So, lets spec out and set up a system of LiFePo4 batteries and inverter

[Willy T]

Thanks for sharing, it's the only way we will learn. Successes and Failures will help us all understand where we need to go.

I know you removed the boards and thats ok. A couple tips for those using them, there are two issues that can arise when you install them that people should be aware of. Yours were connected correctly. Cell #13 it's hard to tell, I wonder if it did hit the balance voltage at some point ?? I did test mine with a bench top supply and numbered them.

1) They are piggybacked on to the connecting terminal, meaning the interconnect / cable goes on to the battery terminal first and the board connectors are last closest to the bolt / nut and below the washer.

2) When you make the last connection to the battery bank make sure the output cable is cut off to the bank and load. If you do not have a breaker disconnect the board can end up carrying the whole load. It will fry the board or damage it. A fused disconnect will not work and the inverter will begin to boot as soon as the last connection is made.

[dax]

[QUOTE=PNjunction;182070Anyway, courses for horses I guess. I wish you all the success in the world, but we have to admit that lead-acid isn't going away tomorrow. Or the next decade. When it appears at the corner gas-station is when it will have truly arrived.[/QUOTE]

It does seem you yanks are way behind in this technology. Lifepo4 off grid is very well established in this country and growing rapidly, also in Rv and increasingly marine installations. The economics have been proven over and over if you steer clear of corporate brands, not expensive at all. You can buy lifepo4 starter batteries in this country easily, from a number of importers including us and a growing number of battery retailers who are now selling lifepo4 starter packs, we supply some retailers in Tas and Vic. We will also be supplying other types of lifepo4 batteries and cells, simply because the demand is so high and brand names so expensive. Every day now we get asked about different types of applications for lifepo4 and so are expanding our inventory of them, as people see what they can do in person. At least once a week we have a search for lifepo4 cell configurations that will suit a particular application we've been asked about, it opens doors for new business and introduces new technology to others, saving them time and money. Now we are about to produce a 20amp mains lifepo4 charger, because of the demand for them.

This is the 21st century, you don't find many batteries at garages any more in advanced countries, you get them from battery shops and on line, especially lifepo4.

The balancing voltage I posted of 3.2v we use, is not a typo but a fact. It's the one we've found allows for good balance between the voltages we use, 3-3.5v per cell and it works. As I've said before, these are not lead acid and the charing/discharging/balancing theories you are stuck in, don't work for lifepo4 unless you want a short life from them. When you can understand and accept the majority of energy is stored around 3.2v, it only logical that being the case, balancing at these points, means you are balancing at the highest concentration of energy. Above and below that and all you're doing is fiddling around at the edges, which in the long run creates imbalances.

Don't know the technical reasons why this is, we tried balancing at just about every voltage point there was and found 3.2v is the best. Except for my own house pack, the longest off grid installations we have are now over 5 years old and none have suffered from going out of balance, by more than .02v per cell.

[karrak]

The balancing voltage I posted of 3.2v we use, is not a typo but a fact. It's the one we've found allows for good balance between the voltages we use, 3-3.5v per cell and it works. As I've said before, these are not lead acid and the charing/discharging/balancing theories you are stuck in, don't work for lifepo4 unless you want a short life from them. When you can understand and accept the majority of energy is stored around 3.2v, it only logical that being the case, balancing at these points, means you are balancing at the highest concentration of energy. Above and below that and all you're doing is fiddling around at the edges, which in the long run creates imbalances.

Don't know the technical reasons why this is, we tried balancing at just about every voltage point there was and found 3.2v is the best. Except for my own house pack, the longest off grid installations we have are now over 5 years old and none have suffered from going out of balance, by more than .02v per cell.

I thought the majority of energy >60% was stored between cell resting voltages of between 3.3 and 3.2 volts. The 50%SOC level being around 3.28 volts. Is your figure of 3.2 volts under load?

I am puzzled by your balancing voltage of 3.2 volts, I thought the accepted wisdom was to balance at or below a cell resting voltage of 3 volts or above 3.5 volts. I would like to know more about your balancing procedure.

Simon

[karrak]

Try it - it's fun. Take a spare cell, and charge at less than .05C. Watch it rise to 3.45v (which at this point is fully charged). It will plateau there, but keep watching. A while later - voltage zoooooom from electrolyte heating / parasitics. Be safe of course. Aside from being slower than watching paint dry, this is one reason I either set my upper limit to 3.45v, or use a higher charge current >.05C and change the upper limit to something like 3.55v for absorb.

