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Old 08-11-2011, 02:08   #106
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Quote:
Originally Posted by T1 Terry
Hi Dave,
I've got to disagree with you on this one. If all the cells are in parallel and charged to 3.45v per cell then as soon at a cell reaches full copacity it can't accept any more charge without the terminal voltage climbing higher than the 3.45v setting. The charge current will simple flow across the link of that cell to the next cell. Charge voltage limiting is the key, constant current charging is for EV's constant voltage charging is for house batteries and 3.45v is a safe voltage. Even in series a 4 cell 12v battery can't have a single cell go over the deadly 4v mark without the oher 3 cells all being below 3.3v. The way the charging voltage curve maps out, that would be a huge out of balance.


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Terry I'm agreeing you. I agree that for safety, long cell life and practically speaking at fractional C charging, constant voltage charging. is the way to go. ( you must be using yittrium cells btw).

I was making the point that BMs arnt really necessary at fractional C charging and discharging

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Old 08-11-2011, 02:27   #107
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Originally Posted by Sparohok

I wish I had a better understanding of Li cell physics. However when I hear things like this I start to wonder if the problem isn't my understanding of batteries, but rather others understanding of electricity.

If all cells are in parallel, all cells are at the same voltage. How can one cell in a parallel configuration be at a dangerous voltage while the others aren't?

Also, I thought "terminal voltage" was defined as the voltage potential between positive and negative terminals of a battery. By definition a battery is "at" its terminal voltage; it doesn't "reach" its terminal voltage.

You seem to describing the risks of cells being charged in series, not parallel.

Help?

Martin
This is just terminology. What I mean is the by terminal is the voltage the cell is at at it's full SOC.

The problem with Li is that in dome cases you really need cell charging rather then bank charging. In boats you will in effect have a series parallel battery bank in most cases.

What I was saying that in this situation ie the common " bank" charging regime , some cells reach the terminal voltage first. Ie they exhibit full SOC in parallel charging the charger must now be disconnected hence other cells can get left undercharged. bMS system balance at a cell level by allowing the charging process to continue by controlling at the cell level. ( sorry this all easier to show on my Fluke Scopemeter them write it down)

What I was trying to convey. as opposed to Genasuns very over the top BMS. In current boat environments, with typically banks in the 400-10000 Ah range. It's very difficult to charge at greater then 0.5 to 0.7 C.

Equally discharge currents are typical 0.1 to 0.5 C. In this environment cells dp not get put of balance. ( other then initially from the manufacturers. ) cell balancing in this environment could be done maybe annually or monthly using a separate system.

Anyway small imbalances just result in a slight loss of total " theoretical " capacity. In practice it's irrelevant. High voltage cutoff is best as I aggreed with T1 done by constant voltage charging. ( or more correctly voltage limited charging ), low voltage cutoff is not needed but simple to rig up . Mainly because in a boat there will be monitoring of batteries anyway. ( low voltage cutover are available commercially anyway ).

There's no mystery to charging Li. In fact it's vastly simpler then LA as there's no reason to try and achieve 100% SOC, which is the main reason for complex IUI or IUU charging in LAs. HVC and LVC protection is easy to arrange or design out of a boat system.

Dave
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Old 08-11-2011, 11:24   #108
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by goboatingnow View Post
The problem with Li is that in dome cases you really need cell charging rather then bank charging. In boats you will in effect have a series parallel battery bank in most cases.
Yes, but Bill was specifically asking about parallel balancing... not series/parallel or series. In parallel, by definition, all cells are at the same voltage.

I just wanted to verify that you are in fact talking about series banks, not parallel.

Martin
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Old 08-11-2011, 17:26   #109
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Simple form of one-off or occasional balancing

Quote:
Originally Posted by Sparohok View Post
Yes, but Bill was specifically asking about parallel balancing... not series/parallel or series. In parallel, by definition, all cells are at the same voltage.

I just wanted to verify that you are in fact talking about series banks, not parallel.

Martin
Martin (Sporahok), hope you don't mind if I elaborate on a related question:

- To get 12v from LifePO, we always connect 4 cells in series (each cell around 3.4v) when in use or being charged.

- If you have a simple 4-cell battery, and want to balance these 4 cells (once-off, or occasionally) without using a cell-balancing BMS, can you simply charge them up, then connect these 4 cells in parallel with zero load, let them settle. Then disconnect and measure individual cell voltages and compare. Then maybe charge a bit more (in series) if necessary, and re-connect in parallel to let settle again until individual cell voltages come up close enough?

Question is whether such a 'primitive' approach is viable for fraction-of-C load usage, or whether it does not make sense for LifePO?

Secondly, if it is viable, what would "close enough" be when comparing individual cell voltages? For example, would readings of 3.45v, 3.35v, 3.30v and 3.40v for the four cells be Ok?

