End of Battery Life Replacement Strategy

mcgivor said:

Welcome to the forum 
What is the 32 degrees reference to? Currently 24*12 V would be 288 cells no? Just trying to understand, I've a few thoughts just need some clarification.

mcgivor said:

Welcome to the forum 
What is the 32 degrees reference to? Currently 24*12 V would be 288 cells no? Just trying to understand, I've a few thoughts just need some clarification.

mcgivor said:

Perhaps thinking about a different chemistry is an alternative, LFP for example, given the current bank is not dead but dying, this would afford some time to study options. There are two approaches, a manufactured system or DIY, the latter dose involve more time the former more $$, the DIY is not as complicated in practice as it is in theroy, with assistance it's less so. Naturally I've no idea of your knowledge base so this may, or may not, be beyond your skill set. Based on price alone 15K $ could buy a very similar Ah capacity LFP battery with the advantage of being able to use a much greater depth of discharge without premature degredation associated with lead acid. This would allow a smaller Ah capacity to do the work of a larger lead acid bank, bearing  in mind even LFP's life expectancy is decreased the harder they're used, operating between 95% and 20% SOC is preferable, 75% capacity versus 50% at best,  to get 10 years with LA your DOD must have been less, one would assume. So without rambling on too much, have you ever considered a change?

mcgivor said:

Perhaps thinking about a different chemistry is an alternative, LFP for example, given the current bank is not dead but dying, this would afford some time to study options. There are two approaches, a manufactured system or DIY, the latter dose involve more time the former more $$, the DIY is not as complicated in practice as it is in theroy, with assistance it's less so. Naturally I've no idea of your knowledge base so this may, or may not, be beyond your skill set. Based on price alone 15K $ could buy a very similar Ah capacity LFP battery with the advantage of being able to use a much greater depth of discharge without premature degredation associated with lead acid. This would allow a smaller Ah capacity to do the work of a larger lead acid bank, bearing  in mind even LFP's life expectancy is decreased the harder they're used, operating between 95% and 20% SOC is preferable, 75% capacity versus 50% at best,  to get 10 years with LA your DOD must have been less, one would assume. So without rambling on too much, have you ever considered a change?

Estragon said:

If I was to replace my bank, I'd consider Lifepo, but in the end might stay with LA based mainly on cost, but also on complications around low temp charging.

Staying with LA, I'd definitely go with 2v individual cells sized for my needs (~500ah in my case) if available at reasonable cost. For me, the extra cost for VRLA isn't worth it, but it might be for some applications.

As for what to do with the existing, at 10 years old, the bank is near end-of-life. Although it's only showing its age with capacity loss for now, there's a decent chance one or more cells will suffer a more catastrophic failure. I wouldn't size a new bank on the assumption the old one will stay in service long term as when it ultimately fails, you might not want to add new batteries to the now-older new bank. Keeping it in service in a separate application where if it fails and it takes time to replace it's not an issue might be an option. If you had an application where a 12v bank would work might be able to keep it going for some time by swapping out batteries as they fail.

Personally, I'd probably just consider them near EOL, and use the luxury of having a serviceable (for now) bank to take my time looking for replacement, and apply the savage value of the old bank to offset the cost of the new one.

Estragon said:

If I was to replace my bank, I'd consider Lifepo, but in the end might stay with LA based mainly on cost, but also on complications around low temp charging.

Staying with LA, I'd definitely go with 2v individual cells sized for my needs (~500ah in my case) if available at reasonable cost. For me, the extra cost for VRLA isn't worth it, but it might be for some applications.

As for what to do with the existing, at 10 years old, the bank is near end-of-life. Although it's only showing its age with capacity loss for now, there's a decent chance one or more cells will suffer a more catastrophic failure. I wouldn't size a new bank on the assumption the old one will stay in service long term as when it ultimately fails, you might not want to add new batteries to the now-older new bank. Keeping it in service in a separate application where if it fails and it takes time to replace it's not an issue might be an option. If you had an application where a 12v bank would work might be able to keep it going for some time by swapping out batteries as they fail.

