Scott54: Costco battery bank discharging too quickly?

System · February 2017

This discussion was created from comments split from: Different SG in each of the batteries in my bank.

scott54 · February 2017

I have 8 new 6v golf cart batteries from Costco. Wires in series (2 banks @ 24v). 1500w solar producing @ 30 amp. What I don't understand is how fast they draw down. I only have 2 freezers connected which draw @ 180w (including my inverter) which they seldom run together. I monitor with 2 shunt meters. Without the sun the batteries draw down in 30 minutes below a safe level. How can that be ? I watch the shunt meter during the day. It can vary from 29v to 25v in a short time. This is a concern because I understand it's important to keep the batteries above 60% charge. How can 8 batteries fluctuate with one freezer drawing 110w for 15 minutes ?

mike95490 · February 2017

scott54 said:

I have 8 new 6v golf cart batteries from Costco. Wires in series (2 banks @ 24v). 1500w solar producing @ 30 amp. What I don't understand is how fast they draw down. I only have 2 freezers connected which draw @ 180w (including my inverter) which they seldom run together. I monitor with 2 shunt meters. Without the sun the batteries draw down in 30 minutes below a safe level. How can that be ? I watch the shunt meter during the day. It can vary from 29v to 25v in a short time. This is a concern because I understand it's important to keep the batteries above 60% charge. How can 8 batteries fluctuate with one freezer drawing 110w for 15 minutes ?

Your batteries might be toast. If they charge up rapidly, and discharge rapidly, its often a sign of suflation, Might be able to EQ a couple of cycles and see if the SG comes up. You really need a hydrometer.
Daytime charging should be up to 28-29V and hold there a couple hours for absorb. What is your charge controller ?

littleharbor2 · February 2017

What panels (including specs) are you using? Charge controller? Battery history including any accidental abuse and equalizing?

mcgivor · February 2017

scott54 said:

I have 8 new 6v golf cart batteries from Costco. Wires in series (2 banks @ 24v). 1500w solar producing @ 30 amp. What I don't understand is how fast they draw down. I only have 2 freezers
connected which draw @ 180w (including my inverter) which they seldom run together. I monitor with 2 shunt meters. Without the sun the batteries draw down in 30 minutes below a safe level. How can that be ? I watch the shunt meter during the day. It can vary from 29v to 25v in a short time. This is a concern because I understand it's important to keep the batteries above 60% charge. How can 8 batteries fluctuate with one freezer drawing 110w for 15 minutes ?

It would be better to start a new thread regarding your problems rather than derailing a thread in progress, your needs would be better addressed, should you decide to do so.

Estragon · February 2017

@scott54 - definitely check SG on every cell in the bank. Also, I would check voltage on individual batteries while the freezer is running, if you don't have immediate access to a hydrometer. Could be a bad connection, lug crimp, etc., or maybe a defective battery.

A "typical" freezer will use around 1500 watt hours per day, so your two may draw ~3kw/day, plus maybe another kw for inverter/wire losses. You have ~400ah@24v or 9600wh of storage so you could hit 60% SOC after one day. Do you have a generator or other means of charging for cloudy days?

softdown · February 2017

Freezers and fridges consume major amounts of energy. A 1500 watt system is probably not the stuff to support two of them. Each freezer likely came with a yellow KwH rating tag inside of them. That rating may be for something that is wisely installed and rarely opened for long...if at all.

Electrical fridges were generally avoided until panel prices came way down and made them economic. Propane was the way to go in bygone years.

Costco is the retailer. Appropriate info would be Ah rating of battery and manufacturer.

scott54 · February 2017

Thanks for all the replies. I didn't know to start a new thread...... My batteries are brand new 6v 220Ah. My freezers are small and my shunt meter shows they each use @100w and my inverter uses @ 13w. My batteries do get up to 29v on a good day. My panels seem to produce 30amps. I'm using a ultra high efficiency Flexcharge controller.
I monitor the batteries all day and the freezers are disconnected before the sun goes down. Even one freezer connected using 100w plus 13w inverter draws the 8 batteries down quickly.
Watching a drop from 28.5V to 26V in no time when 113w is being pulled can't be right.
$3000 system and I get to run the 2 7cubic chest freezers only during a good day.