Lastly, when comparing cell voltages, do these cells first need to be rested for 24h after charging like Lead-Acid types, or is that not relevant?

Thanks.
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Old 08-11-2011, 18:53   #110
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Re: Simple form of one-off or occasional balancing

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- If you have a simple 4-cell battery, and want to balance these 4 cells (once-off, or occasionally) without using a cell-balancing BMS, can you simply charge them up, then connect these 4 cells in parallel with zero load, let them settle.
One thing I would be concerned about here is current limiting. If the individual cells are at different state of charge, how much current will flow as the cells balance? Will that exceed the charging current limits of the batteries? I guess there are a few approaches. One way is to do the math, calculate the current using the voltage difference and the batteries internal resistance. Another way is to add power resistors between batteries for current limiting (e.g. 0.1 ohm 10 watt). But I wouldn't mess with something like this without making sure you know exactly what you are doing.

As for your questions, hopefully the LiFePO4 experts here can comment, I don't know.

Martin
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Old 08-11-2011, 19:14   #111
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Re: Simple form of one-off or occasional balancing

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Originally Posted by Sparohok View Post
Another way is to add power resistors between batteries for current limiting (e.g. 0.1 ohm 10 watt).
Martin
Excellent point - when paralleling the cells to balance, inter-cell current surges can be avoided via small ballast loads (12v globes, wirewound resistors, etc.).

Given that sensible precaution, how about the rest of the question?
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Old 08-11-2011, 19:39   #112
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Quote:
Originally Posted by Sparohok

One thing I would be concerned about here is current limiting. If the individual cells are at different state of charge, how much current will flow as the cells balance? Will that exceed the charging current limits of the batteries? I guess there are a few approaches. One way is to do the math, calculate the current using the voltage difference and the batteries internal resistance. Another way is to add power resistors between batteries for current limiting (e.g. 0.1 ohm 10 watt). But I wouldn't mess with something like this without making sure you know exactly what you are doing.

As for your questions, hopefully the LiFePO4 experts here can comment, I don't know.

Martin
You cant actually cell balance by paralleling the cells.

Let's look at what happens. Cells in parallel are being charged by a common charger. When the cell limit voltage is reached ( ie less the HVC ) the charger must disconnect. At that point the common terminal voltage will be basically the limit voltage but individual cells may have different SOCs. Leaving them sit will in fact do nothing. ( this is due to differences in internal impedances)

Breaking the parallel set apart you will then see that each cell has slight ( ie 0.01 v) differences. Hence each cell that's below the original limit needs individual charging.

This problem also occurs in LAs, but because we seek 100% and in doing so we drive excessive voltage and drive into absorption ,we mask the problem as LAs tolerate the overvoltage.

In Li we can't do that. So in practice cell balancing can only be sensed on series sets. Hence for large prismatics that you wish to balance the battery should be built from series configurations only.

Dave
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Old 08-11-2011, 19:47   #113
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Quote:
Originally Posted by Marqus

Excellent point - when paralleling the cells to balance, inter-cell current surges can be avoided via small ballast loads (12v globes, wirewound resistors, etc.).

Given that sensible precaution, how about the rest of the question?
No none of this works Li terminal voltage remains almost flat until near full SOC is reached. Hence there's little or no voltage difference , it's only when you teach near the end that you get a cell voltage rise and then you stop charging.

Connecting up a Li cell with say 80% charge in parallel with one with 90 % will not result in both cells arriving at 85% SOC. All that happens is both cells basically sit at there respective SOCs.

Dave.
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Old 08-11-2011, 20:26   #114
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

As Dave has said, parallel balancing can only occur while charging but the voltage must be limited to 3.45v otherwise the cells that fill first will overheat and die.
Simple connecting the cells together when at rest will achieve very little, there is only0.4v difference between 1o% charged and 100% charged when the cells are at rest, that 0.4v adds up to 13.6volts fully charged and 12.2 volts near flat, enough difference there but not enough at individual cell level.
I have a heap of appointments this afternoon but this evening I'll post a few charts and a bit of an explination regarding the importance or lack of it when it comes to cell balance and just where is the balance measured.

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Old 08-11-2011, 20:32   #115
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by goboatingnow View Post
Connecting up a Li cell with say 80% charge in parallel with one with 90 % will not result in both cells arriving at 85% SOC. All that happens is both cells basically sit at there respective SOCs.

Dave.
I guess that's a point I've been missing. You're saying that we can have two cells with identical voltages, but they can harbor a 10% difference in SOC. Quite a significant point.

Given that's the case, how does a BMS then equalize SOC across multiple cells? The BMS cannot rely on cell voltage measurement, because (per above) it cannot rely on a cell's voltage to reflect it's SOC. So what does a BMS measure in order to be able to equalize a set of 4 cells?
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Old 08-11-2011, 20:39   #116
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Quote:
Originally Posted by Marqus

I guess that's a point I've been missing. You're saying that we can have two cells with identical voltages, but they can harbor a 10% difference in SOC. Quite a significant point.

