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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?

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  • \$\begingroup\$ No, that’s not true. If you gas an AGM battery, most of it will recombine as water but the ~1% that doesn’t is lost forever. In a normal flooded battery you can just add more destilled water and you are back where you started again. Hence it’s more crucial in AGM batteries not to outgas them. \$\endgroup\$ – winny Jan 12 at 9:52
  • \$\begingroup\$ A car manufacturer who want to sell maintience free cars will think the opposite, 99 % capture of outgassing versus 0 % capture and no end users will bother refilling with water. Do you see the difference between that case and yours? \$\endgroup\$ – winny Jan 12 at 9:54
  • \$\begingroup\$ Also, what final float voltage do you propose for your AGM? \$\endgroup\$ – winny Jan 12 at 10:54
  • \$\begingroup\$ I already stated 13.6V for my AGM batteries but it is not "float" mode it is trickle charge mode. That is, there will always be some current flow, even when the battery is "fully" charged. However, it usually drops down to less than 100mA on a healthy AGM battery. I use a laboratory power supply to confirm this. It seems batteries "prefer" being charged slowly since it gives time for the chemistry to work its "magic". I haven't had any issues (it seems) with this slow charging method and the good part is if there is a power interruption, unlike smart chargers, my method recovers nicely. \$\endgroup\$ – David Jan 12 at 13:03
  • \$\begingroup\$ You will not reach 100% SOC using that method, but outgassing should be near zero at that low voltage. \$\endgroup\$ – winny Jan 12 at 15:01
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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. lifeline agm battery chart

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.

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  • \$\begingroup\$ I've had good success using 13.6V or 13.7V for typical AGM batteries. It allows me to "set it and forget it". Even if I fall alseep and let it charge all night, when I wake up, it is usually below 100mA. Some batteries will go below 10mA. The beauty of this system is you only have to set 1 mode and it both fully charges and maintains withOUT overcharging. Your point about applying 13.6V to a deeply discharged battery is a very good one. I suspect the power supply would be overloaded, the voltage would sag, and the amps would stay close to 10 on a 10A rated power supply but I will check. \$\endgroup\$ – David Jan 12 at 23:04
  • \$\begingroup\$ Also, the trickle charge/float/maintain voltage is usually show directly on the battery my the manufacturer. Most of the ones I have seen for AGMs are in the 13.5V-13.7V range. If that is the case, I usually select the middle which is 13.6. If they specify 13.5V-13.8V and I am topping off 2 in series, then I would select 27.3V on my laboratory power supply. \$\endgroup\$ – David Jan 12 at 23:11
  • \$\begingroup\$ @David, my comment about current limit is applicable to power supplies. Less so to battery chargers which should be designed to deal with that situation. \$\endgroup\$ – mkeith Jan 13 at 3:28
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    \$\begingroup\$ Yes I agree. Normally for small batteries, I use my lab PSU which has current limiting so on a deeply discharged battery will not draw more than 3A which is the maximum it can output. It just drops the voltage automatically to something like 12.5 to start and then ramps up slowly until it reaches the maximum I set, usually 13.6V, then the amperage trickles down from 3A to as low as as few mA. I've seen lithium iron phosphate batteries eventually draw 0.000A when the lab PSU was set at 14.2V, the maximum recommended for that battery type. Most AGMs have a recommended max trickle about 13.6V. \$\endgroup\$ – David Jan 14 at 6:17
  • \$\begingroup\$ Yes. There are also supplies out there intended for powering mobile radios at home. Typically they put out 13.6 or 13.8. These are not lab supplies. They may not current limit in a nice way like a good CC/CV lab supply does. So, since you said "supply" I wasn't sure what kind of supply you had in mind. That is the only reason I cautioned about the supply. For a labs supply with current limiting, no problem. For a charger, no problem. For a generic 13.6V supply, well, maybe a problem. \$\endgroup\$ – mkeith Jan 14 at 7:41
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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.

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