Float Charging Flooded Lead Acid Batteries?

ligwyd

Is it hard on Flooded Lead Acid batteries if they do not receive a full float charge periodically?

I find when my batteries go through a bulk, absorb and then into float charge, the amps required to float charge slowly drops as the batteries reach full saturation. If I leave the power on them, in float charge, it can take 24 hours or more before the charge amps no longer are dropping. At this point the batteries are fully charged, but doing so regularly when off-grid would not be be possible in the sun and not economical when charging from the generator.

mike95490

Right,  solar sort of breaks the rules, because you only get 5 hours of sun to recharge everything.   In order to put enough back into the batteries, you need to raise the absorb voltage a bit, till your tests show the battery is getting fully recharged at least every 3rd day.

mvas

ligwyd said:

Is it hard on Flooded Lead Acid batteries if they do not receive a full float charge periodically?

I find when my batteries go through a bulk, absorb and then into float charge, the amps required to float charge slowly drops as the batteries reach full saturation. If I leave the power on them, in float charge, it can take 24 hours or more before the charge amps no longer are dropping. At this point the batteries are fully charged, but doing so regularly when off-grid would not be be possible in the sun and not economical when charging from the generator.

You are recharging your battery bank way too slowly.
Do you have AGM or Flooded?

Can your battery bank accept more amps during Bulk Mode?
Can your battery bank accept more voltage during Absorb Mode?

With Solar ...
a) The short time to recharge the battery bank is your enemy.
b) Some days you may not get back to 100% SOC.

Describe your ...
a) Battery Bank
b) Charging Parameters
c) PV Array
d) Daily Depth of Discharge

Photowhit

ligwyd said:

Is it hard on Flooded Lead Acid batteries if they do not receive a full float charge periodically?

It  is hard on flooded batteries if they aren't fully charged 2-3 times week.
Float should have  very little if any charging going on. In float  the voltage is high enough that the charge controller can increase energy from the array before drawing from the battery bank when sensing a load.

ligwyd

So could a FLA battery survive just fine without a float charge at all and just using the 2 stage charge cycle(bulk and absorb)?

There is very little charging going on in float anyway, as Photowhit has mentioned.

Of course if it is sunny and the batteries are full, the batteries can float charge and run loads from the sun.

Mvas,

The batteries and just sitting and are charged up from grid power each month.These are flooded lead acid batteries. The XW+ automatically controls amps and only voltage is adjustable and yes the batteries can accept whatever I through at them. Only time in service will tell when and how much I may need to bump up absorb voltage to make the most of the sun hours.

Estragon

The main purpose for floating is to offset battery self-discharge.  How much depends on battery type, age, and temperature, but it shouldn't be much (eg. maybe 1% of capacity).  If it's taking much more in float, this suggests absorb charge voltage and/or time may be insufficient to fully charge (or batteries nearing end of life).

In cool/cold temps, a fully charged battery can be left with no float (or loads, including controller self-consumption) for weeks/months.  In warm temps or with even small loads, floating would be wise.

If sitting floated, a bulk/absorb cycle is only needed monthly or so to prevent stratification in flooded batteries.

ligwyd

My XW kicks into float at 2% of capacity. 2% of 630amp/hour = 12.6 amps. Then it starts to float, and displays about 5 amps at the lower float voltage. Then amps slowly reduce to around .5 amps after 24 hours or so when charging from grid power. I also give the bank a 1 hour (default EQ cycle) over charge each mth to help mix up the electrolyte.

Off-topic a but, but if the battery where to sit for say 3 months, fully charged with no float, would the concentration of acid become higher at the bottom of the battery and lower and the top?

Estragon

The stronger (and denser) acid will tend to sink, leaving weaker acid nearer the top.  Fully charged though, even the weaker acid on top is still pretty strong.  Apparently tall flooded batteries (eg L16s) are more prone to this happening.

In reasonably cool conditions, 3 months is probably ok.  In hotter, maybe not.

Vic

IMO,   a considerable amount of battery recharge WILL occur in Float,  at any reasonable Float voltage.

To fully charge a FLA battery,  the charge voltage should be raised above the Gassing voltage of that battery.   This is in the range of reasonable Absorb voltages.

Gassing is the friend of FLA batteries,  as it does help mix the electrolyte,   de-stratifying the battery.   Tall batteries require more electrolyte mixing,  than do short ones.

The ideal Float voltage will just barely keep,  a fully charged battery,   fully charged.

On the Flooded banks in use here,   the battery charge current,  an hour after the end of Absorb,  is about  0.1% of 20-hour Capacity.

FWIW   YMMV,   and so on.   Vic

ligwyd

What exit threshold do you use Vic? (Exit - from Absorb to Float) I use 2% of 20 hour Capacity.

On my battery spec sheet it does have slightly higher (.3 of a volt @ 12Volts - so 1.2 volts higher @48 Volts) recommended Absorb and Float voltages for cyclic use. Now i have the V set for standby use.

