Absorb and bulk times

[Vic]

Hi Vic,
Reading your post meanwhile taking my morning coffee so be forgiving ;-).
I understand that batt have a cycle life but doesn't cycles rise depending on DOD?
In order to understand the skip day function, First I need to know how cycles are calculated.
If during a day with a load, batt charging pattern goes from absorb to float several times a day (from let say 80%SOC to 100% SOC), does that means every time batt goes from absorb to float it counts for 1 cycle or some kind of a portion of a cycle? From what I can see on the Trojan cycle curve, I understand that it should count for only a portion of a cycle and the skip absorb function is then  valid only when a minimum load is put on the batt, i.e. WE cabin

Hi dapago,

When you said,  "If during a day with a load, batt charging pattern goes from absorb to float several times a day   ..."   Then,  you must have set ReBulk (RB)  to some voltage above the Default of 8 V,  and somewhat below the Float voltage setting.  The RB function allows for several charge cycles in one day,  if the battery becomes discharged to or below the voltage setting for RB.

If you are seeing more than one charge cycle in a single day,   very often,  then your system is not a candidate for Skip Day charging.

Skip Day charging for Lead Acid (LA) batteries is usually best for larger Capacity banks that are not cycled very deeply on the average day.  Shallow-cycling LA batteries (to a SOC that is not below 90%)  is generally not good for them.   So Skipping charges allows the battery to cycle below 90%,  and reduce the total time above Gassing Voltage  (Absorb is above Gassing V,  which causes positive plate erosion,  which is not so good for those plates).

It is possible that every battery manufacturer has a somewhat different idea of what is a Cycle.   The bottom line for Flooded batteries,  it would seem that every hour spent in Absorb will extract some life from the battery.   AND every hour spent in EQ  will extract even more life from the battery.

BUT,  repeatedly cycling LA  batteries without fully charging them once or twice per week can be more damaging to batteries than Absorbing and EQing them when they need it,  and Absorbing and EQing them,  doing so within the range of voltages for these stages that are recommended by the battery manufacturer is what really counts.

There are other mechanisms which will help determine when the end of battery life occurs.  Batteries that are well cared for,  will still have a Calendar life  --  months since manufacture,  but this also depends upon a number of variables.

Chronically undercharging LA batteries is very bad for battery life.  Not keeping batteries relatively cool also takes much life from them.   The rule of thumb,  that for every 10 degree C rise in average temperature of a battery will remove one half of its life,  also applies.

There are few absolutes when dealing with batteries,  and what is the "best" thing to do in a certain situation ...   many times,  depends  upon many variables.

So,  bottom line,  IMO is,   if your Flooded batteries really need to be recharged more than once per day,  and/or,  your system does cycle to 80% SOC,  or below on the average day,  the system does not the  Skip Day function.

Occasionally,  it can be healthy for Flooded batteries to be cycled down into the 50 -60% SOC range,  and then recharged in a relatively short time period.   Doing this would normally require the ability to charge at a rate of about 10% of 20 hour Capacity,  or having a generator which can help get the battery above 80% SOC within about one day,  and fully charge by the end of the following day  ...   from memory,  believe that you have enough PV power to do this recharging (about 2400 W STC,  IIRC).  Guess that your PVs could charge at about a 37-ish amp rate,  without any loads.   Which may be about 15-ish percent of battery Capacity with no loads   ...

Opinions,    FWIW,    Vic

[russ_drinkwater]

Vic,
these surrettes I have here only get 40 amps max charge ( per 24 volt bank) during the day as the classics go through their cycle.
So 24 volts @ 1350 amps there should not be any concerns for the battery life as the input charge will never get to 10-20% of rated capacity?
Even though I have a lower input amperage daily charging (cycle) will reduce the life of the cells?
I have noticed the temps of the surrettes never seem to get higher than the ambient temp here.
I am assuming this is because they are not getting the recommended 100 plus amps of charge per bank.

[dapago]

Vic,

That is a complete answer to my question. For the moment I haven't all the load  I will have when place is fully equipped and I believe my SOC now is between 80/90% in the morning. Can not be sure until a batt meter is installed, hopefully in a few week. I bought a device to check gravity and try to know the SOC in the morning but not sure it is of good quality as the meter inside sometime seems to get stuck at the bottom.

ReBulk set shows <8v so that should be ok and eq being at 64.8v it gives 8.1v per batt so >8v. On that side everything seems ok.

[Vic]

Vic,
these surrettes I have here only get 40 amps max charge ( per 24 volt bank) during the day as the classics go through their cycle.
So 24 volts @ 1350 amps there should not be any concerns for the battery life as the input charge will never get to 10-20% of rated capacity?
Even though I have a lower input amperage daily charging (cycle) will reduce the life of the cells?
I have noticed the temps of the surrettes never seem to get higher than the ambient temp here.
I am assuming this is because they are not getting the recommended 100 plus amps of charge per bank.

Hi Russ,

There various opinions about many of these aspects of battery charging.

The charge rate on your standalone system would be a bit below 3% of 20 hour Capacity of 1350 Ah (folks like me,  believe that these 4KS25Ps have an actual Capacity of about 1280 Ah IMO).

For batteries with some loads while being charged,  3-ish percent of C charge rate might not be enough on a number of days, to finish a charge,  and one could get into deficit charging  (never, or almost never fully charging the batteries).

But,  if you are monitoring SGs,  and your DOD is not too great,  this rate can be just fine.

Most Flooded battery heating during charge comes when the charge voltage is above the Gassing Voltage.  The deeper the discharge,  the longer will (or should) be the Absorb time,  and therefore greater heating of the batteries.  EQing produces even more heating than does Absorb,  as water in the electrolyte is broken down ah a faster rate,  for Flooded batteries.  This breaking down on the water is very inefficient,   and creates a lot of heat).

Generally there is very little heating of Flooded batteries,  until the Gassing Voltage is reached.  The more that the charge voltage exceeds Gassing voltage,  the greater the rate of heating.  So,  most of the Bulk charge stage generates little heat.

I DO believe,  that occasional high rate (high current) charging (10 - 13% of Capacity),  can be healthy for batteries,  just as are occasional discharges into the 60 to 50 percent DOD range for FLAs,  and a fairly rapid full recharge (within one day) .

If you can fully-charge your Surrettes when they need it,  with your 40-ish amp max charge rate,  then this should be OK.

With this 40 A charge rate,  you might have trouble EQing your battery,  even with your probably wall-to-wall sun,  on many days per year.

FWIW,  the main battery banks here are charged at about a 70 - 75 amp max rate,  which has worked fine,  on most days.  EQing on a partly cloudy day can need a smallish genset for a bit of help.

Just my opinions,   Vic

[russ_drinkwater]

Thanks Vic,
The 2 x 24volt banks are less than 10% discharged from sunset to 5 am in summer.
Each array has another 4 panels to go into them.
So once I get organised the bank I will be keeping will be getting close or better than 50 amps charge at peak.
The classics show on average around 5kw into each bank per day.
Have had a new satillite dish for the internet installed so I can install the rest of panels now as the shading issue I had has been resolved!
Now that the days are longer the sg's on the cells all sit around 1275 by 1200 hrs.