Electronic – “Safe” (but slower) charge method for AGM batteries

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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?

Best Answer

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.