Heavily boiled FLA battery

[SunEagle]

.......

Now if we can go back to the original subject of this thread, I have few more comments to add.

Okay, you're perfectly right: series battery topology is the more preferable by far. But, if the parallel strings "does't work", how come they actually.. work?!

Well, there is a self regulating mechanism due to variable internal resistance of the battery, which is mainly related to its SOC.

That's it, when the batteries are used in a S+P configuration, there will be a self-ballancing mechanism (ohm low) as to get equal internal resistance for each string (which roughly means equal SOC).

These physics laws could only be "canceled" by the chemistry; if a battery has been abused (sulfation, overcharging) it will act erratically, due to inconsistent current path. In this situation, you can no more count on "internal resistance <=> SOC" relationship.

Anyway, by using an equalisation charge more frequently, the actual SOC of every battery of the bank will be set to 100℅ hence any cycling unballance will be removed.

The key factor when using series/parallel configurations is to keep all the batteries in a healthy condition (by avoiding sulfation or deep under/overcharge) as to allow identical chemistry behaviour.

Well a parallel wired battery system does work but it is not the best way to use them. So the idea is to avoid doing something that can reduce the lifespan of the components.

It is almost the same reasoning as not keeping a bad apple in a bunch of good ones. That bad boy will hurt the other ones and soon your entire bunch is bad. Cleaning and wiping each apple individually does not really help the bunch because the bad boy continues to rotten and affects the other apples that are in contact with it. The best way to keep the good fruit longer would be to remove the bad boy from the bunch.

[santaclaws]

That's an interesting analogy. Anyway, a shorted cell will affect a series bank, too.

A shorted cell might be fooling the charger. It might not be so "dangerous" for a large series bank (>48V) but, for a smaller one, it could be lethal (if the charger isn't "smart" enough.

Individual cell voltage (and/or temperature) monitoring could avoid a disaster though.

[SunEagle]

That's an interesting analogy. Anyway, a shorted cell will affect a series bank, too.

A shorted cell might be fooling the charger. It might not be so "dangerous" for a large series bank (>48V) but, for a smaller one, it could be lethal (if the charger isn't "smart" enough.

Individual cell voltage (and/or temperature) monitoring could avoid a disaster though.

I agree that individual cell monitoring should head off a major battery failure and a cell failure will hurt the system no matter how it is wired but the chances of a series wired system failing is less then a parallel wired system due to the additional connection points and paths for the current to flow in a parallel system.

If the system was a series wired type then there would be no question that the bad cell would need to be replaced to get the system working again even if doing so would hurt the replacement cell due to the older series wired cells.

With a parallel bank the failed cell will quickly bring down a "string" but not the whole system so it would not be noticed until the bank was asked to provide output akin to it's design. The result of replacing the bad cell with a new one is the same result in a parallel or series system.

It is still the Bad Apple policy.

[santaclaws]

With a parallel bank the failed cell will quickly bring down a "string" but not the whole system so it would not be noticed until the bank was asked to provide output akin to it's design.

Speaking of that, I've just put in the circuit one current transducer for every battery string. I'm also monitoring the voltage of every battery of the bank, so now I have realtime informations regarding individual battery status.

However, I could only use those informations for an early warning alarm, as I don't have (yet?) an automatic disconnect switch for every string (thus I'll have to do it manually).

The result of replacing the bad cell with a new one is the same result in a parallel or series system.

I hope my cousin Sunking is reading this. He yielded at me that only on parallel (actually, mixed) connection you need to replace all the cells because of a damaged one.

Okay, to be correct: on a series topology, you have the chance to automatically isolate the battery bank (thus the individual cells) by shutdowning the charger and the load (inverter). This way, the rest of the batteries are saved, even if you have to replace them all to restore the bank.

On the other hand, in a mixed connection, the damaged string will still affect the rest of the batteries, despite of the bank being disconnected.

[Sunking]

Sun Eagle I know you will understand this, Santafraud not so much.

Let's say we have two parallel strings of batteries. Any voltage or size you want, makes no difference. Lets call it 12 volts. One string has a shorted cell so we have 5 good cells and one bad one. The string with the shorted cell will not go any higher than 12 volts. What is the good parallel cell voltage? Easy right 12 volts.

So what is happening. The bad string is sucking all the charger current, and has discharge the good string through the bad cell. So now you have a hot string sucking all the current from every source boiling away. Very easy to spot and tell what is going on.

So why is the bad string boiling and the good string doing nothing?

Real simple the string with the 1 bad means the 5-cells still working are exposed to 2.4 volts which is GASSING VOLTAGE and boiling like crazy. Not only are you loosing water, but corrosion is super accelerated. In the mean time your good string is setting at 12 volts (25% SOC) and all the sulfate crystals have hardened off destroying it.

