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Keep batteries for load shedding or cycle daily with solar?


iops
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Not even sure if this needs so much thought, but over the last few days I've been trying to get the best out of my system. I'm trying to decide whether it's worth using my batteries alongside my solar panels daily or instead extend the life of my batteries and just use them for load shedding and other power outages.

 

A little bit about my system:

Axpert Synapse 3.0+ 24v inverter

8x Deltec BR-12V100 AGM batteries. Specs here.

6x 330W Enersol solar panels

I've also included my current inverter settings below.

Averaging 495 kWh per month, paying R2.01 per kWh. This is my first full month on solar though.

Load at night is ~500W with loads during the day averaging ~800W.

 

My thinking to reserve my batteries for load shedding is to extend their life, giving me a better ROI.

My (very limited) thinking about cycling daily is to save on some electricity and cycle the batteries in the evening, using the grid to charge at night.

I've seen that the batteries are used some time to time if my power requirements exceed that of what the panels are currently producing.

 

Any thoughts on this?

 

Axpert settings that I'm currently using, if a setting isn't mentioned here it's using the default value.

01 SBU
02 30A
03 APL
05 AGM
11 20A
12 24.5V
13 27v
16 CSO

 

Edited by iops
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1 hour ago, iops said:

I'm trying to decide whether it's worth using my batteries alongside my solar panels daily or instead extend the life of my batteries

When you cycle them you can recover a bit of your money. Even if you keep them charged, they only have a certain chemical design life and they will seldom go past that lifetime. 

Being AGMs, I wont recommend discharging them to much, but letting them die from old age without recovering at least some of your money is also not wise. When there is loadshediing keep them charged, in times like now, cycle them even if its only 10 or 20%. 

I hope this makes sense. 

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2 hours ago, Jaco de Jongh said:

When you cycle them you can recover a bit of your money. Even if you keep them charged, they only have a certain chemical design life and they will seldom go past that lifetime. 

I agree fully with this. Most batteries of the UPS variety lasts 5 years max, even if you leave them on float permanently. AGM batteries usually give you somewhere between 600 and 1200 cycles to 50%, depending on quality, which is essentially 2 or 3 years.

So I would essentially take the 3-year plan, look at their specs, and cycle the such that they die in around 3 years or so. It's a bit thumbsuck, but in that 3-5 year bracket you're essentially just one cell failure away from replacing the whole thing anyway, so I'd say it is pointless to cycle them so shallowly in an attempt to make 4 years or more.

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4 hours ago, Jaco de Jongh said:

When you cycle them you can recover a bit of your money. Even if you keep them charged, they only have a certain chemical design life and they will seldom go past that lifetime. 

Being AGMs, I wont recommend discharging them to much, but letting them die from old age without recovering at least some of your money is also not wise. When there is loadshediing keep them charged, in times like now, cycle them even if its only 10 or 20%. 

I hope this makes sense. 

Thanks, this does make sense.

2 hours ago, plonkster said:

I agree fully with this. Most batteries of the UPS variety lasts 5 years max, even if you leave them on float permanently. AGM batteries usually give you somewhere between 600 and 1200 cycles to 50%, depending on quality, which is essentially 2 or 3 years.

So I would essentially take the 3-year plan, look at their specs, and cycle the such that they die in around 3 years or so. It's a bit thumbsuck, but in that 3-5 year bracket you're essentially just one cell failure away from replacing the whole thing anyway, so I'd say it is pointless to cycle them so shallowly in an attempt to make 4 years or more.

Makes sense to not just leave them in float, this is what initially got me thinking about how long they'd last sitting there vs being used.

I linked the spec sheet in my original post, DOD @ 50% seems to be on the lower scale of things at 600 cycles. Today there's been one cycle earlier this morning, probably two more tonight going down to 24.5V. I think that's roughly 60% DOD. If that really is a cycle am I relying on 3 cycles per day? 

That seems low considering I'd hit 600 cycles in under a year if I had to keep the settings as is. I'm a little confused.

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I agree absolutely with what has been said. I have 8x200Ah Naradas which are going into their fourth year. I had your dilemma and 4yrs on they are on SBU. I think I discharge them around 30-40%. They have served me some money and I am yet to see them start showing signs of age really. I have 12 x 260W panels and they are made to work to recharge them and run the load which is about your load. Hell, during the day the 1.5HP sprinkler pump is used to good use on free solar and the 1HP booster to overhead tank also runs to utilise the panels. Life is good.

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Thanks Iops, I have been thinking about this as well. Please let me know how your panels are set up? 

I have the Must equivalent of your inverter with just 4 panels wired 2S2P and have purchased 2 more however was concerned that 6 panels of 330w would be too much for the inverter?? 