Estragon · February 2017

Your batteries, with no loads or charging sources, have a "resting" voltage of 25.6v fully charged. Voltage above this is a surface charge being absorbed by the bank while charging.

When a load is applied, some or all of this surface charge will go to the load. In the case of your freezers there will be a momentary starting current needed to overcome inertia in motor and compressor and to build initial pressures, which may be several times running current. In other words, seeing voltage drop when applying load to a charging bank may not, in itself, indicate a problem.

That said, you mentioned running a freezer after dark runs voltage down to "below a safe level" in 30 mins., and that is a problem. A "safe level" to run down to is around 24.2v (resting), which is about 50%, as long as the bank can be recharged promptly. It may drop below this with load running, but should recover to above 24.2 after the load cycles off.

I highly recommend you do the diagnostics suggested above. If you can catch a problem with a bad cell or connection now, there's a decent chance you can save the bank.

Estragon · February 2017

Also, can you post model numbers for your inverter and controller? I looked up a Flexcharge controller and it had an odd (to me) charge algorithm that could be part of the problem.

westbranch · February 2017

scott54 said:

.1..... My batteries are brand new 6v 220Ah.
2 My freezers are small and my shunt meter shows they each use @100w and my inverter uses @ 13w.
3 My batteries do get up to 29v on a good day.
My panels seem to produce 30amps. I'm using a ultra high efficiency Flexcharge controller.
I monitor the batteries all day and the freezers are disconnected before the sun goes down.
4 Even one freezer connected using 100w plus 13w inverter draws the 8 batteries down quickly.
5. Watching a drop from 28.5V to 26V in no time when 113w is being pulled can't be right.
$3000 system and I get to run the 2 7cubic chest freezers only during a good day.

A few questions...
1. Did you do a commissioning charge before using them? The bank capacity should be 50% of 440Ahr = 220Ahr
2 The size is not important, the Amperage draw on startup and then running IS. What is the startup surge in Amps?
Lots of people mistakenly associate the size with power consumption.... See if you can record the startup surge...
3. what happens on a bad day? what voltage do they get up to?
4 Yes, you are right.
5. What is the time frame you are referring ''in no time'' to? Is that measured in minutes? or 3 hrs? 4 hrs?
5a. Is this while the pv are producing power? If so there is a big problem somewhere...

Have you checked ALL the connections? You will need to re-torque them if they have not been done since assembling the bank.
Have you checked the SGs of each cell?
What are the specs on the panels?
How is the array configured ? How many hrs of full sun do you get on the panels>
Do you have ANY SHADE on those panels?

softdown · February 2017

westbranch said:

scott54 said:

.1..... My batteries are brand new 6v 220Ah.
2 My freezers are small and my shunt meter shows they each use @100w and my inverter uses @ 13w.
3 My batteries do get up to 29v on a good day.
My panels seem to produce 30amps. I'm using a ultra high efficiency Flexcharge controller.
I monitor the batteries all day and the freezers are disconnected before the sun goes down.
4 Even one freezer connected using 100w plus 13w inverter draws the 8 batteries down quickly.
5. Watching a drop from 28.5V to 26V in no time when 113w is being pulled can't be right.
$3000 system and I get to run the 2 7cubic chest freezers only during a good day.

A few questions...
1. Did you do a commissioning charge before using them? The bank capacity should be 50% of 440Ahr = 220Ahr
2 The size is not important, the Amperage draw on startup and then running IS. What is the startup surge in Amps?
Lots of people mistakenly associate the size with power consumption.... See if you can record the startup surge...
3. what happens on a bad day? what voltage do they get up to?
4 Yes, you are right.
5. What is the time frame you are referring ''in no time'' to? Is that measured in minutes? or 3 hrs? 4 hrs?
5a. Is this while the pv are producing power? If so there is a big problem somewhere...

Have you checked ALL the connections? You will need to re-torque them if they have not been done since assembling the bank.
Have you checked the SGs of each cell?
What are the specs on the panels?
How is the array configured ? How many hrs of full sun do you get on the panels>
Do you have ANY SHADE on those panels?

