Cycle Life - Is time a factor?

[jt_retro]

Hi Folks

I have 6 x 80AH 12V AGM batteries in parallel.

I run my washing machine off the solar and I've got myself into a situation where I'm discharging from 70% to 60%, and then back up to 70% within 90mins or so maybe 2 or 3 times a day (due to multiple washes).

I know LA batteries have a much more limited cycle life (compared to Lifepo4), but I was wondering how much damage am i doing to my batteries given I'm recharging so fast? Is my damage more limited because I'm recharging so quickly?

I'm not sure if it's relevant to my question above, but I think I bought the "wrong" type of batteries when I started my project. My AGM's are car starter batteries, and not branded as deep cycle or leisure batteries. I've never got a good answer on if there's really a difference, but I guess it's possible my batteries have thinner plates than would be ideal.

Thanks

JT

 

[snoobler]

6 * 80 = 480Ah @ 12V

Consider that the "standard" charge for 12V is typically 0.1C or 10% of capacity. In your case, this is 48A. Assuming you're charging with solar, that's about 700W.

You're moving 10% in 1.5 hours, which is actually LOWER than the standard charge rate. Also, the actual source of the % is important. If you're reading this value off a charge controller display, it's worthless. This number must come from a battery monitor that counts current in and out and computes SoC based on net Ah in/out and programmed battery capacity.

For longevity, you would be better served by "cycling" between 80 and 90% rather than 60-70%.

There is absolutely a difference between deep cycle and starter. Deep cycle have thicker plates and can't deliver as high a current as starter batteries. Their thicker plates also make them more tolerant of deep cycles. Starter batteries have thin plates. Greater surface area makes them able to deliver higher currents, but they don't tolerate deep cycling very well. There is also more surface area for sulfation resulting in faster capacity loss/degradation and potential for fracture of the plates.

 

[jt_retro]

Thanks Snoobler

My % is from my Victron BMV 712. I'm struggling to get it calibrated correctly, but that's another story. For the sake of argument, let's assume that 70% down to 60% is a correct reading.

Thanks for the numbers, I now know that my charge rate is below "standard" so not as good as a thought. Getting around 500W into my battery bank is a bit of a novelty for me, as I've recently upgraded my system.

I really do struggle to get current into my bank. It seems I hit my boost voltage sooner than I would like. Maybe my bank is charged more than I think it is? For example, just after my above wash cycle (which according to my poorly calibrated Victron was 70% - 60%), I would be able to charge at approx 500W-600W, then after about an hour, the bank would hit 14.8V (my controller's set boost voltage), after which the controllers will limit the current so it will drop and drop and drop. Am I doing something wrong here? I would happily so 90% - 80% if I could (or maybe I already am??)

My question on the chemistry still stands - does recharging a LA battery quicker increase longevity (compared to leaving is at 60% overnight, for example)

Thanks

JT

 

[snoobler]

Thanks Snoobler

My % is from my Victron BMV 712. I'm struggling to get it calibrated correctly, but that's another story. For the sake of argument, let's assume that 70% down to 60% is a correct reading.

Good. I have a 702. Battery monitors can be finicky due to the variable nature of solar charging. It is important that the battery be fully charged regularly to ensure the accuracy is retained.

Post your settings for review.

Thanks for the numbers, I now know that my charge rate is below "standard" so not as good as a thought. Getting around 500W into my battery bank is a bit of a novelty for me, as I've recently upgraded my system.

More upgrades are needed. You are likely over-using power relative to your generation capabilities.

I really do struggle to get current into my bank. It seems I hit my boost voltage sooner than I would like. Maybe my bank is charged more than I think it is? For example, just after my above wash cycle (which according to my poorly calibrated Victron was 70% - 60%), I would be able to charge at 500W, then after about an hour, the bank would hit 14.8V (my controller's set boost voltage), after which the controllers will limit the current so it will drop and drop and drop. Am I doing something wrong here? I would happily so 90% - 80% if I could (or maybe I already am??)