Thanks 

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17 hours ago, iops said:

I linked the spec sheet in my original post, DOD @ 50% seems to be on the lower scale of things at 600 cycles. Today there's been one cycle earlier this morning, probably two more tonight going down to 24.5V. I think that's roughly 60% DOD. If that really is a cycle am I relying on 3 cycles per day? 

Cycles don't quite work like that. Not every discharge event (of a few minutes or seconds) counts as a cycle. Of course such "micro cycles" do wear the battery (just like permanently floating it), but not as much as the macro cycles. By macro I mean that generally the battery comes down over night, and goes back up during the day. That is to say, most people do one macro cycle per day, with a bit of up and down during the day.

You also cannot use voltage to estimate this. To use an example we all know: When you start your car, for that few seconds the car battery pulls down to 10V, but within seconds the engine fires up and the battery jumps right up to 13.5V or higher... does that count as a cycle? Not really... 🙂

Looking at the spec sheet, I'd say you can easily cycle them to 30% DoD while losing little of the overall life of the battery. How to cycle to 30% without a good SoC measurement is left as an exercise for the reader 😉

 

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On 2020/04/13 at 2:04 AM, iops said:

 

A little bit about my system:

Axpert Synapse 3.0+ 24v inverter

Hi iops,

Slightly off topic, but I would like to ask you about your Synapse 3.0+ 24V inverter, as the residential complex I live in on the KZN South Coast has a few of these units installed, together with many CBSolar 24V inverters and a handful of Mecers

The CBSolar inverters have been the standard for 2 years and apart from a few failures have performed well. We are still getting to know the Synapse and Mecer.

One issue we have noted with the Synapse 3.0+ is that the fan runs continuously, this is not the case with the CBSolar and Mecer.

Do you know if the Synapse is a Voltronics, or a clone?  Who supplied the unit to you?

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On 2020/04/13 at 6:36 PM, Kalito said:

I agree absolutely with what has been said. I have 8x200Ah Naradas which are going into their fourth year. I had your dilemma and 4yrs on they are on SBU. I think I discharge them around 30-40%. They have served me some money and I am yet to see them start showing signs of age really. I have 12 x 260W panels and they are made to work to recharge them and run the load which is about your load. Hell, during the day the 1.5HP sprinkler pump is used to good use on free solar and the 1HP booster to overhead tank also runs to utilise the panels. Life is good.

That's awesome to hear, cheers for your feedback!

Can I ask how you're measuring SOC

12 hours ago, plonkster said:

Cycles don't quite work like that. Not every discharge event (of a few minutes or seconds) counts as a cycle. Of course such "micro cycles" do wear the battery (just like permanently floating it), but not as much as the macro cycles. By macro I mean that generally the battery comes down over night, and goes back up during the day. That is to say, most people do one macro cycle per day, with a bit of up and down during the day.

You also cannot use voltage to estimate this. To use an example we all know: When you start your car, for that few seconds the car battery pulls down to 10V, but within seconds the engine fires up and the battery jumps right up to 13.5V or higher... does that count as a cycle? Not really... 🙂

Looking at the spec sheet, I'd say you can easily cycle them to 30% DoD while losing little of the overall life of the battery. How to cycle to 30% without a good SoC measurement is left as an exercise for the reader 😉

 

Cool, I think I'm getting there slowly but surely.

I just need to figure out how to measure SOC on these batteries properly, a few posts in on this forum and I'm wanting to expand my whole systems :) I've read great things about the Victron BMV - I think I need the 702 to measure the midpoint since I have 4x 24V banks?

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On 2020/04/13 at 7:21 PM, Dougiedanger said:

Thanks Iops, I have been thinking about this as well. Please let me know how your panels are set up? 

I have the Must equivalent of your inverter with just 4 panels wired 2S2P and have purchased 2 more however was concerned that 6 panels of 330w would be too much for the inverter?? 

Thanks 

I've asked the installer how they're installed, it is within the max limitation of my inverter of 1500W though. When PV is in use the inverter reports 70V at max. I'll get back to you on this though.

Also just a heads up, I believe the Must is a clone of the Axpert.

1 hour ago, Solo said:

Hi iops,

Slightly off topic, but I would like to ask you about your Synapse 3.0+ 24V inverter, as the residential complex I live in on the KZN South Coast has a few of these units installed, together with many CBSolar 24V inverters and a handful of Mecers

The CBSolar inverters have been the standard for 2 years and apart from a few failures have performed well. We are still getting to know the Synapse and Mecer.

One issue we have noted with the Synapse 3.0+ is that the fan runs continuously, this is not the case with the CBSolar and Mecer.

Do you know if the Synapse is a Voltronics, or a clone?  Who supplied the unit to you?

Good question, I believe the Synapse isn't a clone - perhaps someone more knowledable than my can confirm?

I've had a read through another forum thread which shows you how to identify if it is a clone, I've done most of them apart from hooking up that dreadful WatchPower app. I've included a picture of the side labels below.

