Connecting lifepo4 cells to make a battery.

[tasman]

G'day, putting together a lifepo4 battery of 640 ah x 12 volts, using 40ah x 3.2 volt lifepo4 cells, which means 64 cells have to be connected. Using 16 cells in parallel and then the 4 banks in series.

Can I connect each 3.2v 640 ah cell line, to the other lines in series and parallel. By connecting as follows. Four lines of 16 cells

+ - x + - x + - x + -
+ - x + - x + - x + -
+ - x + - x + - x + -
+ - x + - x + - x + -

Taking the power from the first + & last – across the lines

Connect the first + and last – lines on single bars connecting each of the 16 cells and the 3 inside lines, connected together in series down the cell line with bars and hopefully give me 12v x 640ah.

The other way is to connect each line in parallel, then the ends in series, which means lots of separate bars and the associated connecting hassles. The cell lugs are aluminum, so intend using long aluminum bars for connecting and going to rivet the bars to the lugs, to overcome the problems of nuts and bolts connections. Someone told me to use silicone lubricant on the lugs before riveting, so it provides a proper connection. Not really sure about that though and unless someone with experience of lifepo4 recommends it, won't bother.

The BMS I'll be using, is capable of adjusting all parameters of the batteries, including charge and discharge limits.

This is my first go at making a lifepo4 battery, havge lifepo4 starter batteries which are great, so changing our house and the mobile home over the lifepo4 now we have a reliable cheap supply of cells. Any advice, or alternative methods would be appreciated.

[Mike90250]

The cell lugs are aluminum

That's pretty interesting. Aluminum oxidizes instantly on exposure to air, making a very tough film of Aluminum Oxide (used in sandpaper). Aluminum wiring uses a special grease with very sharp conductive particles in it to cut thru the oxide layer and restore connection (remember hearing about house fires caused by aluminum wire?)

If done right, the aluminum has a layer of tin plate over it to solve the oxide problem. I would suggest maybe use aluminum as a test fit for the buss bars, but to actually make them from copper, which has half the resistance of aluminum.

[PNjunction]

Update - removed some irrelevant material .. too hot here to think properly..

G'day, putting together a lifepo4 battery of 640 ah x 12 volts, using 40ah x 3.2 volt lifepo4 cells, which means 64 cells have to be connected. Using 16 cells in parallel and then the 4 banks in series.

That is the right way to do it. Parallel to get your capacity first, and then group those sets in series to get your voltage. I would have reduced the cell count from the start by using larger capacity cells in the first place, the 40ah cells are the smallest of the lifepo4 line with CALB.

The other way is to connect each line in parallel, then the ends in series, which means lots of separate bars and the associated connecting hassles.

And a guaranteed nightmare for balance. Absolute worst way to do it.

Someone told me to use silicone lubricant on the lugs before riveting, so it provides a proper connection. Not really sure about that though and unless someone with experience of lifepo4 recommends it, won't bother.

This comes from the dissimilar metals used for interconnects which can be a combination of aluminum, copper, brass, stainless. After a light sanding with emory cloth, a SMALL amount of No-Alox or Penetrox is useful. Be sure to torque the connections properly - not too loose, and certainly not too tight. A real torque-wrench set to the manufacturer's setting is valuable.

This is my first go at making a lifepo4 battery, havge lifepo4 starter batteries which are great, so changing our house and the mobile home over the lifepo4 now we have a reliable cheap supply of cells. Any advice, or alternative methods would be appreciated.

Make sure the cells are physically tight - that is you don't want the cells to be putting any undue stress or torque on the cell-links and terminals themselves. Unless abused, these batteries do not swell, however, strapping is still a good idea to reduce the mechanical stress.

Can you give more information on your cells? (make / model) Are they new or used (hopefully NOT used).

As per the BMS - which one have you chosen, or been hoodwinked into using? Aside from common battery protection like an HVD or LVD, a bms on a solar house bank is overkill and another point of failure.

[tasman]

Thanks for the replies, I'm new at this lifepo4 stuff. Was going to use aluminum because someone said it was fine to use, but will get copper and use that for connections. So it will work fine with connecting it the way I propose.

