Considering upgrades, is my math correct?

[HalfBaked]

Longtime lurker, finally made an account.

I will begin just to express my gratitude for finding this community. Most everything worthwhile solar diy related came from reading here.

I have built what I intended to be an offgrid home that makes sense. It took me many years to build, meanwhile I bought a small second-hand solar setup about 4 years ago. 4 6v lifeline 300ah agms wired 2s2p for 600ah 12v and 300w panels, which I have upgraded to 3 strategic arrays totalling 3000w, strongly overpanelling my tsmppt60.

After enough completion of the dwelling, we moved in. It's been 3 months and now I can see we are extremely tight on making the solar work. A few mornings, like today, we wake up to the beeps of our plug-in CO/gas alarms telling me the power went out.

Enough rambling with backstory. I'm thinking about getting 1 (cuz poor) eg4 100ah 48v and getting a 48v inverter or aio. Looking for confirmation that this would take me from 600ahX12v=3600whX50%=1800wh
To 100ahx48v=4800whX80%=3840wh?
Over double my current system?

Follow up Q:
If this is correct, is it a problem to add more of these batteries later, like 1 per year?

Thank you for your time.

 

[1201]

Why not diy your battery and save even more money?

 

[Hedges]

What price range are you considering?
What watt peak and watt-hours per day can you see yourself growing into? What motor loads?

You can get an inverter (stand-alone or system) from about $700 to $15k. Battery and PV can span a similarly large range.
Now that you have experience, good to plan so no new purchase gets set aside, unless you intend to do that.

 

[JoeHam]

If this is correct, is it a problem to add more of these batteries later, like 1 per year?

Absolutely not, I started with a 60Ah battery a few years ago and a critical loads panel.

As I add more battery and solar I continue to move more circuits from the main panel to the CL sub panel.

I’m up over 300 Ah now and continue to grow the system. Perhaps the older batteries don’t share the load exactly, perhaps they do.

It’s not enough of a concern to quantify for me, it just works.

 

[littleharbor2]

Do you know the true condition of the batteries? How old are they?
Good to hear you are going to change your system voltage. Going from 12 to 48 volts will, if nothing else make your charge controller much more efficient in you will no longer be over paneled and will be pushing lower current through your wiring

 

[Mattb4]

...

Enough rambling with backstory. I'm thinking about getting 1 (cuz poor) eg4 100ah 48v and getting a 48v inverter or aio. Looking for confirmation that this would take me from 600ahX12v=3600whX50%=1800wh
To 100ahx48v=4800whX80%=3840wh?
Over double my current system?

Follow up Q:
If this is correct, is it a problem to add more of these batteries later, like 1 per year?

Thank you for your time.

Bit of an error in your math. 600ah at 12v nominal is 7200wh. As a practice you might use 50% of it but this is not a hard limit.

If you find that you are hitting low voltage shutdown overnight you really ought to figure out your loading and if you are getting an adequate charge. It is not hard to kill AGM's and they often are not good for much more than 3-5 years even using good practices. You will be much better served with the LiFePO4. Make sure your SCC is able to handle lithium charging.

 

[HalfBaked]

Why not diy your battery and save even more money?

I began my deep dive on this site by researching that angle. It seemed to me the window when that made sense has passed. The consensus was that the savings had shrunk due to the price reduction of preassembled, warranteed lithium batteries. I could be wrong.

 

[HalfBaked]

What price range are you considering?
What watt peak and watt-hours per day can you see yourself growing into? What motor loads?

You can get an inverter (stand-alone or system) from about $700 to $15k. Battery and PV can span a similarly large range.
Now that you have experience, good to plan so no new purchase gets set aside, unless you intend to do that.

2000ish watt peak, 3600ish watt-hours
What motor loads? Currently just a 20cf fridge, fans and occasional shop vac. One day I plan to add a small chest freezer as a keezer.

Absolutely want the new system to be something I can add batteries to as funds permit.

