replacement battery bank

stillchillin · July 2023

Hi, I have a 12 year old Rolls battery bank which has performed perfectly, I am trying to plan ahead for its replacement. The current bank is 1350 AH @ 48V, I have the AGS set to turn the generator on at 68% SOC and off at 90%. I have an array that can put out 4.2 KW,
Looking for some input/ experience with replacement options. It looks like I am under paneled for lithium replacements. Any thoughts/ advice?
Thanks in advance. Still

Vic · July 2023

Hi still..,

I am running two separate systems with the same Surrette batteries, in their 18th year of service.

It sounds like you have taken care of your batteries. Guess that you feel that the batts are nearing end-of-life. Mine use a LOT of water, but seem to still be working well.

As you probably know, Li batteries have much greater charge efficiency, than our FLAs. My Surrettes, have a charge effficiency of about 75%. LiFePO4 (LFP) batts are about at about 98-99% charge-efficient.

You should probably have some additional PV power, but, think that LFP batteries may well be happier than your FLAs. LFPs must not be charged when cold, and large peak currents (usually on discharge), can cause the internal BMS to switch off, which would be bad, if one was away from the site, etc.

I bought some LFP batts, last year, as lead-times from Surrette for the 4KS25s was quite long, at that time. I just did not want to get "stuck" without some batteries, for each of the off-grid sites, here.

This is not really any adivce, but you might get a few more years from your Surrettes. Are your batts losing Capacity, or what are you seeing with them? Vic

stillchillin · July 2023

Hi Vic, thanks for your response. I have been diligent with my battery service hoping I would get 15 years out of them. The water usage is still about the same as always 2.5 gallons every 30 days for the bank. It has been a tough summer for eqing here in northern Vermont and I have noticed SG readings are dropping and harder to bring up than years ago. I would have to add a charge controller to add panels to my system. This is our full time residence so having the system drop out is not a concern. I have been looking into LFPs and see that they are
pretty pricey compared to FLAs I think I would be looking at 30KW of storage to have the same capacity as I have with the FLAs. Can you say how many KW of panels I would have to add to keep the LFPs happy? Thanks Still

Vic · July 2023

Still,

Well, IMO, you did not really have enough PV power for the present FLA bank, but you appear to have made that work fairly well.

If you had a 30 kWh LFP battery, and you were to remove 70% of its capacity each day (which you probably would not do), then you would need more PVs, and some more CC power.

You probably get 3 - 4 hours per day of good sun, but the number of days of Autonomy, desired etc, etc all matters.

We are beginning a new heat-wave here, and am needing to actively shed loads, and control the amount of A/C that remains for the power room (that is not enough).

Am too distracted right now, I need MORE PVs, myself, and need to do some more noodling on that ... Know that you folks have some serious WX issues in the NE of US, so good luck on that. Later, Vic

BB. · July 2023

Just to run some numbers... Assuming 5% minimum rate of charge, and 10%-13%+ rate of charge suggested for full time off grid:

1,350 AH * 58 volts charging * 1/0.77 panels+charger deratings * 0.05 rate of charge = 5,084 Watt array minimum
1,350 AH * 58 volts charging * 1/0.77 panels+charger deratings * 0.10 rate of charge = 10,169 Watt array nominal
1,350 AH * 58 volts charging * 1/0.77 panels+charger deratings * 0.13 rate of charge = 13,219 Watt array "max" cost effective

So your 4.2 kWatt array is really on the small size for that battery bank for a full time off grid system... In Vermont, if system is used during the winter, even >13 kWatt array is probably justified to reduce genset/fuel usage.

What is your actual energy usage (Watt*Hours) during summer/winter/etc...?

The 10%+ rate of charge number is useful for any system... But especially for Lead Acid batteries which really need enough current to keep the plates in good condition (as well as watching electrolyte levels, keeping up with distilled water, etc.), well as enough charging current to get the batteries full in a day or two of use...

