Battery charging - single vs parallel pair

[Paul_NJ]

I had a confusing situation the other day from one of our solar installations in Africa - (we're a volunteer group). One of our systems is small with two identical (110 amp.hr) batteries connected in parallel. Currently it is underpowered (compared to load and battery capacity) with only 1250 watt.hrs of panel capacity installed so far. The parallel batteries are charged by a MPPT controller, and until we install additional panels, it's taking two days of sunlight to fully recharge the batteries. At the time of testing (during daylight hours) the system voltage read 13.0 volts. This would suggest the charge controller was in Bulk mode – absorption voltage is set at 14.3 vDC, so apparently the battery bank wasn’t fully charged yet.

Due to other concerns, during our troubleshooting I had them remove one battery and operate with the other, and vice versa. In both cases, operating with a single battery the system voltage read 13.6 and 13.7 vDC. I always thought Bulk mode was at constant current, so I don’t understand why the system voltage with two batteries in parallel would be less than with one battery. If I'm "sinking" more current into two batteries, given constant power, I can see how the voltage would be dragged down. So is the current not constant, or could something else be going on?

I'd appreciate any thoughts.

Thanks

[Sunking]

You simply do not understand how charging works.

Read these two threads and tell the mods to make one a Sticky. [Done - Mod]

Bulk, Absorb, Float. What does it mean?
Are you Killing your Batteries?

One you have read both of those threads them come back with questions. The answer to your question is when you removed the battery, you doubled the resistance, Read the two threads and you will understand the answer.

[Paul_NJ]

Sunking,

I’'ve read your postings, and found them most enlightening, to say the least. Thank you. Testing my understanding of what you are saying, is that the voltage a controller is sensing is the battery’s OCV plus the voltage drop across the battery by virtue of the charging current. When the battery is fully charged (100% SOC), the OCV would equal the bulk-to-absorption stage setpoint and the charging current would have dropped to zero. The point you are making is there is simply not enough time to accomplish this, given the solar energy available. The bulk-to-absorption setpoint and the 100% SOC-OCV don't converge - there is still current being sent to charge the battery, so when the setpoint is reached, it isn't all the battery charge. Am I "on track"?

1. I suppose a possible worst case scenario is that the bulk-to-absorption setpoint isn'’t reached at all, and the controller stays in bulk mode ??. . . .
2. One thing that confuses me is that I usually see published 100% SOC voltage levels on the order of 12.8 volts. Here’'s the spec on the batteries I’'ve got. Does OCV level ever really get to a bulk-to-absorption setpoint, eg 14.3?

AGM Battery 1.jpg

OK, here’s my system. Location: Africa, solar irrad = 5.0 / Controller: Morningstar MPPT-45. / Batteries: sealed MK 8A31 AGM, 12v.

MK specs list charging parameters for these batteries:

Cycle: 2.4 to 2.43 vDC = 14.4 to 14.58 vDC
Float: 2.25 to 2.30 vDC = 13.5 to 13.8 vDC

Here’s the controllers options:

MPPT Charge settings.jpg

I’ve been using the controller settings for Battery Type 3 so far. Per your arguments, and the MK specs, seems I should bump up to Type 4 or 5 to at least raise the bulk-to-absorption setpoint from 14.3 to 14.6. But there's nothing I can do to extend the constant power bulk stage other than raising the voltage setpoint (at least I don't think I can). As you said, the Absorption stage is constant voltage.

Morningstar's controller manual states:

mppt 3.jpg


I believe I can use custom option to raise the Float voltage from 13.5 to 13.8. I assume it safe with a sealed battery to do this, in terms of gassing, etc as long as I stay within MK battery guidelines?

The MK battery specs list the resistance at 3 milli-ohms (0.003 ohm), considerably lower than your example for a FLA battery at 0.01 ohm. Is that significant/correct? Seems like it might bring the battery OCV and bulk-to-absorb setpoint closer due to less volt drop across the battery? Maybe I'm dreaming.

The final question I have, and maybe most important judging from your comments: I hear what you are saying about using a hydrometer to accurately determine the SOC. But I’'ve got sealed batteries. So with sealed AGM batteries what option do I have to gauge SOC other than voltage?

