My New Battery Bank Seems to Sag in Voltage

[Matrix]

i would:
a) increase the delay to 2 min (from 30 sec)

Thanks ... that seems to have helped. I am still getting the warning because the LBCO is set to 23.6v now but the inverter is reporting the voltage drops below that 23.6v under load for what the inverter thinks is 10 seconds ... but I do not see a drop anywhere near that ... more like from 25v down to 24.2v when watching the volt meter on the inverter ... But I am not shutting off because of this 2 min delay setting.

I am going to put a DVM on the battery posts and engage the well pump so I can watch the surge and see if it does indeed drop as low as the inverter is reporting in it's warning.

b) re-confirm you ARE getting a full charge, gradually increase absorb time (15 min chunks) to see if it helps your state of charge.

Just got a nice glass with glass float Hydrometer, so I am headed home to test my SG of all cells and record them. I have been charging the batteries all morning (not with the solar charger) according to the recommended Trojan specs for Bulk, Absorb with a 180 min time cutoff and Float.The batteries have been being used for 8 days and were produced at Trojan in Oct 2017 according to the date code... so I will consider this first SG reading the New Battery reading for all cells fully charged (that is if the SG numbers indicate full charge). Then I am going to do an Equalization charge as Trojan says I should have done before putting them into service for the first time, and test again and record those numbers as well. I will keep all these records for baselines in the future.

[Matrix]

I also got a WhizBang jr for the Classic 150 today. So I will be installing that this afternoon as well.

[Matrix]

Also ... What would be a "normal" Current(amp) reading for a charge in the beginning of the Absorb stage? I am seeing 80-90 amps going into my L16H-AC 435 batteries. Do I need to adjust something else? I have the Volts set to 29.4v for absorb which is the manufacture spec.

Trojan is saying "As a general rule of thumb, the total amps from your PV panels should be sized between 10% and 20% of the total amp-hours (Ah) of the battery pack." And in another Trojan page I found, "In applications where battery recharge must be accomplished within 8 to 10 hours, a three stage, automatic charger, rated at 20% of the battery capacity, may be required."

So for my 435 ah battery that would be at most 87 amps? Should I set a max amps at the CC for this number?

[Sunking]

Also ... What would be a "normal" Current(amp) reading for a charge in the beginning of the Absorb stage? I am seeing 80-90 amps going into my L16H-AC 435 batteries. Do I need to adjust something else? I have the Volts set to 29.4v for absorb which is the manufacture spec.

There is no answer to the questions. Let's see if I can shine some light on it. Current is going to be whatever it needs to be. Charge current is limited by how much current the charger can supply, the battery Internal Resistance (Ri), and the battery Open Circuit Voltage (OCV).

Any battery charge has a maximum amount of current it can supply. For example 10-Amps, and you will never get more than 10-Amps from a 10-Amp charger, simple enough. So how does the charge limit the current. Simple it limits the voltage.

So let's build a simple model of a 12 volt 100 AH battery and 10-Amp Charger. The battery OCV = 12 volts, Ri = .01 Ohms, Charge Voltage = 14.6 volts we connect the charger. What is the voltage of the battery and how much charge current. That is super easy to calculate. We already know 10 amps maximum, DUH! But what is the voltage? I can tell you it is not 14.6 volts.

For a battery under charge, Battery Voltage = Battery OCV + (Ri x Charge Current)
So 12 volts + (.01 Ohms x 10 amps) = 12.1 Volts despite the fact you set the voltage to 14.6 volts. So what is going on here? Simple you have a 10 amp charger, and it cannot supply more than 10 amps. It had to Fold Back the voltage from 14.6 volts down to 12.1 volts to limit current to 10 amps. FWIW this is the Bulk Stage aka Constant Current. Now as the battery charges up, the voltage goes up. The charge will supply Constant Current (Bulk) of 10 amps until the battery voltage reaches 14.5 volts. When the battery voltage reaches 14.5 volts, current starts to Taper down from 10 amps down to 0 amps when the battery voltage reaches 14.6 volts. This is the Absorb stage aka Constant Voltage and in this model current is some value between 0 and 10 amps. It is what it is and you have no control of it.

