Thanks, Kermit2!

Yes, that's exactly what I was thinking: to change the absorption voltage to 14.1V, because that seems to be a safe limit for gassing.

I wish to thank you all, Guys! You have opened my eyes to a real problem and made me learn about it. You basically saved my batteries before I've done too much damage to them. Thank you!

 

It's possible that your charger is a poor match to that particular battery capacity or type. I've had expensive, fancy multimode chargers overcharge a battery where a simpler, cheaper dual mode one would not.

My simpler charger is like the proverbial everlasting broom.

It started off as a very old Halfords metal cased type, first to fail was the moving iron current meter, that got replaced by a very fancy surface mounted admiralty type. Since then the mains transformer has been replaced by one from an (allegedly) portable TV. The rectifier in it is an old motorcycle type.

Since then; I've added capacitors across one AC arm of the rectifier to make it a low current doubler - does very nicely for sulphated batteries.

As it stands, its a bit brutal for routine charging - so it gets used as a last-ditch attempt to recover what the Optimate can't.

 

I would suggest that you lower the absorption charge voltage while monitoring the battery voltage after charging stops and after a few hours rest to determine its state of charge (fully charged is around 12.6v per battery with no current).
I think you will find the proper charge voltage to fully charge the batteries will be close to 28V for the two batteries in series.

 

I am glad we could educate you! (We see way too many people here that refuse to become educated.)
I feel sure you can lower your charge rate and measure the end voltage as crutschow just said.
I have seen many batteries that sit there, dead quiet at the beginning of charging, and end up with a tiny soda-pop fizz. That is not the same as bubbling. If the fizz had to sneak around in a gel (like your battery) it would accumulate a bubble every few minutes, but not a steady stream of bubbles.

Keep up the good work.

 

Thanks Guys!

crutschow, are you sure that 12.6V is a good value for a full charge for AGM batteries? I seem to remember reading somewhere that for AGMs the full charging voltage after rest is much higher than conventional floaded lead-acid. Right now my batteries are sitting at 13.2V after 24 hours of rest. That may be a bit much because of the high absorption that took place. However when I got them from the seller, they had 12.93V.

 

Guys, you won't believe this!

Finally got my cable and have reprogrammed the charger to have an absorption voltage of 14.1V and gave the charging another try.
Currently the charger is in bulk mode and is pumping 3.5-4.0A into the two 150Ah sealed AGM batteries. The batteries are right now at around 13.5V (27.1V in total). I can already hear a light but steady stirring in them, especially in one of them (but also in the other to some extent). This stirring (fizzing) is not as string as I heard at 14.4V previously (during absorption), but it's definitely there. I wonder what will happen as the voltage rises to 14.1V... how can it already be gassing at 13.5V?

I measured the voltage of the batteries individually during charging and the one which produces less sound is at 13.53V, the one which produces more sound is at 13.52V. When they were fully charged, they had identical voltage...

I'm puzzled...

 

crutschow, are you sure that 12.6V is a good value for a full charge for AGM batteries? I seem to remember reading somewhere that for AGMs the full charging voltage after rest is much higher than conventional floaded lead-acid. Right now my batteries are sitting at 13.2V after 24 hours of rest. That may be a bit much because of the high absorption that took place. However when I got them from the seller, they had 12.93V.

According to this, the open circuit voltage of a fully charged 12V AGM battery is about 12.8V-12.9V which is about 100 to 200mV higher than a flooded cell battery.
So an AGM voltage is some higher than a flooded cell, but I wouldn't say that it is "much higher".

 

I'm starting to wonder if my batteries are really AGM and should be charged as such or are they some kind of gel batteries...
This is what the manual says:

Construction Component ......Raw material
Positive ........Lead dioxide
Negative ........Lead
Container ........ABS
Cover ........ABS
Sealant ........Epoxy Resin
Safety valve .....EPDR
Terminal ........Copper
Separator ........AGM
Electrolyte ........Hybrid Gel Solar Se

What's confusing is that the separator is AGM but the electrolyte is some kind of gel...

I asked the guys at the local store who sold me the batteries about the charging voltages. They say not to go above 13.8V ever, not even in the absorption phase, but to always charge between 13.6V-13.8V, not only in the float phase. I think they might be exagerating and they probably don't have a clue, since they sell mostly car starter batteries, they just rarely resell these sealed solar batteries.

 

Yes, those would be considered a GEL type battery, and the sales people are correct that they should never be charged to fast.

Charging is a chemical process despite the fact you are using electricity. It is not one of those, "If a little bit is good, then more is better" type of situations. The electrolyte is the choke point in the charging process. It must convert the incoming electrons into chemicals and the change takes place at the surface of the lead plates where the electrolyte meets it. Once all the available chemical elements at the plate/electrolyte interface have undergone the chemical change, MORE electrolyte has to migrate into the area so that it too can undergo the chemical conversion process. When you feed the electrons in to fast of a rate, the movement of the fresh electrolyte cannot keep pace with it. The excess electrical power then proceeds to work on the only element left, which is the water. The production of Hydrogen and Oxygen is the result of sending electrons into the exhausted solution of electrolyte, since it is mostly water to begin with.