I'm changing 1000ah of lead acid over to the 640ah lifepo4, the LA has been in for years and failing. The house is 12v, but runs on 2000w x 24v panels and a 350w wind genny. Going to installed a Gamber BMS, which I've seen in action. It allows you full control over cells for balancing, input and output voltages. The previous owners just disconnected the 240 and connected 12v and two power used to run a 600w inverter, which I've change to a 3000-6000w 12v inverter.

I know someone who has lifepo4 and didn't use a BMS, sadly it cost them heaps of money because they installed them just like you would LA and were advised a BMS was not needed. So they just connected the panels to a sealed charge controller, then left it. Wasn't long before they packed it in and the supplier told them lifepo4 can't be allowed to constantly receive charge when they are full, or what's called float, as they overcharge and collapse. That's why I'll use a BMS so I know what's going on, can control it myself and the reason I'm doing it all. Want to know and understand my system as best I can. I've thrown out their charge controller, because I couldn't work it out and installed 2 x 60ah MPPT controllers.

I've built an aluminum box for the cells and will secure and insulate them with silicone.

The cells are new, got 40ah because of the price, as they were wrongly labelled as 20v 4400milliamps, when they are 3.2v 40ah. At about 1/3 the price, it made sense and some things I've read say smaller cells provide a better solution, because if one cell breaks down, it's not a big deal and can be replaced cheaply, compared to large 100-200ah cells.

Once this bank is up and running, will build another for our converted bus, which runs 380ah of gel currently, to 400ah of lifepo4. Can get 400ah 12v batteries already setup, for less than $1500, they are order over runs. Was lucky to come across a supplier who deals in over runs and badly labelled cells, with a guarantee of quality.

[PNjunction]

I know someone who has lifepo4 and didn't use a BMS, sadly it cost them heaps of money because they installed them just like you would LA and were advised a BMS was not needed. So they just connected the panels to a sealed charge controller, then left it. Wasn't long before they packed it in and the supplier told them lifepo4 can't be allowed to constantly receive charge when they are full, or what's called float, as they overcharge and collapse.

Treating lifepo4 like a drop-in replacement is a recipe for disaster. Too bad the owner did no research on his own, bought that hook line and sinker, and the seller backpedaled. Common story. While there are too many anecdotal variables to consider, it is true you don't float lifepo4. However, if your charge controller has no way of disabling it, then the best thing to do is drop the float voltage to well under 13.8v to make it more or less benign.

I'll bet the guy had his controller set for 14.4 to 14.6v, did absolutely no sanity checks on voltage, and ran a high float voltage. No wonder. If anything, one needs to drop the bulk/absorb voltage for a nominal 12v lifepo4 pack to something reasonable, like 14.0v. (3.5v per cell)

That's why I'll use a BMS so I know what's going on, can control it myself and the reason I'm doing it all. Want to know and understand my system as best I can. I've thrown out their charge controller, because I couldn't work it out and installed 2 x 60ah MPPT controllers.

That will be fun. At first.

At about 1/3 the price, it made sense and some things I've read say smaller cells provide a better solution, because if one cell breaks down, it's not a big deal and can be replaced cheaply, compared to large 100-200ah cells.

The reason they fail is either due to poor initial quality, treating them to drop-in replacement voltages, or a bms failure. No kidding. Ships at sea might consider this.

Was lucky to come across a supplier who deals in over runs and badly labelled cells, with a guarantee of quality.

That is usually an oxymoronic red flag. Tread carefully. Sometimes that can be an indication that lesser-quality unknown cells have to permanently live on artificial bms life support.

Seriously, I hope your project goes well, just don't be tempted all the time for deals too good to be true.

[tasman]

DSCF1814.JPGThe cells have turned out great, no problems at all with them and they came with a 2 year full warranty and 5 years parts. Been using them for well over a month and they function excellently, but do have one problem with charging, one cell line always goes over 3.6 volts during charge and the others are still at 3.3-4v.

Not being the good at understanding electrical maths etc, feel it is the way I have the cells connected. The cell that goes over is the end one which the negative charge connection and it reaches 3.6 quickly, the positive side cell line is also higher than the two in between and the one closest to the negative end is the one that gets the least charge.

I”ve included a photo of the cell pack and would like to know if I can connect a bar across each cell line between the positive and negative ends and connected to each cell line so they get a more even charge. The negative side is on the left of the photo of the pack and those are remote active balancers comected to each cel line

Other than that, can anyone tell me a better way to connect the 4 cells line together or where within the pack to introduce charge and discharge to even things up. You can do something like that with LA, but not sure if it would work with lifepo4 the way I have them connected.