PNJunction, I am a little puzzled by this statement. I assume the test you were performing was with a current limited power supply set to a voltage greater than 3.45 volts? If so, could you tell me the supply voltage and current limit.

Thanks
Simon

[Willy T]

I am going to give some cover on this one. I had 8, 260 amp hr cells, in series, with a bench top power supply ( Mastertec 3030 ) and was checking the voltage every 10 minutes. My intention was to bring the pack up from 3.40v to 3.50v. To this point they were more or less the same voltage, as the voltage started to climb past 3.47 or so I noticed the amperage they were accepting had changed. I tested the voltage of each cell again and one cell had taken off like a rocket and was at 3.72v while all the rest were still down below 3.50v. I cut off the charge at that point, paralleled the pack and let the rest of the pack pull the voltage back down and quit. The next day they were all equal again.

In hind sight it was that lowest capacity cell that reached full charge before the other 7 and without intervention would have gone way over the high limit. This is where a balance board would have saved the day ( maybe ) if you were watching it and saw the lights change. It would have started limiting the amperage at 3.55v, would it have been enough ??, I didn't test it.

[Mike90250]

I am going to give some cover on this one. I had 8, 260 amp hr cells, paralleled them, with a bench top power supply ( Mastertec 3030 ) and was checking the voltage every 10 minutes. My intention was to bring the pack up from 3.40v to 3.50v. To this point they were more or less the same voltage, as the voltage started to climb past 3.47 or so I noticed the amperage they were accepting had changed. I tested the voltage of each cell again and one cell had taken off like a rocket and was at 3.72v while all the rest were still down below 3.50v. I cut off the charge at that point and let the rest of the pack pull the voltage back down and quit. The next day they were all equal again.

In hind sight it was that lowest capacity cell that reached full charge before the other 7 and without intervention would have gone way over the high limit. This is where a balance board would have saved the day ( maybe ) if you were watching it and saw the lights change. It would have started limiting the amperage at 3.55v, would it have been enough ??, I didn't test it.

EXCEPT. What charge rate were you hitting the pack with ? 15A? Balance Boards generally only shunt 0.5 - 1 amp when they kick in, so unless you were charging at 2A or less, even with BB, you would have fried a battery. Manual intervention saved the day.

[karrak]

I am going to give some cover on this one. I had 8, 260 amp hr cells, paralleled them, with a bench top power supply ( Mastertec 3030 ) and was checking the voltage every 10 minutes. My intention was to bring the pack up from 3.40v to 3.50v. To this point they were more or less the same voltage, as the voltage started to climb past 3.47 or so I noticed the amperage they were accepting had changed. I tested the voltage of each cell again and one cell had taken off like a rocket and was at 3.72v while all the rest were still down below 3.50v. I cut off the charge at that point and let the rest of the pack pull the voltage back down and quit. The next day they were all equal again.

Did you mean you connected the cells is series rather than parallel? If they were in parallel the should all be at the same voltage.

In hind sight it was that lowest capacity cell that reached full charge before the other 7 and without intervention would have gone way over the high limit. This is where a balance board would have saved the day ( maybe ) if you were watching it and saw the lights change. It would have started limiting the amperage at 3.55v, would it have been enough ??, I didn't test it.

An audible alarm can be useful for this situation. I have had an audible alarm wake me up for a Low Voltage event.

Simon

[Willy T]

Did you mean you connected the cells is series rather than parallel? If they were in parallel the should all be at the same voltage.



An audible alarm can be useful for this situation. I have had an audible alarm wake me up for a Low Voltage event.

Simon

Hi Simon, your correct, I change it, thanks for the correction.

[Willy T]

EXCEPT. What charge rate were you hitting the pack with ? 15A? Balance Boards generally only shunt 0.5 - 1 amp when they kick in, so unless you were charging at 2A or less, even with BB, you would have fried a battery. Manual intervention saved the day.

It was about 12 amps at the time, thats why I said maybe. If it started shunting at 3.55 it would have at least slowed it down. That would be a good test for PNJ to do on his bench top, .5 amps won't do much.

[Sunking]

In hind sight it was that lowest capacity cell that reached full charge before the other 7 and without intervention would have gone way over the high limit. This is where a balance board would have saved the day ( maybe ) if you were watching it and saw the lights change. It would have started limiting the amperage at 3.55v, would it have been enough ??, I didn't test it.

So when are you going to learn Top Balance is death?