How many amps are displayed just after your charger starts to float? (Just so I can compare to my set up)

Estragon

FWIW, my setup ends absorb at closer to 1% of C (5a on 370ah bank IIRC).  Current drops to zero at the end of absorb while voltage drops from Vabs to Vfloat.  This can take some time with no loads.  When not actively cycling  I currently have the system set to do a bulk/absorb cycle done ~every 14 days.

As Vic notes, absorb voltage should be high enough to get some gassing.  Not boiling but more of a fizzing with an occasional "burp".

mcgivor

The LA 1%, 2% rule is largely dependent on batteries being in good health, as they age they may however they may never actually reach such a low current, thus resulting in overcharge, and or gassing unnecessary. The fine line of ballancing the charging regime to suit the demand, this  is something each and every system differs from one another,  the  one size fits all rule dosent apply, 

Keep in mind however the most common battery failure mode with lead scid is undercharging, as previously stated it's a fine line, lead acid, so simple in concept but complicated in reality.

Estragon

Good point about the aging bank.  Mine are still getting to end-amps, but I expect they'll start timing out at some point and incremental increases in end-amps will be needed.

BB.

If your "end amps" stable current is >~2-2.5%, it is time to replace the battery bank. Your bank is actually running the risk of overheating or even worse.

-Bill

ligwyd

I've got a 630amp/hour battery bank. Max charge rate presently set to 10% - 63Amps max. XW transitions from absorb to float at 2% of 630 amp/hour - 12.6 amps. Then in float, the current displayed on the XW, slowly decreases from approx 5 amps, over 24 hours or more, to .5 to .75 of an amp after 24 hours or more, of being in float charge (from grid power).

Vic

ligwyd said:

What exit threshold do you use Vic? (Exit - from Absorb to Float) I use 2% of 20 hour Capacity.

On my battery spec sheet it does have slightly higher (.3 of a volt @ 12Volts - so 1.2 volts higher @48 Volts) recommended Absorb and Float voltages for cyclic use. Now i have the V set for standby use.

How many amps are displayed just after your charger starts to float? (Just so I can compare to my set up)

Generally,  the bank that gets more regular cycling here,  uses an EA of about 17 A on a 1280 Ah bank that is in its fifteenth year of service.   These banks are old Surrettes  --  classic Lead-Antimony batteries.

The proper EA on these banks  varies considerably vs DOD in the previous discharge.   This is one reason that I'd like to see CCs like the MN Classic have the ability to use Rate-Of-Change in battery charge current as an option to end Absorb.

The deeper the battery discharge,  the higher the EA setting needs to be,   or else,   Absorb will not be terminated by the EA setting,  but (with most good CCs)  the Max Absorb time will cause the termination.

We try to choose an EA setting that is far out on the diminishing battery charge current curve,   where one wonders if the average finishing current will EVER drop any more.   Measured finishing current can have a reasonable amount of current Jitter.  This Jitter can make the human averaging a bit difficultto see,     and so on. 

Of course,   there is some Surface Charge effect at the very beginning of the transition to Float,  from Absorb,  so,  this results in the CC providing 0.0 A at that time.   The battery charge current,  early in Float is a bit difficult to describe,   should really graph it  ... (but,  not now).

More later,   FWIW,   Vic

ligwyd

Next time I give my batteries a charge, I might try to reduce the End Amps (EA) from 2% (of 20 Hour Capacity rating) to 1%. So in my case 630 amp/hour X .01 = 6.3 amps. Of course any time I make any adjustment I monitor everything very closely. We'll see how it effects the end stable current in float, and time it takes to get there.

softdown

Inferior battery chargers are prone to quitting before any absorb cycle. They hit ~ 12.7 volts and call it quits. If you use such an inferior battery charger you will need to charge stored batteries a lot more frequently. 

It is why I keep raving about my new Duracell chargers. Only 4 amps but they keep charging for ~half a day after hitting the proper voltage. So I wind up at 13 volts instead of 12.7 volts. 

Interesting that so many posters are whipping out math formula's this year. I know a professional solar installer with years and years of experience. I don't think he uses a single formula. 

ligwyd

What formulas are you referring to?

softdown

ligwyd said:

What formulas are you referring to?

STEM degree + DOD = Formulas

ligwyd

Anyone care to expand?

Estragon

IMHO, a bit of simple math is unavoidable.  You don't really need to get deep in the weeds of charge/discharge curve math.  Setting EA at 1% is just picking a point on the curve that seems to work well for lots of people.  Bank capacity times 1% isn't exactly advanced calculus.

Maybe Softdowns installer friend can divine cold Voc somehow, but mere mortals like me have to do some arithmetic (or use a string calculator).

softdown

My suspicion is that dealing with the same suppliers gives him a few options: 
Small
Medium
Large
XLarge
Etc. 

So you do the math and think that you need a 968 watt inverter with a 2349 watt surge. 1441 watts of panels. 731 pounds of deep cycle batteries. You find stuff that is close yet it is still way off for several reasons. Then the wife has twins and your brother develops cancer and has to be taken care of - along with his family. 