GEL cells have an even SLOWER rate of movement in the electrolyte than the Absorbed Glass Mat type. Even in flooded cell batteries, the ions in solution(electrolyte) move at a much slower rate than the electrons. As I said, this is the choke point. If you feed more electrical power into the battery than the rate at which chemical conversion occurs, the ions in the immediate area of the plates is used up and only water remains there, which means water GAS is produced.

The BEST type of battery for a situation that requires extremely heavy and fast recharging(which solar most definitely is) would be flooded cell, since ion migration in the liquid solution is faster than in the other two types.

 

Thanks, Kermit2! So then I should charge them as GEL batteries? I'll have to look up the appropriate voltages...

 

 

So in other words you're saying that for my 150Ah batteries the charging current should never exceed 2.25A...
I still don't understand why the manufacturer says that it can be charged at 14.4V-14.8V and 37.5A when used in cyclic applications... Perhaps I'm not discharging it enough to be considered cyclic use...

 

So in other words you're saying that for my 150Ah batteries the charging current should never exceed 2.25A...
I still don't understand why the manufacturer says that it can be charged at 14.4V-14.8V and 37.5A when used in cyclic applications... Perhaps I'm not discharging it enough to be considered cyclic use...

Gassing is one of the indications of a lead-acid battery reaching full charge - when it gets to gassing, the conversion of one form of lead to another is about complete, after that; the energy you put into the system is increasingly splitting water into hydrogen and oxygen.

 

Can I be sure that the stirring or bubbling that I'm hearing is really gassing? I guess there's no other thing that can cause these sounds...
Oh, boy, this whole battery charging is a headache... I just wish there were some clear indications from the manufacturer about how to charge them . Well, there are... but if I do it according to those indications then the batteries are bubbling...

 

Can I be sure that the stirring or bubbling that I'm hearing is really gassing? I guess there's no other thing that can cause these sounds...
Oh, boy, this whole battery charging is a headache... I just wish there were some clear indications from the manufacturer about how to charge them . Well, there are... but if I do it according to those indications then the batteries are bubbling...

The SLA batteries have vent caps - just an upside down rubber cup that fits over a small funnel in the top of the cell, if any pressure develops it gets released safely. Presumably AGM batteries are not all that different. You probably shouldn't be able to hear any fizzing/bubbling - the pressure builds up to a very low value then the rubber cup expands slightly and lets it go.

As there's (supposed to be) no way of topping up the distilled water, any gassing should be kept to the absolute minimum - I believe this type of cell has some sort of gas recombination technology to minimise water loss.

For flooded cell batteries, conditioners like the Optimate bulk charge to about 14.4V, then drop back to about 13.6V to minimise water loss for extended period float charging/maintenance. Terminal voltages may be slightly different for SLA & AGM, but there's the general idea.

 

Thank you, Ian!

I'm really trying to understand the following charging characteristics graph from the manual of the batteries, but I just can't wrap my head around it:

Can anybody help me out?

 

Thank you, Ian!

I'm really trying to understand the following charging characteristics graph from the manual of the batteries, but I just can't wrap my head around it:

Can anybody help me out?

Its not difficult - if you leave the full bulk charge voltage applied after the battery is fully charged, it'll gas all the water out of the electrolyte.

You need a comparator and a latch to switch about 0.8V reduction in applied voltage when fully charged is sensed.

 

Ian,
My charger does switch from bulk to absorption and from absorption to float automatically. What really worries me is that I hear stirring/fizzing already at 13.5V! That's a voltage I cannot go below. Even the batteries' manual recommends a float voltage of at least 13.62V. The only thing I can think of is to limit the charging current. Perhaps it's pushing current into the battery too fast. For example at around 13.5V it was putting into it about 4A. Unfortunately, if I do limit the charging current then I'm wasting energy produced by my solar panels even when the batteries are far from fully charged.

 

You should charge these batteries at a minimum of 15 amps or C/10 (1/10 of the AH rating.) These are 150 amp hour batteries. When I said 15 amps minimum charging current, you should be able to go 2 to 3 times that. Call the battery manufacturer to verify charging rates.

I downloaded the charger specs and the bulk/absorption/float thresholds are set for a flooded lead acid battery, not a typical AGM. Again, call the battery manufacturer.

 

Thank you, Lestraveled! I did try contacting the manufacturer, but they are some huge Chinese company and they don't really give a ... about small clients like me. They never bothered to answer me. So my only hope lies in figuring it out myself...

It's not a problem that the charger's voltage thresholds are not set correctly from factory because I can change them. I just don't know what values to use. The 13.6V for float and 14.1V for absorption seemed a good choice, until I realized that I hear these sounds even at 13.5V.