Duracell Ultra AGM, Cold Weather, Low SOC

WRFP399

Question:

What time frame are we looking at for sulfation to set in with AGM batteries? Does the cold weather, approximately 0-10 degrees F assist slowing the rate of sulfation or no?


Background:
Over in the Solar Beginners Forums I have a post where I am working on powering a small mountain top radio repeater here in Alaska. The radio and controller draw about 90 mA when in stand-by and about 2.15 A when transmitting. When the controller shuts down the load it draws only 13mA. Here in Alaska we get plenty of sun to keep the system moving 24/7 on 80 watts of panels during Spring, Summer and Fall. It's tjhe Winter, when we our sunlight gets down to only a few hours a day, that it gets more challenging.  Adding more panels are not an option at this point. Having the repeater shutdown due to low power isn't a deal breaker as long as it recovers on it's own.

I currently am using a 35 amp hour Duracell Ultra AGM battery. I have opted to use a solar controller that allows me to have it cut the load off at 11.8 volts (may bump that higher, haven't decided).  This leaves enough energy in the battery to run the controller for about 30 days before it gets down to 10.5 volts which is the lowest SOC recommended by Duracell. 10.5 volts is still well within the range of keeping the controller running. This 30 day number is assuming there is ZERO solar generated during that time but that is just not accurate. Even on cloudy days the panels generate enough to offset the controller use so it's likely the voltage will stay around 11.8 or slightly climb until it gets real sun. I have tested this out using a 30 watt panel and 9 Ah battery over the past week. Without the load it has been slowly gaining power, but it's so slow I consider it a wash. We often have 2-3 weeks of clouds during the winter, so it's possible the battery will stay in this low SOC for a fair amount of time.

Marc Kurth

I do not know who makes the Duracell battery that you have, but here are a couple of thoughts that are accurate for another manufacturer of AGM batteries:

- 10.5v under a light load is "dead" = 0% SOC and the battery will have a freezing point of +21 degrees F. But 11.6v measured with no load or charging for 6-8 hours is exactly the condition = 0% SOC. So, +21F freezing point.

- 11.8v under a light load is around 75% SOC and has a freezing point of -50F.

I do have additional thoughts on sulfation rate, but sulfation may not be your biggest problem when it is cold.

WRFP399

Side note on freezing. Tried as a might I could not get an AGM to freeze on me. I dragged one down to a standing voltage of less than 9.0 volts and it would not freeze up in sub zero temperatures after several days of exposure.  I am not saying it won't happen, just I couldn't get it to in my tests.  

Either way I chose 11.8 volts as the cut off to keep the freezing point lower and also give it plenty of time to run the controller while waiting on a sunny day. 

Based on what I am reading as long as I get a full, 100% complete charge, in 30 days the sulfation should be "soft" enough to dissolve back into the electrolyte. The charge controller normally charges at rates set at the low end of Bulk and Float rates.  However I set the charge controller to charge the battery at a voltage of 15.0 once every 28 days. This is the max charge voltage recommended for cycle use.  Hopefully that gives the battery a soft charge and an easy float normally with a once a month "cycle" charge to make sure it is topped off.

Yes? No?

Marc Kurth

I would assume that you froze your battery after several days at sub-zero temps at 0% SOC. Did you run a capacity to test to find out?

With sulfation, so much depends upon the time vs. SOC curve. It is not a simple on/off switch, the sulfation forms far more rapidly at 10% SOC vs. 75 SOC. You can get away with a LOT if you expect to replace a battery every year or two, vs. a 10-year design life. I teach my customers to always reach full charge at least every 6-8 days. If your battery sits at 75% SOC for a month, then you charge it at the maximum voltage you may well get away with it.

WRFP399

I did not run any kind of specific capacity test. I just hooked up a light bulb that pulled .5 amp to see if it would light and it did. I figured if it was frozen there would be little to no electrical activity?