 

[1201]

I began my deep dive on this site by researching that angle. It seemed to me the window when that made sense has passed. The consensus was that the savings had shrunk due to the price reduction of preassembled, warranteed lithium batteries. I could be wrong.

Well it depends on the scale I think. I can get a 30kwh rack for $9k. Or a 71kwh diy for about $10500.

However if you just need 100a 48v, that I have not researched

 

[HalfBaked]

Do you know the true condition of the batteries? How old are they?
Good to hear you are going to change your system voltage. Going from 12 to 48 volts will, if nothing else make your charge controller much more efficient in you will no longer be over paneled and will be pushing lower current through your wiring

They were allegedly 1yr old and lightly used in a camper when I got them 4yrs ago. According to the live view web page served by the tsmppt60, they don't seem to hold a charge over around 12.7v, I know this isn't a true reading, just what I'm seeing with loads after the sun is off the panels

 

[HalfBaked]

Bit of an error in your math. 600ah at 12v nominal is 7200wh. As a practice you might use 50% of it but this is not a hard limit.

If you find that you are hitting low voltage shutdown overnight you really ought to figure out your loading and if you are getting an adequate charge. It is not hard to kill AGM's and they often are not good for much more than 3-5 years even using good practices. You will be much better served with the LiFePO4. Make sure your SCC is able to handle lithium charging.

What a dope, thanks for the math check, this is what I came for. So I would need 2 of the 48v 100ah batteries to double my capacity.

Just last night I put the starlink, tv and fire stick on a timed power strip. I believe they were the straw breaking my back. No beeps this am

 

[Mattb4]

They were allegedly 1yr old and lightly used in a camper when I got them 4yrs ago. According to the live view web page served by the tsmppt60, they don't seem to hold a charge over around 12.7v, I know this isn't a true reading, just what I'm seeing with loads after the sun is off the panels

Probably your AGM batteries are suffering from diminished capacity. Without a load test it is hard to guess at capacity left. You might find that going to one new 100ah 48v Li battery will give you a lot more run time.

 

[HalfBaked]

Probably your AGM batteries are suffering from diminished capacity. Without a load test it is hard to guess at capacity left. You might find that going to one new 100ah 48v Li battery will give you a lot more run time.

This is certainly possible/probable.
I have looked, but not finding much, can you point me towards a good method of load testing these batteries?

I believe Li batteries would solve some problems, one being that they don't need to be recharged fully daily, as I live in the middle of a forest and the sun plays hard to get and I'm often forced to run the generator to fill in

 

[derekisastro]

Enough rambling with backstory. I'm thinking about getting 1 (cuz poor) eg4 100ah 48v and getting a 48v inverter or aio. Looking for confirmation that this would take me from 600ahX12v=3600whX50%=1800wh
To 100ahx48v=4800whX80%=3840wh?
Over double my current system?

No, this is not correct. Not sure how you did it but it looks like you double 'halved' the LA.

600ah @ 12V (I'll even use the 12.6V nominal for a little more kick) is 7560wh. At a usable DOD of 50%, your usable capacity is 3780wh or 3.78kwh. It looks like you already applied the 50% initially, then did it again for good measure ... unless I've missed something?

Likewise, with a 48V lithium option, we are talking about a nominal voltage of 51.2V. So with a 100ah battery, we are talking 5120wh and if you are looking at a DOD of 80%, that leaves 4096wh or a tick over 4kwh. A very modest 8% increase in battery capacity. Even if you pull a little more for the lithium, say 90%, you're looking at almost a 20% increase in capacity. Not bad but not what you are planning I guess?

As mentioned, look at your loads/what you use and go from there. Yes, you can add batteries quite easily to increase your system as time goes by. So maybe the 8-20% increase initially will be enough for you? Maybe it won't. Without knowing your loads it's hard to tell. You have assumed I guess, that you're using/getting the 3.78kwh from the LA at the 50% DOD, maybe you're not even getting close to that?