I'd greatly appreciate your further advice.

Thanks

[Sunking]

Yes you are on the right track.

Bulk/Absorption charge voltage is set at the electrolyte Gassing Voltage. That varies a bit from one manufacture to the next and has to do with plate/grid alloys and paste used by the manufacture. Also varies with temperature. 2.4 volts per cell is the common Gassing Voltage.

Absorption stage ends when the Charge Current tapers off to around 2 to 4% of C. For a 100 AH battery would be around 2 to 4 amps. However they may or may not bring the battery to a full 100% SOC. Only possible way to measure SOC is with a Hydrometer. There is no other way.

In Solar applications you really do not have any need hard voltage set points. Unless you have more panel wattage than required or excellent location, chances are you will never reach 100% SOC. You have to use a Temp Corrected Hydrometer to find the right Voltage Set Point. In summer time it may be 14.1 volts will get you there. In Winter shorter months there is no voltage high enough to ever get to 100%. If that is the case you want to force your controller to stay in BULK MODE from Sunrise to Sunset to pump as much energy in the battery as it possible can. Then once a week or as required run the generator to get the batteries up to 100%.

There are two killers of Lead Acid Batteries.

1. 95% of all premature battery failures are from chronic under charging, or leaving the batteries at less than 100% SOC. Anytime the battery is less than 100%, lead sulfate crystals begin to develop. If let go or deeply discharged the soft lead sulfate crystals harden on the plates. Once hardened, there is no way to dissolve them The damage is accumulative.

2. The less frequent failure mode is Corrosion from excessive Over Charging. However it is a better death then undercharging. Undercharged batteries are sluggish, slow to charge, and when loaded have excessive voltage sag. Corrosion only affects capacity.

Take away here is charging Lead Acid batteries is Dancing on the Knife Edge. One side is Sulfate, and the other is Corrosion. Ideally you want to be balanced, but that is impossible. So you want to flirt with the Corrosion side of the Dance Hall. You know the side with the Naughty Girls. Only way to know and get there is with your hydrometer pecker. .

[Mike90250]

Sadly, a drawback with AGM batteries, is that they are sealed, and cannot use a hydrometer. That's why a starter set of cheap golf cart batteries is good to use to learn
how your system works in your configuration.
And I think with Morningstar, you have to have a computer with a cable that fits the socket on your controller, to program the custom voltage with their MS-View program.

http://www.morningstarcorp.com/msview/

[Paul_NJ]

Sadly, a drawback with AGM batteries, is that they are sealed, and cannot use a hydrometer. That's why a starter set of cheap golf cart batteries is good to use to learn
how your system works in your configuration.
And I think with Morningstar, you have to have a computer with a cable that fits the socket on your controller, to program the custom voltage with their MS-View program.

http://www.morningstarcorp.com/msview/

Thanks for the reply. I do have the adapter to connect into the controller. Do you agree with increasing the set points, without being able to confirm %SOC, up to the battery manufacturer's limits?

[Mike90250]

I'd start at the lower voltage of the mfg's setpoint, and as you approach the termination of the absorb cycle, check the batteries - make sure they are not heating up, or starting to swell or bulge. you also want to listen to see if you hear a faint whistle or clicking of the integral vents popping open or bleeding off pressure. If you think you are still not getting well charged, extend the time, OR raise the voltage a notch over the next couple days. Beware, it may take several days to fully recharge chronically undercharged batteries, and then, if you cranked up the power too much, you are in overcharge land and venting electrolyte.
If you have a BTS, this is the time to use it to monitor the battery bank temperature.

[PNjunction]

Paul - I'm glad you are getting info, and from what I see right now, I would leave controller settings as they are, but strive to get more panel power to those East Penn / Deka / MK agm's.

Search for this Deka document in pdf form: "1913 Renewable Energy Charging Parameters". 1913 is the document number, not the year it was published!

You will see your #3 selection in the charge controller fits those parameters for these Mk/Deka's well, which includes the little 28-day mild-eq. Middle column for agm please!