Personally, I'd probably just consider them near EOL, and use the luxury of having a serviceable (for now) bank to take my time looking for replacement, and apply the savage value of the old bank to offset the cost of the new one.

Estragon said:

If I was to replace my bank, I'd consider Lifepo, but in the end might stay with LA based mainly on cost, but also on complications around low temp charging.

Staying with LA, I'd definitely go with 2v individual cells sized for my needs (~500ah in my case) if available at reasonable cost. For me, the extra cost for VRLA isn't worth it, but it might be for some applications.

As for what to do with the existing, at 10 years old, the bank is near end-of-life. Although it's only showing its age with capacity loss for now, there's a decent chance one or more cells will suffer a more catastrophic failure. I wouldn't size a new bank on the assumption the old one will stay in service long term as when it ultimately fails, you might not want to add new batteries to the now-older new bank. Keeping it in service in a separate application where if it fails and it takes time to replace it's not an issue might be an option. If you had an application where a 12v bank would work might be able to keep it going for some time by swapping out batteries as they fail.

Personally, I'd probably just consider them near EOL, and use the luxury of having a serviceable (for now) bank to take my time looking for replacement, and apply the savage value of the old bank to offset the cost of the new one.

Marc Kurth said:

richardimorse said:

I have a 10 year old Dual Outback 80 configuration (32 degrees) so it is time to replace the batteries, currently using 24 * 12V 230AH batteries gel VRLA, so now comes reality instead of theory, everything on the internet is theory on this subject aimed at people starting out in their journey with solar power, I have been doing it for 10 years already.

So what to choose I am thinking higher capacity batteries to reduce the number of "real" 2V cells in the configuration from 144 to something lower more like 72 at 460AH or 48 at 690AH or just replace like with like

2nd Question . . . what to do with my near end of life batteries, here I am not having problems right now, so they probably just operate normally as 180AH batteries due to degradation over the years, what do you think I should do with them, one option is to look at having 2 battery banks to reduce the cost of upgrade, could be an option to split everything, currently the everything is split apart from the battery bank, I could just get a second battery bank the existing one has 66,000 Watt Hours lets assume that has degraded to 50,000 Watt Hours

If I add a second battery bank sized at 40,000 watt hours brand new would that be a good approach, no advice anywhere on these strategies
currently have 9 strings of 3 panels 175W each split in a ratio of 5:4 between the two MPPT charge controllers

Or is it better to take the old batteries to my second home as a new installation for grid resilience.

What to do when your battery bank costs USD 15,000

You said that you are not having any problems now. My approach would be to continue to use your existing battery bank until it becomes unusable. I often suggest that as capacity falls off, simply discharge them a bit deeper as needed to serve your loads. you may have another 3-5 years of usable life. Because you are exploring other uses for these batteries, I suspect that you feel that there is more life in them.

With six parallel strings of VRLA's running for 10 years, my experience makes me assume that your battery bank is shallow cycled and spends a lot of time in float.

Whether you replace them with fewer, larger cells or changing chemistry - will come down to economics.

If you lose an individual battery, you can bypass that string and continue on with that 16% missing. Then you could use the three other batteries from that string as replacements for another failure.

Marc

richardimorse said:

softdown said:

What does "(32 degrees)" communicate? 

Thats the temperature in centigrade, the batteries are in Mozambique in an outbuilding that is not cooled, the outside temperature is 32 degrees in peak season and drops to a freezing cold 25 in July 

Dave Angelini said:

I would buy one $ 7.5K group and see how you do. Then within a year buy the second group if you need them. If you can still get them.

Also look into the 2V 1,100 AH cells. They are at my cost about $500 each so even more money. Two strings of the 400AH would be OK, if you can get them. Maybe just one 400AH string and use the old as bank 2.

It looks like you are shallow cycling to get that kind of life and your virtual tracked solar is a big reason also that you got this kind of life.
Tracking is the best way offgrid to make a battery last. Good Luck

richardimorse said:

Dave Angelini said:

I would buy one $ 7.5K group and see how you do. Then within a year buy the second group if you need them. If you can still get them.

