Switching from Wet to Lithium?

Converting units:

• 672 AH * 24 volts = 16,128 WH = 16.1 kWH of Lead Acid storage capacity
• 2 * 3.8 kWH = 7.6 kWH of Li Ion storage

It sounds like you have received a "reasonable" life from your Lead Acid Trojan bank... Overall, well treated.

Using our "rules of thumbs"--We can make some estimates of energy usage:

• 672 AH * 24 volts * 0.85 AC inverter usage * 1/2 days of storage * 0.50 max planned discharge = 3,427 Watt*Hours of average daily loading (2 days of usage to 50% discharge rule of thumb)

For Lithium Ion, let's start with with some assumptions.

• Capacity range (best life): 90% State of charge to 20% SoC = 70% of battery capacity
• 2 days of "no sun"
• 70% capacity available * 1/2 days = 35% usage over two days of "no sun"
• 7,600 kWH capacity * 0.35 daily usage = 2,660 Watt*Hours per day

That is a bit less than your "rule of thumb" usage estimated from your present Lead Acid bank...

Next, for the lead acid bank, we suggest 50% minimum planned discharge--But you can go down to 20% SoC on occasion--With the suggestion that the "next day" you have good recharging (sunny weather, genset running, etc.)... Does that "extra 30% capacity" help you for those random times during bad weather that you use that "cushion" of battery capacity?

With the above Li Ion bank suggestion, at 2 days--You are out of battery power and will need sun and/or genset power for the "third day".

Solar charging wise... 6 * 250 Watt panels for a 1,500 Watt array. For a lead acid bank, suggest 5%/10%/13%+ rate of charge with 10% minimum for full time off grid usage (technically 10% is recommended by most FLA battery mfg... Li Ion do not have a "minimum rate of charge" specification):

• 672 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,531 Watt array "nominal"

Array size, you are at quite a bit less than 10% suggested rate of charge:

• 1,500 Watt array / 2,531 Watt 10% charge = 0.59 of 10%
• 59% * 0.10 rate of charge = 5.9% rate of charge

You are a bit under paneled--But with your battery bank life, you have been keeping it relatively happy (state of charge wise)... So you "average daily load" is probably less than 3,427 WH per day (at least during winter).

The "break even" hours of sun per day:

• 3,427 WH per day loading * 1/0.52 off grid FLA AC system eff * 1/1,500 Watt array = 4.4 hours of sun "break even"

For somewhere in Vermont, hours of sun per day:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Burlington
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 46° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.08	4.00	4.49	4.51	4.50	4.72
Jul	Aug	Sep	Oct	Nov	Dec
4.89	4.77	4.33	3.41	2.46	2.40

Your break even power usage is only barely exceeded in summer months... Unless you are using a fair amount of generator run time, 3,427 WH per day is probably too high of estimate of your energy usage.

Lets look at your 2 day available power from Li Ion bank of 2,660 WH per day estimate... And instead of the 52% FLA AC off grid system efficiency, FLA which average at 80% efficiency vs Li Ion of 98% efficiency or overall system eff of ~0.64 overall system efficiency:

• 2,660 WH per day loading * 1/0.52 off grid FLA AC system eff * 1/1,500 Watt array = 3.4 hours of sun "break even" FLA bank
• 2,660 WH per day loading * 1/0.64 off grid FLA AC system eff * 1/1,500 Watt array = 2.8 hours of sun "break even" Li Ion

Again, guessing at your daily loads as being 2,660 WH per day--A lithium bank will be more efficient at charging/discharging, so it will make your solar energy harvest "larger" (makes array 1.25x "larger" due to less battery bank losses). And gets you pretty deep into winter before needing genset usage.

Also, the charging ability of Li Ion is generally "better" than FLA (higher rates of charge, no "wasted fuel" running for 2-6 hours to "absorb" charge the battery bank... Li Ion typically take "full charge current" until they are at 90%+ SoC, then turn off--Saving you fuel and wear on genset.

The only functional "down side" at this point--You have a "hard" battery cutoff of 2 days where the FLA had a "soft cutoff" of 2 days and a hard cutoff of 3 days of storage (at ~3,427 WH per day energy usage).

