AGM as a starting battery that lives at a relatively high state of charge, is never going to be able to accpet huge amperages for long, and will not heat up much, nor be likely to release pressure, as the pressure really only builds when the battery has been accepting high amperages for a long time( meaning a well discharged state) and in that situation then exceeding 15v is to be avoided.
In my opinion, the ONLY time an AGM owner using it solely as a starter battery has to worry about the vents opening, is in hot ambient temperatures, and the owner has run the battery flat, jump starts the engine, then gets on the highway, and the vehicle's voltage regulator holds 14.7v+ the whole time. Which is extremely unlikely. Note idling the engine to recharge a well depleted AGM is very hard on the alternator, especially if it is being told to seek and maintain high voltages. Underhood airflow and alternator fan rpm is required to keep it cooler.
Even if the vents do open and release some pressure, it is not the end of the battery, far from it. Dozens and dozens of such events would likely be required before it would become 'dried out'. A flooded battery whose plates are exposed actually holds higher voltages than one would expect, and will appear to be working abnormally well. It is when it is refilled that the voltages plummet after loading it and even long trimes held at higher voltages do not seem to raise specific gravity, and the battery behaves like a goner, depending on just how much plate material was exposed to air, and for how long.
What Lead acid batteries are NOT tolerant of, is chronic undercharging, living their life in a partial state of charge The lower the average state of charge and the longer they stay there the faster they lose capacity and ability until its CCA has lowered to the point where it cannot start the engine, likely in the first cold snap. When they lose capacity and cranking amps, and the procedures for capacity recovery actually require 'abusing' the battery, discharging it hard and heavy, high amp recharging it, getting it hot( sub 110f), and holding it at higher voltages until amperage no longer tapers. I put abusing in quotes as so many people think that all batteries like to be slowly charged, when AGMS especially, can be tickled to death with too low a charging rate and traditional often parrotted internet hearsay is total BS.
Odyssey AGM basically says lather rinse repeat in their reconditioning procedure:
Quote
Performing more than one discharge and recharge cycle is beneficial to
increasing restored capacity.
I was recently noticing my 5+ year old 90 AH 930CCA Group 27 Northstar AGM that has over 1000 deep cycles, the amps at absorption voltage were not tapering to 0.4a, but would bottom out in the 0.9amp range before they would start rising again. I performed Odyssey's reconditioning procedure, and now amps will taper to 0.3 amps, before they start rising again. Voltage held during discharge/ engine cranking is now higher than before the procedure, and I have not repeated it again, yet.
I have not deliberately taken this battery much above 15v for very long, it perhaps has spent a total of 10 hours above 15 volts, in its 5+ year lifespan of 1000+ deep cycles, and high amp recharges.
The reason this battery has lasted so long, performed so well, is because I can perform high amp recharges, and can hold high absorption voltages for as long as is required for amperage to taper to 0.5% of capacity(20 hr rate). Meaning fully charged, something most garage chargers cannot and will not do, despite their marketing of them saying they will also fellate the owner once the green light comes on after their special proprietary 13 stage charging algorithm has completed.
