Solar charge controller, what for?

[J.P.M.]

I won't comment until I see the stuff, but what I want you to say when you get the package and rip it open with beating heart is to:

SLOW DOWN

You've got 1000ah of battery there. Here is a tip: if for some reason you end up having to debate yourself on a wire you can't see, like "did I put the positive lead on?" and end up spending ANY time trying to remember, STOP! Do not try to figure out the answer, and just physically check. As you run around, if you ask yourself this same question again - stop again, even if you feel foolish. Ie, it is not the question itself, but the fact you are spending ANY time debating yourself is the danger.

Recognizing that the start of an internal debate as a safety trigger can take some practice for guys like us who like to figure things out. Here - don't figure it out. Just do the check, even if you have to do it multiple times for the very same thing.

Just sayin' ...

FWIW: BIG ++1 on that one.

[SunEagle]

I won't comment until I see the stuff, but what I want you to say when you get the package and rip it open with beating heart is to:

SLOW DOWN

You've got 1000ah of battery there. Here is a tip: if for some reason you end up having to debate yourself on a wire you can't see, like "did I put the positive lead on?" and end up spending ANY time trying to remember, STOP! Do not try to figure out the answer, and just physically check. As you run around, if you ask yourself this same question again - stop again, even if you feel foolish. Ie, it is not the question itself, but the fact you are spending ANY time debating yourself is the danger.

Recognizing that the start of an internal debate as a safety trigger can take some practice for guys like us who like to figure things out. Here - don't figure it out. Just do the check, even if you have to do it multiple times for the very same thing.

Just sayin' ...

Measure Twice and cut once is a good safety protocol in carpentry.

For electricity. The circuit isn't dead until it is grounded. (keeps you from getting shocked or worse)

And never try to remember how a circuit it is wired. Make a document and refer to it before you energize it.

[PNjunction]

I've opted for the quick delivery as I'll go nuts waiting any longer.

I won't comment until I see the stuff, but what I want you to say when you get the package and rip it open with beating heart is to:

SLOW DOWN

You've got 1000ah of battery there. Here is a tip: if for some reason you end up having to debate yourself on a wire you can't see, like "did I put the positive lead on?" and end up spending ANY time trying to remember, STOP! Do not try to figure out the answer, and just physically check. As you run around, if you ask yourself this same question again - stop again, even if you feel foolish. Ie, it is not the question itself, but the fact you are spending ANY time debating yourself is the danger.

Recognizing that the start of an internal debate as a safety trigger can take some practice for guys like us who like to figure things out. Here - don't figure it out. Just do the check, even if you have to do it multiple times for the very same thing.

Just sayin' ...

[bungawalbyn]

We have voltage cut off.

I finally received enough of the bits in the mail to get stuck into the control board, unfortunately not yet all I'm waiting on, so It's no finished set up for you guys, or me.

I have all the heavy cable work done, save the main fuse, but I'm waiting for bus bars to finish all the thin wire work for the controls. While waiting I'm making a poly carbonate cover for the batteries and covers for the circuit boards.

I finally hooked up the HiLo voltage cut off sensor and programmed it. Works a treat. Run the power supply up to cut off 13.8V, main relay cuts current, run the power supply back to 13.3v relay cuts back in. I still have to do the HWS relay that I'll set at Hi 13.7V and Lo 13.4V. I'll start with these parameters and adjust as/if necessary.

A slight change of plans. Turns out after giving it more thought, the idea of individual volt meters for each cell turns out to be not terribly practical. A rats nest of wires and can't power them from cut off side of main contactor. Fortunately whilst browsing Ebay for bits I came across a RC cell sensor.

http://www.ebay.com.au/itm/321502999136?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

$8.50 in Aus, a couple of days delivery, or about $2.40 from Hong Kong with 3 to 4 weeks postage

I've opted for the quick delivery as I'll go nuts waiting any longer.

From what I've seen of how it's hooked up I can power it from the main circuit. ie will cut out if main contactor is tripped.


I'll post some pics when it's finished.

