I like the quality of your equipment, not many can afford to buy Victron off the bat.

 

However, I am a bit concerned regarding your battery set-up.

Currently you have 3 strings, but no fusing (protection) between any of the strings.

You did install fuses for each MPPT, but not each battery bank

 

I would suggest you install a seperate fused connection for each battery bank.

Also make 100% sure you keep each cable length the same on each bank.

You really should also do that for the MPPT controllers.

 

It would be interesting to find out why you have gone with 3 banks, rather than a single large one?

Multiple banks normally have multiple problems...

I decided to go for the lead carbon batteries, due to them handling PSoC better than flooded lead acid. They also quote rather impressive cycles. At the time I was buying the batteries, I didn't know that Narada do make 2v cells. The nice thing about the Nerada Batteries is that they come with cables that is exactly the same length.

 

To be honest, I didn't think to put fuses between the banks. Would be rather easy to do, I'll look into it. Thanks for the advice.

I like the quality of your equipment, not many can afford to buy Victron off the bat.

 

However, I am a bit concerned regarding your battery set-up.

Currently you have 3 strings, but no fusing (protection) between any of the strings.

You did install fuses for each MPPT, but not each battery bank

 

I would suggest you install a seperate fused connection for each battery bank.

Also make 100% sure you keep each cable length the same on each bank.

You really should also do that for the MPPT controllers.

 

It would be interesting to find out why you have gone with 3 banks, rather than a single large one?

Multiple banks normally have multiple problems...

Would you care to elaborate on the "multiple problems..." ?

I think the main problem with multiple strings -- which really means multiple current paths -- is that one always works harder than the other, and one accepts more of the charge than the other. In theory, if they were perfectly matched, you'd have no problem, but in practice even banks of the same batteries are not identical. So one string is cycled deeper than the other, and the other is never properly charged... so you have a worst-of-both-worlds scenario. So goes the theory.

There's usually also talk of so called circulating currents. This has been proven to be nonsense. Once the strings equalize to the same voltage, the voltage difference between them falls to zero and no charge moves. If there's higher self-discharge in another string, some charge might move from one string to another, but generally speaking, the whole thing behaves exactly as you would expect.

 

Even on discharge, the problem is usually not as bad as it's made out: The internal resistance of a string goes up as it discharges, so there is a natural balancing going on, with the strongest string delivering most of the current. So on discharge, no problem (or at least nothing that can't be dealt with).

 

The biggest problem is on charging, because one bank has to be slightly overcharged (the one with the lower internal resistance) to ensure that the other one gets a proper absorption charge.

 

Think about it this way, if you combine two 100Ah strings into a 200Ah string, upon recharge one string might accept 102Ah (gassing away 2Ah because it's already full) and the other only 98Ah (ignoring for the moment the fact that lead acids are only about 70% efficient when close to 100% full). So to recharge this bank, you need to put in at least 203Ah to ensure that the second string gets fully charged. This is a very simplistic example.

 

So I wouldn't say that parallel strings are definitely wrong... just that they propose challenges and that you need to compensate for that. And you have to make peace with the fact that your total life is going to be slightly less.

My info on this is mostly academic. And controversial. This is the paper I got most of it from... but it makes sense. I did study some physics after all :-)

 

http://neuralfibre.com/paul/wp-content/uploads/2007/05/can-we-now-sin.pdf

I have always been under the impression that batteries only charge at the rate they need, i.e. it will only draw the amount of Ampere needed. And since both battery banks should (maybe in theory?) have need the same Ampere, they won't necessarily charge at different rates? The same rule applies to solar panels, if you put a 5A and 4A solar panel in parallel, you'll get 4A 

The same rule applies to solar panels, if you put a 5A and 4A solar panel in parallel, you'll get 4A 

 

If you put a 5A and 4A solar panels in parallel you will get 9A. The voltage will average out between the two dissimilar panels.

 

If you put a 5A and 4A solar panels in series then the voltage will additive. The current output will only be slightly higher than the current produced by the panel with the lowest current output in the series string.

 

Plonkies info on the parallel battery bank operation makes sense.

This does make sense, but a 48v battery bank is still made up of 24 x 2v cells, so it is also not a single battery, even though they are connected in series. They also have their problems. I have found on the battery banks we use that over time you get issues with your connector cables, which causes some of the cells to work harder than others. Also if you have one cell failing in a bank, it is only a matter of time before the rest of the cells die. You can also not mix old and new cells, somehow the new cells get killed very quickly. I suspect this has something to do with the fact that they accept charge easier and get overcharged which leads to quick failure.

 

Both types of banks have their own problems, but I still suspect a new bank with 24 x 2v cells of the correct ah rating would last longer than a bank consisting of multiple banks of 48v batteries. While you are still figuring out what you want to do, the banks consisting of 48v parallel banks are certainly much easier to work with, as you can add (or subtract) banks as needed. Once you have a bank consisting of 24 x 2v cells, you are pretty much fixed in the way that you are set up. Mixing they types of banks is not recommended and mixing the ah ratings of batteries I don't think is a good thing either.

I am not sure about this: "If you put a 5A and 4A solar panels in parallel you will get 9A."

Is it not 8amps, for the panels will standardise on the lowers panel's specs?

I am not sure about this: "If you put a 5A and 4A solar panels in parallel you will get 9A."

Is it not 8amps, for the panels will standardise on the lowers panel's specs?

Hi TTT see Carl's post above.

