# What type of BMS



## DBCox (Jun 27, 2012)

Hi everyone,

I am very new (like just started looking into Lithium Iron batteries 2 hours ago) to lithium batteries. I am considering using them for a DIY ATV instead of gel cell batteries.

Finding the cells does not seem to be a problem, nor does it seem to be too difficult to wire them to the voltage I need (~48 VDC).

Battery managment is confusing me though. I am not used to BMS since I am a gel cell user.

Do I need to have an on-board BMS, a charger that will charge each cell individually, or both???? Battery life is critical as I want to make sure my investment will pay off in the long run.

At this point, I am looking for a typical continuous draw of 30-40 amps, but peak current draw could be in the 400 amp range.

Thanks!

David Cox


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## frodus (Apr 12, 2008)

what batteries?


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## Elithion (Oct 6, 2009)

> Do I need to have an on-board BMS, 

yes.

> ... a charger that will charge each cell individually, 

no

> or both?

Just the BMS and a standard bulk charger 
(http://liionbms.com/php/charger_options.php).


Mostly, you need to make two choices:

1) Do you want a BMS that protects the battery, plus it can tell you which cells has an issue, what issue, and how big? Or is it OK to have a BMS that just protects the battery? In the first case you want a digital BMS, in the second case you want an analog BMS.

2) Do you prefer one electronic assembly with lots of wires to the cells? Or lots of electronics assemblies mounted on the cells, with just a couple of cables to a master? In the first case you want a non-distributed BMS, in the second case a distributed BMS.

Then go here:
http://liionbms.com/php/bms-selector.php
and enter your 2 choices from above, plus your number of cells in series, and the utility will spit out which BMS are available, and how much it would cost for your system. You can also answer more questions to refine your search: type of cell, where the BMS is made...

I hope that helps.


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## onegreenev (May 18, 2012)

> Do I need to have an on-board BMS


NO. With LiFePO4 Cells you don't need a BMS to safely use them or monitor them. Monitoring with a small pack like that can be as easy as just checking the cells with your multimeter. Bottom balance your cells and don't over charge them and you can forgo the confusing tangled mess of BMS systems that prevail. Everyone will tell you that you need one but guess what, You Don't NEED one. You may choose to use one but you don't NEED one. There are plenty of users of these types of cells that DON'T use BMS's at all and I am one of many. 

Contact off list if you need more information. It really is not difficult to do this safely with no BMS. 

Now if you decide on cells like Li Cobalt then you really should have a BMS as they are a different beast. I don't disregard BMS systems but for the LiFePO4 cell you don't need one. It is good to monitor your cells if you must but not let your BMS control your batteries. I trust my charger more than any BMS for these cells. I trust my controller more than any BMS for these cells. I trust my own checking from time to time and would rather use the extra money for extra parts or batteries. 

Pete


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## DBCox (Jun 27, 2012)

Thanks for the help guys!

To answer the first question, I am considering using 16 CALB 40AH cells in place of 4 MK MU-1 SLD G batteries. Basically, I will get 25-30% more range with the LiFePo4 with half the weight.
However, I have some concerns after reading all of your responses.

First, I will have to have some sort of BMS, whether it be on the charger or on board. Most of you indicate on board is better, and I would agree from a constant monitoring standpoint. I will not have the time or motivation to regularly check the batteries with a multimeter, so I need some sort of automation. That is a lot of cost and complexity. This setup will be used daily, so I do not have time to chase minor issues to keep it operating. I can just see one cell going bad and delaying the whole day to fix it. Also, this setup will be used by people with NO technical training/skill. If it doesn’t work, they will complain or start replacing major components before finding the real problem.

Also, from a cost standpoint, it does not seem feasible. Typically, the gel cells will last 1.5-2 years. A set of gel cells costs $240, so we are looking at $120-$160/year. The LiFePo4 cells will cost $928 + BMS. If I neglect the cost of the BMS by assuming I will never have to repair or replace it, the cells will need to last 6-8 years to break even from a cost standpoint. The improved performance and lower weight could possibly justify a slightly higher costs, but not much.

I think I need to wait a little longer until the price and reliability improves. Please correct me if my assumptions and numbers are incorrect.

-David


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## frodus (Apr 12, 2008)

Yeah you could go without, but if you're not going to maintain your cells and watch them from time to time as onegreenev (Gottdi) says, you'll have issues with balance over time. If you do have time to bottom (or top) ballance the cells and watch them, you may be ok. But Bare minimum IMHO, you need to monitor and provide an output from your monitor to shut down the charger or controller if it goes too high or too low, respectively. 