Now lets say I have a Infinite Current Charger and set it to 14.6 volts and connect it to the same battery. What happens? For 1 we eliminate any Constant Current aka Bulk Charge stage. We just have a Constant Voltage charger which we can call Absorb, Float, or Equalize. It is just a silly name and marketing terms. So what would happen if we connected the Infinite Charge to our test model?

Charge Current = Charge Voltage - Battery OCV / Battery Ri. So 14.6 - 12 volts / .01 Ohms = 260 Amps. So initially you would see 260 amps, but it would immediately start to Taper Off because the battery would charge very fast and the battery OCV is rising fast. About 6 to 10 hours later charge current stops because battery voltage = charger voltage, and the battery is Fully Absorbed which is marketing mumbo jumbo the technical term is FULLY SATURATED aka Fully Charged. You have no control other than sizing the charger to the battery, and setting the voltage.

OK now for the bad news and how to work with it. Solar is NOT CAPABLE of fully charging a battery. The sooner you accept this fact, and gain a basic understanding, the better off you are. That Absorb stage you are so worried about is NOT a timed event. It can take 6 to 10 hours and there is not a place on earth with enough Sun Hours to do that. Forget it not going to happen ever.

So what do you do?

1. Accept the fact and deal with it. If anyone tells you solar can fully charge a battery, just nod your head and think to yourself what an IDIOT, he/she drank the liberal Kool-Ade.

2. Buy a good generator and a battery charger of C/8 to C/6 so you can fully charge your batteries once a week or so. You need at least C/8 to minimize generator run time because even at C/8 can take 12 hours run time

3. Buy a good Temperature Compensating Battery Hydrometer and learn how to use it because a hydrometer is the only way to measure SOC. Voltage does not work.

4. Last and most important is wake the hell up and throw away your solar charge controller manual. A 3-stage solar charge controller is as useless as tits on a boar hog if you follow instructions. Last thing you want is 3-Stage Charging Algorithm. 2 and 4 stage is only useful if you are using AC power with unlimited time and power. You do not have the luxuries of unlimited time and power. What you want to do is make a Bulk or Constant Current Charger out of your Solar Charge Controller. The way you do that is simple, set Bulk = Absorb = Float = 14.2 to 16 volts.

5. At the end of the day, measure specific gravity using your hydrometer. If SOC is low, raise the voltage up and repeat next day. If it happens to be too high, rare, lower the voltage a bit and repeat the next day.

6. At the end of the week, top off batteries with your generator to maximise your investment and battery cycle life. Let the Kool-Aid drinking fools line the pockets of the battery manufactures with their cash.

[Matrix]

OK now for the bad news and how to work with it. Solar is NOT CAPABLE of fully charging a battery. The sooner you accept this fact, and gain a basic understanding, the better off you are. That Absorb stage you are so worried about is NOT a timed event. It can take 6 to 10 hours and there is not a place on earth with enough Sun Hours to do that. Forget it not going to happen ever.

So what do you do?

1. Accept the fact and deal with it.

2. Buy a good generator and a battery charger of C/8 to C/6 so you can fully charge your batteries once a week or so. You need at least C/8 to minimize generator run time because even at C/8 can take 12 hours run time

Perfect. That is good to know. Thanks so much for this excellent response. I had this idea in my gut about solar charging and CC's, but wasnt even sure how to frame the question.

So I have actually been charging the batteries from the AC side of my inverter today, using the AC inverters Charger.