[Sunking]

It was about 12 amps at the time, thats why I said maybe. If it started shunting at 3.55 it would have at least slowed it down. That would be a good test for PNJ to do on his bench top, .5 amps won't do much.

So if you are running 12 amps on the charger and you Vampire Boards can only shunt .5 amps leave 11.5 amps still flowin in an over charged cell. Slowed it down is one heck of a over statement. At .5 to 1 amp is not slowing anything down. Take your blinders off.

[PNjunction]

PNJunction, I am a little puzzled by this statement. I assume the test you were performing was with a current limited power supply set to a voltage greater than 3.45 volts? If so, could you tell me the supply voltage and current limit.

T1 Terry from Australia tipped me off to this early on. He's not too active here lately.

With my small cells of only 20 and 40ah, I can use smaller things - like 700ma single cell 3.7v wall-wart chargers, and watch them stall. Or I can use my 6ah single cell chargers and watch them finish. Batteryspace is a good source for these.

I do plan to get a Mastech like Willy, and if I did I'd be leaning towards the "EX" series. Maybe later.

For now, I can control EVERY aspect of charging LFP, non-lfp, Pb, nimh, etc with an iCharger 306B, not to mention being able to make internal resistance measurements. While this is primarily directed towards the hobby / RC crowd, it does work and work well for testing my larger stuff. All that configurability however means that one needs to know what they are doing. I prefer it to the Revolectrix Powerlab series because I don't have to do discharge tests to get the IR. You can pump some serious amperage through these things.

Still - in the end I want to run my LFP as simple as possible, so that means I use pack-level equipment rather than hobby chargers for daily use. Of course I verify the iCharger with my Fluke gear to make sure I'm not being misled, and they are doing what they say they are doing.

In the end, we are talking about diffusion - kinda' sorta' similar to Peukert, but not really the same. Thus, when charging at a very low current, you stop at a lower voltage. If you are hammering these things like above .5C, then stop at the higher end for the same desired capacity. Fortunately the sane solar housebank user won't be charging at .5C !

[Sunking]

Fortunately the sane solar housebank user won't be charging at .5C !

Not sure I agree with that. You keep talking Sub-C with respect to solar systems. That is not always the case, depends on where it is used, and how many days of autonomy.

On the charge side, solar systems charge at higher rates than an EV, and FLA solar systems. Two reasons for that:

1. EV batteries are much higher capacity cells and voltage. To buy a charger that can exceed say C/10 gets real expensive and requires a dedicated 240 volt 30-amp or larger circuit to power such a beast.

2. With solar you have a very limited amount of time to recharge, so you do not have the luxury of time to charge slower. If you tried would be in deficit charging thus destroying your batteries, or be in the dark most of the time.

In a properly designed Solar System with a 3-day reserve capacity forces to charge at least @ C/6 rate which is way faster than an EV will charge. C/2 is a very real possibility where Solar Insolation or Sun Hours drop to 3 hours or less. Example Seattle December/January drops to 1.2 Sun Hours. At that low of Solar Insolation you are looking at charging around C/2. Before anyone says "I do not use 3 day, I only use 2-day". That argument does not hold water because with less than 3 day autonomy means you have to charge at even higher rates approaching 1C or higher.

So therefore I do not agree with the Sub C charge rate argument. On the discharge that statement is mostly true, but there are exceptions to that also but rare.

[PNjunction]

I think I was expressing myself poorly. On the whole, I agree with you.

My thought was for those who see the 80% DOD spec, and design around that with no autonomy. Basically a small battery that could be hit with .5C with the usual handful of panels.

Or a large system with 3 days or more of autonomy - to do that with .5C means a pretty large array if they are doing 50% DOD daily.

It is the exceptions to the lead-acid 100-xx% mindset that blew me away originally and how many differing setups one can have.

For instance, there is no penalty for making a big mistake and oversizing your battery needs (other than upfront in your wallet), when you could easily just live in say a 40-60 PSOC window. Heh, if you do that, you may never have to even think about balance at all, aside from an initial sanity check on the first charge just to get relatively close.

Lightbulb time! For those that want to spend gobs on balancers and such, just put that money into a larger battery instead, run psoc, and fuggeddaboutit.

[Willy T]

An audible alarm can be useful for this situation. I have had an audible alarm wake me up for a Low Voltage event.

Simon

My boards do give a alarm at 3.65v and a hvd disconnect after 30 seconds, so I am very happy with them. The only problem is I didn't have it on the cells when I was balancing them, lesson learned. Anyone that has boards knows that it is a whole system, not just a balance board.