Steca Solarix 2525

eliosfederico

Hi everyone!
Just for my hobby I have the following setup: 1 50W mono solar panel hooked up with a Steca Solarix 2525 and a 145AH AGM battery.
I use this system with an inverter, that draws about 5A every day (total). 
My doubt is the type of charge of the Steca controller. The controller will boost charge the battery (14.4v) only when the SOC is less than 70% (or less than 12.7v). If the SOC is more than 70% the controller will "normal charge" the battery at 13.8v (lowered it myself, it was 13.9), which is float. 
In my eyes this is stupid because I normally never discharge the battery lower than 70% and I think that the "normal charge" will waste precious solar energy because the amps will be limited if the voltage rises above 13.8.



What do you guys think?

Thanks!

BB.

In general, 13.x volts will not full/quickly charge a battery bank. And there is theory that Lead Acid Battery bank will last longer if discharged below 80% or 70% state of charge before recharging (i.e., deep cycle flooded cell lead acid storage batteries "like" to be cycled).

So--eventually your battery bank will walk below 7x% state of charge and then get a "good" 14.4 volt bulk/absorb charge every few days.

My other question... 12.7 volts is the "resting voltage" of a fully charged flooded cell lead acid battery at room temperature. The charger will only go into Bulk/14.4 volt absorb charging if the battery is even slightly discharged--That is normal behaviour. You do not want to charge an already fully charged lead acid battery--And especially not AGM which could cause them to generate too much gas and vent/overheat (AGMs really do not like to be over charged--Flooded cell batteries can have water added, AGM's are sealed and you cannot make up lost water--And can overheat the catalyst in AGM batteries that have catalyst to recombined hydrogen and oxygen gasses).

-Bill

eliosfederico

Hi Bill,

Thanks for your fast and very helpful answer!

After some googleling I found that the resting voltage of a fully charged AGM battery should be 13.0, maybe AGM and flooded have different resting voltages?

The Steca controller can be set to work with voltage or with SOC. It has a "self learning" SOC calculation so in teory it will only bulk when the battery is indeed 70% or less. I have it currently on SOC. The Steca also has a "recondition" function which will trigger itself every 30 days, but only if in the past month the battery had no bulk charge. 

My main goal is to draw 5Ah in the evening and get the 5Ah back during the day (weather permitting) and so on. Of course, occasionally I may use more than 5Ah, but that's an average. 

Another question: What do you think about the float voltage of the controller? Stock it was 13.9 but the datasheet of my AGM battery (http://www.beaut.nu/fileuploads/zks_agm145-12.pdf) says that the float voltage should range from 13.6 to 13.8 at 25 degrees C so I lowered it to 13.8 but today my battery was full and I saw the voltage going sometimes past the 13.8, with peaks on 13.94 (specially when for a while the sun was pretty strong, bringing in 2.2A).
Should I lower it to 13.7 or it doesn't really matter? I live in North Europe and I doubt there will be that much sun to actually overcharge a 145Ah battery.
P.S.: The Steca has temperature compensation on the float charge. 

BB.

I would keep float voltage on the low side.

There is a limit on the amount of charging current (AH) for the life of the catalyst.

AGM batteries have very low self discharge, so they do not need much float current.

-Bill

PNjunction

eliosfederico said:

>>After some googleling I found that the resting voltage of a fully charged AGM battery should be 13.0, maybe AGM and flooded
 >>have different resting voltages?

That's correct.  NEW, properly charged AGM's will rest after about 12 hours typically from 12.95 to 13.10v.  A few days later, they may drop to 12.9v or so - even less as the cycles wear on.  After a few hours off the charger, about 13v is the norm.

>>My main goal is to draw 5Ah in the evening and get the 5Ah back during the day (weather permitting) and so on. Of course, >>occasionally I may use more than 5Ah, but that's an average. 

Your battery is actually too large for your application.  Unless you draw more than 5% from the battery, and even better 10% capacity, you are "micro" cycling it.  This results in uneven sulfate coatings which eventually lead to hot-spots when a truly large load is placed upon it.  Recharge eventually becomes weird, especially if you have done a deeper discharge - say a few day's worth of outage.

At the very minimum, you should be drawing at least 7.25ah daily from that 145ah bank.

Heh, a very rare problem indeed - you need to be drawing *MORE* from it to keep it healthy!

Also, your panel is too small - even though you are micro-cycling it.  Unless you supply an agm at least .05C current, much of the recharge - even at your highly elevated soc's, is going into heat (although you'll never feel the battery getting hot!).

>>Another question: What do you think about the float voltage of the controller? Stock it was 13.9 but the datasheet of my AGM >>battery (http://www.beaut.nu/fileuploads/zks_agm145-12.pdf) says that the float voltage should range from 13.6 to 13.8 at 25 >>degrees C so I lowered it to 13.8 but today my battery was full and I saw the voltage going sometimes past the 13.8, with peaks >>on 13.94 (specially when for a while the sun was pretty strong, bringing in 2.2A).

Most battery specs are NOT based upon the solar recharging environment where one is doing daily cycling.  The elevated float voltage here (13.9) is actually a very good thing for AGM's, because in reality, there is not enough sun during the day to really get them fully charged as they should be.  The best you can do in a *cycling* application, even at your elevated soc levels, is to ride the high end of the spec because battery specs don't take into account the sun going down!  Thus you try to get as much charge in as fast as you can.

A standby/backup application for agm's is a little different.  But for you, don't namby-pamby that agm.

Here's the big catch with agm's - they need a LOT of float time, or you will progressively walk down their capacity.  It's just the physics of the beast.  In fact, that is what the slightly elevated 14.7v monthly charge level is all about.  It is an attempt to catch up in a solar realm to try and truly get your agm charged properly.  It is not a flooded-type EQ, but just a slightly elevated absorb voltage.

If you only want it to last 3 years tops, then follow the recommendations made by vehicular manufacturers.  But you aren't a vehicle application, so why not make it last MUCH longer?!!!

>>Should I lower it to 13.7 or it doesn't really matter? I live in North Europe and I doubt there will be that much sun to actually >>overcharge a 145Ah battery.

If you aren't cycling it, but just leaving it in the sun for months, then yes, by all means lower the float.

This is a common problem when one uses agm's:  they require different charge parameters based on doing either standby or cycling service.

I kind of see what you did here - thinking that micro-cycling a huge battery will make it last longer - at some point that is actually detrimental.

eliosfederico

Hi PNjunction,

That's kind of bad news! My battery it's 2 days old, replacing my AGM 60Ah because 5Ah draw was just too much for it (60/20=3Ah) and I had quite a large voltage drop. That 60Ah battery it's about a week old and now I use it as "backup" battery.

I was going for a 120Ah which would fit for my draw, but the store had a discount for the 145Ah and it fitted also better in the space (taller rather than larger). Also the store told me that a bigger battery it's not a problem.
At this point I'm not sure what to do. I do not have space for an extra panel and the store surely won't take the battery back.