I did an experiment where I used a "regular" smart-charger set at 12A max AGM mode on a 12V 100Ah rated AGM battery to what it told me was 100% charged. The charger then automatically went into trickle charge mode of 13.5V. I immediately took the battery off of that charger, and put it on a laboratory power supply unit (lab PSU), manually set at 13.5V (and allowed to go up to 1.5A max).
What I am wondering is why does it take so long for that battery to "level off" at about 1/2 watt of charge power going in? For example, when I used the lab PSU, it started at about 3 watts of charge power. Over the course of about a week, it slowly went down to about 1/2 a watt then didn't go much lower.
So what I am thinking is it is very difficult to get those last few % of charge in the battery, so many modern smart chargers don't bother and maybe stop at 90-95% charged.
So does anyone know what exactly is happening in the battery and why those last few % take "forever and a day" to go in the battery and "stick"?
Just as a related comment... it is interesting to see what a particular battery will "bottom out" as, as far as charge power. 1/2 watt at 13.5V is only 37mA. I think many smartchargers may be programmed to stop at 1% of the Ah rating of the battery, so in this case, stopping at 1% of 100Ah = 1A. However, at that point, the battery still seems to want to take more charge.
One experiment I want to do very soon is after charging the battery for a week until it is what I think is a true 100%, drain some fixed amount of power out of it (let's say 1/2 KWh) using a reasonable load (such as a 150W incandescent lamp) and a Kill-A-Watt meter, then measure how much AC wall power it takes to get it back to that true 100% State of Charge about a week later, using that same Kill-A-Watt meter. I suspect running the initial charger a few hours plus running the lab PSU for about a week 24/7 will add up to maybe 3x to 4x the power (I am guessing 1.5 to 2 KWh). If that is correct, then something seems "out of whack" that it takes that much more power to get the battery fully charged.
Also let's assume the first charger really does stop at a true 90 to 95% state of charge. At that point, the wall power consumed is probably much less than 3x to 4x the power we got out of the battery, and I will record this number. It might take as much (or more) wall power to trickle charge for a week, then it does to get the battery to "100%" by the smartcharger's "definition". This should be a very interesting experiment.
I've had the battery on the lab PSU for about a week now and it is very close to drawing 1/2 watt (37mA at 13.5V). It is around 41mA. It must be very difficult to get that last little bit of charge in there, however, I suspect keeping the battery at a true 100% SoC is healthier than what a typical smartcharger (that is much quicker) calls "100%". It would be interesting to do a capacity drain test using both charge methods (smartcharger only vs. smartcharger for bulk + 1 week on lab PSU). I wonder if the difference in capacity will be 5% to 10% more using the lab PSU to top it off.
The last few mA that it drops are VERY slow. It looks like 37mA might be the lowest it will go without reducing the voltage. 13.5V is where it stayed. It must be very close to the battery's self discharge rate, but I wonder if the self discharge rate is a function of how much the battery is charged. For example, if a battery has 1.2 KWh of capacity, and the self discharge rate is 1/2 watt per hour, then in 2400 hours is should be totally dead but that cannot be right cuz that is only 100 days (24/7). The self discharge likely tapers based on SoC.
Here is a pic of four large 6V 230Ah rated AGM batteries in series/parallel configuration (so 12V 460Ah rated). 124mA at 13.5V is only about 62mA per battery (at 6.75V) which is LESS than 1/2 watt per battery of charge power going into each battery. The total charge power is 1.674 watts so that is about 0.42 watts per battery.
For the benefit of those of you wondering, that laboratory DC power supply is Instek brand, model GPS-3030DD. It has a range of 0-30V (actually about 31.4) and current limiting from 0 to 3A (actually about 3.1). It is rated at a maximum of about 90 watts output but can be coaxed into about 100 watts. I usually run it at 1.5A max (about 50% of its max rating) and I usually blow a fan across the heat sink if the charge current is 1A or more.