If the current didn't decrease, the battery voltage would continue to rise and be damaged. The 14.8V + tapering current is the "absorption" phase of the charge cycle.

My question on the chemistry still stands - does recharging a LA battery quicker increase longevity (compared to leaving is at 60% overnight, for example)

Your question needed the additional context. Recharging a battery "quicker" implies the speed of charging, and that's the question I answered.

It's important that AGM batteries be kept above 80% SoC when at rest and charged to 100% daily whenever possible. The more shallow the discharges, the better.

I suspect you have other issues. 6 12V wired in parallel is sub-optimal. Most manufacturers recommend no more than 4, and how they're wired together is important.

Does the 14.8V on the controller correspond to the voltage reported by the BMV?

 

[jt_retro]

Post your settings for review.

More upgrades are needed. You are likely over-using power relative to your generation capabilities.

When I'm using my washing machine, that is correct. I'm using my battery bank as a buffer for high current appliances. Otherwise, I'm within my generation capability. Is this an ok thing to do or am I going about things wrong? I literally don't have any more space for more panels unfortunately. I do use about 7% overnight for standby loads, as well as for the freezer.

If the current didn't decrease, the battery voltage would continue to rise and be damaged. The 14.8V + tapering current is the "absorption" phase of the charge cycle.

Yep, I'm aware of this but I'm just hitting it quicker than I'd expect.

It's important that AGM batteries be kept above 80% SoC when at rest and charged to 100% daily whenever possible. The more shallow the discharges, the better.

i guess my question is: is leaving a battery at a low SOC for a long time (eg overnight) worse for it, than leaving it at the same low SOC for a shorter time (eg 90 mins)

I suspect you have other issues. 6 12V wired in parallel is sub-optimal. Most manufacturers recommend no more than 4, and how they're wired together is important.

Does the 14.8V on the controller correspond to the voltage reported by the BMV?

They are wired in a single string, with the master + and master - at the 2 batteries on the opposite ends.

When the controller is showing 14.8, the BMV shows about 14.7. Voltage drop in the wires and fuses.


I really appreciate your help here.
Thanks
JT

 

[snoobler]

When I'm using my washing machine, that is correct. I'm using my battery bank as a buffer for high current appliances. Otherwise, I'm within my generation capability. Is this an ok thing to do or am I going about things wrong? I literally don't have any more space for more panels unfortunately. I do use about 7% overnight for standby loads, as well as for the freezer.

Fine

Change tail to 3.0%. Lead-acid is typically fully charged @ absorption voltage and .01-.03C (4.8 to 14.4A). 14.4A @ 14.8V is 213W, so I suspect you're getting fully charged a lot more than you think you are.

Change Peukert to 1.25 unless the 1.15 came from your battery specification.

i guess my question is: is leaving a battery at a low SOC for a long time (eg overnight) worse for it, than leaving it at the same low SOC for a shorter time (eg 90 mins)

Since you're discharging at night, you're not really "leaving" it at a low SoC, you're using it. Ideally, you should start your no-solar discharging period at the highest possible state of charge. If you're doing 3 loads, and they take an hour each, probably best to start them at 10:30am and be done by 1:30pm to only use it during peak solar.

They are wired in a single string, with the master + and master - at the 2 batteries on the opposite ends.

Good. You might want to review "Wiring" in link #2 in my signature. "Diagonal" connection as you describe is the worst of the best. You may find one of the other methods more beneficial.

I recommend you check the current flowing through each battery with a clamp DC ammeter while under a heavy-ish load to confirm you have good shared current.

When the controller is showing 14.8, the BMV shows about 14.7. Voltage drop in the wires and fuses.

Good. I wanted to verify it wasn't worse.