What I'd like to see is a charge controller which feeds each individual cell line with a 3.7v charge at high amps, with cell balancing and cut offs, then most charge problems would be solved. If the same was done with load, it would make life easier for everyone. Sadly I have no idea how to go about designing and building one.

[PNjunction]

That is EXACTLY what is not recommended to begin with for this very reason. But what's done is done I suppose.

How are you measuring the voltages - are you using a shirt-pocket voltmeter, or what you should be using at this stage of the game, like a Fluke? And when reporting voltages, it is best to get more precise, but I understand what you are saying.

Yeah, you are badly out of balance.

Given your unfamiliarity with it all, I feel the SAFEST thing for you to do at this time is to charge each cell individually with something like this, aka a "single cell charger". WATCH YOUR POLARITY when you connect the clips. No adult drinks during this laborious procedure!

http://www.batteryspace.com/Smart-Charger-6.0A-for-3.2V-1cells-LiFePO4-Battery-Pack-100-240VAC-CE.aspx

Instead of bank-charging at the end of the bus bars, I'd move whatever those connectors are to the MIDDLE of the bus bars. But I'd charge them individually first with the small single-cell charger and see how it goes from there.

[tasman]

Thanks, I have active cell balancers on each cell line and it is only when charging that one cell gets badly out of control and if there is no load. When there is load, the cells don't reach capacity and stay pretty evenly balanced.

The BMS gives cell voltages and I use a normal multi, which does differ from the BMS. Fluke are extremely expensive in Aus and been told by a couple of auto electricians who have them, they aren't worth the money. They say testboy and textronix are better and much cheaper, but I don't know and figure between the BMS and multi, it should be right.

[PNjunction]

Thanks, I have active cell balancers on each cell line and it is only when charging that one cell gets badly out of control and if there is no load. When there is load, the cells don't reach capacity and stay pretty evenly balanced.

That is a problem monitoring only each line with so many cells in parallel. You could easily have a dead cell, and one or more of them is compensating with an extremely high SOC to give you a false reading to the bms. You've GOT to monitor each cell during this charge procedure with a battery with so many cells. During the charge, have you actually measured the voltage of each and every cell, and not just the line itself?

If one or more cells is not dead, it may be an issue of a high-resistance contact in your lengthy buss-bar screw-tap setup. Measuring each cell individually may help you narrow it down.

Fluke are extremely expensive in Aus and been told by a couple of auto electricians who have them, they aren't worth the money.

Most auto electricians aren't working with 640ah worth of lifepo4. We are. Look at the Fluke 114. It is basically a matter of trust, which you get out of the box with Fluke. It has enough accuracy, but also as important is the repeatability and SPEED of the measurement, which lifepo4 needs when nearing top of charge. Not only that, they are designed to FAIL GRACEFULLY, instead of in your hands. Seriously, look at the inexpensive model 114. I have an 87V which is overkill for just lifepo4 measurements, and my 115 serves as an excellent tool for my lifepo4 jobs. Hopefully, this won't turn into a multimeter tangent.

Again, just be careful. What I detect is the desire to cut corners anywhere possible, and if taken too far or listening to those who tell you what you want to hear without any actual hands-on experience, leads to poor results long term.

[tasman]

Thanks, I understand what you are saying and do check each cell in the lines to see where they are at. So far haven't found any that are out at all, it's only when they are charging that one cell line seems to over charge and the entire cell line goes over. When there is no charge, they settle down quite well.

Not trying to cut corners, I live on a small pension, it took me well over 2 years to save for my cells and over 1 year to get my panels. If I had the money, would have the best of everything and would love a fluke, as well as other diagnostic gear.

I agree regarding auto electricians, they don't have a clue about lifepo4, however asked our local bloke to see what he is using and about fluke.

I listen to everyone and then do my own research, do understand the problems which may arise from such long cell lines, but so far haven't found the problem and looking for a way to charge each individual cell line, if possible.

[yewsuck]

I thought this was a good read for someone using paralleled banks like you are. It might explain some of your balance issues as well or at least give you a idea what could happen. Here is the link reference link.

http://www.smartgauge.co.uk/batt_con.html

-YS

[tasman]

Thanks, I'm aware of that link and looked at it a year or so ago. My lifepo4 pack is a series/parallel connection, not just parallel and 56 x 40ah cells, in 4 x 14 lines. Which is the problem I face and not knowing much about the science of battery connecting, especially when using a pack of 3.2v- to make 12v nominal, it has confused me a bit.

