Hybrid battery system - crazy idea?

[dc/ac]

I'm considering my options for my first real system. 7 kWh.

My 'needs' (freezer, lights, fans, misc) are about 2kWh
My 'wants' (fridge, coffee, pc, tv, clothes washer, toaster oven) come to about 5kWh.

At 50% DoD, with 2 days of autonomy I figure I'll need approximately 28kWh (7x4) of batteries. Both the fridge and freezer will run on DC.

Panels: (2kW = 8 250w) / 4 hours insolation)

Here is where the crazy starts.

I was wondering if I could not run 2 separate battery systems / circuits - one SLA and one LifePO4.

SLA 12V 800Ah =~ 10kWh = Needs
LFP 24V 800Ah =~ 20kWh = Wants

Why?

To take advantage of the strengths and weaknesses of each battery type and the context in which the loads will run:

Needs / Wants
- low / high voltage
- consistent / intermittent
- predictable / unpredictable

SLA strengths
- Cost
- History
- Predictable
SLA weakness
- Maintenance - need to be 'topped off' regularly
- Peukert

LFP strengths
- No need to be 'topped off' regularly
- Not as much effected by heavier loads

LFP weakness
- Cost more
- Not as much history
- More hardware (bms)

Each's strengths and weaknesses are pretty much inverted, as are my needs/wants.
That is, my needs are predictable and consistent and my wants are unpredictable with potentially high current.

The problem (among many?) of course is my solar controller (Outback?) will have to charge both or I'll need 2 controllers with a (hopefully automatic) switch/relay.
Or, maybe I can run the 'overflow' to an LFP charger?

One of the things I find appealing about this strategy is that my SLAs will have priority so they will _more_ regularly get fully charged (as opposed to thy entire system being SLA - the panels will effectively be oversized for them). Whenever the SLAs are topped off it will start the LFPs charging, which I don't care as much about getting topped off. When I need to run some high voltage utility I can just make sure the SoC of the LFPs are OK without worrying about the SLA.

So, what do you think, crazy?

PS, Admins: I got an error while posting: "Missing human verification information." I am not an animal! I think the session timeout could be increased and perhaps a more helpful error.
PSS: Followed by "Empty response". Haha, I used to be a sw developer, that's why I got out :P

[Sunking]

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So, what do you think, crazy?

Yes I do think you are crazy.

First last thing you want to use is SLA batteries. You would use FLA batteries because they cost half as much and last twice as long as SLA making them 75% less expensive long term. But if you only run 2 days anatomy you will destroy any lead acid battery within a year or two, and leave you dark several months out of the year because you have to shut down for cloudy days plus one more day to recharge after the sun comes back out. Go with 2 day Antinomy and your Kwh cost is roughly $1.10 per Kwh or 10 times mote than the power company will charge you. That is only battery cost and replacement never ends.

If you use the minimum recommended 5 day antinomy your battery cost drop to around 60-cents per Kwh or 5 times more than the Power Company will charge you. Expect to replace them every 5 years for a good quality RE battery like a Trojan Industrial or Rolls 5000 series. Cost is roughly $200 Kwy So if you are using 7 Kwh per day you need a 35 Kwh battery costing $7000 every 5 years. The POCO will charge you roughly $1500 for the same amount of power.

If you go with Lithium well you can cut antimony down to 3 days and last the same amount of time a good FLA battery if you are lucky and know exactly how to run Lithium batteries. One mistake and you destroy them. So with 3 days and 7 Kwh per day you are looking at a 21 Kwh battery at $500 per Kwh or roughly $10,000 every 5 years. That works out to about 7 times more than the POCO would charge you.

So you tell us if you are Crazy or not. You can spend $1500 for the POCO to deliver you the same amount of power that will cost you $7000 to $10,000 with a battery system. IMO you are out of your fricking mind. Kep in mind that is just the battery cost alone. Add another $10,000 for the panels, controllers, inverters, wiring, and materials required to make it work.

Lastly if you go off grid you forgot one thing. Actually 3. Generator, 500 gallon LPG tank, and a 2 Kw battery charger for another $6000.