[karrak]

Hi bungawalbyn,

Looks like you have given this quite a bit of thought and done some research on this, will be interested to see how things turn out.

Simon

[bungawalbyn]

Hi Simon,

I take your point, only I am not relying solely on one of these. I have a BMS for Hi Lo emergency cut offs should everything else fail for both pack and cell voltages. The module shown is switching charge on and off. I'll have another one in the system set at lesser values to switch on and off my HWS as a load dump/diversion when approaching charge cut out. Plus I'm wiring in cell and pack voltmeters and low cell voltage alarms. I have manual cut offs on both PV in and Battery in.
Everything will be cut off if low cell voltage is reached. All supply to meters etc will come from circuit after the main low voltage magnetic latching contactor (Doesn't need current to hold open)

If a cell goes far enough out of balance to cause a problem without me catching it going there on the meters, then it's probably going to be a cell fault. Any drift should be evident well before.

Until I'm comfortable with it's reliability, I will switch it all off when I go out, and even when comfortable I will probably switch it all off if I'm going away over night or longer.

Hey, life's a risk and the high end craps out too, which can be worse if you believe it won't.

[karrak]

http://www.ebay.com.au/itm/331509855...%3AMEBIDX%3AIT

That looks like a good start, just a word of warning, remember that your expensive battery relies on this part. A large part of engineering design looks at what sort of things can fail or go wrong with components of/in the design and what impact this will have. If the failure of a part can cause a safety issue or cause expensive damage, steps have to be taken to stop these faults or make sure they can't cause a problem.

When you buy cheap parts from an anonymous manufacturer you do not know how well designed they are, if they uses components that will do the job, or if the components are fakes. Now most of the time these parts work OK although I would always derate their specifications, so if an inverter says it will deliver 150W, assume it will only really safely deliver 100W. If I use parts like this, I assume they will fail and ask myself what will happen if they do fail, actually I also do this with components from well known companies who have a reputation to upheld as well.

In the case of this part, if it fails and does not disconnect the solar panels from the battery when it is full, it will very quickly, and I mean very quickly drive the battery voltage up to nearly the open circuit voltage of the panels of around 20 volts which could cause irreversible damage to the battery. If it fails and does not connect the panels to the battery, the battery could be damaged by being over discharged.

Whenever you are designing or putting together anything always ask yourself what will happen if any part of the system should fail and what the consequences of this failure will be.

While on this subject, do you have any plans to provide an alarm if any individual cell voltages go out of the safe operating range?

Simon

[bungawalbyn]

[QUOTE=karrak;164450]

I am interested to know what controller you are using for this.

http://www.ebay.com.au/itm/331509855668?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

There are guys selling load dump/diverters for wind/hydro in the States who use these.

- It needs more than 4 volts to operate so you will need to connect more than one cell to each device.

-

The little volt meters I'm using for individual cell monitoring are 3 wire, + - 12V with sensor wire

My system is a "Work in Progress" and I would be pounced on if I posted pictures of it. I might post pictures of a system I have installed for a friend when we reinstall it in its new and permanent home in the next few months. Here is a description of my system from a previous post http://www.solarpaneltalk.com/showth...l=1#post154470, One thing not mentioned in that post is that the batteries are from Winston.

I wouldn't worry about that. My set up will be something out of Professor Ratbagy's Manufactory

[SunEagle]

That's a great idea. I run the great risk of turning this into an endless multimeter thread, but I think the simplest tool to get *that you can trust out of the box* is of course a Fluke. It doesn't have to be expensive. I run a recent model "low end" Fluke 114 since there is no need to use my more expensive 87v for my lifepo4 monitoring. The measurements are the same. There are many multimeter alternatives, but only the Fluke comes with a level of TRUST for an out of box experience without intensive calibration.

Man, I hope this reply wasn't a mistake ...

I agree with PNjunction. A reliable multimeter is an important tool to have.

While I have professionally used a Fluke meter for many years I have found that the Extech line is as reliable and accurate as the Fluke at a lower cost.

