LiFeP04 Batteries for Solar & BMS

[karrak]

Is this you Sunkink?

Typo that I can't seem to edit, I meant Is this you Sunking

http://www.diyelectriccar.com/forums/showpost.php?p=629377&postcount=1

Anyone know for sure if Tesla BMS is Passive or Active.

I would think Active Mid Balance shuffling power from higher energy state Bricks to Lower state Bricks. At least I think that is what I have read before. I just don't see Tesla claiming to have the "Most Advance BMS in the World." Bleeding power off as lost waste heat is not advanced IMO. But then again Musk is full of himself.

Anyone know or have links?

[Sunking]

I mean a solar user having lithium batteries normally sizes their batteries for 2 or 3 days rather than 5 days like lead acid. It will be frequent you are in the lower 1/3 of the operating range of the batteries with a cloudy day or two. And if you are not there you need some fail safe protection from over discharge. All I am saying is you do not need a BMS to protect yourself form over charge and over discharge. Fine if you want to use one.

I use a BMS but nothing like you are thinking. I have no vampire boards. My BMS is Battery Monitor System, not Battery Management System. I mimic Middle Balance by using a Bottom Balance reference. I don't even need the monitor system. Like I said Willy I use to be in the Top Balance park. I got my Orion Jr long before I built the EV. By the time I built it I flipped to Bottom Balance and worked with a couple of guru's to reprogram my Orion to use as Middle Balance. I reall do not need it as the charger can handle the charge part, and motor controler can handle the LVD. The Orion is mostly a Gas Gauge, Motor RPM, Speedometer, and system monitor. It interfaces with the motor controller. The majority of Bottom Balance EV guys do not use anything. All they do is periodic voltage checks.

When it gets right down to it. Lithium is not ready for Solar. Too expensive right now and using Chi-Com LFP cells is a huge gamble no one if they will hold up. So far their track record is HORRIBLE. To get the good stuff like A123, Enerdel, Panasonic is 4 to 6 times more than Chi-Com. Chi-Com is 2 to 3 times higher than top end Pb which is proven 5 year service. Nlo Chi-Com has made it that long yet. CALB has been around the longest since 2007 and on their 3rd generation released last year.

[Willy T]

I do not know what Utopia you live in, but the rest of us live in places with cloudy days and our batteries spend most of their life around 50%. Then we have to take vacation once in a while and cannot be there every day to take corrective action. I guess you guys down under never leave home or have a cloudy day?

You have a Golf Cart charged on grid, how does this fit you or is this some general statement meant to be about solar users ??

[Sunking]

Why would bottom balance be any more valid than top balance for any other reason than to get a more accurate representation of 0%SOC (largely irrelevant in an off-grid system)

BS. A solar system you do not design for 5 day reserve capacity, more like 2 to 3 days. I do not know what Utopia you live in, but the rest of us live in places with cloudy days and our batteries spend most of their life around 50%. Then we have to take vacation once in a while and cannot be there every day to take corrective action. I guess you guys down under never leave home or have a cloudy day?

Very strange you and Karrak came here on to take part in this thread. You both came at the same time and only one or two threads. Very Interesting.

Here is the bottom line. I don't care how you manage your batteries. Karrak already admitted he does nothing and has no BMS so i have no idea what he is squaking about. If you want to use Top Balance have it. I prefer to get the most out of my investment. What you do with your investment is your problem. But no commercial EV uses Top Balance.

Neither of you can explain why you can buy 12, 24, and 48 volt LFP batteries right now today and no need for any BMS. If drift was a real problem they could not give you 2 to 5 year warranties. I know why you 2 do not want to acknowledge that. Right here is 13 of them sized from Motorcycle to RV House battery. NO BMS of any kind or Balance Boards. It is a FACT. They are Drop In replacements. Feed then no more than 14.2 volts and do not let them get below 10 volts.

So stop the nonsense, you do not need BMS.

[Sunking]

Oops, I have made a mistake, using the terminology that SK uses, Tesla use 'Passive Middle Balance'. All this means is that they keep the charging voltage of all the cells in the battery within a narrow range of each other for the whole charging cycle. Fully charging a lithium battery using this technique is equivalent to top balancing a battery.

You are nucking Futs Simon. No EV manufacture goes anywhere near the Top or Bottom. They could not offer 8 and 10 year warranties doing that. Top Balance is for amateurs and novices. Chevy volt operates 80/20, Tesla and Nissan 90/10. Tesla's controller never turns off, which is why it cannot set for long periods without being plugged in. It use 1 Kwh per day doing nothing but monitoring batteries and shuffling power, heat, and cooling around Brick to Brick.

[Sunking]

BMS is NOT expensive, my system cost less than $400AUD excluding the contactors (which are required regardless of top or bottom balancing).

LOL. You do not call $400 plus support hardware expensive. Good luck convincing anyone.