If you put a 5A and 4A solar panels in parallel you will get 9A. The voltage will average out between the two dissimilar panels.

 

If you put a 5A and 4A solar panels in series then the voltage will additive. The current output will only be slightly higher than the current produced by the panel with the lowest current output in the series string.

 

Plonkies info on the parallel battery bank operation makes sense.

Yes, you're right. 

 

Amps add up in parallel, not in series. It was quite late last night when I made that reply. 

So in this case you have 9A. 

But the point I was making is that Ampere is drawn, not pushed. The question is, will the batteries use the max ampere available, or only what is needed? If it uses only what is needed then it should still equalize out over two battery banks with slightly different Ampere ratings. I don't have two batteries and two different chargers + 2 oscilloscopes to test this though. 

 

Edit: If there's 1Ah for 10 hours, then the batteries will draw 1A for 10 hours, with some "falloff" due to peukert's law. 

But if there's 200Ah and the battery is only a 100Ah  battery, will it draw the full 200Ah for on hour, or will it draw 10A for 10 hours / 20A for 10 hours?

P.S. I'm not a chemical engineer so I don't actually know the answer. I am asking an educational question here. 

Will the batteries only use what is needed? I don't think they are clever enough to do that, so the answer is probably no. Let me attempt an explanation.

 

Solar panels are constant current devices. So a panel that can do 8 amp will usually do so at any voltage down to zero (if you short-circuit it). So your average 100W 12V panel will push 8A of current into any load willing to accept it up to 18V and even more. For the purposes of this discussion, you have to look at the battery as a load.

 

So if you wire this directly to a 12V battery with no charge controller, the panel will continue to push 8A into the battery until it reaches 20V... at which time the panel can no longer do 8A and start to taper off. Eventually you will find an equilibrium point where the amount of energy the battery consumes (basically by boiling it's electrolyte at this voltage!) equals the amount of energy the panel can provide.

 

A lead acid will always accept more charge. It won't do anything useful with it... it will just boil away electrolyte, but as long as there is a potential difference (ie the panels operate at a higher voltage) current will flow from the higher potential to the lower potential.

Ok, so you're saying Ampere gets pushed, and not pulled?

I have never seen amps being pushed out of a solar panel via the wires if they are just lying there ...   

But if you add any form of resistance, then the magic starts happening, as if it is being absorbed, used.

Eish. Funny how difficult it becomes to explain stuff you take for granted. Like my physics 114 class where it turns out that centrifugal force is really "center seeking" force. So now like some geek I snigger whenever someone talks of centrifugal force :-)

 

Anyway... drawn... pushed. Point is, if there is a potential difference, and a path for the charge to flow, then the current will be pushed from the higher potential side to the lower potential side OR it will be drawn from the higher potential side to the lower potential side. It really depends on your frame of reference.

 

Think of two water tanks. If the water level is higher in the one than in the other, and you have a pipe running from the one to the other, water is going to flow. How fast it flows depends on how thick the pipe is, and the difference in water level (aka water pressure, analogous for voltage here). As the two tanks near the same water level, the flow slows down. When they are at the same level, the flow stops.

 

So in terms of appliances, say in a 230V application, amps are "drawn", the flow depends on how thick the pipe is (aka the resistance of the appliance). In terms of solar panels, the amps are "pushed" into the battery, if you will, because the solar panel always sits at a higher potential, just by it's very nature.

 

You have to understand that V = IR :-)

Was on a business forum (before they closed) where you had the general population and then the few really clever ones.

 

It was difficult for them to explain some things in such a manner that the general pop could understand the first time ... and then you had the jokers in between. 

But the clever ones became very good at explaining things. It is a fine art, explanations and jokes.

 

O by the way, the day I find that darn water tank in my solar panel, that is the day I will cut off the municipal water supply, I swear!!!     

So, its been a month. Quite happy with the system's performance. I have used 37 units last month, mainly because of cloudy days, also a few because I have drawn the inverter into overload 

 

Since Ekurhuleni gives me 100 units a month no charge, I'm quite happy with that performance.

On 2015/08/29 at 11:11 AM, McWidowmaker said:

Finally finished mounting the panels and getting everything connected. Switched over yesterday and it looks like everything is working. Solar yield this far is a little less than expected. Its 11am now and I'm getting about 2750w from 4500w worth of panels. May need to investigate and see if this is a problem or this is usual yield. Don't have enough data yet, but I was expecting about 3.5kw around now.

 

 

Just a small note from my side:

This fuse in the pic is an AC fuse and not a DC fuse
AC and DC fuses are not the same
http://solarhomestead.com/difference-between-ac-and-dc-fuses/

Read it please for your own peace of mind

AC fuses are cheaper and I think that is why many installers use them

My installers did exactly the same and I replaced the AC fuses with DC fuses this week
Its a small price to pay compared to the consequence it can bring should something go wrong...

Edited May 14, 20206 yr by Bobby Kaucic

1 hour ago, Bobby Kaucic said:

This fuse in the pic is an AC fuse and not a DC fuse

It is rated for 500V though, and sometimes a 500V AC fuse will also be rated for 60VDC... but you must look at the spec sheet and make sure this is the case. DC fuses have a kind of silica (looks like sand) that fills the area around the melting wire, to arrest the arc. AC fuses often don't have that and rely purely on creating enough space in a short enough time.

I spent a little bit of time perusing the catalogue at gave.com, but there is no clear indication that these have an eplicit DC rating. So I would also recommend swapping that out.