Some AGM questions

[EasternPromise]

Hi all. I'm sure some of these Qs have been addressed in other threads, but I could not find specifics.

I have a 48V bank of 24x 2V Rolls AGMs. I'm trying to do whatever possible to prolong the life of the batteries, as much as is practical.

1. Ambient Temperature.
How sensitive are these models to temperature? My bank is stored outside in an insulated box, which has a fan running in from inside to keep the temps up. I I don't yet know how cold it will get in winter, but in summer it has gotten up to 33C in the box. How much life am I shaving off the batteries with every degree north of 25C and should this be a concern? I can probably rig something up to blow in cold hallway air as opposed to warm/hot mechanical room air, but if it's all for naught - I'd rather not fuss with it.

2. Cycling and full charge

By definition, in a renewable PV setting, does one cycle = a 24 hour period? E.g.: one charge and one discharge?

The manual states: To maintain good health, VRLA AGM batteries should be brought to a full charge on each cycle or, at minimum, once every 6-7 days.
The manual then later states: It is very important that VRLA AGM batteries are brought to a full charge frequently to prevent capacity loss.

Anytime but winter - full cycle recharge isn't an issue, but becomes problematic during winter - especially around the solstice. Where is the trade off between cost of fuel and maintaining battery longevity? The last 10% or so take a while to charge, and that's a lot of generator run time. How much battery life do I save by a full daily recharge vs. full recharge every 6 to 7 days? What about recharge to 90% vs. 80%, etc.?

I appreciate these are difficult questions to answer with precision, but it would be helpful to get a feel for this as I get into the first winter on this set.
Big thanks!

 

[Wellbuilt]

Those are nice battery’s , I would keep them as cool as possible .
My garage stays 67o all summer and my battery’s hit 78o on a warm day after charging witch is not to bad .
Your bank is seen 91o so you are hot for sure .
may be get them under ground ?
Winter time is good for them they have less capacity but stay happy
my bank can hit 10o just sitting .
Once they start to charge and discharge my temp will come up 5o a day until they hit 47o then they stay there all the time.
I use about 10% of my battery over night, and about 13% if my panels are snow covered and no sun
I run a 5 day cycle , day one I skip charging day 2 I’ll charge for a hour day 3 I’ll charge for 11/2 hours day 4 I’ll charge for 2 hours day 5 I charge for 3 hours
I figured that I need to absorb down until my bank is taking in 500 watts . I have a 430 ah battery at 48 volts .
I can replace 2000watts a hour +- and I use about 3500/4000wats in 24hours
I try to do a 3 hour absorb every 5th day .
The thing is charging with lead ac batterys the lower you discharge the longer it takes to charge back to full
if I use 10% of my battery over nite I can replace that power in 11/2 hours .
If I use 20% over nite it takes me 31/2 hours so dis charging and charging is not equal .
agms can be charged faster with more power in a shorter time .
I do not go below 65% it takes a lot of time to re charge 20/25% seems to be ok.
I try not to EQ when it’s cold my charger dos not have temperature compensation
I can bump up my Voltage from 62v to 64 or 65 volts if it’s cold .

 

[Hedges]

Battery temperature sensor connected, and charge controller set according to specs from Rolls?

My Sunny Island has a relay to enable fan for batteries based on temperature, so you could do something similar.
Lead-acid produces heat while charging.

Are you able to hit and maintain optimum charge current?

 

[Substrate]

Temperature is important, especially during charge. Your manual should show how the CV voltage needs adjustment/ compensation for optimum life. This means a charger or solar controller that has a REMOTE temperature sensor probe attached to one of the battery terminals or at least inside the box.

Obtaining a 100% charge is very important, otherwise you will walk down the capacity with sulfation. Ie, if you undercharge and that last 1% of capacity isn't recharged, it hard sulfates - and as such is no longer rechargeable, and you now have a 99% capacity battery. Repeated undercharging reduces your capacity even further.

Some users with huge banks will use a generator for the majority of the bulk mode of charging, and then switch to solar for the absorb - which you've noticed takes a long time and is quite costly from a petrol standpoint. Popular application for mariners and others who need to conserve fuel.