Am going to try taking charge from neg from one end of the pack and positive from the other. Hopefully this will even up both charge and discharge, as in the 2nd connection in the link you provided, but a series connection.

No one has yet given me an easy way to connect all my cells together and get a better result, all have either said nothing or said there is nothing wrong with connecting them in that way. Had thought it may be a crook cell or bad cell connection, yet every time I test each individual cell in the lines, they are all fine and equally representing the total cell line voltage.

In my simple understanding, the answer is a solar charge controller that switches off charge/load at user set, pack voltages and charges each cell line with 3.7v until it hits the user set, cell voltage. In my case, 3.6v.

There are those chargers around, at huge prices, but they have a float stage and from what I've learnt, floating lifepo4 is a recipe for disaster. However that has come from one source only, who claims to know all there is about lifepo4 and everyone else is wrong or knows nothing. I don't have the knowledge or experince to say one way or the other, so am ooen to understanding regarding float and stage charging, compared to bulk charging to switch off with lifepo4.

Wouldn't slowing of charging allow cells to balance themselves nearing the upper voltage limit.

[Sunking]

Well for LFP 12 volts is what is called 4S, and at 100% SOC is 3.65 volts and a BMS must be used if you intend to take to 100% SOC which gets real expensive real fast. But 100% SOC is really only a goal of EV's to get every mile they can out of a pack. For solar there is no reason to go to 100%, 90% is more than good enough and does not take an expensive BMS, just some simple monitoring to make sure no cell goes to 100% or goes below 25%. So all you need to do is charge using Constant Voltage of 3.6 volts per cell (14.4 volts @ 4S), and never allow any cell to go below around 2.25 vpc or 10 volts @ 4S.

[PNjunction]

Your biggest problem right now is overall balance of the bank. Lets tackle that first.

If it is only ONE cell, and not one group of cells going over 3.6v, you can manually discharge that cell easily enough with a typical 12v incandescent RV bulb. Not critical that the bulb is rated for 12v when you hook it across a single cell since we're only using it as a dummy load to burn off excess charge. Or in the case of one of the entire 3.2v sets going high, then use an automotive headlamp across it. Discharge it a bit until the voltage drops near the others, but don't try for perfection the first time out. It will take a few iterations of manual discharge, followed by a normal small discharge of the entire battery, and then watch again as they charge up and see if you have made any progress. Repeat if necessary.

I sure hope you are limiting your charge voltage to no more than about 14.0v at this stage of the game.

Because lifepo4 has a very flat charge discharge curve, except near the ends, means that once you put the battery to work, just because they all seem to read 3.2v does NOT mean they are balanced. They could actually vary from 10% SOC to 80% SOC individually. So SOC voltages during use are pretty much useless as far as determining balance goes except in extreme cases. That why a balance is only done at the top or bottom, and not in the middle so to speak.

No, you should not float lifepo4. There is no need to as once it is fully charged, there will be no more current flowing. However, if you keep a constant charge voltage on it, say at 13.8v or above, then you merely start to heat the electrolyte. If you can disable float, then do so, but if you have no way to disable it, consider ...

If your system falls back to a float of about 13.2 to 13.6v or so, then that would be somewhat benign after a full charge. There would be no float current flowing since the battery is already sitting well above these levels, and all it will serve is to catch a small parasitic load from draining your pack during long periods of storage. At 13.2 to 13.6v, that's not enough voltage even with no current flowing to degrade the electrolyte. Above 13.8v is another story.

Right now, you need to manually balance your battery bank with some sort of dummy load, either across rogue individual cells, or the entire large "cell" that is made up of many paralleled ones.

The quick skinny on a solar charge controller:
Disable temperature compensation
Set voltage to no more than 14.0v especially if you do not have individual cell bleed stuff.
Turn off float, or not worry about it if it only is set for 13.2 to 13.6v or so. At that voltage, it is benign.

Seriously consider taking whatever voltmeter you are using now to someone who has some better gear to at least be able to determine if you need to manually compensate. Using a non-rechargeable 3.3v button cell is ideal for this test among differing voltmeters or just take one of yours and compare.