Looking for opinions on my solar diagram!

[tictag]

So i got 4 sets of 24v 100 batteries(two 12v 100ah in series). So i should supply no more than 40 amps to the busbar? c10 x 4 sets= 40amps. is that correct?

You do need to check your battery manufacturer's recommended charge rate. C/10 is a 'safe' maximum charge rate for lead-acid deep cycle batteries. But that said, yes, if one battery has a C/10 recommended charge rate then each battery 'leg' (i.e. 2 x 100AH 12V in series) would need to be limited to 10A (C/10 = 100AH / 10 = 10A), so you should not allow more than 40A to flow through your battery bank and you must never allow one battery or battery leg to become more or less discharged than the other, which is as mentioned already, very unlikly if thay are all connected to the same busbars.

I have two 12V 110AH AGMs connected in parallel. Each AGM has a C/5 maximum charge rate, so I have to limit my charging current to 44A (C/5 = 110AH / 5 = 22A, multiplied by two because my batteries are in parallel = 44A).

 

[tictag]

1C= 400amps? so i can give my battery busbar 400amps and i'll be fine?

Most Lithium-ion batteries have a maximum charge rate of around 1C e.g. for a 100AH 12V LiFePO4 battery, 1C = 100A. Ability to charge and discharge at such high rates is one of the (many) benefits of Lithium-ion battery technology. For us lead-acid dinosaurs, C/10 is good, C/5 is great. Anything higher is, frankly, magical.

 

[tictag]

I just found out that the scc restricts the amps with different charge settings. Is that correct? Bulk gives more amps then absorbtion and float?

Yes. Google CC/CV charging profile.

The CC (constant current) bit is often called the Bulk stage, this is where a constant current is supplied to the battery - for a PWM SCC, this is just whatever the panels can produce (don't exceed the SCC specs!). The CV (constant voltage) bit is often called Absorption and this is where the battery voltage is maintained until the current flowing through it reduces to a threshold value (usually around C/100 Amps, e.g. 1A for a 100Ah battery). A PWM SCC achieves this by going through a connect > disconnect > measure cycle multiples times per second where it connects the array, disconnects after a certain time, measures the battery voltage, then repeats. As the battery voltage increases, the time the array is connected to the battery decreases (mark/space ratio) until eventually it disconnects the array permanently when the average current flow is around C/100 Amps.

It then switches into a Float mode where the SCC will maintain a set voltage, usually 13.8V. It does this in the same way as in Absorption, by connect > disconnect > measure, varying its connect/disconnect mark/space ratio in order to maintain a constant 13.8V. Float is required for lead-acid batteries because they have a higher internal discharge rate than Lithium-ion.

There are rules-of-thumb parameters for Bulk, Absorption and Float but I would encourage you to check your battery manufacturer's specs. Typical parameters are:

• Bulk - whatever your PV array is cable of providing, within the specification of your SCC (maximum current) and battery (maximum charge rate).
• Absorption - typically 14.6V for a FLA, 14.4V for AGMs and a little lower I think for gells.
• Float - typically 13.8V

Edit: Corrected typo.

 

[tictag]

though with lead acid I believe the slower you charge and discharge the more capacity you have, so a battery might be 100ah at C/20 but only 90ah at C/10, but I don't know much about lead acid)

@Abel Google 'Puekert Effect' for further information on this phenomenon. Basically, the heavier you charge or discharge, the less capacity you end up with.

The reason is to do with the chemical reactions that take place in a lead-acid battery. Imagine dunking a biscuit in a cup of tea - if you dunk quickly, you don't get so much of the tea soak into the biscuit, dunk more slowly, you get more tea soaking in. Tea in this analogy is a metaphor for battery capacity. Don't take the analogy too far though!!

 

[tictag]

I'm not sure if this is what he is referring to or not, and as I said, I don't know a lot about lead acid.

Lead-acid batteries generally have two specification: one for charge, one for discharge. A typical 'safe' charge rate is C/10 Amps, though some can handle a greate charge e.g. my AGMs are rated for C/5 Amp charging. The discharge limit is often called CCA, or Cold Cranking Amps and is less useful for PV applications. Lead-acid discharging is more impacted by the Puekert Effect.

Confusingly, C-rates (because they very usefully allow a variable to be based on the capacity of a battery) are ALSO used for capacity ratings, but the two are distinctly different. One 100AH at C20 rated battery could have a charging rate limit of 10A, another 100AH at C20 rated battery could be limited to 50A. Same tool used (i.e. c-rate) for two completely different purposes. Like one multimeter can measure both current and voltage.

 

[tictag]

I'm just theorizing idk if its 40a or 80a. I dont know anything. I do know i got 80 amps going to my battery busbar from 10 panels all wired in paralell for 36v and 80a.

Sorry if this has been a bit of a learning curve for you! But, hey, if life were simple it would be boring! ;-)

As mentioned in my responses to your PMs, you need to find out from your battery manufacturer what their recommended charge rate limit is. C/10 is a 'safe' charge rate for most lead-acid batteries - yours could be C/5 or even C/2 which would mean you could throw 50A into it!

But if the answer is C/10 then the fact is your system as configured will fry your batteries. Boiling everything down, a 2,500W array will dump 104A into a 24V battery bank, meaning each of your 5 'legs' will receive 20.8A, which is twice the recommended limit for a C/10 rated battery (and, funnily enough, exceeds your busbar terminal stud rating too...). It will absolutely work, your batteries will charge and everything will look good, until you eventually realise that your batteries are getting hot, water will begin electrolysis and produce a flammable mixture of oxygen and hydrogen, the water in the electrolyte will evaporate (if open) and/or build vapour pressure (if 'maintenance free') and potentially blow its protection valve, spilling out explosive gasses and corrosive acid...

And.....RELAX! None of that is going to happen because you are going to:

1. Determine the maximum charge limit as recommended by your battery manufacturer and
2. Design your system so this is not exceeded

Tic tac said:
Yes. Lead-acid batteries generally speaking have a C/10 maximum charge rate (some AGMs allow up to C/5), which for your battery bank that's 80A (800AH / 10 = 80A). You should not regularly exceed this to avoid premature aging.

This is correct but requires context: a typical 800AH battery bank can be charged safely at 80A assuming a C/10 maximum charge rate. You don't have an 800AH battery bank, you have a 400AH at 24V battery bank.

Edit: Corrected typo.

 

[tictag]

I never realized C-rate was such an issue with Lead Acid

It's a bitch.

 

[tictag]

And that's me, mentally and physically exhausted now! Going to rest my head in dark, cool corner for a bit...

 

[tictag]

oh, just realised, all I've done is tell you what the problems are with your system as designed, sorry, the solution is to simply connect your battery bank as a 12V system i.e. 8 batteries in parallel. Then you can throw 80A into it (assuming a C/10 maximum charge rate ... which you need to check )

... or reduce your array size to around 1,000W (40A x 24V = 960W, plus losses)

... or use MPPT SCCs that have a configurable charge current setting

... or replace your existing batteries with Lithium-ion and throw a disgusting amount of current into them

... or design a current sensing switch that switches in/out your PV panels depending on solar radiance / current generation


OK, I'm getting a bit giddy now. Back tomorrow.

 

[Abell]

oh, just realised, all I've done is tell you what the problems are with your system as designed, sorry, the solution is to simply connect your battery bank as a 12V system i.e. 8 batteries in parallel. Then you can throw 80A into it (assuming a C/10 maximum charge rate ... which you need to check )

or add 16 batteries and it would solve the problem.

 

[tictag]

An extreme solution, but nonetheless true!