Author Topic: The best voltage to regulate AGMs to?  (Read 3204 times)

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Xan

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The best voltage to regulate AGMs to?
« on: January 25, 2015, 04:59:45 PM »
Since I have discovered this forum a few days ago I have been spending much time here because the wealth of knowledge here is incredible.

Now, I feel time to start a thread asking a question that I have been curious about for some time...

I have just been reading a thread where Ghurd says that it is best to just have a single stage controller on these. So I presume this means switching from the bulk charge to the regulated voltage that you wish to hold them at while you have an energy input to do so.

What then, is the best voltage to regulate AGM batteries to in a solar power setup?   

There will be no issues with sulphation due to not putting them through some kind of "absorb" stage?

gww

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Re: The best voltage to regulate AGMs to?
« Reply #1 on: January 25, 2015, 05:03:57 PM »
You really need to contact the battery maker for your spicific battery maker for your set points.  This is critical for you type of battery as over voltage and too many charging amps could damage it.  Welcome to the forum.
gww

Xan

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Re: The best voltage to regulate AGMs to?
« Reply #2 on: January 25, 2015, 05:30:24 PM »
Thanx for the welcome gww..!  :)

The voltages are actually written on the battery:

Standby Use: 13.5-13.8
Cycle Use: 14.5-14.9 

However they the Standby ones seem a little low to me for use in a RE system and the Cycle ones seem a little high. Not so comfortable on holding those batteries at 14.5 all day once they are charged.

That's why I am curious to see what other voltages people are running in general with this construction of battery.




mab

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Re: The best voltage to regulate AGMs to?
« Reply #3 on: January 25, 2015, 06:57:24 PM »
sealed batteries are a bit of a compromise on a solar system - if you use the cyclic voltage then you're going to dry them out fairly quickly (assuming you're not cycling them down to 80% dod every day - in which case using the cyclic voltage would be appropriate).

If you stick to the standby voltage but partially drain them every day then you may find they don't get fully equalised -  as it usually takes 16-24hrs continuous charging to fully equalise the battery at standby voltage which is difficult to achieve with solar alone. That said, on long summer days with light overnight use they will equalise over several days.

When I had only solar (I have hydro too now) I would periodically (say once a week in the winter and once a month in spring & autumn) switch to grid power with a float charger for a night so the AGM would have 2 days and a night of continuous charging to equalise.

I also tended to run at13.8 to 13.9v (rather than the 13.56 recommended in the manufacturers datasheet) to speed things along a bit, but this does risk drying the electrolyte out faster, especially when the battery's getting old. That said those batteries have survived for several years and are still working (though they rest at 12.6v and are bulging slightly on the sides now - signs they are getting old).

following some advice from a fieldlines member I'm currently experimenting with adding water to my AGMs but I'm stating with some old ones to see how they take it... so far so good....


joestue

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Re: The best voltage to regulate AGMs to?
« Reply #4 on: January 25, 2015, 09:21:55 PM »
I believe it is more reasonable to regulate float current, not float volts.

Someone here told me it is reasonable for the battery to absorb .01C charge rate (100 hour charge current) without losing any water.. I would run it lower than that, at a C/200 rate.

As the battery temperature changes the float voltage will change of course but this wont change the amps.
Trying to find the right float voltage is fundamentally backwards in my opinion.

If you don't want to go through the trouble of setting up a constant current system, then hook up your battery to a laboratory power supply, set the amps to c/200, put it in a box with the rest of the system to keep the temperature the same, then come back in a week and read the voltage across the battery and set that voltage in your system.
My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.

Xan

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Re: The best voltage to regulate AGMs to?
« Reply #5 on: January 26, 2015, 03:31:36 PM »
It seems I may have been thinking along the right lines, at least. The voltage I have been currently running is a compromise between the standby & the cycle voltages, which is 14.0 volts. But the system doesn't hold it there all day, once the bulk charge is finished it tends to drop down to 13.5 or so and then slowly rise back up to the 14.0 where it is clamped for the rest of the solar day (depending on sun & loads etc of course).

If I feel the batteries have  had a deeper discharge I shift the regulator "up a gear" so it clamps at 14.5 volts and I'll leave it there for the day.

So far this all seems to be going very well. I just a little concerned that there may be some insidious drying out happening that may not be noticed until it's too late.

That idea of regulating for current rather than voltage is interesting, as we are usually told that lead acid chemistries require a constant voltage charge rather than a CC. But I suppose for floating it wouldn't matter and regulating the current may be better?

It would not be difficult for me to arrange a current regulating circuit. This battery bank in total is 520AH and C/100 would only be 5.2 amps, which seems quite low...but maybe not? The other issue I am thinking of here is that this bank is 12volts so all the batteries are in parallel, which could potentially cause issues with a current regulating scheme as there would be no way of guaranteeing they share that current evenly or "fairly". 

Flux

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Re: The best voltage to regulate AGMs to?
« Reply #6 on: January 26, 2015, 04:51:52 PM »
I think your 14v is a reasonable compromise with the occasional boost to 14.5 as long as you make the correction for temperature. Unless you hear them venting they should not dry out but long periods above 14.5 probably should be avoided. AGM need higher voltages than gel.

Flux

Xan

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Re: The best voltage to regulate AGMs to?
« Reply #7 on: January 28, 2015, 03:52:28 AM »
Thanx Flux, that's good to hear as they seem to be going well like this and no, I haven't heard (or seen) any evidence of venting at all...unlike a naughty gel battery I have, but I am not too concerned about that one right now as I suspect it has a dicey cell.

I haven't made a temperature compensation circuit on the regulator, but it has been very hot & sunny here lately (we're just on half way through our summer season) & the bank has not been discharged very much so it has pretty much been on the 14 volt setting the whole time.

Although having said this I would like to add some temp compensation to it. I guess there are plenty of tables around for helping to design this.....or does anyone have a suggestion as to what would be a good figure to use. From what I understand the voltage tends to decreased by a certain increment for every degree over 25 Celsius & increased for every degree below.


mab

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Re: The best voltage to regulate AGMs to?
« Reply #8 on: January 28, 2015, 02:26:16 PM »
IIRC it's -3mV per cell per degree Celcius.

Personally, I tend to seasonally adjust my setpoint rather than automate it, but them my batteries are indoors where the variation is small anyway