Great responses. I'll probably sound mean but I like the idea of filling out a form before the *oh so common* "my loads are bigger than my energy generation". Why.....because there are at least 20 different reasons why solar equipment will deliver a disappointing output. Not everybody will later admit something along the lines of "oh by the way, my panels are lying on the ground in a yard full of trees."

Sorry man....I may well be wrong but something doesn't seem quite right with installing two small freezers right off the bat. Then injecting into another threa. Rant over....sorry I was mean.

What other loads are there?

mcgivor · February 2017

scott54 said:

Thanks for all the replies. I didn't know to start a new thread...... My batteries are brand new 6v 220Ah. My freezers are small and my shunt meter shows they each use @100w and my inverter uses @ 13w. My batteries do get up to 29v on a good day. My panels seem to produce 30amps. I'm using a ultra high efficiency Flexcharge controller.
I monitor the batteries all day and the freezers are disconnected before the sun goes down. Even one freezer connected using 100w plus 13w inverter draws the 8 batteries down quickly.
Watching a drop from 28.5V to 26V in no time when 113w is being pulled can't be right.
$3000 system and I get to run the 2 7cubic chest

Expect a useage of around 2Kwh per day per freezer, yes you might be thinking that's not possible but experience has shown me that running a medium refrigerator/freezer alone uses this amount, 80Ah at 24V . Readings of daily solar production are almost identical for 90 days recorded, being that is the only load, including inverter, this would include all losses. The point being, don't simply use running wattage to estimate the amount of energy required, your panels need to not to only produce enough to run the freezers during the day, but also to charge the battery, and 12 hours of daylight is not 12 hours of sunlight.

Things may not be what they first appear to be, in the solar world.

scott54 · February 2017

softdown I already said I wasn't aware I needed to start a new thread. I looked for a "conversation" that was a related topic. Most forums don't have a problem with that........ I have worked in basic electrical for 40 years. I studied solar systems for a year+ before even initiating this. I am almost anal in my approach. Every wire is correct.
My reasonable mind tells me a golf cart runs all day on 4 batteries. So my concern is a freezer drawing 100w for 15 minutes is drawing an unreasonable amount from my batteries.
My 10 panels are actually 2000w total (24v). There are no trees. My inverter is a Samlex 2000 (mppt). My controller is a Flexcharge NC 25a. You might all research these as they are definitely the most efficient, problem free controllers available. I started with a couple BZ mppt 100 and they were crap (another topic for another day)
Softdown...... you made an interesting comment.....Please tell me why after completing my solar system I shouldn't plug in 2 small chest freezers ?

scott54 · February 2017

softdown I already said I wasn't aware I needed to start a new thread. I looked for a "conversation" that was a related topic. Most forums don't have a problem with that........ I have worked in basic electrical for 40 years. I studied solar systems for a year+ before even initiating this. I am almost anal in my approach. Every wire is correct.
My reasonable mind tells me a golf cart runs all day on 4 batteries. So my concern is a freezer drawing 100w for 15 minutes is drawing an unreasonable amount from my batteries.
My 10 panels are actually 2000w total (24v). There are no trees. My inverter is a Samlex 2000 (mppt). My controller is a Flexcharge NC 25a. You might all research these as they are definitely the most efficient, problem free controllers available. I started with a couple BZ mppt 100 and they were crap (another topic for another day)
Softdown...... you made an interesting comment.....Please tell me why after completing my solar system I shouldn't plug in 2 small chest freezers ?

mcgivor · February 2017

scott54 said:

Thanks for all the replies. I didn't know to start a new thread...... My batteries are brand new 6v 220Ah. My freezers are small and my shunt meter shows they each use @100w and my inverter uses @ 13w. My batteries do get up to 29v on a good day. My panels seem to produce 30amps. I'm using a ultra high efficiency Flexcharge controller.
I monitor the batteries all day and the freezers are disconnected before the sun goes down. Even one freezer connected using 100w plus 13w inverter draws the 8 batteries down quickly.
Watching a drop from 28.5V to 26V in no time when 113w is being pulled can't be right.
$3000 system and I get to run the 2 7cubic chest freezers only during a good day.