Schneider SW 4024 Charger Specs:
Output current: 90 A
Nominal output voltage: 24 Vdc
Output voltage range: 12 - 32 Vdc
Charge control: 2 or 3 stage
Charge temperature compensation: Yes - BTS included
Optimal efficiency: 90%
AC input power factor: > 0.98
Input current: 13 A
Input AC voltage: 120 / 240 Vac split phase

I have the Charger set up as a custom setting to accept all the voltage values from Trojan:
Bulk: 29.6v
Absorb: 29.4v with a 4 hour timer
Float: 27v
Equalize: 32 (although Trojan wants 32.4, 32v is the Schneider's Max)

There is no settings for Min or Max amps

Did a 3 stage charge this morning and it lasted from Bulk - Float for just over 3 hrs. The Absorb never reached the timer limit. I saw over 90amps going in at one point during Absorb.

At the end my voltage ... about 30 minutes after I turned the charger off was 26.2v. resting. This is the highest voltage i have seen on the battery bank since putting it into service last monday 8 days ago.

3. Buy a good Temperature Compensating Battery Hydrometer and learn how to use it because a hydrometer is the only way to measure SOC. Voltage does not work.

My Temp compernsation glass Battery Hydrometer arrived today. I did my first test of all cells after the charge from the Schneider this morning. Voltage was at 26.2v

Trojan says the SG for 100% charge is 1.277 and at 90% it is 1.258. When I tested with my new Hydrometer after a full charge cycle from the AC charger with the above noted parameters set ... My SG with my Hydrometer was 1.260 +2 for Temp Compensation. Basically about 91-93% charged I am thinking?

So ... either my Hydrometer is off ... or ... I had just over 90% charge at a resting voltage of 26.2v. I then proceeded to do an EQ charge for 60 minutes (the Schneider's set time limit for Equalization). Still waiting on that to complete.

But ... Now ... is my Hydrometer off ... or am I really still at only 90% + charged. I have a friend from a battery store bringing me a different Hydrometer to test with side by side and try to determine a baseline. What other way could I possible know if I am at 100% charged?

4. Last and most important is wake the hell up and throw away your solar charge controller manual. A 3-stage solar charge controller is as useless as tits on a boar hog if you follow instructions. Last thing you want is 3-Stage Charging Algorithm. 2 and 4 stage is only useful if you are using AC power with unlimited time and power. You do not have the luxuries of unlimited time and power. What you want to do is make a Bulk or Constant Current Charger out of your Solar Charge Controller. The way you do that is simple, set Bulk = Absorb = Float = 14.2 to 16 volts.

So ... just so I am sure I understand ... for a 24v system where Trojan recommends bulk being set for 29.6v .... just set all the 3 stages in the Midnite Classic CC to 29.6v ... including float?

5. At the end of the day, measure specific gravity using your hydrometer. If SOC is low, raise the voltage up and repeat next day. If it happens to be too high, rare, lower the voltage a bit and repeat the next day.

And again for my own clarity ... to raise the SOC as measured from a Hydrometer ... raise that 29.6v Bulk / Absorb in the charger settings? Correct? Until the SG agrees at 100% and call whatever that charging voltage is that I discover after testing the actual Bulk/Absorb voltage to raise my batteries to 100% using an AC charger.

6. At the end of the week, top off batteries with your generator to maximise your investment and battery cycle life. Let the Kool-Aid drinking fools line the pockets of the battery manufactures with their cash.

Great info. Thanks so much for your time.

[Sunking]

Matrix battery manufactures charging instructions are made assuming an AC charger with unlimited time and power available. Take a C/10 AC charger, set the voltages to the manufactures ranges, a fully discharged battery, connect it, and 16 hours later you have a fully charged battery on Float Voltage and will hold for years if you want.

I cannot tell you what voltage to use. I cannot tell anyone what voltage to use. Only your hydrometer can tell you that. For 90% or more solar users, there is no voltage high enough because 90% or more of the DIY systems and even consumer level professionally installed systems are grossly undersized. If most people know what it would really take, would not go off grid.

FWIW your hybrid inverter like all hybrid systems use the built-in AC charger to charge the batteries. During extended outages, you would need a generator to recharge the batteries every few hours and nightly.

