Has this been discussed somewhere? Are there inverters that automatically start up when connected to 12V?

[A.Justice]

Keep it simple. More parts, more to fail later, more places for mistakes to be made.

 

[Rick Unplugged]

Overpaneling question:

Take 3x160W panels @12V and the EPEVER Tracer-BN 40A as the baseline.

In full sunshine, those panels pretty much exactly max out the 40A that the charge controller can actually put into the battery.

So if I add 1 or 2 more panels to that, I'm over paneling - meaning, if I understand this correctly, that there will be no actual extra power arriving in the battery during times of full mid day sunshine; however at all other times, more power will be harvested and put into the battery than with just the original 3 panels.

My question is:

What is a half reasonable rough estimate how many more Wh 1 or 2 extra panels might actually give me (as in, actually arriving in my battery) when they are added in that way; keeping in mind that this is over paneling?

That would decide whether I even try phase 2 (over paneling) as per above post, or jump straight to phase 3 (going 24V) if it should turn out that phase 1 isn't meeting my power needs.

If it wasn't over paneling, each additional panel would give me roughly 400Wh per day in my location, mid winter.

But for over paneling, should I expect 100Wh a day from each additional panel? 200Wh?

What is a half reasonable guesstimate here?

 

[Rick Unplugged]

A. Justice:

"Keep it simple. More parts, more to fail later, more places for mistakes to be made."

I hear you, I really do...

The "three phases" approach as per above is about the simplest way to tackle this that I can think of at the moment.

 

[mmame]

+1 on trying to keep the system as simple as possible ?

Using 24V makes only sense when using 24V inverters, too. So, starting with 12V and then moving to 24V requires new inverters.
What are your power needs for 12V-only systems? 10..100...1000W? Maybe a simple 24/12V stepdown converter would do the trick...

Concerning lot if inverter on/off cycles: mine cheap-o inverter has a soft-start, so I don't expect much stress on power on cycles. And a well dimensioned relay (for inverter power switch bridging) will survive 100'00s of cycles.

What I don't know for sure, is, if that inverter softstart will make your coolers/freezers struggling. Although I don't expect that, who knows....

 

[Rick Unplugged]

Mmame:

Thank you for pointing that out! I plain overlooked that changing to 24V later would mean buying a new inverter. Bummer.

So in fact I am at the crossroads right now already where I need to decide whether to stay with 12V or go 24V - before I buy the inverter.|

It seems to be like this:

Either I do 12V, in which case all I can do if it turns out that that doesn't quite cover my needs is overpaneling and getting a little bit more that way.

Or I go 24V, which basically doubles the ceiling of how many panels I can install later on should I wish or need to; an increase from a total of 3 panels to 6.

I guess I'll have a look around for 24V inverters that might come into question...

Are 24V inverters a lot less common? How much harder than a 12V inverter might they be to find in future in a scenario where potentially everything is harder to come by?

My 12V power needs will be in the vicinty of 100W to 200W. A laptop, a couple phones, a little bit of lighting. I'll go and have a look how much such a step down converter would be. Although that's another component that might be harder to source if it should need to be replaced in future?

I would guess that a soft starting inverter is mainly to protect the battery from capacitor charging surge load. And yes who knows what it does on the AC end. Also, even if I find a specific inverter that works like that, that's another item that is very specific and harder to replace in future if that should be needed. I'd prefer a setup where nearly any inverter will plug in and work, so I can use whatever inverter I may be able to find in future.

Which once again points towards leaving the inverter on 24/7, I think... and just making sure I have enough power for that.

This is a bit of a journey...

 

[rin67630]

As to the inverter on/off issue: If most inverters just start producing power when connected to 12V then I guess all I have to do is find a suitable high current relay, put it between the battery bank and the inverter, and trigger that relay based on the temperature inside the fridge.

Any suggestions for a suitable relay, or some device that may already exist that comes complete with temperature sensor?

You ought to use two relays: one to switch on the inverter, another one to switch, say, 10 seconds later the fridge so to avoid having the inverter starting against the compressor, which has notoriously a huge power peak upon starting.
Additionally you could greatly reduce the impact on the battery by forcing the cooling at the afternoon to when you still have sunlight and and your battery is full, so you have unused energy. Then you can just let the chest freezer unpowered at night.