 

[Hedges]

Lead Acid batteries are better off being recharged to full soon, not left sitting at a low state of charge for a long time.
They also need to be properly given a long absorption charge, which takes 2 to 4 hours, at least occasionally.
AGM is supposed to be good for being left disconnected for months at a time, after being fully charged. They self-discharge somewhat during those months. This suggests partial state of discharge doesn't hurt them so much. Although for starting application they can still do their job with a fraction of original Ah capacity.

If you can over-panel with PV such that your laundry runs directly off sunshine without draining the batteries, and batteries only supply starting surge, I think that is best. PV panels are about 1/10th or 1/20th the price of AGM batteries (in terms of watt-hours usable life.) That means you can better afford to waste available PV production than to store it in batteries.
I think DIY LiFePO4 is presently about 2x the price of PV panels (so panels are 1/2 the price of the batteries.)

I have a quality deep-cycle AGM bank which should deliver 650 cycles to 70% DoD. That is something they would only have to do overnight during a power failure (grid-backup). During the day with grid down, PV normally exceeds household consumption.

 

[jt_retro]

Change tail to 3.0%. Lead-acid is typically fully charged @ absorption voltage and .01-.03C (4.8 to 14.4A). 14.4A @ 14.8V is 213W, so I suspect you're getting fully charged a lot more than you think you are

Ooooo now this is interesting! And very exciting! So my BMV calibration is way off and I'm indeed reaching full charge almost? If I hit around 213W, what would be a reasonable % to set the BMV to manually if I wanted to calibrate?

I guess I've been manually calibrating my bank at 75% when the resting voltage hit 12.5V. But perhaps that is just really wrong because voltage is a terrible terrible way to measure SOC?

Since you're discharging at night, you're not really "leaving" it at a low SoC, you're using it. Ideally, you should start your no-solar discharging period at the highest possible state of charge. If you're doing 3 loads, and they take an hour each, probably best to start them at 10:30am and be done by 1:30pm to only use it during peak solar.

Sorry, yes your correct about the overnight part. But I'm referring to my daytime laundry usage where I recharge quickly.

Yeah, I always try to do the washing during peak solar and your proposed times match up with mine roughly.

Regarding the DC clamp test, are you able to help me with some example maths for what each link should be? I tried this test before and got very confused with my numbers. For a 112A load, I guess my current readings on each link should be ~16, 32, 48, 64, 96, does that sound correct?

Thanks

 

[snoobler]

Ooooo now this is interesting! And very exciting! So my BMV calibration is way off and I'm indeed reaching full charge almost? If I hit around 213W, what would be a reasonable % to set the BMV to manually if I wanted to calibrate?

I guess I've been manually calibrating my bank at 75% when the resting voltage hit 12.5V. But perhaps that is just really wrong because voltage is a terrible terrible way to measure SOC?

Likely out of whack. Next time you see 14.8V AND 14A charging on the BMV, recalibrate.

Sorry, yes your correct about the overnight part. But I'm referring to my daytime laundry usage where I recharge quickly.

If you have available solar, and your batteries are less than 100%, ALWAYS charge.

Regarding the DC clamp test, are you able to help me with some example maths for what each link should be? I tried this test before and got very confused with my numbers. For a 112A load, I guess my current readings on each link should be ~16, 32, 48, 64, 96, does that sound correct?

Each battery should SHARE the load, so 112A / 6 = 18.7A EA.

 

[jt_retro]

Likely out of whack. Next time you see 14.8V AND 14A charging on the BMV, recalibrate.

Just to clarify, recalibrate to 100%, right? Which would be just over 200W going into my AGM bank.

I'm still curious to learn if that chemistry wise, leaving a battery at 60% SOC for 12 hours is worse than leaving a battery at 60% charge for 90 mins.

Thanks

 

[snoobler]

Just to clarify, recalibrate to 100%, right? Which would be just over 200W going into my AGM bank.

Prefer you do it to 100% based on the BMV reported voltage (14.8V) and current (<14A). Honestly, set the BMV as indicated, and it should do it automatically.

I'm still curious to learn if that chemistry wise, leaving a battery at 60% SOC for 12 hours is worse than leaving a battery at 60% charge for 90 mins.

