Question About State Of Charge

[AL1952]

Before I knew any better many years ago I bought some crappy 12 volt, 35amp AGM batteries to use with my 4, 100 watt solar panels. I have always been puzzled by the fact that when under full continuous load the system draws the batteries down from 12.7 resting volts to 10.3 volts (measured at the batteries) under load, over a period of 4 hours of continuous operation. The moment I remove all load the voltage goes up to 12.3 volts and stays there until I turn on the solar panels to recharge them. I was told that the state of charge should be measured at full rest, no load, no charging. If the state of charge is not supposed to go below 50%, or 12.1 volts for these batteries at rest, is it safe for me to continue operating at full load until the voltage drops to 12.1 at rest, even if the operating voltage under load drops as low as 10.0 volts or less?

[SunEagle]

Well measuring the voltage is not the best way to know what the SOC status of your battery system but with AGM you do not have a lot of options.

I guess my question is how many of those 35Ah batteries do you have an how are they wired? Also what type of CC are you using? A PWM type will maybe get your 67% of the panel wattage while an MPPT can get you up in the 90% of the panel wattage.

So before I give my opinion if you should continue to use your batteries as you mentioned I really need to know how they are being charged but I will say that a battery that shows less than 12V is being over discharged and may not provide the number of cycles the manufacture has stated.

[AL1952]

I have 6 batteries. Wired in series. All equal length # 2 wire. PMW 30 amp controller. When you say "a battery that shows less than 12V is being over discharged" do you mean at rest or when under load?

[SunEagle]

I have 6 batteries. Wired in series. All equal length # 2 wire. PMW 30 amp controller. When you say "a battery that shows less than 12V is being over discharged" do you mean at rest or when under load?

First off those 6 batteries create a 210Ah 12v system (6x35Ah = 210Ah). It needs about 21amps of charging which 4 100 watt panels going through a PWM CC yields a little over 20 amps which is good.

What I meant was that a 12V battery that measure less than 12V at rest we being hurt. But few people wait the hours with no loads connected to perform that measurement.

I do not know why your system is showing low voltage under load except that it might be too much for the batteries. I will say that 6 batteries wired in parallel end up charging and discharging unevenly so you might have one or more bad batteries in the system which is lowering the overall voltage measurement.

Have you disconnected each of the 6 batteries and performed a voltage test well after they have been under no load. You might find a bad apple in the bunch which is causing the problem.

[AL1952]

Have you disconnected each of the 6 batteries and performed a voltage test well after they have been under no load. You might find a bad apple in the bunch which is causing the problem.

Thank you Sun Eagle for your suggestion. It was very helpful. I tested all batteries individually and each one was exactly 13.3 volts. They are however 8 years old and about due for replacement. I don’t want to make the same mistake I did with these less than ideal batteries. I want something better that has less possibility of trouble. You are obviously very knowledgeable so if I may, I’d like some advice from you about your recommendation for new batteries given these parameters. My space is VERY limited. I can currently fit up to 8 smaller 35 Amp batteries, like my current ones, in the space I have, which is on a shelf 4 feet up from the floor. I now have 6 batteries. The space is not ventilated very well and somewhat close to an ignition source, which is why I chose sealed AGM batteries in the first place. I am also old and partially disabled so lifting and moving larger 50+ pound batteries isn’t possible for me. Can you suggest any batteries (6 or 12 volt) and how to set them up (series or parallel), that would give me the same (210 AH system) or better, up to the best capacity of my solar panels, that won’t break my bank account? I currently have 4, 100-watt panels on the roof of a shed next to my house, with just enough space and budget for 1 more. If you think I need it if you recommend a bigger battery storage capacity let me know. I’m open to any of your informed suggestions. Thank you for your help. It is much appreciated.

