End current for AGM batteries

[SolarMusher]

Hi all,
Does someone know about end current and/or basic absorb time that could be set for AGM batteries (especially Rolls S-460 AGM)?
Is that about the same than FLA?
Battery bank would be 415Ah @ 24V, Absorb 29.4V, Float 27.6.
Have never used AGM battery.
Thanks for your help,
Erik

[zoneblue]

I dont know specifically for your batterys, but AGM in general have a much lower final acceptance current, and 0.5% of the C20 rate is an oft recomended EA figure. You will need WBJr to work with this fine degree of accuracy.

But as always check your datasheets, and best of all do some observation. After a long sunny day , when the bank has been in float all afternoon, with your inverter off, what does WBJr say. Thats how low your acceptances goes, and can guide the figure you select.

Also its a good idea to set WB EA about a half amp higher than your calc to allow for the noise floor detection to work right.

If you dont have WB, then EA may not be that good of an idea, as the loads (depending on how predictable they are) will screw it up.

Lastly youll need a firmware 1769 or better, thats when WB smoothing was introduced.

[SolarMusher]

Thanks ZB,
Ok I will start with 0.5% EA, no problem with the Wbjr. Just curious, what would be the voltage in the evening when not charging and with the inverter on (small loads)?
Any idea of your personnal absorb time based on your average deep of discharge ?

[zoneblue]

Thanks ZB,
Ok I will start with 0.5% EA, no problem with the Wbjr.

If you start at 0.5% watch it the first few days to make sure it trips.  And set max absorb time as a fallback.

Just curious, what would be the voltage in the evening when not charging and with the inverter on (small loads)?

Healthy batterys (assuming 24v nom), will come to rest around 25.2V in the early evening. After drawing them down 10% overnite, rest voltage will be around 24.8V. Those numbers will vary a little across batterys but not a heck of a lot.

Any idea of your personnal absorb time based on your average deep of discharge ?

Around 2 hours here for our shallow discharges. I only do it once every 5 days for that reason.

[Vic]

Hi Erik,

Some AGMs appear to have astonishingly low EA values,   some at something like 0.01% of 20 hour C.  Believe that some others spec end of Absorb when the Rate Of Change of Charge Current is below some value -- both are very difficult to do in RE applications.

Surrette Battery has been very responsive to telephone info requests that I have made in the past,   and they may do well via e-mail as well.   AGMs are a fairly new line for them,  so there may not be too much EA info in the battery manual yet,  but please try pinging them for that data.   Good Luck,   Vic

[SolarMusher]

Hello Vic and ZB,
Hey Vic, hope you're fine!
Thanks a lot to share your personnal experience with AGM.
This is a system upgrade for a customer next spring, problem is that this guy already owns a FM60 . I visited his cabin yesterday in 4 feet depth of snow and the system was 100% down, FM60 was blank because of FLA batteries which were near dead (-30°c for the past three days, yes spring is coming soon!). I've no idea how to set Absorb for AGM in the FM60 when this cabin is only used for spring/summer, I hope that I'll be able to use EA with inverter system Off and no DC loads in winter and a basic time value when this system will be used. I've contacted Steve at Rolls to know more about end current for these AGM batteries, we'll see.
Budget is going to be limited so not a single chance to change this Outback for a Classic/WBjr (it would be too easy...).
Erik

[Vic]

Hi Erik,

WOW  4 feet of snow!    High temps here have been in the 70s for a couple of weeks.  Glad that you can take the cold and snow/ice (I cannot!).

Have you seen the latest Rolls Surrette  Battery Manual ?  It covers AGM and Flooded,  and appears to have been re-written somewhat:
http://support.rollsbattery.com/support/solutions/articles/654-battery-user-manual-flooded-

There is a set of charge curves which might help,  but hearing from Steve,   with specific RE charging advice should be the best bet (of course).

Temperature compensated Resting voltage is a very good way to determine SOC,  but it takes rest time of about three hours,  usually.  You may not have that kind of time to spend,  and obviously,  if  changes to charge parameters need to be made and monitored,  this COULD take a week or so.

AGMs seem like a great idea,  as self-discharge is so low,  but very cold Flooded batteries also have fairly low self discharge.   Always tradeoffs.

Good Luck,  and enjoy Spring.    Thanks,   Vic