60% SOC and "battery at 60% charge" - to me those are the same thing.

12 hours is worse than 90 minutes.

 

[jt_retro]

Prefer you do it to 100% based on the BMV reported voltage (14.8V) and current (<14A). Honestly, set the BMV as indicated, and it should do it automatically.

Got it, thanks!

12 hours is worse than 90 minutes.

Good to know! I'm assuming this is because 12 hours gives more time for sulfication to occur?

 

[snoobler]

Got it, thanks!

Good to know! I'm assuming this is because 12 hours gives more time for sulfication to occur?

Yep.

 

[jt_retro]

Thank you Snoobler for your help, I really appreciate it and am grateful for this community.

@Hedges thank you for your help too. I completely agree with you however I have maxed out my room for more panels. I have about 2kWp (rated) of panels (12V in parallel) which bring in about 1.2kWp given angle to sun, losses and charge controller limatations. Bringing in a total of about 7.5kWh per day at present.

Cheers

JT

 

[jt_retro]

@snoobler i manually synced to 100% when I saw the current being limited as indicated.

However, after some evening loads where the BMV has registered 10% usage, here is my readout:



Doesn't the voltage seem quite low? I know that you're supposed to remove all loads and wait a few hours to use voltage as SOC, but aren't I quite far off regardless?

I will admit that my bank hasn't had the kindest life, especially at the start of my project (about 1 year ago) when my system was much smaller. Have I damaged my batteries and if so, am I able to tweak my BMV settings to compensate?

Thanks!

 

[snoobler]

Yes. Voltage is low. That's about 70% resting, but if the BMV didn't recalibrate automatically, then you might have been premature.

Given that these are starter batteries, it's very possible a year's worth of heavy use has done a number on them. With 6 in parallel, certain batteries may have deteriorated significantly worse than others.

Reduce your BMV capacity to 384 (80% health) and see how it correlates.

 

[jt_retro]

Thanks, I'll give that a go and see if it's more accurate.

Btw, I've reduced my charged voltage in the BMV to 14.5 as due to losses, temp compensation etc it might not see 14.7 for 50mins. Is this a reasonable adjustment?

Thanks

 

[jt_retro]

Also, if I manage to calibrate my BMV by reducing the Ah in the settings, are my batteries still safe if I stay above 80% on the calibrated scale?

Thanks

 

[snoobler]

Thanks, I'll give that a go and see if it's more accurate.

Btw, I've reduced my charged voltage in the BMV to 14.5 as due to losses, temp compensation etc it might not see 14.7 for 50mins. Is this a reasonable adjustment?

Thanks

Yes. Actually, I missed that point. Victron recommends "charged" voltage 0.2V below absorption.

I also missed the 50m setting. WAY too high. Your battery is fully charged AT absorption voltage when current has dropped below 0.01-0.03C. IIRC, the default for that is 3m. Change to 3m. This explains why your BMV didn't reset to 100% automatically.

Summarizing:

Also, if I manage to calibrate my BMV by reducing the Ah in the settings, are my batteries still safe if I stay above 80% on the calibrated scale?

Thanks

If your batteries are degraded compared to their rating, the SoC against 480Ah is reported as artificially high. Reducing the rated capacity in the BMV means the SoC should be closer to actual.

 

[jt_retro]

Thanks for the config summary very useful!

If your batteries are degraded compared to their rating, the SoC against 480Ah is reported as artificially high. Reducing the rated capacity in the BMV means the SoC should be closer to actual.

Yeah, but I'm wondering what "100% - 80%" means for battery health in the context of degraded batteries and a calibrated scale. I guess I'm doing the best I possibly can for them, save for not using them at all!

I was also thinking....given my batteries are starter batteries, do you think it's possible that my batteries aren't actually degraded, but should be derated for solar cycle usage? I.e. 80Ah is only applicable in the context of starting an engine. Just a thought.