Rewinding a little, when you observed the voltage being pulled down from 28.5V to 26V, what time of day would that be? If it is early or late and indirect sunlight, this would be normal as the panels cannot cover the current required to support the load. Fully charged without loads or input would be 25.4V, even a 100W load will sag the voltage but once it turns off, the voltage will recover. Your system appears to be large enough to support your loads, if there are no others, the important thing to note is the rest voltage in the morning before sunrise with no loads, if you have around 24.4V you are fine. I'm thinking perhaps you are having voltage anxiety like I had at first, until I got comfortable, a year later the the rest voltage is spot on plus or minus a few tenths. Always remember if you are not using your battery you're loosing your battery, don't get hung up on momentarily snap shots, take a step back and look at the overall picture. Let them run overnight and see what the aforementioned rest voltage is, it takes less energy to maintain a set-point than it dose to recover lost cooling.

This being said however, dose not include an extra day of autonomy, for that you would need a larger bank, roughly you have 4Kwh of usable reserve and 4Kwh of load, for a 40% depth-of-discharge, for a healthy battery , so double the battery capacity to achieve this, off grid.

Refrigeration is a killer, it seems benign at at first, but really it's a killer, it never rests and is the single most energy hungry appliance, off grid, baring anything heating related, which is why I built a separate system just for the purpose, to understand it's requirements. My thoughts are not intended to discourage, but rather to assist, it's been a learning experience and has cost much in the form of underestimating it's requirements.

Estragon · February 2017

I'd be interested in thoughts from the board on the charging algorithm used by your NC25a Flexcharge controller.

As I understand it, the controller does what we would call a normal bulk, charging to a default 14.25 (12v system, x2 for 24v). When it hits this voltage, it opens the charging circuit and lets voltage drop to about float voltage of 13.6v, then does another "pulse" to 14.25. These values are not temp compensated.

Most controllers hold the higher voltage for a number of hours or until current tapers to some low (eg. 1% of capacity), called "absorb". They often adjust voltages for temperature, ideally using a temp sensor attached to the bank. A bank below 25C will get a higher voltage charge.

The charge regime of this controller helps explain the OP's issue with voltage dropping quickly under load during the day. The panels are supplying zero current to the load until voltage drops to float. I also wonder if 14.25 (28.5) is high enough for GC FLA batteries, particularly if they're cool, using this pulse algorithm. Any thoughts from the board?

To the OP, I again suggest you do the checks (specific gravity of each cell, and voltages under load of each battery).

scott54 · February 2017

Thanks, very informative. This is a very knowledgeable forum. The Flexcharge controller works as you describe. Shutting off until it hits 27.25v. Incessantly off and on. The speed of the drop (10-15 seconds) is what was disturbing. If a single freezer is pulling only 100w how can 8 batteries drop that quickly ? I will check all battery cells.
Because the controller shuts off and on continually (and thus not utilizing the solar) I considered dumping to my water heater (4' away). I know that's a poor use of solar energy but understand it's diverting only when not needed.

westbranch · February 2017

I did not read all of the detailed specs on that controller but I have a feeling that it is not a good fit for your application. It seems that you chose it primarily on efficiency and not on features. Not knowing exactly which model you have.. I am suspicious of that CC's ability to handle all the amps ( ~83A) from your now bigger array at 2000W... at that level you are getting up into Classic 150 territory
You need a bigger charge controller......I just read this ditty on their website:

This controller can easily and inexpensively be expanded to handle one or more banks of 100A charging sources. (1000A max charging current in this configuration). Multiple battery banks can be charged with the use of a standard battery charging isolators.

Photowhit · February 2017

scott54 said:

Thanks, very informative. This is a very knowledgeable forum. The Flexcharge controller works as you describe. Shutting off until it hits 27.25v. Incessantly off and on. The speed of the drop (10-15 seconds) is what was disturbing. If a single freezer is pulling only 100w how can 8 batteries drop that quickly ? I will check all battery cells.
Because the controller shuts off and on continually (and thus not utilizing the solar) I considered dumping to my water heater (4' away). I know that's a poor use of solar energy but understand it's diverting only when not needed.

It's not really dropping that much, it's the system voltage, NOT the battery voltage!

Go back and read Mcgivors post.