To move the discussion, say you use 25% (1/4) of the bank capacity daily (during bad weather, over night, etc.)... That would be:

1,350 AH * 48 volts * 0.25 of capacity * 0.85 AC inverter eff = 13,770 Watt*Hours per day (pure guess)

And lets look at what the average hours of sun per day would be for Montpelier VT would be over the year. Guess 45 degree array tilt (optimum year round) facing south, assume 52% overall solar efficiency (48% losses for calculator):
https://pvwatts.nrel.gov/pvwatts.php

Month	Solar Radiation ( kWh / m2 / day )	monthly AC Energy harvest ( kWh )
January	3.24	217
February	4.29	258
March	5.14	329
April	4.85	284
May	5.05	292
June	4.78	263
July	5.28	299
August	5.14	292
September	4.79	268
October	3.22	195
November	2.72	167
December	2.53	166
Annual	4.25	3,030

Lets "toss" the bottom three months of sun--Assume lots of genset usage because of location/weather. That gives us January at 3.24 hours of sun as the "break even" month.

13,770 WH per day * 1/0.52 off grid AC lead acid system eff * 1/3.24 hours of sun per day (January) = 8,173 Watt array "break even" for January

If you use a 65% "fudge factor" for solar (50-65% suggested) to allow for bad weather and other issues, then you could justify an array for your system (again, numbers are all my guesses as to your actual usage):
8,173 Watt / 0.65 solar fudge factor = 12,574 Watt array "comfortable" and minimal genset usage

Again, the above numbers are based on our typical system configuration/rules of thumbs. Suggest 10,169 Watt array (10% rate of charge) as "minimum" for your present FLA battery bank. And upwards of a 12.5 kWatt array could be justified.

With Lithium Ion batteries... You (typically) do not have a minimum rate of charge (L ion batteries just don't want to have their charge rate requirements exceeded). And they certainly charge much more efficiently than FLA batteries do (since there is no "absorb" time needed for Li Ion batteries, they do much better in northern climates with short days).

However, they pretty much need to be kept over ~40F or warmer for proper operation. If they get to freezing or below (32F), they should not be cycled until the bank is warmed up again (conditioned space). Lead Acid batteries are "better" for sub freezing climates because they can be cycled well below 0F just fine (other than they have less usable AH/WH capacity when cold).

The battery bank and array sizing for Li Ion (and even lead acid), still need to know your daily energy usage (WH per day, etc.) to properly size the array.

If you do continue to use Lead Acid batteries and add solar panels... You might look at "virtual tracking"... Facing some panels to the South East and others to the South West--Get more "hours of sun" per day--Makes Lead Acid batteries much happier. With Li Ion batteries, you do not need virtual tracking for more hours of sun per day--They will simply charge "faster" with more current (up to the max charging specifications for the bank/BMS).

The above numbers are a first pass guess on my part... Use your "actual" WH per day energy needs to do the calculations. The 25% per day of battery capacity is just another "rule of thumb"... In general it usually is a good "optimum" for system performance. battery life, and overall costs.

-Bill

Anawa · August 2023

Hope y’all don’t mind that I join in this thread as I am facing a battery replacement for my system also. My system 6.1kw PV with 800 Ah FLA’s (GB Forklift’s). I opted to go the forklift batteries route from advice received on this site in 2014 and I’m glad I did. The advice was this: if this is your first serious off-grid system, then buy the least expensive batteries to use as “training wheels”. The road to hell is paved with good intentions and even though I was gonna baby-my-batts, my effort was a C+ in spite of my good intentions. Learned a hell of a lot about batteries in 8 years, but still got the slacker blues from time to time.

Symptoms include: tremendous water usage, difficulty in getting full SG charge even on my “hot” cells, a few cells will never charge above 82%, rapid decline in Ah out and slow to increase even when gennie is adding additional load. The worst is that my batteries do not reach full charge when I’m running my AC during the day. It’s Georgia man, and I only run AC during the day. I live in the woods and nighttime temps are manageable.

Additional: off-grid and primary residence, retired and getting older, enthusiastic energy level not as high as it was 8 years ago, 875 Ah GB replacement batteries about $8k plus hookup, bank account can handle this, but would a bit of a strain to go higher.

Vic and Bill, so glad y’all are still around, this joint wouldn’t be the same without you. You’ve been helpful over the years in many different ways.

I guess this is what I’m looking for from the forum. In a thumbnail, what would be the pros/cons of the current battery types for my replacements? And, my equipment is out-of-date and should I consider some upgrades?

Paul in Georgia

replacement battery bank

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