Here is the game Installer play. I want your biz, thus I must have the lowest bid. That means I grossly undersize the batteries and solar power. Wrks great for a couple of years until the batteries weaken. Guess what? Cha-Ching I get to sell you more batteries.

[Matrix]

I cannot tell you what voltage to use. I cannot tell anyone what voltage to use. Only your hydrometer can tell you that.

Perhaps I misstated or was not completely clear in my question.

I do have a hydrometer. If I set my charger to 29.6v which is the Manufacture Charge spec, but after AC charge my hydrometer tells me I am only at 90% ... do I adjust for that by raising the voltage little by little and retest charges with the hydrometer until I find the voltage of charge that will charge the batteries to the correct manufacturer 100% charge SG? And if so could it be said that Charge voltage is not as set in stone as SG. My battery 100% SOC in SG according to the Manufacturer is 1.277. Should that be my goal regardless (within reason) of charging voltage from an AC charger?

And if charging voltage is fairly relative (?) ... is there any way short of comparing a few different hydrometers to know if it is giving you accurate readings?

Also, After I did the Equalization charge for 1 hour at 32 volts ... the batteries for the first time hit what my hydrometer called a full charge at 1.277 SG. (the batteries were at 92% according to SG before Equalization charge)

[Sunking]

Perhaps I misstated or was not completely clear in my question.

Or I could have missed something. So lets take it point by point.

I do have a hydrometer. If I set my charger to 29.6v which is the Manufacture Charge spec, but after AC charge my hydrometer tells me I am only at 90% ... do I adjust for that by raising the voltage little by little and retest charges with the hydrometer until I find the voltage of charge that will charge the batteries to the correct manufacturer 100% charge SG?

Yes, that is 1 of 2 things you can do. The other thing you can do, not practical for daily cycle service is just more time. You can fully charge a 12 volt Pb battery at 13.6 to 13.8 volts aka Float Voltage. The catch is it takes up to 24 hours despite the fact the charger is capable of 1C charge rate. With solar, you do not have the luxury of TIME. You cannot change it, but you can change a Voltage Set Point to make your charger stay in Bulk Mode. In an AC charger Bulk Mode is constant current regardless of the voltage which means the charger pumps in a specified amount of current. In a solar charge controller Bulk Mode is Constant Power meaning the controller will extract every bit of power the panel can produce and send it to the battery and/or load.

So by setting the Voltage Set Points high you force the controller to stay in Bulk or Constant Power from sunrise to sunset harvesting every watt possible. In any Constant Voltage like Absorb or Float, charge current will be less than Bulk and going down minute by minute on its way down to zero if the batteries ever saturate. You loose a lot of power doing it that way. Actually you did not lose it, you just did not utilize all the power possible.

And if so could it be said that Charge voltage is not as set in stone as SG. My battery 100% SOC in SG according to the Manufacturer is 1.277. Should that be my goal regardless (within reason) of charging voltage from an AC charger?

Correct. Voltage is only an indicator and a poor one at that. A temperature compensated hydrometer is the only tool that can be used in real time and is the most accurate method of determining SOC. Manufacturer recommendations are just that. All of them give a window or range, not a specific number. Example one manufacture may recommend

Float = 2.2 to 2.25 vpc. On a 12 volt battery is 13.2 to 13.5 volts
Bulk/Absorb = 2.33 to 2.43 or 14 to 14.6 on a 12 volt battery.

So as you can see, it is a range or window. However the manufacturer likely assumes you are using a commercial AC charger.

And if charging voltage is fairly relative (?) ... is there any way short of comparing a few different hydrometers to know if it is giving you accurate readings?

Also, After I did the Equalization charge for 1 hour at 32 volts ... the batteries for the first time hit what my hydrometer called a full charge at 1.277 SG. (the batteries were at 92% according to SG before Equalization charge)

There you go, you just answered all your questions. You had to up the voltage to get the battery fully SATURATED. All EQ does is raise the Voltage Set Point and is a Constant Voltage mode just like Absorb, and Float.