Might read and understand how battery charging works. There is good info in the FAQs;
https://www.solar-electric.com/deep-cycle-battery-information-faq.html/

When charging, the system voltage will rise until it reaches absorb when the system voltage would run away with out the charge controller. The charge controller will hold the voltage at roughly 29 volts. At this point your batteries are roughly 80% full. If you disconnected the charger and loads, and waited a few hours the battery voltage would be roughly 24.8. The system voltage is higher to facilitate the charging of the battery bank. When you add a load greater than the charging input (actually greater than about 70% of the input, this has to do with the switching the charge controller does) the voltage will drop to reflect the lack or extremely reduce charging. Since it only dropped to 26 volts (at rest a 24 volt, fully charged batter bank should be about 25.4 volts) it would indicate that there is still some charging going on, or at least very minimal discharging.

If you have a single 30 amp charge controller, or are only capable of charging at 30 amps, this would be undersized for a system in constant use with a 440 amp hour battery bank, allowing for a maximum @ 7% charge rate. I could only find a

If your only seeing 29 volts on good days, your system might well be under charging on a regular basis. It might be good to know what size array, the exact charge controller and inverter and see if we can evaluate your complete system. A modified sine wave inverter might be using more power than you are measuring due to the power factor (a concept I'm just learning about but others understand better)

BB. · February 2017

I believe that the Flexcharge NC25A is a very old, relay based, charge controller. It is not high frequency--It is very low frequency (how ever long it takes the surface charge to dissipate)--And if relay based, no high current relay would be used in high frequency application (if I recall correctly, the "high frequency" description is a relatively recent addition, I don't remember this from a decade ago).

It also looks like you may need to add a blocking diodes between the solar panels and the Flexcharge controller--Or the panels may discharge the battery at night (see data sheet for Flexcharge):

http://www.flexcharge.com/custom-1/NC25a Manual 2012.pdf

This is an older style controller... If you are using flooded cell batteries, check the specific gravity and see how well they are really charging.

https://www.solar-electric.com/midnite-solar-battery-hydrometer.html
https://www.solar-electric.com/brin1bahy.html

-Bill

Estragon · February 2017

@Photowhit

The controller is 25a not 30. Default max voltage appears to be 28.5v with no temp compensation. OP may have adjusted max voltage higher though as it appears to be adjustable.

Unlike most controllers, it supplies NO current for either loads or charging during the off cycle (i.e. until voltage drops to float voltage), so any loads are drawing down surface charge from the last cycle. It does NOT hold voltage at absorb and taper. It just opens the charge circuit until voltage drops to float. Not surprising voltage drops quickly from absorb to float voltage with load.

scott54 · February 2017

Thanks everyone. I'm beginning to understand. I had 2 different BZ 50amp MPPT controllers that didn't work and found the Flexcharge controller to be basically problem free though it has it's shortcomings (problem free was high on the list because this is a back-up shtf system).
It basically turns on below 27.25 and shuts off @ 28.75. That was one reason I considered dumping into my water heater, thus eliminating much of the cycling. I'm not as concerned now and will do the system checks everyone suggested.
Also going to pull out my back-up BZ 50 (repaired) and try to learn some more.

mvas · February 2017

scott54 said:

Thanks everyone. I'm beginning to understand. I had 2 different BZ 50amp MPPT controllers that didn't work and found the Flexcharge controller to be basically problem free though it has it's shortcomings (problem free was high on the list because this is a back-up shtf system).
It basically turns on below 27.25 and shuts off @ 28.75. That was one reason I considered dumping into my water heater, thus eliminating much of the cycling. I'm not as concerned now and will do the system checks everyone suggested.
Also going to pull out my back-up BZ 50 (repaired) and try to learn some more.

The engineers at Trojan state, the #1 issue for under-performing batteries is sulfation from Under-Charging. I do believe that you are habitually under-charging your batteries. In response to this under-charging evidence, Trojan has recently increased the transition voltage from Bulk to Absorb to 29.6 Volts @ 77°F. You are terminating all charging at 28.75 volts (too low). Then you are not even doing the Absorb Charge by holding the voltage constant at ( 28.2v - 29.4v ) until the amps decrease to 1% - 3% of the C20 rate (several hours). Then there is a Finish Charge (that you also are not doing) where you hold the current constant at 1% - 3% of the C20 rate until the voltage reaches 32.40 Volts. Per the engineers at Trojan, the battery is 100% charged when it finally reaches 32.40 volts.