So now you have 2 useless strings of batteries beyond repair. It is extremely easy for the operator to detect a shorted cell because he has batteries that never charge despite taking full charger current..

[Sunking]

Speaking of that, I've just put in the circuit one current transducer for every battery string. I'm also monitoring the voltage of every battery of the bank, so now I have realtime informations regarding individual battery status.

Please explain how you can monitor the voltage of every battery in parallel strings individually.

[santaclaws]

I'll do it for you:

I'm monitoring the bank voltage (24V) and each node (12V, acriss the low-side battery) of the parallel strings.

By substracting the low-side battery voltage from the bank voltage, I get the high-side battery voltage, too.

So basically I'm reading five voltages and, after computing, I got eight (plus one, the bank voltage).

Later edit:

You're talking about something else (parallel only). I was talking about my configuration (2S4P).

[santaclaws]

It is extremely easy for the operator to detect a shorted cell because he has batteries that never charge despite taking full charger current..

If you read my first post, that's exactly what happened: I've realised, after a day or two, that my bank was discharging internally (because the load current was quite small, as usual).

I had no indication of that issue during charging time as I have a lot of charge power available (solar + wind) to overcome any extra current required by a damaged string (shorted cell).

I must have been reading the daily log to look for unballanced charging/discharging ratio but I was on a vacation.

Murphy's law..

[Sunking]

Take this FWIW. Shorted cells are caused by one thing and one thing only:

CORROSION from Overcharging abuse. That causes plates to Flake and Shed material that gets lodged between the plates and shorts out the cell. That tells me two things. You have to much power and your charge voltage set to high. Shorted cells are usually only seen in Automotive applications because voltage regulators in the alternators are set to 14.2 volts.

There is no reason to ever use parallel strings in a Solar System especially 2S4P is insane. It tells me instantly you are stuck inside a 12 volt toy box because you used 12 volt batteries. I do not know what size you used, but if you needed say 800 AH, then use 800 AH batteries like a Rolls 6CS25P a 6-volt 820 AH battery.

But 24 volts into a 5000 wat Inverter is just plain ignorant and dangerous. Not to mention expensive.

[santaclaws]

Now we're talking!

Well, thanks for the shorted cell explanation but I suspect a wrong manufacture design & build.

My batteries (12V/225A) are made by a local manufacturer. They were using the same case for a large range of their products, this one being the best "performer".

Actually, to allow the increase in capacity, they have used very tall plates, so there was a very small space left at the bottom (and at the top, for that matter).

From what I could see through the watering holes (few days after I put them in charge), the plates looked very loose and misshaped.

That's it, I bet they put some leftovers inside those cells, too. To be honest, I've been waiting for this day since then; I'm actually wondering how did they resist such a long time.

Regarding my bank voltage: it was a "temporary" solution, as I already had some commercial 24V inverters available for free at the time.

My current (main) inverter that I've made has a LF toroidal transformer with two secondary windings (30V). For now, I'm running them in parallel (for actual 24V bank) but they only need to be wired in series to get a 48V inverter.

That's supposed to be the next step (to switch the battery bank from 24V to 48V), but I didn't have time to make it (as long as it was working perfectly on 24V).

Of course, it's efficiency will improve for 48V operation but so far so good.

Btw, being home made, the price was quite low (I mainly paid for the toroidal transformer only).

And I have to tell you once again: I knew it from the beginning that it will be a temporary solution (so I kept the investment low) as I'm going to relocate (abroad) during this year.

[jflorey2]

Okay, you're perfectly right: series battery topology is the more preferable by far. But, if the parallel strings "does't work", how come they actually.. work?!

Oh, they will work for a little while. And in narrow applications (like UPSes that can charge for days) you can even parallel a lot of strings with only a minor loss in reliability. It's not generally done simply because it's less reliable, and large UPS customers are willing to spend the money for a more reliable system.

Well, there is a self regulating mechanism due to variable internal resistance of the battery, which is mainly related to its SOC.

Resistance DECREASES when a string is badly discharged. Then, the next time you go to charge it, the good string takes all the charge current.

Now, let it sit there for a week on a float charge and eventually the trickle of current it is getting charges it, its gravity goes up, resistance goes down and it comes back into balance. Most off-grid people can't let it sit on a charger for a week - so it dies.

That's it, when the batteries are used in a S+P configuration, there will be a self-ballancing mechanism (ohm low) as to get equal internal resistance for each string (which roughly means equal SOC).

There is actually a self-UNBALANCING mechanism. Which is why multiple strings are a problem.

The key factor when using series/parallel configurations is to keep all the batteries in a healthy condition (by avoiding sulfation or deep under/overcharge) as to allow identical chemistry behaviour.