Tom it is real easy to know if you have drift, and even easier to fix if it should ever occur. Once you get down to 10% SOC all th evoltages are equal. If not you have had drift. I have never seen any drift in 6 months of operation. I also know of about 136 EV's made by Jack Rickard, all Bottom Balanced and in operation from recent to 4 years ago and no drift.

If they do, we all have a very simple RC Hobby chargers to fix any battery problems. Most use a Powerlab 8. They will charge/discharge, balance, measure any characteristic you can name of battery of today and tomorrow. Rated at 1200 watts max. Just drive until you get near the bottom, then clip it on a cell, set to discharge to whatever voltage you want at any current you want within its limit. On a single cell can go to 30 amps charge or discharge. If you want to Top Balance it will do that with computer accuracy up to 8 cells.

[Sunking]

All this means is that they keep the charging voltage of all the cells in the battery within a narrow range of each other for the whole charging cycle. Fully charging a lithium battery using this technique is equivalent to top balancing a battery.

There is a good description of the different balancing techniques here.

http://liionbms.com/php/wp_passive_active_balancing.php

Simon

I know the author quite well. His name is Davide Andrea. He owns Elithion who makes BMS, assemble LFP batteries, solar systems, and makes custom EV's. His company is in Boulder Colorado and hangs out on a couple of EV forums. I have done contract work for him, and was offered a position as Executive Engineer at his company 2 yyears ago before I moved to Panama. Known him for 7 years now and learned a lot from like BOTTOM BALANCE. He even has two Integrated Circuits he designed and made for TI to use in his BMS systems.

[karrak]

Now maybe I am missing something and have made a horrible mistake as I have only skimmed the thread and only got to page 38 but it looks to me like top balancing using resistors to bleed off power.
Simon

Oops, I have made a mistake, using the terminology that SK uses, Tesla use 'Passive Middle Balance'. All this means is that they keep the charging voltage of all the cells in the battery within a narrow range of each other for the whole charging cycle. Fully charging a lithium battery using this technique is equivalent to top balancing a battery.

There is a good description of the different balancing techniques here.

http://liionbms.com/php/wp_passive_active_balancing.php

Simon

[tom rickard]

BMS is NOT expensive, my system cost less than $400AUD excluding the contactors (which are required regardless of top or bottom balancing). The main reason i am posting in this thread is to make sure people are aware of that!

When you have run your bottom balanced system long enough for drift to occur (which will happen), three things will happen:
1: you will risk the highest voltage cell going into the knee when you are at CV charging at 55V
2: you will have lost useable capacity of your batteries
3: you will no longer have any idea (without returning to 48V) if your pack is still bottom balanced!

Kind of defeats the purpose of balancing in the first place doesn't it..

If you balance your cells each cycle at their working voltage (eg 3.4V), you have none of these disadvantages, and no other disadvantage that i can think of especially if you never plan to run your pack close to 0%SOC.

Basically Sunking it comes to this:

1: you insist a BMS that is programmable to balance at working voltage is expensive, i have the receipt to prove to anyone that is interested that it cost me less than $400AUD
2: you insist that your cells will never drift in voltage. My experience shows this is not true (i can only speak for my system).

If your cells never drift in voltage, then just balance them at any voltage within either knee one time and be done with it. Why would bottom balance be any more valid than top balance for any other reason than to get a more accurate representation of 0%SOC (largely irrelevant in an off-grid system)

[Mike90250]

...Thanks for giving me the neon rope to hang myself, Dereck!

That's an LED neon rope

[karrak]

Yawn!

Do yourself a favor and read. You are really getting quite boring. If you read through no one can find any power resistors to do bleeding. Post 82

Keep reading you haven't gone far enough

Then in post 135 we have:

The Tesla pack as I understand it ....

As I understand it are the operative words

So quit your sqauking. You lost the debate.

How about post 245 http://www.teslamotorsclub.com/showt...l=1#post754273 with detailed picture of the bleed resistors and the following text

Great pictures!

Below are some enlargements of one of the repeating pattern of 6 bleed circuits, one for each group of paralleled cells; one enlargement for each side of the board. Don't get fooled by the bottom set; all the components are there, just rearranged a little for board layout.

Simon

[Living Large]

Yawn!

Do yourself a favor and read. You are really getting quite boring. If you read through no one can find any power resistors to do bleeding. Post 82

So quit your sqauking. You lost the debate.

God bless you for having the strength to go browse a 51 page thread. Thank you for putting your reply here, and not sending me elsewhere to try to figure out what you're talking about.

I'm here in this thread for solar power in a house, and how LFP and BMS may be used with it. I believe that is the purpose of the thread. Your scheme of what you call "middle balancing from the bottom" + monitoriing + LVD and HVD makes perfect sense to me, though as you have said and recognized there are "many ways to skin a cat." Maybe you've brainwashed me, but I am an EE and I don't think so.

I am today putting an offer on the off grid house that brought me to this site in the first place - so I may go to the dark side with solar and LFP after all. Thanks for giving me the neon rope to hang myself, Dereck!