I personally have a multimeter and clamp on ammeter made by Extech and feel very safe using them on energized equipment. Although if I can afford it my first choice would still be a Fluke.

[PNjunction]

Regarding calibration, you can check how accurate your multimeter is with a low cost voltage standard from this crowd http://www.voltagestandard.com/, for checking LFP batteries I think accuracy down to 10mV is probably enough.

That's a great idea. I run the great risk of turning this into an endless multimeter thread, but I think the simplest tool to get *that you can trust out of the box* is of course a Fluke. It doesn't have to be expensive. I run a recent model "low end" Fluke 114 since there is no need to use my more expensive 87v for my lifepo4 monitoring. The measurements are the same. There are many multimeter alternatives, but only the Fluke comes with a level of TRUST for an out of box experience without intensive calibration.

Man, I hope this reply wasn't a mistake ...

[PNjunction]

13.8 to 14V seemed to be the target high point at the start of this thread but now it looks like maybe 13.6 is the go.

I don't normally run 13.6v, but no more than 14v. The only time I run up to 13.6v rested is when I'm doing a discharge capacity test to vet the cell performance down to 12.8v rested. It must meet 80% of rated capacity for me. Normally I'm conservative like others.

I've noticed a lot of individual cell voltage anxiety. I intend to alleviate this by putting volt meters on each cell. This way I'll have a fairly accurate(ebay digitals) at a glance of how each cell is performing, backed up of course by a couple of layers of safeguard. I guess this option is not very practical for banks with lots of small cells.

Precisely the reason we use large prismatics to keep the cell count and balance complexity down! No one-off enthusiast buckets full non-lifepo4 cells pulled from crashed cars or laptops here.

The importance was made of using a voltmeter you TRUST to prove the adequacy of other components, such as those ebay cheapies and calibrate or make chart-conversions for. It really does pay to get a good voltmeter, and one you can trust. Did I mention trust?

Also be careful of stuff hanging off your batteries, which can be potential points of failure or UNbalancing devices if they go wrong. Many guys use JUNSI cell monitors, but to do that right, you run that from it's OWN SEPARATE battery. At 1000ah, even if the cell monitor starts to fail, you may not notice it, but why not do it right from the outset and run it from it's own battery / supply? And oh yeah, the JUNSI or devices like them benefit from being vetted by .... wait for it ... a voltmeter you can trust!

[karrak]

We seem to be getting more conservative.
13.8 to 14V seemed to be the target high point at the start of this thread but now it looks like maybe 13.6 is the go. I guess it all gets a bit rubbery with variables such as at rest/ load, temps, charge voltage, current, constant values from mains charging and variable values from solar. And of course there is wether they are left standing at voltage or are in constant use.

You are right, as with most things the devil is in the detail. Winston still specify 4 volts as the maximum and if my memory serves me correctly the electrolyte will stand around 4.3 volts until the rate of the breakdown chemical reactions will damage or destroy the cell. These cells are primarily designed for high rate charge/discharge usage. Under this regime there lifespan is only around a couple of thousand cycles or 5-7 years. In our application with low rate charge/discharge usage (in my case max charge rate 0.1C and max discharge rate 0.5C) we should get more cycles out of them and we want them to last as long as possible. My understanding is that when charging, higher charge rates and higher voltages will decrease the lifespan of the cells, so my goal is try to keep the voltages down but still charge the battery to as full as possible to get a good economic return. I am not sure if it will make much difference to the lifespan if one limits the voltage to 3.4 volts compared to 3.6 volts or if one limits the SOC to say 90%, but as you can get an LFP battery to ~95% full with only 3.4 volts I don't see any reason to push things.

Fortunately, my cut off controller is programmable, so I guess the thing to do is start conservative and adjust if necessary. I sized the bank so that I can do this. I'm not too concerned with the low end, under normal usage I don't expect to ever go there.

I am interested to know what controller you are using for this.