The fans do run a lot. I think the only time it isn't running is when the batteries are in float and PV isn't being used. I had my panels installed mid-month last month and have noticed that the fans run a lot. It doesn't really bother me since the setup is in a room outside of the main house.

The Synapse has been running as a loadshedding backup solution with batteries connected since December, I haven't had any issues... apart from posting here and wanting to upgrade to Victron gear ;)

 

IMG_20200414_221935.jpg

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On 2020/04/14 at 11:07 PM, iops said:

I've asked the installer how they're installed, it is within the max limitation of my inverter of 1500W though. When PV is in use the inverter reports 70V at max. I'll get back to you on this though.

Also just a heads up, I believe the Must is a clone of the Axpert.

Thanks Iops that is appreciated.

Yes I am aware that they are a clone, wasn't at the time, but a reputable installer in Zim recommended them so I went with it. I have been very happy with its performance over the past 2 years but ultimately would like to be able to have the better monitoring options that come with the Axpert and Victrons.

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On 2020/04/13 at 7:21 PM, Dougiedanger said:

Thanks Iops, I have been thinking about this as well. Please let me know how your panels are set up? 

I have the Must equivalent of your inverter with just 4 panels wired 2S2P and have purchased 2 more however was concerned that 6 panels of 330w would be too much for the inverter?? 

Thanks 

I have my 6 330W panels wired in 3S3P.

My inverter doesn't report that the voltage for PV goes over 80V so I might be able to add more panels. Someone else probably needs to weigh in though, I'm a bit out of my depth here.

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On 2020/04/14 at 10:21 AM, plonkster said:

Cycles don't quite work like that. Not every discharge event (of a few minutes or seconds) counts as a cycle. Of course such "micro cycles" do wear the battery (just like permanently floating it), but not as much as the macro cycles. By macro I mean that generally the battery comes down over night, and goes back up during the day. That is to say, most people do one macro cycle per day, with a bit of up and down during the day.

How does the shelf life (or using a smart charger) of a battery compare to keeping it on float?

I notice that the suppliers don't float their batteries whilst they are on the shelf.. 

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2 hours ago, Richard Mackay said:

I notice that the suppliers don't float their batteries whilst they are on the shelf.. 

Self discharge is something like 10% a week or thereabouts. So I assume suppliers just move them fast enough so they don't have to float-charge them. Or (and this I've actually seen in a battery shop), the charging bench is in the back and they do cycle the stock from the front room to the back occasionally 🙂

 

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On 2020/04/14 at 10:21 AM, plonkster said:

Cycles don't quite work like that. Not every discharge event (of a few minutes or seconds) counts as a cycle. Of course such "micro cycles" do wear the battery (just like permanently floating it)

Are you implying the permanently floating the batteries by the inverter is detrimental (like a regular old battery charger)?

Would a smart charger be a better bet??

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57 minutes ago, Richard Mackay said:

Are you implying the permanently floating the batteries by the inverter is detrimental (like a regular old battery charger)?

No, not really. The battery has a certain design life and even when you don't use it or use it only very lightly, it tends to degrade and fail around that time.

The same question is often asked of laptops. People ask: Should I keep the battery charged, or should I use it? The answer is that if you keep it permanently charged, you will have to replace the battery in about 5 years  or so. If you use it, you will have to replace it somewhere in the next 5 years. Whatever you do... it's going to get you.

There are ways to make the battery last significantly shorter, but in the end there aren't many ways to make them last longer.

Regarding battery chargers: Good ones have a "storage" mode. The reason is that floating voltage is still a "gassing" voltage and tends to slowly cook away electrolyte. Storage mode doesn't do that... but storage mode uses a voltage that is too low to keep the battery from self-discharging, it merely slows the self-discharge. So such smart battery chargers will do a quick absorb-float loop once a week before going back to storage. Does it significantly extend battery life? I don't know... it probably helps, but I doubt it gets you much past the design life.

Let me just add a disclaimer: There are lead-acid batteries designed to last 20 years. When I say 5 years, I talk about the low-end UPS-ey types 🙂

 

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This tendency of lead acid batteries to self destruct over time has put me off them. One pays a lot of money for big capacity batteries and the nest time you look they are kaput!

There is also the other issue of temperature that isn't fully appreciated. The attached graph shows the relationship. I can't recall ever seeing a battery charger (in whatever form) taking this into account.

In our hot climate the float voltage should be reduced 

volt-temp.jpg

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3 hours ago, Richard Mackay said:

In our hot climate the float voltage should be reduced 

So that's why we made STS (shared temperature sense) for the Victron setups. If you have a temperature sensor on any device that supports it (Multi, old CAN-bus solarcharger, directly on the Venus-GX, on a BMV 702/712 with a temperature probe on the aux input), that temperature can be shared with other devices that don't have a sensor, and they can do temperature compensation.

Edited by plonkster
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