Also look into the 2V 1,100 AH cells. They are at my cost about $500 each so even more money. Two strings of the 400AH would be OK, if you can get them. Maybe just one 400AH string and use the old as bank 2.

It looks like you are shallow cycling to get that kind of life and your virtual tracked solar is a big reason also that you got this kind of life.
Tracking is the best way offgrid to make a battery last. Good Luck

Not many people would have noticed the virtual tracking using additional panels on a wide arc, or perhaps we can attribute that to the local labor being creative when building the solar panel support stands out of fence posts.

24 * 2V batteries seems the way to go this time.  What are your thoughts on getting 25 and rotating one out every 2 months.

richardimorse said:

BB. said:

In series strings, the "weakest battery" is usually what limits the AH capacity of the string. So "match" the batteries in a string (weak batteries with with batteries; strong batteries with strong batteries in each series string). The "strong string" will provide/sink more Amp*Hours and the weak string will provide less during discharging/charging.

"Open batteries" (failed open) batteries will stop current flow through a string (and the other batteries will not see any charge/discharge current) (high resistance batteries will certainly limit current flow).

"Shorted cell batteries" will drop the voltage of a string by ~2 volts per shorted cell--That will limit or prevent that string from providing load currents, and will sink way more current during charging. Obviously, not a good thing (open cell, the rest of the batteries in the string just sit there and slowly self discharge--With a shorted cell, that string gets way more than its share of charging current and usually will over charge the rest of the batteries, and discharge the rest of the bank at night).

Generally, AGM and GEL batteries have very low self discharge and can go 6 months between charging cycles (~24 hour charging cycle). However, in hot climates/storage conditions, self discharge current goes up (typically for every 10C over ~25C, self discharge current goes up by 2x and the batteries will last 1/2 as long (faster aging)). At 30+C, you should probably place them on charge every 3 months (1-3 months between charging cycles). You can check the resting voltage... Ideally, you do not want the batteries to be spending any time "resting" below 75% state of charge (sulfation happens much faster below ~75% SoC).

Yes, rotating batteries sounds good--But I am not fan of doing that. Lots of work and wear&tear on the cables/batteries/your back. Get a spare battery and setup a nice float charger/charge once every 1-3 months...

-Bill

I think the current batteries are gel+agm hybrid, they have a 12 year float life at 25 deg and 10 year at 30 deg so lets assume we get 11 years, also at a 20% DoD cycle the 12 years drops to 10 years, so lets call that 9 years due to temperature, at 30% DoD thats 7 years hence my worry that they are at end of life, however if they were only 6 years old, a do nothing strategy might suffice for now.

So if they are 6 years old, and they want more power, a good strategy might be to add three more panel strings = 9 panels at 175W

Lets assume original sizing is good, so 27 panels feed 66,000 Watt Hours, lets assume a 33% battery bank degradation to 44,000 Watt Hours so 18 panels can manage that same as before, the inverter is going to work harder on this battery bank needs a quick sizing check

then installing 9 new panels and a 2nd battery bank at 44,000 Watt Hours might sound a good proposition, MPPT/Inverter redundancy is 50% same as before, but they might want that increasing with more power to 75% with a four of each perhaps using outback 60's, well for the sake of sameness that could just as well be two more of the 80's give or take a few hundred dollars. Better not to change any of that to keep costs down on the electrical work and the ease of upgrade.

So we could double their power usage and cover the battery end of life story with the same upgrade just by adding 9 more panels

BB. said:

Check the maximum charging current for your present & proposed battery bank.

In the US, GEL batteries are generally used for standby power systems and have something likea C/20 (5%) rate of charge (100 ah battery, 5 amp charge rate).

Generally, for a full time off grid system, a 10% to 13% rate of charge for solar charging makes life easier.

And most deep cycle lead acid flooded cell and AGM types have a recommended 10% minimum rate of charge.

Your system at 5% rate of charge (and shallow cycling) is probably helping your present bank and its long life so far.

Bill