If all cells stay perfectly balanced, then you can parallel as many strings as you want.

In the real world, all cells don't stay perfectly balanced.

[Sunking]

Oh, they will work for a little while. And in narrow applications (like UPSes that can charge for days) you can even parallel a lot of strings with only a minor loss in reliability. It's not generally done simply because it's less reliable, and large UPS customers are willing to spend the money for a more reliable system.

Resistance DECREASES when a string is badly discharged. Then, the next time you go to charge it, the good string takes all the charge current.

Now, let it sit there for a week on a float charge and eventually the trickle of current it is getting charges it, its gravity goes up, resistance goes down and it comes back into balance. Most off-grid people can't let it sit on a charger for a week - so it dies.


There is actually a self-UNBALANCING mechanism. Which is why multiple strings are a problem.

If all cells stay perfectly balanced, then you can parallel as many strings as you want.

In the real world, all cells don't stay perfectly balanced.

See we agree on a lot of things technically. You beat me to it. Santa just does not understand batteries, or power when it comes right down to it.

Parallel strings are used exclusively by telecom where I come from. Not because it is a good idea, because you cannot find 10,000 to 60,000 AH 2 volt cells. However in Float Service you can use parallel strings for 1 good reason, they never get cycled. They just sit there waiting for a day to be used. When used only for a few minutes until generators come back on. Plus it allows telcos to take a string off-line for maintenance. Bu tif you do that they must be equalized before being brought back on line.

UPS are special purpose and batteries are usually in cabinets. They mostly use special purpose AGM batteries that can be discharged at 4C or 15 minutes. Like Telecom they Float and rarely ever used.

But in cycle service, parallel cells are the last thing you want to use. When you do use 12 volt batteries is out of ignorance. Batteries are not 12 volts.

Now there is one way you can parallel Lead Acid batteries that work just fine. It requires the user to use 2 volt cells in a Ladder Connected circuit. But there is a catch. 2 volt cells start at 1200 AH and go up to 6000 AH. In any solar system there is never a reason to use anything larger than 800 AH. If you need something larger than 800 AH batteries means your voltage is to low. That keeps charge and discharge current to 80 amps or less. That means nice small inexpensive wire that does not require hydraulic compression tools to terminate and test with DRLO meter. Only DIY make that deadly mistake.

[santaclaws]

Okay, let's try to have a look at this equivalent circuit of four strings in parallel:b4p.png

At any time, if any of the parallel strings have a lower voltage (V1 - V4) than the bank voltage (Vbank), there will be a charging current flowing through that string. For string #1, by example, the charging current would be (Vbank - V1)/R1. A string will only supply current to the load if its voltage is greater than the bank voltage thus it can't be discharged (externally) at a value lower than the bank voltage.

If the load and the charging current are zero (isolated bank), there will be a self-ballancing process as to get zero currents through R1-R4 hence you end up with identical string voltages (V1-V4) which are further identical with the bank voltage (Vbank). Anyway, the self-ballancing process remains active despite the load/charge current.

During normal operation, the flowing currents through each string would be more complex (dynamically changing in and out) but you can't have large variations in individual string currents as long as all the batteries are in a healthy condition and, obviously, they are of the same type.

[Sunking]

Okay, let's try to have a look at this equivalent circuit of four strings in parallel:b4p.png

At any time, if any of the parallel strings have a lower voltage (V1 - V4) than the bank voltage (Vbank), there will be a charging current flowing through that string. For string #1, by example, the charging current would be (Vbank - V1)/R1. A string will only supply current to the load if its voltage is greater than the bank voltage thus it can't be discharged (externally) at a value lower than the bank voltage.

If the load and the charging current are zero (isolated bank), there will be a self-ballancing process as to get zero currents through R1-R4 hence you end up with identical string voltages (V1-V4) which are further identical with the bank voltage (Vbank). Anyway, the self-ballancing process remains active despite the load/charge current.

During normal operation, the flowing currents through each string would be more complex (dynamically changing in and out) but you can't have large variations in individual string currents as long as all the batteries are in a healthy condition and, obviously, they are of the same type.

You have had 4 engineers telling you do not understand and only fooling yourself.

[santaclaws]

That's all you have to say in your defense?! So I'm an electronics engineer, too.. what's the point??

I kind of knew you will deny the laws of physics too. If you're an electronics engineer, please apply the Kirchhoff's laws on the circuit above and let me know if/where I go wrong.

Btw, are you a professional in electronics design or just a simple user? Have you ever designed a SMPS or any other power circuit (solar/off-grid related) or you're just setting up Xantrex/Midnite equipments?

(no offence, I'd really like to know if I can ask you to help me choose between some power circuit topologies for a new project)