Bulk (CC) = up to 85% to 90% recharged
Absorb (CV) = up to 95% recharged
Finish (CC) = 100% recharged

Float (CV) = 27 volts

Are you recharging your batteries to only 80% or less, at 28.75 volts max ?
That could explain why your battery voltage decreases to 25 volts, so quickly.

mcgivor · February 2017

mvas said:

scott54 said:

Thanks everyone. I'm beginning to understand. I had 2 different BZ 50amp MPPT controllers that didn't work and found the Flexcharge controller to be basically problem free though it has it's shortcomings (problem free was high on the list because this is a back-up shtf system).
It basically turns on below 27.25 and shuts off @ 28.75. That was one reason I considered dumping into my water heater, thus eliminating much of the cycling. I'm not as concerned now and will do the system checks everyone suggested.
Also going to pull out my back-up BZ 50 (repaired) and try to learn some more.

The engineers at Trojan state, the #1 issue for under-performing batteries is sulfation from Under-Charging. I do believe that you are habitually under-charging your batteries. In response to this under-charging evidence, Trojan has recently increased the transition voltage from Bulk to Absorb to 29.6 Volts @ 77°F. You are terminating all charging at 28.75 volts (too low). Then you are not even doing the Absorb Charge by holding the voltage constant at ( 28.2v - 29.4v ) until the amps decrease to 1% - 3% of the C20 rate (several hours). Then there is a Finish Charge (that you also are not doing) where you hold the current constant at 1% - 3% of the C20 rate until the voltage reaches 32.40 Volts. Per the engineers at Trojan, the battery is 100% charged when it finally reaches 32.40 volts.

Bulk (CC) = up to 85% to 90% recharged
Absorb (CV) = up to 95% recharged
Finish (CC) = 100% recharged

Float (CV) = 27 volts

Are you recharging your batteries to only 80% or less, at 28.75 volts max ?
That could explain why your battery voltage decreases to 25 volts, so quickly.

As your batteries are not Trojan but Costco, and unlikely supplied by Trojan,could be wrong, it would be advisable to find out who manufacturers them and follow their recommendations, as all are not created equal, perhaps one of the smart guys here knows who supplies batteries to Costco. @mvas is however demonstrating the importance of fully charging, it appears the controller you are using is not, up to current technological standards for off grid. A much better PWM controller would be a better choice and is much cheaper than replacing the batteries, check the host's store, there are many to choose from, Schneider, Morningstar, Midnight etc. There is always MPPT but more expensive.

mvas · February 2017

Also, if your batteries are not fully charged, and I doubt they are, then your electrolyte may have stratified.

Low battery capacity ....
1) Battery charged to only 80%
2) Sulfate on the plates
3) Stratified electrolyte

It appears that you do not have an hydrometer & cannot measure the SG of the electrolyte
therefore you must charge your battery past the point of making a few small bubbles.
I seriously doubt that stopping all charging 28.75 Volts is enough.

You need to recharge your batteries to 100% SOC, ASAP.

There is nothing "special" about your batteries from Costco = Lead, Sulfuric Acid & Water
Chemistry is chemistry.
Probably from a factory in Mexico, where the same manufacturer makes 10 other re-branded batteries.
Actually, there are very few battery manufacturers, just many different labels.

Both Trojan and Rolls specify higher charging voltages = higher performance
Costo provides a 2 year pro-rated warranty.
They have no incentive to help you get 3 - 5 years out your battery.

Interstate GC-2 ( aka Johnson Controls ) recommends 29.2 Volts for Bulk, then Absorb.
Exide GC-2 recommends 29.4 Volts for Bulk, then Absorb.
Depending on the location of your Costco, it may receive batteries from Johnson Controls or Exide ( minimize ship $ )
Unfortunately, Costco does not purchase from just one vendor ( lower price wins the bid )

I prefer the higher voltage before transitioning from Bulk to Absorb.
It makes the Absorb time shorter.
Our responsibility is to get the battery back to 100% SOC, as quickly as possible using reasonable voltages and amps.
Shutting off all charging at 28.75 Volts will never get the battery to 100% SOC.

scott54 · February 2017

Photowhit stated " at rest a 24 volt, fully charged battery bank should be about 25.4 volts ". That made me feel better. Then I looked at a chart I got with my Samlex inverter. It stated a full 100% charge is 12.63 (12v battery). 12.63 X 2 (24v) is 25.26v. I just looked at my "resting" batteries. Shunt meter says 25.27. That made me feel better (for a moment).
Maybe the Trojans allow for a higher charge ? mvas mentions the Interstate GC-2 charging to a 29.2v. I believe that's my brand.