Other than higher battery costs and a BMS (battery management system requirement/complexity) for Li Ion, the other tripping point for Li Ion is that they need to be kept >~40F bank temperature for cycling... Below that temperature, Li Ion batteries should not be cycled. You will have to keep the banks "warm" during winter for daily usage... FLA batteries, as long as you keep them >50% SoC, they will function well at below 0F very nicely. And cycling them will tend to keep them warm (Li Ion batteries will need heaters/warm room... Not just an insulated battery box).

In a cold location like Vermont--Winter battery temperature could be a major issue for you.

-Bill

Rob,

OK, I missed on the size of your array... Should be 3 arrays * 6 * 250 Watt panels per controller: = 4,500 Watt Array

Yea--That is a lot of panels for your battery bank (672 AH @ 24 volts)... Do you have a remote battery temperature sensor setup on one of the batteries? You want to make sure that you do not cook them with that high of charging current (batteries get hot, their charging voltage drops, charge controller thinks batteries need more charging current, batteries get hotter... potentially overheated bank or worse).

I like to "do the math" to make sure that all is makes sense:

• 4,500 Watt array * 0.52 off grid FLA AC system eff * 4.33 hours of sun per summer day (September) = 10,132 WH per average summer day FLA
• 4,500 Watt array * 0.64 off grid Li Ion AC system eff * 4.33 hours of sun per summer day (September) = 12,470 WH per average summer day Li Ion

9-12 kWH per day--You are pushing it--But it is possible during sunny summer weather.

Now you have to look at the battery bank "useful" capacity of 3,427 WH (FLA) to 2,660 WH (Li Ion) per day... If most of your loads are during daytime (water pumping, cooking, laptop usage, even some Mini-Split time)--And you reserve your lesser loads to evening/sun down (lights, entertainment, cell charging, overnight fridge, etc.)--Then you would be OK... And your present FLA bank 7.5 year life seems to indicate that you are not badly treating your present bank...

You are not going to see a huge improvement in harvest with Li Ion over FLA with a "small battery bank"--Most of your loads appear to be "day time" which has zero battery cycling (i.e., 100% efficient) and only a (relatively small amount) of night time/bad weather usage.

The smaller Li Ion battery bank would function for your needs as they are (i.e., can supply the surge current much better than even your larger FLA battery bank)--But look at what happens when battery bank hits bottom (~20% SoC) when clouds roll in--Is that going to be an issue for you (2,660 Watts--Then hit BMS shutdown)--Or when you get the warning, either cut power usage to near zero (lights, cell phone charging) and/or crank up the genset to carry you through to the next sunny day (and topoff the battery bank).

That is something only you can decide... Watching your energy usage and placing loads in "base loads" (those that must run 24x7 (fridge/lights) and those that can be put off to next sunny day or use genset (well pump, A/C, cooking, washing machine, etc.)....

-Bill

I am certainly no Lithium Ion expert...

I was "derating" for better battery life, not pure capacity. As I understand, charging to 100% shortens Li Ion battery life (as does storing >90% SoC)... There is also the issue of not exceeding maximum cell voltage (as you said--Lead Acid batteries are much more "forgiving" with cell/battery over voltage/over charge conditions--But they have their limits too).

And the 20% SoC at the bottom was to prevent any one cell from going to 0% or negative SoC which destroys most rechargeable chemistries.

There are a wide variety of Lithium rechargeable batteries with lots of variation in chemical and physical make-up. The above was just a back of the envelope starting calculation. And will change based on actual batteries chosen. And what type/brand/model of BMS is used.

-Bill

No problem... I also try to give a bit of cushion too... As cells wear, their capacity drops, colder temperatures, etc... Don't want to see a "normal daily load" "hit the limits" when new or a year or two old/cold weather/etc...

But I always try to include my "deratings" numbers and reasoning, so that the orginal poster can adjust them for their user profile.

-Bill

Rob,

I think everyone would be very happy to see the details of your new system (and anything you have learned along the way)... What batteries/BMS/what your plans are for winter/cold weather operation with Li Ion/etc... Some photos and such too.

-Bill "always learning" B.