[karrak]

Pretty sure we have had an aussie company build us a custom BMS I will learn more about it in the next week or so, cheers

I wonder if this is the engineer you are using? He is from Australia.

http://www.teslamotorsclub.com/showthread.php/34934-Pics-Info-Inside-the-battery-pack?p=789299&viewfull=1#post789299

Obsevations from an Engineer

I've just completed a BMS for a commercial product and thought I would offer some observations.

Balancing.
Shunt balancing is the same method I'm using. Once a pack has been top balanced there is usually very little else to do. Total pack capacity is limited by the lowest capacity of a paralleled cell group.
There are ways to shuttle power around using switched capacitor, and or inductive (dc-dc) methods, but these are costly and add a whole lot of complexity to working out your failure mode analysis.
In fact, once initially balanced, future balancing has nothing to do with different cell capacities, but different internal cell resistances and leakage rates.

Reliability.
There would have been some noisy debates in the engineering team about this one. The tradeoffs of using higher capacity prismatic cells vs low capacity cylindrical cells is a hard problem.
eg. If one cell out of 7000 fails open circuit (or blows the protection fuse with an internal short circuit), the capacity of the entire pack is reduced by the number of seriesed cells. So for 16S system if one 5Wh cell fails, it reduces the pack by 16 x, so 80Wh.
Worst case is if one cell became leaky. This prematurely discharges a group (requiring constant re balancing) but with no hope of isolation the faulty cell.

There was a suggestion of having an ASIC per cell. This could disconnect the faulty cell, but on a 7000 cell pack this would add a lot more failure modes and significant cost that would be worse than the occasional cell failing.
The ASIC idea might have legs for a prismatic high cell capacity system, but not for cylindrical cells.

Effort is better spent on making high quality cells, rather than trying to work around them the complex electronics.

Future
If I had a crystal ball of what the future would be. I think the anode/cathode materials will improve to reduce impedance (so less heat in high charge/discharge), increase cycle & calendar life, and improve combustion safety to a point where where larger prismatic cells will be more viable. Having a prismatic with 100 times the capacity of a cylidrical makes it more economic to manufacture, maintain, and with more opportunity for electronics to intervene.

Wearing my systems engineering hat, the current Tesla solution 'feels right' given the current state of tech.
I think the genius of Musk is having both a good understanding of engineering AND the ability organize capital & labour. It is rare to find both skills in one person.

Simon

[Sunking]

Wrong, wrong, wrong, wrong wrong. Do us and yourself a favour and read the thread http://www.teslamotorsclub.com/showt...e-battery-pack at least up to page 40 and see if you change your mind.

Yawn!

Do yourself a favor and read. You are really getting quite boring. If you read through no one can find any power resistors to do bleeding. Post 82

Aside from the passive switched-resistor method, active circuits can use switched capacitors, switched inductors, etc. to "shuttle" charge between the different series-connected cell groups as needed. Or even use small DC-DC converters to shuttle charge back/forth with the auxiliary battery. Many many ways to get the job done.

I'm not at all surprised that there are no power-dissipating resistors to be found -- wasting energy isn't really Tesla's style

Then in post 135 we have:

The Tesla pack as I understand it has a BMS to do centralized monitoring, but the actually balancing is done brick only brick to brick by using a rail capacitor to shuttle charge. The max potential the circuit between bricks "sees" is only one brick to the next brick in series. By shuttling you can move charge to any brick in the pack eventually. This variable impedance balancer can act totally independent from the BMS and is a very simple circuit. It doesn't need a sleep or off mode because if it is not shuttling it is using almost no power

So quit your sqauking. You lost the debate.

[karrak]

Which is pretty much what I told you. What that author misstated is the power is not bled-off which was just a poor choice of words, it is transferred to weaker bricks. Balancing is either Active aka Distributive taking energy from higher cells or blocks to weaker cells or blocks. The opposite of that is Passive aka Dissipative (vampire or bleeder boards) that burn off power in stronger cells as waste heat. But take note they DO NOT MONITOR CELLS only groups of series/parallel cells called Bricks. They could not Top Balance if they tried, it would be a nightmare and never work with over 6800 cells. The dam thing would glow in the dark burning all that power off.

Wrong, wrong, wrong, wrong wrong. Do us and yourself a favour and read the thread http://www.teslamotorsclub.com/showt...e-battery-pack at least up to page 40 and see if you change your mind.

EV's don't waste energy, that is the job of novice DIY Solar users. Commercial EV's move energy around so it can be used to get that extra mile if needed. That is the heart and sole of Middle Balance systems. .

The burning off of energy will only occur when you are charging so isn't going to make any difference to the range. You could move energy around from one set of cells to another while charging but I would think that Tesla has decided not to bother due to the extra complexity and cost for the very good reason that balanced cells do not drift very far so the amount of energy needed to be moved around is negligible and not worth worrying about. This agrees with my experience that with a period of 11 months between balanaces that the amount of power 'wasted' to do the rebalance was less than 10Wh. Hardly enough to get excited about.

Simon