Mixing a 4 year old FLA battery with a new one in Series 24Volts

[vivekgangwar02]

Hi, I'm from India using Flooded lead acid batteries, I have the battery so called a solar c10 150AH battery, where they claims 60months warranty and 96Months of lifetime.
I recently moved to a 24 volts system, due to budget issues I mixed the 4 years used lead acid with a new one of the exact same make.
I was pretty sure on this because on a 100amp load tester I tested the condition(the load bearing capacity) of the old FLA, it showed in excellent condition.

But i'm sure the old FLA has increased internal resistance comapared to the new one. I am concerned because when I'm charging my batteries on MPPT, I see a big voltage difference in both individually. That difference is around 0.8 volts. I dont see this big difference during discharging.

My question is, is there any metric to decide based on the voltage difference between them to know if I really need to replace the old battery from the system.
I'm currently have 550watt of panels and Im planning to move to 1100watts, which will output 40amps for each battery at 24volts.
Thank You

 

[sunshine_eggo]

Perhaps you meant 96 months lifetime? 96 years seems a bit long.

4 years is a long run UNLESS you were absolutely fanatical about your maintenance, i.e., you fully charged daily, checked specific gravity regularly and equalized as needed. Did you do that?

How to the specific gravities of the old vs. new batteries compare?

Did you fully charge each 12V to true full before placing them in series @ 24V?

 

[vivekgangwar02]

Perhaps you meant 96 months lifetime? 96 years seems a bit long.

4 years is a long run UNLESS you were absolutely fanatical about your maintenance, i.e., you fully charged daily, checked specific gravity regularly and equalized as needed. Did you do that?

How to the specific gravities of the old vs. new batteries compare?

Did you fully charge each 12V to true full before placing them in series @ 24V?

I appreciate your response to my post and the corrections.

1. I was not aware about this thing that I need to fully charge each of them before connecting. Is it because I need to see the full charge voltage of them?
2. About maintenance, in the initial time I was not much paying attention and sometimes discharging it completely as the PV system was not then installed, I always kept it full of distilled water and never had shortage of in it. Used the float voltage of 14.4 for that period during daytime. Never did equlization charging in intial years, but later did when I got the mppt.
3. I checked SG, and it was as printed on the battery using an hydrometer. Since I checked it long ago, now I have two batteries, I will compare both of them and will let you know. Question is what does specific gravity tells about health of the battery? I though it is just about charge stored?
4. about the 1st point, if I plugged them not fully charged, but now they are connected to the system and are on float, will not they get fully charged in series?

 

[sunshine_eggo]

1. Both batteries need to be 100% at the same time. If one is at 90% when the other is at 100%, charging continues pushing the 100% battery over 100% with the 90% battery never quite reaching 100%

2. These batteries were not maintained. I would classify this as neglect.

3. SG tells you a LOT. It tells you the cells charge level, and it tells you if the cells need to be equalized.

4. They can, but it tends to damage both batteries. It depends on how far off they were to start, your charge voltage, current temperature, etc.

 

[vivekgangwar02]

1. Both batteries need to be 100% at the same time. If one is at 90% when the other is at 100%, charging continues pushing the 100% battery over 100% with the 90% battery never quite reaching 100%

2. These batteries were not maintained. I would classify this as neglect.

3. SG tells you a LOT. It tells you the cells charge level, and it tells you if the cells need to be equalized.

4. They can, but it tends to damage both batteries. It depends on how far off they were to start, your charge voltage, current temperature, etc.

I think you're going too much towards the ideal way, in the first place this practice was to save costs.
1. Does load testing a battery with a 100Amps load doesnt tell about the condition of batteries? I checked and the voltage dropped no less than 11.65. you can call it cranking load test.
2. Specific gravities are fine, but the older battery seems to evaporate more water from it, I noticed this.
3. By equalized I thought you mean equalization charging, is it about adding acid to the cell? I checked both the batteries are having same specific gravity around 1.2

 

[sunshine_eggo]

There's "ideal" and then there's worst case. The entire idea is poised for premature failure of BOTH batteries. If you don't care, keep doing what you're doing.

1. not in a way that's important to your system. Did you test BOTH batteries at identical states of charge and compare values?
2. "fine" is not an answer. Specific gravity is a numerical value. Each 12V has 6 cells, so you need 12 values.
3. Equalization is charging. You never add acid to a battery.

"around 1.2" is meaningless. 12 values of 1.2XX is meaningful.

Again, if you don't care about maximizing the life of your worst-case design, keep doing what you're doing. If you want to get the most out of it and learn how to maintain batteries, we can keep talking. Essentially, you need to learn how to maintain FLA batteries. If you don't want to, again, we can stop now.

 

[vivekgangwar02]

There's "ideal" and then there's worst case. The entire idea is poised for premature failure of BOTH batteries. If you don't care, keep doing what you're doing.

1. not in a way that's important to your system. Did you test BOTH batteries at identical states of charge and compare values?
2. "fine" is not an answer. Specific gravity is a numerical value. Each 12V has 6 cells, so you need 12 values.
3. Equalization is charging. You never add acid to a battery.

"around 1.2" is meaningless. 12 values of 1.2XX is meaningful.

Again, if you don't care about maximizing the life of your worst-case design, keep doing what you're doing. If you want to get the most out of it and learn how to maintain batteries, we can keep talking. Essentially, you need to learn how to maintain FLA batteries. If you don't want to, again, we can stop now.

I would like to keep talking on it sir, I want to learn, but the purpose of it is to be efficient to save some costs:

1. I'm 90% sure the new battery that came out of box was also charged from the factory too. But the load tester meter has readings mentioned on it, based on that I can confirm the old one FLA was in excellent condition. I'm afraid if we do load testing on a fully charged battery?
2. Actually today due to less sunshine, the specific gravity was 1.190 for all 12 cells, I was avoiding it because ik batteries are not fully charged today. but I can confirm that all 6+6 cells were same s.g.
3. So by equalization charging, you are referring to the process of de-sulfation by charging 12v battery to 15-16volts? If yes, then how does it corresponds to "You need to equalize each cell"?
4. I could not find many vlogs related to connecting different AH capacity battery in series, but I somewhere came to read that when the smaller one is discharged, it will get start charging from the negative terminal and this process leads to explosion of battery. I'm referring to my old battery as the battery with reduced AH capacity. is this correct?
5. I see you posted some video on bifacial panels, I'm also going to upgrade to 680*2 bifacial panels removing my current 190*3 panels.

 

[sunshine_eggo]

1. FLA can lose 1%/day since last charge, so it was not likely fully charged. Load tester markings are essentially only good for what you need to start a car, not draw power from and re-charge daily.
2. 1.190 is about as low as you ever want to go. They are at about 50%. It is important that you used the correct method. You should draw in and expel fluid three times before taking a reading. Now they need to be checked after a full charge and have sat at float for 2 hours.
3. I did not say "each cell." The equalization process applied to the battery equalizes the cells.\
4. You want to avoid connecting batteries of different capacities in series, which you have done as you correctly identified. Appropriate management and regular monitoring is required to confirm that the lower capacity battery isn't being abused. Abuse can lead to outgassing, and Oxygen+Hydrogen can equal boom. You essentially need to treat the entire battery as though it's the capacity of the weaker battery.
5. Cool.