Estragon · February 2017

Fully charged will be about 25.2 - 25.6v so you're in the range. This is encouraging. How long had the batteries been resting?

It's still important to check SG on each cell though, as voltage is only indicative. It would also be a good idea to check voltages on each battery while charging.

Are the batteries located where temps are much above or below room temp (~25c)?

mvas · February 2017

scott54 said:

Photowhit stated " at rest a 24 volt, fully charged battery bank should be about 25.4 volts ". That made me feel better. Then I looked at a chart I got with my Samlex inverter. It stated a full 100% charge is 12.63 (12v battery). 12.63 X 2 (24v) is 25.26v. I just looked at my "resting" batteries. Shunt meter says 25.27. That made me feel better (for a moment).
Maybe the Trojans allow for a higher charge ? mvas mentions the Interstate GC-2 charging to a 29.2v. I believe that's my brand.

Simply measuring the Resting Voltage is not an accurate way of determining 100% SOC and here is why ...
Is your Shunt Meter calibrated & verified to better than +/-1% accuracy ?
Because just a +/-1% variance in your Volt Meter equates to a huge +/-10% SOC variance.

A +/-1% voltage variance ...
12.7 Volts = 100% SOC
12.6 Volts = 90% SOC
12.5 Volts = 80% SOC

So, with a +/-1% accurate meter, all we know is, your battery is somewhere between 80% SOC and 100% SOC.
That is a huge range of uncertainty.

My Fluke Voltmeter was verified against an Ultra-Precision 5.000 Volt Power Supply having +/- 0.02% accuracy.
When tested, my voltmeter displayed 5.01 Volts, which is 0.2% accuracy or +/-0.01 volts
At 100% SOC, my battery voltage is between 12.74 Volts - 12.76 Volts.

If my batteries measured only 12.63 Volts, after a 24 hour rest, I would finish recharging them, ASAP.

Two consecutive SG readings of the same value while charging is the only way to know that your battery is 100% SOC.
Actually the battery is 100% SOC when the first max / high SG reading is taken.
The battery is then slightly over-charged for the second identical reading. ( You have never done this )
Then after a 24 hour rest, you let the batteries tell your voltmeter what your 100% SOC voltage actually is.
Now I have facts, not any assumptions, regarding my 100% SOC Voltage ( +/-0.01 volt ), on my meter, with my batteries.

You have incorrectly used a voltmeter, of unknown calibration, to estimate your 100% SOC Voltage.
If you do not have a hydrometer then must perform a full Bulk, Absorb & Finish Charge to get your batteries to 100% SOC.
I don't think your batteries have ever been at 100% SOC (PSOC leads to Sulfation & Electrolyte Stratification)
Next, rest your batteries for 24 hours.
Finally, use your fully charged batteries to tell your voltmeter, what your 100% SOC voltage really, at one specific temperature.
Now you know a fact about your system.
You cannot precisely determine your 100% SOC Voltage by reading any vendor literature - too many variables.

You asked ...
How can 8 batteries fluctuate with one freezer drawing 110w for 15 minutes ?

And I will repeat ...
Shutting off all charging as soon as 28.75 Volts is obtained, will never get your batteries to 100% SOC.

softdown · February 2017

We need to forget about the Costco store and possibly even the Interstate label. The manufacturer is probably Johnson Controls. The OP appears to have this figured out.

It is a bit frustrating that manufacturers of off grid solar equipment are still, on occasion, supplying equipment that does not maintain optimal battery health. We have a long way to go...

Sounds like the problem has likely been identified. CC cut off bulk charging prematurely while employing insufficient voltage...I better go check some stuff.

Scott54: Costco battery bank discharging too quickly?

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