 

[Rick Unplugged]

I just realized that it might be a smart idea to buy 2x 12V 100Ah batteries instead of 1x 12V 200Ah battery or 1x 24V 100Ah battery.

With the former, I can put them parallel for a 12V system and in series for a 24V system, making the battery investment less limiting.

Are there any real downsides? Would most integrated lithium battery BMS be happy with both two identical batteries in series or in parallel?

 

[Rick Unplugged]

Rin67630:

"You ought to use two relays: one to switch on the inverter, another one to switch, say, 10 seconds later the fridge so to avoid having the inverter starting against the compressor, which has notoriously a huge power peak upon starting."

That's one option I am considering, yes

"Additionally you could greatly reduce the impact on the battery by forcing the cooling at the afternoon to when you still have sunlight and and your battery is full, so you have unused energy. Then you can just let the chest freezer unpowered at night."

That's not a stupid idea, within reason obviously. It might help quite a lot to stretch out the battery life even more - I am already dimensioning the battery so that it should rarely get discharged past 50%. I'll see how far I can push your suggestion and still maintain low enough temperatures for the food. Since this is a chest format fridge it lends itself well to that - all the expensive cold air doesn't instantly fall out when you open it, as opposed to a standup fridge.

 

[rin67630]

Rin67630: "You ought to use two relays: one to switch on the inverter, another one to switch, say, 10 seconds later the fridge so to avoid having the inverter starting against the compressor, which has notoriously a huge power peak upon starting."

That's one option I am considering, yes

The other advantage ist that you won't need high amps relays.

Rin67630: "... forcing the cooling at the afternoon "

That's not a stupid idea, within reason obviously. It might help quite a lot to stretch out the battery life even more

You can also fill the unused room in the freezer with salt water bottles, that is an extremely efficient way of storing energy.

 

[mmame]

Are there any real downsides? Would most integrated lithium battery BMS be happy with both two identical batteries in series or in parallel?

As a general rule (at least when using battieries with integrated BMS), only identical can/should be put in series or parallel. Depending on used BMS, there could be some limitations (i.e. only putting in series permitted).

My 12V power needs will be in the vicinty of 100W to 200W. A laptop, a couple phones, a little bit of lighting. I'll go and have a look how much such a step down converter would be. Although that's another component that might be harder to source if it should need to be replaced in future?

100..200W is easily achieveable with some generic 24/12V step-down converter. So I'd propose using a 24V system and 24V inverters plus one (or multiple...) 24/12V stepdown inverter for 12V.

Concerning switching arcs: capacitive loads will create "inrush current" when powering on. Inductive loads will create high voltage when powering off. Both kind will stress (-> arcing) switches/relays when powering on/off. This has to be taken into account when choosing proper relay stuff. Check datasheets and add big safety margins when using that approach.

Btw, imho, many modern inverters have soft-power start functionality so this shouldn't be a big issue when replacing stuff.

 

[Rick Unplugged]

Rin67630:

"The other advantage ist that you won't need high amps relays."

The way I understand it, I would still need high amps relays - because of the surge current that the charging up of the capacitors inside the inverter causes when the inverter is connected to DC. Correct me if I'm wrong?

Re unused room in the freezer: Good point. In our case, there hardly ever is any. If there is I usually just freeze drinking water in plastic bottles - the more thermal mass in the freezer, the longer food will stay ok if the freezer ever fails.

 

[Rick Unplugged]

Mmame:

Thanks, yes I would only use identical ones - not just in Ah rating and other specs, but identical make and model as well, so identical BMS too.

So is it safe to assume that most any lithium batteries with integrated BMS will be happy if I put two in series or two parallel? I'll still check specs of the manufacturer, just trying to get a feel for how easy or hard it might be to find ones that would be ok with that in future when after a long time of service they need replacing (95% of the time, the batteries will never go below 50%, I'm aiming and hoping for a very long life).

