Used bank of batteries

[Mudhutwarrior]

Used batteries are a big gamble mostly related to how much they have been cycled.
Invest in new as the money you save on used could be zero.

[PNW_Steve]

Originally Posted by john61ct

Are they FLA?

Deka / EPM makes great FLA as does Trojan

AGM from both are meh, more so from Deka.

But the Trojan FLA L16 are crazy great, longer lasting than GCs, especially the RE version.

But crazy heavy.

FYI up to four parallel strings should be fine if wiring nice and robust, and three should really be no issue.

Yes they are FLA.

Regarding 4 parallel strings being a good idea..... I am going to have to disagree. To each their own.....

[john61ct]

Originally Posted by PNW_Steve

Regarding 4 parallel strings being a good idea..... I am going to have to disagree. To each their own.....

To be clear I am not saying going over 3 strings is "a good idea".

But people do lots more I've seen x20 !

Yes, each added string making the potential balance issues worst, but there is not some magic hard black and white line between 3 vs 4 strings.

Just like avg DoD vs cycle lifetime, nothing magic about 50%.

Just like getting an AGM bank back to 100% Full as per endAmps only once a week rather than "most cycles".

Or charging LFP at .6C compared to .5C

These are all continuous greyscale gradients of compromise, "departure from optimal", and there is room for informed judgment calls as opposed to slavishly following what are meant to be guidelines.

[PNW_Steve]

Originally Posted by john61ct

To be clear I am not saying going over 3 strings is "a good idea".

But people do lots more I've seen x20 !

Yes, each added string making the potential balance issues worst, but there is not some magic hard black and white line between 3 vs 4 strings.

Just like avg DoD vs cycle lifetime, nothing magic about 50%.

Just like getting an AGM bank back to 100% Full as per endAmps only once a week rather than "most cycles".

Or charging LFP at .6C compared to .5C

These are all continuous greyscale gradients of compromise, "departure from optimal", and there is room for informed judgment calls as opposed to slavishly following what are meant to be guidelines.

If I couldn't find batteries of adequate capacity to achieve the bank I want in a single string I would consider parallel strings. I have seen FLA up to 1100 a/h. I'm sure you could find larger if you looked about. For me, four L-16's in series are plenty.

I tend to agree with Sunking over on Sloarpaneltalk.com when it comes to the peril of parallel batteries.

https://www.solarpaneltalk.com/forum...ry-connections

[john61ct]

I always want at least one pair for redundancy, most of my use cases are full-time liveaboard far from civilization, often mission critical.

Sunking's great but **so** dogmatic, certainly not infallible and man clueless socially, I think been banned on more forums than allowed

> When you parallel batteries, it is almost impossible to balance Battery Internal Resistances, Cable and Connector Resistances. This forces one string to do most of the work. It weakens the strings and results in early failure.

My checking / maintenance routines on a per-cell basis would pick these up and allow correction.

But have to say never seen except when over 3 strings, and bad only higher.

Yes if welding or soldering intra-bank connections I can see the argument, but I don't do that.

[Camalot]

To preface, I'm an electrician by trade and I know my sh*t when it comes to battery systems.

Your panels when upgraded to 400w will gather a reasonable daily max of 1500w. After conversion loss let's call this 700w per day. (lead acid burns like half the power you try to put in them, inverters burn roughly 20% after that... I'm being nice with the numbers)

Unless your fridge sucks 700w/24hours.... 30w per hour and you run *nothing else* and you have good luck with the sun... this isn't going to work. More realistically your 2000w inverter will eat all 30w by itself just running. GO DC with the fridge, go bigger with the batteries... roughly 4x more than that post. I use a 2,200 lb forklift battery for comparison. Get the solar panels off of your roof and park in the shade. No reason to bake the whole bus into a solar toaster so you can keep the milk cold. Solar panels yield a theoretical max of 20% of the sun's energy. After conversion and inefficiencies it is more like 4%. your bus however will absorb more than 4% of the sun's heat.

Sorry to be a buzzkill. Feel free to message.

[john61ct]

Originally Posted by Camalot

To preface, I'm an electrician by trade and I know my sh*t when it comes to battery systems.

Then you should know better than to use phrases like "daily max of 1500w", "700w per day" "700w/24hours".

Wh would be OK, but Ah is even better once we all know we're talking a standard 12V system.

400W of panels will output in ideal conditions, about 140Ah per day, assuming there is at least that much demand.

Usually less, in any case well under 2kWh per day.

That is from measured IRL experience, over many dozens of installs.

Of course, in winter away from the equator, cloudy weather, any shading even dirt will lower, perhaps often as little as the **700Wh per day** level I assume you meant.

An efficient DC compressor fridge can use as low as 15-60Ah / day, more when used in freezer mode or Death Valley ambients, but average of 30Ah per day is common even in the tropics.

That's **~200-800Wh per 24 hours**.

I have seen hundreds of installs where 200W of panel and 100Ah of incremental storage keeps ice cream rock-hard just fine.

A few dozen where half that's just fine to keep the beer cold 95% of the time.


Maybe read some back threads, do some Googling. Sorry to be a buzzkill. . .

[Camalot]

My apologies about the watt vs watt hour discrepancy. For some reason I figure people understand my chicken scratch.

I just can’t see a build with an inverter and AC mini fridge working out without many times the panels and storage capacity.

I also can’t see the system you proposed with a substituted DC fridge running reliably due to a 50% burn off when charging lead acid. Am I wrong on this? The numbers look like they only add up on sunny days, and barely so.

Personally, I imagine people running more than a refrigerator in this build which is why I advise some overkill. More panels, larger battery bank, and DC refrigerator.

[Camalot]

You’re right about the power consumption of high efficiency DC refrigerators.

To clarify my chicken scratch: my numbers of 30w/hour pertained to the 2000w inverter + AC mini fridge setup. I was being conservative with the numbers as it realistically pulls more than that.

[john61ct]

Many shorties just don't have much **room** for multiple kW of panels.

The key is design for very limited inputs, choosing very efficient devices - minimizing inverter usage, cooking with propane,

I know dozens of off-grid full-timers barely use 100Ah per day. Most first-worlders would think, ugh, camping!

Others have the space and inputs to live just like in a S&B home, or have regular access to shore power.

Originally Posted by Camalot

I also can’t see the system you proposed with a substituted DC fridge running reliably due to a 50% burn off when charging lead acid.

Yah that's just abstract book stuff. Spend tons more on LFP banks if you like, but plenty of buses have carrying capacity for a 600Ah bank or more even with old-school FLA.