Trying to put exact numbers on all this is a recipe for endless speculation. Either follow the manufacturers advice or don't - and replace batteries sooner than you have to. For some the cost and practicality of doing it right isn't a concern.

 

[EasternPromise]

Battery temperature sensor connected, and charge controller set according to specs from Rolls?

My Sunny Island has a relay to enable fan for batteries based on temperature, so you could do something similar.
Lead-acid produces heat while charging.

Are you able to hit and maintain optimum charge current?

Yes and yes. Sensor connected and charge controller set appropriately. Also - hitting appropriate charge currents. I'm just wondering whether and how relevant ambient temps are outside of this. Thanks!

 

[EasternPromise]

Temperature is important, especially during charge. Your manual should show how the CV voltage needs adjustment/ compensation for optimum life. This means a charger or solar controller that has a REMOTE temperature sensor probe attached to one of the battery terminals or at least inside the box.

Obtaining a 100% charge is very important, otherwise you will walk down the capacity with sulfation. Ie, if you undercharge and that last 1% of capacity isn't recharged, it hard sulfates - and as such is no longer rechargeable, and you now have a 99% capacity battery. Repeated undercharging reduces your capacity even further.

Some users with huge banks will use a generator for the majority of the bulk mode of charging, and then switch to solar for the absorb - which you've noticed takes a long time and is quite costly from a petrol standpoint. Popular application for mariners and others who need to conserve fuel.

Trying to put exact numbers on all this is a recipe for endless speculation. Either follow the manufacturers advice or don't - and replace batteries sooner than you have to. For some the cost and practicality of doing it right isn't a concern.

Interesting feedback. It may make sense for me to then charge up in the mornings to say 90% and let the PV do the work during the day to hit 100%. Great tip!

 

[Hedges]

Depends on how much PV available, and how low SoC gets overnight.

Some systems have considerably more PV than needed to charge at 0.2C (or whatever you batteries' desired charge rate is), and regulate charge to that target amount while supplying additional current only when used by loads like inverter. Others deliver whatever power is available from PV, so battery gets what's left after consumption, and charge rate varies.

If not that low an SoC overnight, battery would recharge to the point of switching from CC to CV early in the day so even with large PV array and simple SCC, charge current gets regulated to what battery wants.

My AC coupled system has enough PV for about 0.55C charge rate, but it is regulated at 0.2C

 

[Wellbuilt]

My neighbor uses 24 6v battery’s and has minimal solar power .
He runs a diesel generator on corn oil ?‍ Every morning at sun rise
then the sun finishes the job .
I think you need enough solar to charge up and run day time loads for 9 months out of the year .
There is enough generator run time in the winter ?

 

[EasternPromise]

Follow-up questions here re: AGMs.
It's almost winter solstice and hitting 100% recharge has been increasingly difficult. I've been following manufacturer's instructions on getting recharge to 100% at minimum every 6 or 7 days, but I can't now seem to push above 99% despite the bank still absorbing significant amperage (45amps at 48V).
Is my bank capacity now reduced to 99% and that's just what I live with? But if so, shouldn't the bank just go into float mode at 99% and absorb a lower amperage?
I'm concerned that it's absorbing more than it should at capacity and I'm doing damage to the bank by overcharging, if that's a thing.
Appreciate your thoughts.
Thanks,

 

[Wellbuilt]

I use flood battery’s , but my charging dose weird things in the cold weather it takes longer to charge with higher volts
are your battery’s Temperature compensate?
I raise my voltage to 59.5 this time of year but the charger compensates for the cold and bumps it to 62ish or a little more it takes longer to charge cold battery’s
how cold are your battery’s ? Mine start acting funny at around 45 o and below .
I use a infrared heat gun to check the battery temp .
If my battery’s are not hot I’m not losing fluid they should be ok .
I start off cold at 20 o and after a few days of charging an discharging I get to about 50o
after that the charging seams to go better and I get to full in a Reasonable amount of time

 

[Hedges]

Insufficient charge could lead to degradation, not limited to 99% of capacity. Basically shortens useful life.
Can you use a generator early in the day to raise state of charge (when battery can accept higher current), then let PV finish charge and absorption?
(or better yet reduce consumption somehow. Or over-panel so more solar charging.)