What really determines many solar arrays:
Roof space
Battery room
Well pump surge
Budget
Anticipated future
Deals

Life has curve balls. Nothing but curve balls. We are talking about energy creation and adding more can be problematic. If we are going to live in a place it may be a good idea to install more than we theoretically need need. How many people regret an abundance of energy storage or ability to harvest that energy?

If numbers were crunched a 67hp Lada might be recommended. Would I be happy with its limitations? Often it is better to get more than you need. I wonder how many people have lamented to D Angelini that they wish they had less power. Or told Marc K that they wished they had less storage capacity. Does it happen? Of course. Is it the norm? 

I would not want to be sitting here yearning for more storage capacity when dealing with common -15F temps, savage winds, and darkness. Luxury is appreciated at times. 

BB.

In my humble opinion, size the battery bank to your needs... A typical starting point is 2 days of "no sun" and 50% maximum discharge on average (going below 50% state of charge can be OK on occasion, but you need to get the batteries recharged pretty quickly--Don't let them sit for many hours/days below 50% SoC--If they are cycling, that is better than just sitting there).

And spend your $$$ on more solar panels and solar charge controllers... 5% rate of charge (20 hour battery capacity specifications) can work for standby/summer weekend systems--But for daily cycling, start with 10%-13% rate of charge minimum, and adding more panels is very helpful too (especially in Winter for most folks not near the equator). (and sizing for hours of sun per day by season, no shading, etc. too).

Using "cheaper" flooded cell batteries for your first bank or two until you get things working well--Then look at AGM or LiFePO4 batteries longer therm (for fixed/cabin/home installations).

As all rules of thumbs--The above is a good starting point. If you have something "different' (RV, not much room, not much extra weight capacity, tools, etc.), then details on the loads are critical to figuring out what your "optimum system"  may be...

Over-sizing the battery bank, without appropriate charging capability (more solar panels, more genset/AC mains charging capacity, etc.) can be an expensive way to cause a shorter than normal battery bank life.

If you have a good working bank and 2x oversize it... Yes, it may last upwards of 2x longer, but it also cost you 2x as much. Obviously, less battery change outs (6-10 years vs 3-5 years) is not a bad thing for more people... But you are also "risking" your expensive "over-sized" battery bank being killed by other things  that may sneak up on you (guests leave everything on when going out for a couple days, lighting kills charge controller while you are away, somebody leaves inverter on over winter at the cabin, etc.).

-Bill

ligwyd

My battery specs state maximum life span 4500 cycles at 20% D.O.D. The cycles stated for 50% D.O.D. is half that. Would it not be wise to design a system based on 20% D.O.D., albeit more upfront investment, and have lots of extra power when needed?

Estragon

I think it really depends on the application.  A weekend summer cabin would be different than a standby/UPS which would be different than a full-time off-grid, etc.

For more or less full-time off-grid, the rule of thumb is to design for two days no sun to ~50% max DOD.  Mine is designed that way, and the way it works out is it mostly cycles between ~80-100%.  In gloomy stretches I use the genny to cycle between ~60ish and 85-90%.  Having much more bank wouldn't really change that much.  The genny could run less often, but for longer to recharge a bigger bank.

ligwyd

And do you do any float charging from the generator or just bulk and adsorb down to 1% or of 20 Hours capacity?

BB.

In theory (as I have read over the years here), as long as your lead acid battery bank is cycling daily (like 50% to 80%), you really do not need daily Float/EQ charging.

The "standard" reply is to cycle to >90% once per week. However, one major mfg (Rolls/Surrette?) had said that >90% is only needed every 4 weeks (if daily 50-80% cycling).

If the batteries are not cycling (you leave the cabin for a week at 50% SoC and bad weather)--You do really need to get upwards to 90%+ State of Charge before putting the cabin/batteries "to bed"--When there is no immediate solar charging available (days/weeks of assumed bad weather/missing sun) getting the bank charged will help reduce Sulfation (which is, more or less, permanent battery damage).

-Bill "I am not a battery engineer" B.

mcgivor

The cycle life is also temperature dependent, usually at 25°C, above that cycle life is reduced and inversely increased below that value, in theroy at least. Being that lead acid batteries also have a limited lifespan even when not cycled, there would be a point where having a shallow cycled larger capacity bank is less economical than a smaller deeper cycled one, it would be a calculation of energy pass through  over a given time versus initial cost. This would  a complex equation with many variables, which I don't profess to  have answers to, however from observation it would appear that in general, life expectancy all too often falls short of expectations and or of manufacturer’s predictions, outside of laboratory conditions anyhow.   

Estragon

ligwyd said:

And do you do any float charging from the generator or just bulk and adsorb down to 1% or of 20 Hours capacity?

I usually only bulk charge with the generator.  

ligwyd

With the XW+, to get it float charge down to 1% of 630 amp hour capacity would I have to program the capacity to be 315amp hours as opposed to what it actually is, 630 amp hour. This would cause the XW to transition into float at 6.3 amps instead of 12.6 amp like it is doing now. I'll have to read up in the manual again. Don't remember the "transition to float %age of capacity" being adjustable, but I'm sure it is somehow.