[Matrix]

With solar, you do not have the luxury of TIME. You cannot change it, but you can change a Voltage Set Point to make your charger stay in Bulk Mode. ...

So by setting the Voltage Set Points high you force the controller to stay in Bulk or Constant Power from sunrise to sunset harvesting every watt possible. In any Constant Voltage like Absorb or Float, charge current will be less than Bulk and going down minute by minute on its way down to zero if the batteries ever saturate. You loose a lot of power doing it that way. Actually you did not lose it, you just did not utilize all the power possible.

Are you familiar with how this might be done with a Midnite Solar Classic 150? There is no voltage adjustments for bulk, but as you suggest there might be some way to force the charger to stay in bulk longer. Do I want to change the absorb voltage set point ... raise the volt setting so Hi it never switched over to Absorb? Any chance that might be damaging to the battery bank? I have a 24v L16 435 AH battery bank (4 batteries) and 9 285w REC TwinPeak 2 panels facing south at about 16* slope angle in NE Florida. So the Array is cable of over 2500 watts in ideal conditions.

Also, your comments to this thread really helped me wrap my head a little more around my much smaller application. Helped me see several relationships like load draw, etc. While mostly over my head, still very helpful for what I was looking for at the beginning of my thread here.

[Mike90250]

for solar, you alternate between weeks deficit charging, then a controlled overcharge (EQ) . Summers (sunny weather) there is often plenty of power. bad weather, I run generator a couple hours in the am, and hope there is enough sun to finish.
running generator in am, lets you use full charger power, reduces run time, and saves fuel - avoid using generator for float charge

[sensij]

Are you familiar with how this might be done with a Midnite Solar Classic 150? There is no voltage adjustments for bulk, but as you suggest there might be some way to force the charger to stay in bulk longer. Do I want to change the absorb voltage set point ... raise the volt setting so Hi it never switched over to Absorb? Any chance that might be damaging to the battery bank? I have a 24v L16 435 AH battery bank (4 batteries) and 9 285w REC TwinPeak 2 panels facing south at about 16* slope angle in NE Florida. So the Array is cable of over 2500 watts in ideal conditions.

You still aren't getting this right. You should not be trying to avoid absorb. The voltage setting for absorb is the point at which the controller switches from Constant Current, in which it is delivering as much power as it can up to its current limit setting, to Constant Voltage, in which it holds a single voltage and allows current to taper down as the battery saturates.

In an ideal charge cycle from AC power, you would allow absorb to run until current tapers down to something like 0.03C, then drop the voltage to a lower float voltage to avoid drying out the battery. Running absorb on a timer doesn't accomplish this.

Because the solar day for some of the year does not give enough time for a full bulk+absorb+float, you can drive more charge into the battery by raising the voltage settings, eliminating a separate float stage and turning it into an extension of the absorb stage. If you are daily cycling, this can be appropriate. For a weekend use cabin, it is better to have a more normal float, since there should be days of non-use in which the battery eventually saturates.

Controllers that offer a separate setting for bulk and absorb are giving the option to allow the constant current stage (bulk) to just *touch* the bulk voltage before dropping to an absorb voltage to saturate at CV. Midnight Solar recognizes there is no use case where this really makes much sense, and just assumes that the CC to CV transition and soak happens at the same voltage.

A comment on the timed absorb.... Midnight Solar supports using a current threshold to end absorb rather than a timer. Look into the Whiz Bang Jr. If you would like to pursue this. With that capability, you should be able to leave the lower float voltage intact for occasions when you aren't cycling, and allow absorb to run properly over however much of the day is available.

[Matrix]

I did install a Whiz Bang Jr yesterday. I can only find where I can set the current threshold and the timer. But not turn the timer off ... so I set the timer to 5 hrs and set the voltage to 30v for absorb, which I assume means that it will not go into absorb from Bulk until the voltage reaches 30v?

[sensij]

I did install a Whiz Bang Jr yesterday. I can only find where I can set the current threshold and the timer. But not turn the timer off ... so I set the timer to 5 hrs and set the voltage to 30v for absorb, which I assume means that it will not go into absorb from Bulk until the voltage reaches 30v?