Charging the recommended boost voltage and absorption time should not be excessive over-charging, just the best way to fully charge them. I think the cells get some degree of over-charge but H2 and O2 get catalytically recombined.

 

[EasternPromise]

I use flood battery’s , but my charging dose weird things in the cold weather it takes longer to charge with higher volts
are your battery’s Temperature compensate?
I raise my voltage to 59.5 this time of year but the charger compensates for the cold and bumps it to 62ish or a little more it takes longer to charge cold battery’s
how cold are your battery’s ? Mine start acting funny at around 45 o and below .
I use a infrared heat gun to check the battery temp .
If my battery’s are not hot I’m not losing fluid they should be ok .
I start off cold at 20 o and after a few days of charging an discharging I get to about 50o
after that the charging seams to go better and I get to full in a Reasonable amount of time

Batts are temp compensated and it's not a temp issue today. They're probably hanging out at 70F. I've had full day of sun and I can't get the bank above 99%. It's been like that for probably 2-3 hours now and still absorbing 40 amps or so.

 

[EasternPromise]

Insufficient charge could lead to degradation, not limited to 99% of capacity. Basically shortens useful life.
Can you use a generator early in the day to raise state of charge (when battery can accept higher current), then let PV finish charge and absorption?
(or better yet reduce consumption somehow. Or over-panel so more solar charging.)

Gennie in the morning and having PV finish off is my standard routine at this time of the year. Hasn't been an issue to date, and it's my first winter with this bank. But today I've had full sun *all day*... and I can't move above 99%, with the bank absorbing 40+ amps at 48V, and refusing to flip into float.

Charging the recommended boost voltage and absorption time should not be excessive over-charging, just the best way to fully charge them. I think the cells get some degree of over-charge but H2 and O2 get catalytically recombined.

Ok, so if I'm interpreting this correctly - I'll just continue to let it absorb and hopefully I hit my charge? It's a bit confusing though because if it doesn't hit 100%, it seems like I'll just never hit float mode again...

 

[Hedges]

48V?
What voltage exactly? That is far from full.

Do you know how much power you're harvesting and how much you're consuming?

My system is set up to charge at constant current 0.2C, and can maintain that over a range of AC loads. Followed by an absorption period with CV, then dropping to a lower CV for float.


40A ... how many Ah is your bank? Mine is 405 Ah, and I've set CC to about 85A.
For my SunXtender, it says if CC is < 0.2C, then an alternate absorption cycle should be used in order to get them full charged.

 

[EasternPromise]

48V?
What voltage exactly? That is far from full.

Do you know how much power you're harvesting and how much you're consuming?

My system is set up to charge at constant current 0.2C, and can maintain that over a range of AC loads. Followed by an absorption period with CV, then dropping to a lower CV for float.


40A ... how many Ah is your bank? Mine is 405 Ah, and I've set CC to about 85A.
For my SunXtender, it says if CC is < 0.2C, then an alternate absorption cycle should be used in order to get them full charged.

It's a 48V bank. 24 x 2V batts. Under charge load, currently sitting at 56V. I've been consistently delivering 40amps to the bank for the past 3 hours while it's sitting at 99% charged. I'll need to scratch the noggin' to convert the amp hours.

What is the CC?

 

[Hedges]

CC Constant Current, CV Constant Voltage.
Sunny Island is my inverter/charger. It had some defaults, which I adjusted according to tech specs for my battery.

I don't have a lot of experience with the batteries. They are mostly floated on-grid, only cycle during a power failure (if at night). During the day, they just float because PV is usually greater than my loads.

Your system's report of % charge may or may not be accurate. I've heard Sunny Island reports funny numbers. Yours may hold CV for absorption for some number of hours it is programmed for. Not sure if it terminates based on something other than time. 2 to 4 hours is typical. What does your battery manual call for? Is your charge controller programed to match that?