"Safe" (but slower) charge method for AGM batteries?

Question

My understanding of AGM vs flooded lead acid (FLA) batteries is that FLAs like to gas/outgas but AGMs do not. It is pretty much a requirement to increase the charge voltage of FLAs so that they gas and "de-stratify". However for AGMs, that is not a requirement, nor is it recommended.

So my idea is for AGMs, if you are not in a hurry to charge them, why not just set them at the trickle charge voltage and let them slowly charge (such as overnight)? An example would be I used my battery bank during the day and it is resting at 12.3V (a mid state of charge for AGM). Now suppose I have a 13.6V 10A power supply (ps) and just connect it to the battery for many hours. In theory, I don't have to worry about overcharging the battery since 13.6 is already the battery manufacturer recommended trickle charge / maintain voltage.

I have also tried this method with Lithium Iron Phosphate batteries, setting the charge voltage to 14.2V and watching the charge current eventually drop to 0.000A.

For AGMs and LiFePO4 (LFP) batteries, it seems like a good safe technique since you are guaranteed not to overcharge the batteries and it is healthy for AGMs to keep the charge voltage lower so there will be no loss of water.

So my question is are there any drawbacks (other than slower charge times) for using this method? What might it do to the life of the battery charging it ONLY this way?

Answer 1

It is fine to charge 12V AGM batteries with constant voltage of 13.6V or so. The exact voltage may depend somewhat on the specific battery model, and temperature of the battery. At higher temperatures, you will definitely need to reduce the voltage. Otherwise the battery life will definitely be reduced. This type of charging is often referred to as "float" charging, even though in one of your comments you said that it is "not float mode." I get what you are trying to say. Another term for it is "standby mode" or "standby use."

Here is a chart for lifeline AGM batteries which I found Here.

One thing to consider if you are using a power supply is that there should probably be a current limit also. Once discharged, an AGM battery may accept very large currents if you apply 13.6V. What will the supply do? Ideally it would transition to constant current mode until the voltage reaches 13.6V then transition to constant voltage. But it may also shut down, or overheat or who knows what. So, you need to attend to that detail.

Will the AGM battery be fully charged by this process? It may take a long time to actually get to 100% charge. Or it may never get there. But by carefully setting the CV voltage point, it should get very close.

If you are unable to adjust for battery temperature, the safest thing would be to use a slightly lower "float" voltage. Also, read up on your specific battery. This chart is for one brand of batteries. Others may be slightly different.

Answer 2

Actually my favorite mode of charging AGM batteries is to use a smart charger for the bulk phase (usually up to 2/3 to 3/4 charged, then remove it from that and put it on my laboratory power supply (lab PSU for short), at 13.6V and 1.5A max. The last 1/3 to 1/4 charge takes a long time, usually overnight, but it seems to work well and the battery will be at 100% State of Charge (SoC) when I wake up in the morning.

As someone pointed out, this method will NOT work well on a deeply discharged battery because 13.6V applied to a battery at say 12.0V will try to suck too much juice out of the PS and likely exceed its rating.

So to answer my own question, yes it can and does work, provided you do NOT exceed the limit of your PS. This is usually the case on a large battery at a mid SoC or higher, or a small battery at pretty much ANY SoC. It also depends on how powerful your PS is of course. For example, a small 3A PS will likely get overloaded by even a small battery in a mid SoC.

So this is why my favorite method to charge is blast it will 10 to 20% of the rated Ah in the bulk charge phase (for example, a 100Ah battery should get between 10A and 20A in the bulk charge phase), then gentle charge it the rest of the way to 100% SoC using the manufacturer's recommended "float" voltage.

Another advantage of this method is I can use a powerful 12V 25A charger for the bulk phase and then free that up after it reaches about a 70% SoC, so I can then use the powerful charger for the next battery that needs a good "blast". I have several 13.6V PSs so I can "top off" multiple batteries at the same time.

If using a regulated PS for the top off charge, it would be smart to first set the bulk charger to a max charge rate no higher than that of the PS rating. For example, bulk charge at 8A max (for a 70Ah rated AGM battery let's say). Wait for the smart charger to report that it is at 13.6V charge voltage, then you can safely take it off of that charger and put it on the 13.6V 10A max PS, and you will be guaranteed it will NOT exceed 8A. Therefore you are guaranteed you will NOT overload the PS and you will then free up the bulk charger for some other battery that may need it. I actually have a pair of 13.6V 10A rated PS so I can use them for top off purposes.