Thanks for pointing that out - I had a look around, and the 24V to 12V converting really seems almost trivially easy and affordable too.

Also noted that 24V inverters and solar chargers seem to be more common and easy to come by locally where I live than I had thought; in the case of inverters that's in part due to 24V systems in heavy trucks.

So I am in fact now leaning more towards 24V, although not entirely decided yet. It just seems to be less limiting, and while I'll want to buy a fourth panel so I'm able to go two parallel and two in series for 24V, I'll probably save the money for the extra panel in buying less copper, as a very rough guesstimate.

If I'm going with 24V, would you say I should buy a 24V battery, or two 12V ones in series? What would the main advantages and disadvantages of doing either be, if any? Or is this more a case of potato potato?

Arcing: Yes I"m aware of both factors. My (naive?) thinking is that if I buy a high current relay (as per its spec sheet) and use a safety factor of maybe 2 I should be ok? IF I go that route, still not entirely sure.

"Btw, imho, many modern inverters have soft-power start functionality so this shouldn't be a big issue when replacing stuff."

That's good to know, great.

Hm. I'm still not decided on whether to let the inverter run non stop or not. There's quite a number of pros and cons for both options. But I'm getting closer.

 

[Rick Unplugged]

Hey all,

I just want to say that I'm really grateful for all your thoughts and support.

You are really helping me to get clarity on all this. It's a beautiful process to design this kind of system and I'm enjoying it, but you are helping me avoid quite a few mistakes, unnecessary costs, and headaches.

Huge thank you to all of you!

I will definitely pass on what I learn in the local communities that I am part of.

If you have any additional thoughts please do keep them coming, I do read and take in everything.

 

[timselectric]

I would never recommend putting batteries with BMS's in series. If you want a 24v system, use 24v batteries.

 

[Rick Unplugged]

Timselectric:

Thanks for sharing that - not that I doubt you (and I kind of lean in that direction myself as well), but are you able to deliberate on the reason for that a little bit? Not super detail, just bird's eye view level.

 

[timselectric]

The individual BMS's , monitor and balance the cells that they are connected to. Usually by passive balancing. Which means that the higher cells are discharged by adding a small load, which turns the balancing power into heat. This lowers the SOC of the battery. Two or more BMS's can easily get out of sync, on their SOC's. Which brings down the capacity of the entire bank.
Better to have a single BMS, balancing all cells in a series string.

 

[rin67630]

Concerning switching arcs: capacitive loads will create "inrush current" when powering on. Inductive loads will create high voltage when powering off. Both kind will stress (-> arcing) switches/relays when powering on/off. This has to be taken into account when choosing proper relay stuff. Check datasheets and add big safety margins when using that approach.

And don't forget: DC switching arcs are MUCH harder to extinguish than AC ones. The higher the voltage, the harder to switch.
DC breakers have a special design with a magnet to expand the arc into fire resistant comb chambers.

 

[Supervstech]

And don't forget: DC switching arcs are MUCH harder to extinguish than AC ones. The higher the voltage, the harder to switch.
DC breakers have a special design with a magnet to expand the arc into fire resistant comb chambers.

Yes, relays and switches rated for 500A are rated to flow that, not rated to switch that under load... so keep that in mind.

 

[Rick Unplugged]

Timselectric:

Thanks heaps for the explanation, makes perfect sense!

Rin67630:

Thanks, I didn't know that, sounds a bit like something I want to avoid if I can...

Supervstech:

Interesting. What is the point of such a high rating on such a relay then? I can't right now figure out a use case for a relay where a high current needs to go across that relay when closed, but the relay doesn't actually need to switch that current on or off. Unless it is meant for very occasional use, think emergency off functions and the like. Do manufacturers of high current DC relays specify a cycle number for high currents, such as "W times at X amps, Y times at Z amps"?

 

[Rick Unplugged]

Timselectric:

I assume the exception to the "don't put batteries with BMS in series" rule would be batteries that can communicate with each other, from one BMS to the other, right? If they are made for that, basically. I have seen that somewhere, just a simple cable connecting the two BMS so they can "talk" to each other. Can't remember where I saw it though.