I agree that the low end is not such an issue, lowest voltage that my battery has been down to since last November is 24.49 volts (3.06 volts/cell) under load. Most months it wouldn't go below 25 volts (3.125 volts/cell) under load. Again it is an issue of economic return as to where you set your end point. I will turn all loads off when any cell gets below 2.8 volts to stop damage to the cell, this has only happened a couple of times when the battery was new due to the battery not being balanced properly by the supplier.

I guess one can get a good idea about where the knees start on a particular battery by charging into them a bit, and keeping a close eye on how the voltage is going. Like when I was bringing down the balance charge. It was easy to see when the Batteries hit the "Flat"

If you act conservatively and stay within the 80%-20% SOC range as PNjunction suggests you probably don't need to do any close monitoring although I think you should still check to see that the batteries are staying in balance over the longer term. If you want to go outside this range for economic reasons you do have to keep an eye on things. I would always advocate some sort of monitoring and alarm system under any conditions just in case of a fault or stuffup.

I've noticed a lot of individual cell voltage anxiety. I intend to alleviate this by putting volt meters on each cell. This way I'll have a fairly accurate(ebay digitals) at a glance of how each cell is performing, backed up of course by a couple of layers of safeguard. I guess this option is not very practical for banks with lots of small cells.
Also I have one of these from EBAY

RC Lipo Battery Low Voltage Alarm 1S-8S Buzzer Indicator Checker Tester

Something like this should be OK, a couple of things to look for with these sort of cheap devices.
- It needs more than 4 volts to operate so you will need to connect more than one cell to each device.
- Check that the current draw from each cell is the same otherwise in will unbalance your battery
- Being able to calibrate it is a nice feature.

I went for the Cellog 8, as it can be calibrated, the more expensive one will store data that can be downloaded. For use with more than six cells it has a problem which is detailed here http://endless-sphere.com/forums/vie...2&hilit=cellog. With four cells it should be OK.

Regarding calibration, you can check how accurate your multimeter is with a low cost voltage standard from this crowd http://www.voltagestandard.com/, for checking LFP batteries I think accuracy down to 10mV is probably enough.

Simon, is there a link to a description etc of your set up? I'd be interested to see it.

My system is a "Work in Progress" and I would be pounced on if I posted pictures of it. I might post pictures of a system I have installed for a friend when we reinstall it in its new and permanent home in the next few months. Here is a description of my system from a previous post http://www.solarpaneltalk.com/showth...l=1#post154470, One thing not mentioned in that post is that the batteries are from Winston.

Simon

[bungawalbyn]

Thanks for the info guys,

We seem to be getting more conservative.
13.8 to 14V seemed to be the target high point at the start of this thread but now it looks like maybe 13.6 is the go. I guess it all gets a bit rubbery with variables such as at rest/ load, temps, charge voltage, current, constant values from mains charging and variable values from solar. And of course there is wether they are left standing at voltage or are in constant use.

Fortunately, my cut off controller is programmable, so I guess the thing to do is start conservative and adjust if necessary. I sized the bank so that I can do this. I'm not too concerned with the low end, under normal usage I don't expect to ever go there.

I guess one can get a good idea about where the knees start on a particular battery by charging into them a bit, and keeping a close eye on how the voltage is going. Like when I was bringing down the balance charge. It was easy to see when the Batteries hit the "Flat"

I've noticed a lot of individual cell voltage anxiety. I intend to alleviate this by putting volt meters on each cell. This way I'll have a fairly accurate(ebay digitals) at a glance of how each cell is performing, backed up of course by a couple of layers of safeguard. I guess this option is not very practical for banks with lots of small cells.
Also I have one of these from EBAY

RC Lipo Battery Low Voltage Alarm 1S-8S Buzzer Indicator Checker Tester

P309_3_ALL_zpsba55611f.jpg

AU$2.26 free postage. These are programmable for voltage

Simon, is there a link to a description etc of your set up? I'd be interested to see it.

I'm off to town today, hopefully the bits I need to finish off the board will be waiting at the post office and I can actually get to play with my batteries.

[PNjunction]

I was wondering where the nominal cell value of 3.2V comes from?