 

[derekisastro]

This is certainly possible/probable.
I have looked, but not finding much, can you point me towards a good method of load testing these batteries?

I see my original comment on your initial post was far too late.

As to load testing the AGM's I wouldn't worry about it. The bigger thing to measure is to get a grasp on your usage. Whether by using a whole panel meter of some kind (Sense is one brand) or individual circuit/outlet load testing meters/plug (Killawatt, many other smart plug also measure power usage) you can get a pretty good idea of how much you use.

In my experience most people have no idea how much they use, not even close. I'm talking either over or under-estimating by factors of 10. When people ask me if a 4000W generator can 'run their fridge' you know people have no idea of energy use.

 

[HalfBaked]

I see my original comment on your initial post was far too late.

As to load testing the AGM's I wouldn't worry about it. The bigger thing to measure is to get a grasp on your usage. Whether by using a whole panel meter of some kind (Sense is one brand) or individual circuit/outlet load testing meters/plug (Killawatt, many other smart plug also measure power usage) you can get a pretty good idea of how much you use.

In my experience most people have no idea how much they use, not even close. I'm talking either over or under-estimating by factors of 10. When people ask me if a 4000W generator can 'run their fridge' you know people have no idea of energy use.

You're right, I do not trust my mathing, which is one reason why I quit lurking and made the account. I need a little hand-holding. Thank you for the correction.

I do have a clamp meter, a killawatt-type meter and a trimetric meter on a shunt. I believe the trimetric is often way off on it's %full calculations, but is useful to see how many watts are going to charging and how many are used by current loads.

You guys are right about doing a load audit to see what I'm actually using, I did one years ago to estimate my needs, but now that I'm actually living with the system, I need to see what's actually going on.

Thanks everyone for your time and expertise.

 

[Mattb4]

This is certainly possible/probable.
I have looked, but not finding much, can you point me towards a good method of load testing these batteries?

I believe Li batteries would solve some problems, one being that they don't need to be recharged fully daily, as I live in the middle of a forest and the sun plays hard to get and I'm often forced to run the generator to fill in

The simplest way to load test your AGM is to fully charge and than run a consistent load amount like from small space heater. I keep on hand several of those Office types of various wattages such as 200w, 500w up to 1500w units. Here"s the math.

600ah of battery. Run a 500w heater at starting voltage 13.0v down to 10.5v or inverter shutdown. 500w/12v=41.67a, 600ah/41.67a=14.4 hours Of course there is losses in the process from running the inverter and also the inverter should shut down before the battery reaches 0% (most likely with voltage droop under load an inverter will leave around 10% of capacity.) So adding it all up if you are not getting 8 hours you have lost capacity. If you only get an hour the capacity loss is extreme.

 

[HalfBaked]

The simplest way to load test your AGM is to fully charge and than run a consistent load amount like from small space heater. I keep on hand several of those Office types of various wattages such as 200w, 500w up to 1500w units. Here"s the math.

600ah of battery. Run a 500w heater at starting voltage 13.0v down to 10.5v or inverter shutdown. 500w/12v=41.67a, 600ah/41.67a=14.4 hours Of course there is losses in the process from running the inverter and also the inverter should shut down before the battery reaches 0% (most likely with voltage droop under load an inverter will leave around 10% of capacity.) So adding it all up if you are not getting 8 hours you have lost capacity. If you only get an hour the capacity loss is extreme.

Got it, thanks for that. I have done this with my 14,000btu window AC unit. It doesn't cycle the compressor, just constantly runs at 1250w. But I never did the calculations, so 1250w/12v=104.17a, 600ah/104.17=5.76@50%dod=2.88-10%losses=2.59hrs.
When I ran this test, the AC ran for 45min. Sounds like my batteries are at 1/3 original capacity, around 200ah@12v nom=2400wh@50%=1200wh. So a single 48v 100ah (4800@80%=3840wh)would triple my current capacity.
Thanks for helping me see this more clearly.