Given that's the case, how does a BMS then equalize SOC across multiple cells? The BMS cannot rely on cell voltage measurement, because (per above) it cannot rely on a cell's voltage to reflect it's SOC. So what does a BMS measure in order to be able to equalize a set of 4 cells?
When the cells are in series or isolated from each cell imbalance results in small differences in terminal voltage. Hence good BMS top balancers divert charge to the lower SoC cell to bring it to the Setpoint voltage. In some cases a higher voltage right at the HVC point had to be used to cure large imbalances. In this case you charge and then bleed off back to the safe set point voltage.

It's also a factor that Li SOC can only really be determined at the near the full SOC. Ie they exhibit ( after some initial charge) an almost flat constant voltage throughout the bulk of the SOC range. As they approach 80 + point there is a noticeable rise in terminal voltage. Thus is a great feature as it enables easy charge cutoff ( unlike LAs)

This means that balancing can only be at near the top or near the bottom. These are the only events you can detect You simply can't detect a 50% SOC as against a 60% SOC Li as the voltages are virtually identical. ( leaving aside cell manufacturing issues)

Dave
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Old 08-11-2011, 23:30   #117
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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When the cells are in series or isolated from each cell imbalance results in small differences in terminal voltage. Hence good BMS top balancers divert charge to the lower SoC cell to bring it to the Setpoint voltage. In some cases a higher voltage right at the HVC point had to be used to cute large imbalances. In this case you charge and then bleed off back to the safe set point voltage.

Dave
The above says that, ultimately, the BMS relies on cell voltage differences to equalize, even though these voltage deltas are small.

So, if our 4-cell battery measures around 13.6v or above (ie. region nearing full charge), the voltage deltas will be enough to make the BMS do some balancing. At the same time, why would those voltage deltas not be enough to force current flow between paralleled cells (eg. 3.45v to 3.43v)?

Could it be a matter of taking too long?

Saw a figure of 6 - 10 milliOhm on the web for the internal resistance of a LifePo cell (more reliable figure avail?).

For argument's sake, if we assume 10mOhm, and assume a voltage delta of 0.05v between two cells, that would mean an initial equalizing current of 5Amp, reduced by adding a ballast resistor (or lamp).

That doesn't sound too slow. However, the internal resistance may change as SOC increases and that could reduce the current to lower levels. If it rose by ten times, the current would only be 0.5A, and that could be too slow. Or would it? Is that the real obstacle? Possibly not; anyone know?
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Old 08-11-2011, 23:32   #118
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

As an experienced electronic technician but a noob to these batteries, I really appreciate all these posts !

Thanks guys.
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Old 09-11-2011, 01:28   #119
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Internal resistance increases as SOC decreases. Thus, when two cells with unequal SOC's are connected in parallel, there will be less surge current with two low SOC cells then when the cells have a high SOC.

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Old 09-11-2011, 01:48   #120
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

A Li cell is fully charged at 3.4v rested but it requires 0.05v over voltage to create a charging condition. A cell at 3.35v rested is somewhere between 80% and 90% charged, a cell at 3.0v could be anywhere between 100% discharged and 70% discharged. State of charge can be measured by cell volage to a degree but it's in the single milli volt range, a large imbalance has occured by the time there is sufficient difference between cell voltages for it to tranfer. I remember reading some where on the EV DIY forum where someone linked all their cells together and left them for 48hrs, then connected them is series and charged them, they were still all over the place when the 3.45v mark was passed for each cell. This is the method the majority of BMS active cell balancers use, when a cell reaches say 3.8v a controlled rate short circuit discharge across a resistor in switched on and burns the excess voltage down and turns off at 3.5v. Unfortunately manufacturing being what it is some will turn on at between 3.85v and 3.75v and turn off between 3.55v and 3.45v. some drift into the higher end and cells are damaged, some drift under the 3.45v mark and then the balancer is trying to discharge the real charge capacity, this overheats the little units either burning them out or causing them to latch on gradually dragging the cell dead flat.
As balance is really only an issue at the the very top of charging, a cell running away and overheating, or at the bottom and a cell being dragged into reverse voltage. The second one is extremely unlikely to happen in a hose battery situation so top balancing is the choice of methods for house batteries.
Now, if the cells are dragged up to 100% full the over voltage doesn't happen, if the 100% charged point and top end balancing is left to a time the operator can watch what's going on then if a cell goes high a load can be applied to that cell to bring it back down, once all cells are 3.45v to 3.5v and the charge rate has dropped to zero the cells are considerd fully charged, the 100% is reset on the SOC meter and the system returns to being charged to 98% each time till anoher top balance is required. If the cells stay within 50 milli volts while working then a top balance isn't required.

T1 Terry
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