Also see the last paragraph on page 5 of your MK/Deka VRLA charging document you have, where it talks about striving to get close to the 30%, aka 0.3C charge rate of your battery capacity. In this case, with 220ah rated in total, if you can get close to around 65A with a panel array, that would get you out of bulk, into absorb, and perhaps even some vital float before the sun sets every day.

Your agm's are what are known as "conventional" agm's, in that they can accept only up to 0.25 - 0.3C charge continously. Pure-lead agm's can have different voltage parameters, so for lurkers, don't go by these "conventional" agm charts as your sole source of info.

I do agm's, and like others have posted, most are babied to death with constant undercharge - and while you may see references here to "max smoke", unless you know how to do that properly to stay within warranty, you may just be better off getting more panel power to those batts closer to the 30% rate to do a better job of charging in time.

What many don't realize, is that with agm's, is it imperative that you not only complete absorb, but get in many hours of float as possible to actually finish the job! That is hard to do with solar, so try to get in as much float as you can. If you aren't even making it into absorb, then the batteries will be crying soon.

Your upcoming additional panel power is certainly going to help in this regard.

[Sunking]

PN due to Forum Security issues I cannot send you a PM, so I only have this route. You need to do some homework on Pure Lead AGM. What you think is pure lead is NOT pure lead, far from it.

See if this sounds familiar. Pure Lead AGM advantage is extremely low Internal Resistance thus allowing very high charge and discharge currents. Would you say this is true?

You said yes didn't you. Dead arse wrong. Pure Lead batteries have very low Power Density meaning they cannot take high charge rates or high discharge rates. What you think is Pure Lead is actually PbSn alloy. Yep lead and tin. Here is a good paper for you to read. Every battery you call Pure Lead is PbSn or solder. From that paper here are the advantages of each type of Pb batteries.

Plate Alloys

There are five basic plate alloys used today.

1. Pure Lead used in standby long duration batteries with low current demands

2. Lead-Antimony used for cycling applications and often for heavy equipment starting batteries

3. Lead-Calcium (dominates the U.S. market) for flooded standby and VRLA designs

4. Lead-Selenium (dominates the European market) for flooded standby and cycling applications

5. Lead-Tin for VRLA

Pure Lead The pure lead battery has the advantage of being very long lived and reasonably resistant to damage by elevated temperatures and over charging. However, it has very low power density and is not able to produce large currents.

There is only one pure lead VRLA on the market, and it is limited to very small sizes.

Note: There are companies that market “pure lead” VRLA batteries in large sizes. The “pure lead” designation in this case refers to the purity of the lead used to produce the battery4 . In reality, the batteries use a lead-tin plate alloy and pasted plates.

Lead-Tin

In an effort to reduce the potential for thermal runaway in VRLA batteries much research has been done in the area of plate alloys. A recent development is a nearly pure lead plate with a very low quantity of tin added4 . This alloy is often marketed as “pure lead”. The “pure lead” marketing phrase refers to the purity of the lead used to produce the battery.

Sorry PN but Pure Lead AGM designs do not exist. There is only one on the market, and it is for stationary applications only . Pure Lead cannot tolerate any movement. You can buy Pure Lead FLA though and Telecoms are the only one that uses them. They are known as Round Cells and have to be assembled on site. The Round cells were originally designed by Western Electric and service life of 30 years. The product code is KS-20472 made by GE today.

You have fallen victim to Marketing. Odyssey, Optima. Exide, North Star and a handful of others claiming Pure Lead is misleading. What it actually means is th lead they use is 99% pure VIRGIN LEAD straight from the mines and not recycled lead from dead batteries. Recycled lead has impurities and most likely a Lead Calcium or Lead Antimony alloy. So they use virgin lead to make their PbSn alloy. FWIW recycled lead is not a good source for battery lead.

[Mike90250]

.....And I think with Morningstar, you have to have a computer with a cable that fits the socket on your controller, to program the custom voltage with their MS-View program.

http://www.morningstarcorp.com/msview/

UPDATE. MSView comes with a set of firmware updates, but they are 4 years old. You need to go to your product page, and download the updated firmware for your controller.