So are you Crazy or not? I think so.

[jflorey2]

So, what do you think, crazy?

Problems:
You cannot parallel the batteries. So you will have to run from one or the other. That means very heavy switchgear, higher DC losses and transients on the inverter it is not designed for.
Your charging current and discharge current will be 2x higher on each cell in this case, since you will only be charging/discharging one bank at a time. So shorter life and lower efficiency overall.

I'd recommend choosing ONE good battery system and going with that. If, of course, you need batteries at all. If you have grid power, you don't need them.

[dc/ac]

Ok, that looks like 2 votes for crazy.

SLA vs FLA - I did not even know there was a difference . That should disqualify me from even thinking about this sort of thing but when I was looking at the batteries, not being able to decide, I started looking at their qualities. In my defense, when I was pricing them out I had looked Crown 860s (4 of them for about $1200).

Aside from both connecting to the controller, I figured the banks would be separate - 2 different circuits, one at 12v and one at 24v.

Anyway, just another crazy idea, thanks for the feedback.

[Sunking]

Ok, that looks like 2 votes for crazy.

SLA vs FLA - I did not even know there was a difference . That should disqualify me from even thinking about this sort of thing but when I was looking at the batteries, not being able to decide, I started looking at their qualities. In my defense, when I was pricing them out I had looked Crown 860s (4 of them for about $1200).

Aside from both connecting to the controller, I figured the banks would be separate - 2 different circuits, one at 12v and one at 24v.

Anyway, just another crazy idea, thanks for the feedback.

Well here is your wake up call. Trojan makes both the T-105 in FLA and SLA (called AGM) So did you click the links and see the cost. Both are the same 6 volt 220 AH deep cycle batteries. Both are low end batteries. The FLA version maybe 3 years discharged 20%/day. The AGM 2 years if you are lucky.

You want a lithium 6 volt equivalent 130 AH battery will cost you $350.

[Mike90250]

Building 2 different systems will cost a lot more than just 1 fully adequate system. Learn with flooded batteries, many systems destroy the batteries in the first 2 years learning how things work (I'll wait and run the generator tomorrow) In a few years, you will know your system better, and when the batteries need replacing, the LFP batteries will be much more mature and able to make the decision about them.

[dc/ac]

The FLA version maybe 3 years discharged 20%/day. The AGM 2 years if you are lucky.

You want a lithium 6 volt equivalent 130 AH battery will cost you $350.

Wow, I was hoping to be careful with them and get them to at least 5 years.

Building 2 different systems will cost a lot more than just 1 fully adequate system. Learn with flooded batteries, many systems destroy the batteries in the first 2 years learning how things work (I'll wait and run the generator tomorrow) In a few years, you will know your system better, and when the batteries need replacing, the LFP batteries will be much more mature and able to make the decision about them.

I'll start with with my needs and FLAs, oversize the panels and kill my first set first :P and reassess later. Wise words indeed, much appreciate the advice all!

[solar pete]

Wow, I was hoping to be careful with them and get them to at least 5 years.



I'll start with with my needs and FLAs, oversize the panels and kill my first set first :P and reassess later. Wise words indeed, much appreciate the advice all!

Howdy, if your system is designed and installed properly, it is possible to get 10 years out of FLA batteries, my uncle got 10 years out of his in Queensland, Australia.

[SunEagle]

Howdy, if your system is designed and installed properly, it is possible to get 10 years out of FLA batteries, my uncle got 10 years out of his in Queensland, Australia.

Pete. I don't doubt your uncle was able to get an extended life out of his battery system. But I would presume he paid a lot of attention to them and did not over discharge the system.

Unfortunately a lot of people have the idea that a battery is something you plug in and use until the "flashlight goes out" and then replace it.

"Out of sight = Out of mind" for most because they do not know any better.

[inetdog]

Another factor is that battery manufacturers typically state the lifetime of the battery based on 80% of initial capacity being the End of Life point.
For a conservatively designed off grid system the battery bank may continue to be useful well beyond that point.
Beyond that point, though, capacity tends to diminish with time and with cycles faster than it does during the nominal battery life span, so do not expect too much.