Yes, that is right. Transition from bulk to absorb will occur at 30 V (temp adjusted, if set), and the transition from absorb to float occurs when the timer elapses or the current tapers to the value you set, whatever comes first.

[Sunking]

Matrix perhaps a bit of easy math might help.

Discharging Battery Voltage = Battery OCV - (Discharge Current x Battery Ri)

Battery OCV is the voltage of the battery on a Open Circuit (not connected to ANYTHING) that has been allowed to rest for several hours. To make things easier to see and understand let's say Battery OCV is 25.2 volts or 100% fully charged.

Battery Ri is battery internal resistance, and is fixed. In this example lets say Battery Ri = .020 Ohms.

Discharge Current is exactly what it sounds like, how much current the load demands. So in this example it is the only variable which now makes the formula:

Discharging Battery Voltage = 25.2 volts - ( Discharge Current x .020 Ohms.)

So if you look at the formula and assume current is 0 amps.

Discharging battery voltage = 25.2 volts - (0 amps x .020 Ohms) = 25.2 volts.

OK now lets apply a 60 Amp load and see what happens.

25.2 volts - (60 amps x .02 Ohms = 24 Volts.

So despite the battery being fully charged up, when you hit it with a 60 amp load, the voltage will sag to 24 volts. Your Inverter sees that and trips off-line from under voltage on a perfectly healthy fully charged battery. So there is only one problem. The designer screwed up and used too small of a battery. To make matters worse, the designer most likely failed to take the cable loses between the battery and Inverter into account. The voltage at the battery Term Post sagged to 24 volts, but once you add on say 1 volt, the voltage at the Input of the Inverter is now 23 volts and goes dark. Absolutely nothing wrong with the battery. It is all on the designer ignorance. They did not know WTF they are doing.

OK the real issue is the battery and wiring is too small. In a proper design we must limit battery voltage sag and wire losses, and we limit each to 2% or less for a total of 4% between the 2. So how do you do that. A few ways but I will make it KISS (Keep It Simple Stupid).

To start the battery AH capacity should be a minimum of 8 times larger than the maximum load current. In our model max load current is 60 amps, that means the battery minimum AH capacity is 60 amps x 8 hours = 480 AH. AH capacity and Internal Resistance are directly related. The higher the capacity, the lower the resistance. In fact I can tell you the Ri of the required battery with simple math again. I know the voltage loss is 2% of 24 volts is .48 volts. Lets just call it .5 volts to make the math easy. Ri = 0.5 volts / 60 amps = .0083 Ohms.

Just from that I can conclude the first battery AH had to be roughly 2.4 times smaller than 480 AH or about 200 AH. This is a perfect model of a 24 volt 1000 Watt Inverter system. It would require a 1000 watt solar panel, 40 amp MPPT Controller, 24 volt 480 to 500 AH battery, and a 1000 watt 24 volt Inverter. Anything less is going to have issues.

So where do you stand? Work it backwards starting with your Inverter Size and Voltage. Example a 24 volt 1000 wat Inverter requires 60 amps at full load. What is the minimum size battery? I just did it for you above. 60 amps x 8 hours = 480 AH. Oh wait is that a 2000 wat inverter? Guess what? That takes 120 amps x 8 hours = 960 AH battery.

Almost forgot. Wire size and losses. Super simple, use the 3% chart below. Again lets say 60 amps and 1-way distance is 5 feet between Inverter and battery. What did you come up with? Did you come up with #6 AWG copper? You could use larger wire say #4 or #2 AWG, but nothing smaller than 6 AWG.

[Matrix]

Thanks so much all for this brilliant education. Your effort to explain this to a beginner is much appreciated. The Math work really helps a lot. Being a visual learner who learns thru asking questions, I can much more easily visualize the pro and cons of my little system now. Or more specifically , the Do's and Don'ts of what I have and it's limitations.