I'll have to ask Prof. John B Goodenough at the Univ of Texas for that. Tip of the hat to him, as he was the pioneering father of the cells we use, (in addition to core memory and more!). Good history and corporate skullduggery afoot brought us what we have today. Otherwise, it would be all proprietary and locked up.. save that for another thread...

Would I be correct in thinking that it's 50% SOC? and that the Knees at around 20% and 80% start at about 0.4V either side of that?

Close, but temperature and the actual materials used can dictate small changes from brand to brand. And we know that voltage is only an approximation. But we CAN get close with RESTED values, and most importantly, an accurate voltmeter! You spent big bucks on your battery, and if we're going to play the rested voltage to soc game, you've got to use a meter you can TRUST or has been calibrated or all bets are off. With lithium, you really should have a STANDARD to calibrate all your other monitoring / metering gear to. Multimeter threads are elsewhere. Lets just say that even though my battery is small, I don't go cheap on my instruments, but only my Fluke is the standard for all else in the system.

Here is a sampling showing the slight variances according to temperature:

http://www.bestgopower.com/technology/documents/temperature-test.html

I don't use their products, but wanted to point out how not *exactly* accurate it can be.

That being said, here are some common values that EV guys use for rested voltages at normal room temperatures for lifepo4 per-cell, and 4S configuration and I use with my large prismatics.

3.4 / 13.6v = 100 % <--- don't sit here in storage too long - oxidation
3.33 / 13.32 = 90%
3.25 / 13.00 = 50%
3.2 / 12.80v = 80% <----- good conservative value to not go below at rest! EV'ers might, but we won't with properly sized prismatics.
3.0 / 12.00v = 90-100% <-- why rest here? If found ASAP, apply a 0.05C or less charge current to *nudge* them gently back to 3.2v, then apply full current charge.

but I have observed some bounce back in voltage after a load is removed (0.01V/cell),

Yes, that is normal and the reason we need to rest them. Preferably for 12 hours, although in a pinch a few hours will do.

WARNING about "pack voltages": for an extreme mental exercise, lets say I ship you one of my fully charged 40ah GBS cells to replace one of your 1000ah cells and take SOC voltage readings. It is obvious that as we discharge the pack, the pack voltage is no indication of the TRUE soc since my weak little runt cell is the limiting figure and would be destroyed in short order if we relied upon pack voltage alone. Fortunately, at "sub-c" rates, and IF your cells are closely balanced in both capacity and internal resistance, AND we are conservative, simple diy measures from those who are willing to put in the time to monitor closely may be successful. Like WB9K points out, this is not realistic for anything but us battery geeks.

Note that if you want to get closer in accuracy, you'll want to do a real measured discharge capacity test. My GBS cells more or less followed these voltages after testing. Yours may differ slightly.

Here's a tip - are you able to obtain 80% of the fully charged rated capacity of the cells during discharge when you reach about 12.7v under load? If so, that's good enough. No need to go into the basement. Allowing for rest, this will rise a bit above to 12.8 or slightly higher *under rest* at a measly .1c rate ....

[jkirkebo]

I've been putting some thought into where to set my main charge on/off relay.
From looking at graphs etc and briefly observing that LFP seems to hold voltage, rather than like FLA that drops to 12.6V or 2.1V cell at rest, I was wondering where the nominal cell value of 3.2V comes from? Would I be correct in thinking that it's 50% SOC? and that the Knees at around 20% and 80% start at about 0.4V either side of that?
I'm talking here about at rest. Having not done any charging as yet and only having used them a couple of times, I don't have any experience to talk from, but I have observed some bounce back in voltage after a load is removed (0.01V/cell), though minor compared to FLA. I assume it works the other way with charging. I suppose one should be able to work it out from the internal resistance? Are there any formulae out there for such a thing?

Any thoughts?

My understanding of nominal voltage is that it's the energy the cell can hold divided by the number of amp-hours it can deliver. So if a cell can output 37Wh until empty and 10Ah until empty the nominal voltage is calculated as 37/10=3.7V.