[SunEagle]

Thank you Sun Eagle for your suggestion. It was very helpful. I tested all batteries individually and each one was exactly 13.3 volts. They are however 8 years old and about due for replacement. I don’t want to make the same mistake I did with these less than ideal batteries. I want something better that has less possibility of trouble. You are obviously very knowledgeable so if I may, I’d like some advice from you about your recommendation for new batteries given these parameters. My space is VERY limited. I can currently fit up to 8 smaller 35 Amp batteries, like my current ones, in the space I have, which is on a shelf 4 feet up from the floor. I now have 6 batteries. The space is not ventilated very well and somewhat close to an ignition source, which is why I chose sealed AGM batteries in the first place. I am also old and partially disabled so lifting and moving larger 50+ pound batteries isn’t possible for me. Can you suggest any batteries (6 or 12 volt) and how to set them up (series or parallel), that would give me the same (210 AH system) or better, up to the best capacity of my solar panels, that won’t break my bank account? I currently have 4, 100-watt panels on the roof of a shed next to my house, with just enough space and budget for 1 more. If you think I need it if you recommend a bigger battery storage capacity let me know. I’m open to any of your informed suggestions. Thank you for your help. It is much appreciated.

Well for the same Ah rating I would say to get 2 6V 210Ah batteries but they might be heavy as well as not being sealed.

Usually you should build a battery system using lower voltage cells all wired in series. You can get 2V high Ah batteries to build a great system but they are expensive and heavy.

The big thing is not to have batteries wired in parallel but in your case it seems multiple smaller batteries work for you so I really do not have a good solution for you to follow.

[AL1952]

Thank you for your straight forward answer Sun Eagle. I now have a general idea what I need. In order for me to get the right number of 6 or 12 volt batteries, I guess I need to know the maximum number of batteries I need at a given amp hour rating each, to properly charge them each day. Based on what you said earlier with 4, 100 watt panels going through a 30 watt PWM CC yielding a little over 20 amps, then if I don’t add any new panels, 210AH total for new batteries would be about right. If I add another 100 watt panel how much additional amp hours of battery storage would I be able to have? I'm guessing about 260AH total? If no other options will work and I have to stick with 35 amp batteries, 8 of them would equal 280AH. Would this work with 5, 100 watt solar panels or should I go with 7 instead for 245AH? I'm looking for maximum efficiency and appropriate charging capabilities each day.

You also suggested a MTTP controller because it puts more power to the batteries. If I can afford it is it worth upgrading from the PMW to the MPPT with my system or is the 30 watt PWM sufficient and acceptable for 5, 100 watt panels? Would it simply charge faster than the PWM?

Sorry If I'm troubling you with all these details but after getting it wrong the first time I'm doing my best to get it as right as I can this time, given the limitations I have to work with.

[SunEagle]

Thank you for your straight forward answer Sun Eagle. I now have a general idea what I need. In order for me to get the right number of 6 or 12 volt batteries, I guess I need to know the maximum number of batteries I need at a given amp hour rating each, to properly charge them each day. Based on what you said earlier with 4, 100 watt panels going through a 30 watt PWM CC yielding a little over 20 amps, then if I don’t add any new panels, 210AH total for new batteries would be about right. If I add another 100 watt panel how much additional amp hours of battery storage would I be able to have? I'm guessing about 260AH total? If no other options will work and I have to stick with 35 amp batteries, 8 of them would equal 280AH. Would this work with 5, 100 watt solar panels or should I go with 7 instead for 245AH? I'm looking for maximum efficiency and appropriate charging capabilities each day.

You also suggested a MTTP controller because it puts more power to the batteries. If I can afford it is it worth upgrading from the PMW to the MPPT with my system or is the 30 watt PWM sufficient and acceptable for 5, 100 watt panels? Would it simply charge faster than the PWM?

Sorry If I'm troubling you with all these details but after getting it wrong the first time I'm doing my best to get it as right as I can this time, given the limitations I have to work with.

Most FLA batteries need a charge rate between C/12 and C/8 where C = the system Ah rating. With an additional 100 watt panel you will maybe get about 5 more amps or about 25amps total. That may be ok for a 200Ah to 300Ah battery system.

As for a PWM CC, they only work where the Amps in = Amps out. While an MPPT is Watts in = Watts out. Most PWM CC actually lose about 33% of the panel wattage depending on their Vmp rating. So a 400watt system is really only 268watts. You end up adding more panels just to compensate for the watt loss through a PWM. So if you have the money getting a 30amp MPPT will pay for itself without the need to add more panel wattage. But again it depends on what you currently have and to make sure the charging amps falls within the C/12 to C/8 range.

[AL1952]

Thank you Sun Eagle that is very helpful information for determining what to purchase. I will get a new MPPT 30 amp controller, but I have a question about it. I see several listed as negative ground and some as positive ground. I’m confused. I thought all grounds were negative. How do I know which to use? What determines which to use? My current set up came as a package. The panels are grounded to a rod in the earth and the black “minus” wires off the panels go to the negative lug on my controller. Is mine a negative ground set up? Which should I get for the new controller? Is one better than the other?

While looking at batteries I see that the AGM batteries have 600 cycles and lithium phosphate have 7000 cycles. Does this literally mean that these AGM batteries, discharged and recharged every day, are only good for less than 2 years then they lose power and go bad while a lithium battery is good for 19 years?

[SunEagle]

Thank you Sun Eagle that is very helpful information for determining what to purchase. I will get a new MPPT 30 amp controller, but I have a question about it. I see several listed as negative ground and some as positive ground. I’m confused. I thought all grounds were negative. How do I know which to use? What determines which to use? My current set up came as a package. The panels are grounded to a rod in the earth and the black “minus” wires off the panels go to the negative lug on my controller. Is mine a negative ground set up? Which should I get for the new controller? Is one better than the other?

While looking at batteries I see that the AGM batteries have 600 cycles and lithium phosphate have 7000 cycles. Does this literally mean that these AGM batteries, discharged and recharged every day, are only good for less than 2 years then they lose power and go bad while a lithium battery is good for 19 years?

I am not sure which MPPT CC you are looking at but my recommendation would be something like The Kid from Midnite Solar. It may be expensive but it is a higher quality CC.

You are correct. An AGM battery has lower cycle rate then most other batteries (either Li or FLA). While they are sealed and can be moved around easily they usually cost more then an FLA and have lower cycle count.

While Li chemistry batteries have higher cycle count and life time claims, they are usually much more expensive then FLA. I always say "don't take the manufacturers claim as gospel" and "do the math" to determine the actual cost per kWh a battery can deliver over it's life. If cost is not a factor but some other criteria then the decision to go with a more expensive battery is yours to make.

[AL1952]

I'm looking at several different MPPT controllers. My budget only allows $150 max for a new controller given other expenses I anticipate so The Kid is not viable. I'm just trying to figure out which is better for a home solar system, positive or negative ground. I simply don't understand this whole concept. Is one better than the other? I guess if I knew what I currently have it would be easier to simply get the same type ground for the new one but I don't know what determines positive or negative ground for my system.

[Mike90250]

Some PV panels in the past, required positive ground. Some systems that provide electrolysis protection for remote underground pipelines, require positive ground.

But generally, negative ground is much more common.

[SunEagle]

I'm looking at several different MPPT controllers. My budget only allows $150 max for a new controller given other expenses I anticipate so The Kid is not viable. I'm just trying to figure out which is better for a home solar system, positive or negative ground. I simply don't understand this whole concept. Is one better than the other? I guess if I knew what I currently have it would be easier to simply get the same type ground for the new one but I don't know what determines positive or negative ground for my system.

I would not even look at any CC that have a positive ground. I would also be careful of the cheaper MPPT type CC because more than likely they are fake and are really only PWM type.

It is sort of buyer beware out there when it comes to finding quality equipment. Usually the lower cost is a dead give away of something that is fake.

[AL1952]

Sun Eagle I'm looking at a Renogy Rover Li 30 Amp MPPT Charge Controller. It's supposed to be negative ground and will work with sealed batteries, I'm assuming AGM batteries fall under this. It's not $350 and it's not $50 so would this brand of a mid range $139 controller be acceptable?

Mike90250 Thanks for the information

[SunEagle]

Sun Eagle I'm looking at a Renogy Rover Li 30 Amp MPPT Charge Controller. It's supposed to be negative ground and will work with sealed batteries, I'm assuming AGM batteries fall under this. It's not $350 and it's not $50 so would this brand of a mid range $139 controller be acceptable?

Mike90250 Thanks for the information

While I am not familiar with the Renogy Rover it looks like an MPPT type CC. Maybe someone else can comment on its' quality.