If you want everything done automatically, then use a small BMS. BMS for 16 cells is fairly easy to find and are usually fairly reasonable.

Some will say you don't, but they're actually performing the work of the BMS. Some will say you do because they're thinking you won't maintain batteries or be able to watch for LVC or HVC. Both are right that batteries need to stay balanced (whether top or bottom). In each case, the batteries are being watched and maintained by someone (without bms) or some thing (with BMS).

I personally use a BMS, but there are a lot of options out there to do everything from just monitoring, to a full balancing system. Even something like minibms can be a resonably priced solution and would be a good choice.

Also, I don't know where you're getting batteries, but $928 seems high ($57.75 per cell, or 1.45/Ah). Does that include shipping?


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## Yabert (Feb 7, 2010)

DBCox said:


> The improved performance and lower weight could possibly justify a slightly higher costs, but not much.


A swap from 32Ah lead to 40Ah lithium will be a huge improvement in performance considering the power capability and the weight of the calb cells. And you can expect almost double your range because the 32Ah lead are just rated 15-20Ah at high rated discharge (1C to 5C for example).

I used the mini bms centralized in my car and it work well.
Simple and easy to assemble.
http://minibms.mybigcommerce.com/products/MiniBMS-16-cell-Master-Board.html

Hey, don't hesitate to share picture of your DIY ATV.


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## dougingraham (Jul 26, 2011)

DBCox said:


> Battery managment is confusing me though. I am not used to BMS since I am a gel cell user.
> 
> Do I need to have an on-board BMS, a charger that will charge each cell individually, or both???? Battery life is critical as I want to make sure my investment will pay off in the long run.
> 
> ...


You are in luck since LiFePo4 need about the same level of BMS as Gel cells. Actually I consider them less fragile than Gell cells since leaving them discharged for several months doesn't hurt them much at all. And even a few days discharged with a Gel cell is death. They also have essentially zero self discharge which means that once you balance them they stay balanced.

If you are planning on running the batteries dead on a regular basis then bottom balance the cells and set your charger to cut off at whatever pack voltage shows when the first cell reaches 3.6 volts. That is pretty much all you need for a manual BMS. You can check this occasionally but you will eventually get tired of doing so when nothing ever changes.

If know you will never run the pack to dead then you can top balance and set the charger to stop when the charge reaches 3.6 volts times the number of cells. If you ever do run the pack dead you will most likely ruin a cell in this situation. With bottom balanced packs the vehicle will just stop moving and once you charge it up everything is back to normal.

For the amount of money you would spend on a BMS you can buy quite a few extra cells.

Anyone who makes or sells a BMS will tell you that you need one. People who have bought them and use them will tell you that you should have one. This is just human nature. But it also biases their opinion.


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## dougingraham (Jul 26, 2011)

DBCox said:


> Thanks for the help guys!
> 
> To answer the first question, I am considering using 16 CALB 40AH cells in place of 4 MK MU-1 SLD G batteries. Basically, I will get 25-30% more range with the LiFePo4 with half the weight.
> However, I have some concerns after reading all of your responses.
> ...


I think you will see a real range increase of twice what you are seeing with the gel cells. If your gel cells are lasting two years you are not running them down much below half and charging them right away.

All your manual battery management time is done once up front and then you might want to check it once per year. If you treat these the same way as the gel cells you are seeing 2 years of life from you can probably expect 12+ years before they are as bad as the gel cells would be when they are brand new. Unless you abuse them they should last as long as the vehicle.

One of the online resellers has the 40AH Sinopoly or GBS cells listed for $52 each giving a total of $832. I am guessing you can find them for less than that.


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## PStechPaul (May 1, 2012)

I have thought about making a very simple BMS that would just be a tiny device with two or 3 LEDs across each cell, where the LEDs would tell the status of the cell. I would, ideally, use a PIC, but here is an idea using three LEDs, two transistors, and 6 resistors which has one LED change brightness with the voltage of the cell, and the other two would switch (red to green for example) when the cell voltage crossed the threshold of 2.5 volts. I'm just guessing at the threshold for LiPo cells, and that can be changed. It may also be temperature dependent or vary with LED types. But a PIC would be much more accurate, and could flash the LEDs to save power. My simple circuit draws 2.5 mA at 2.5 volts, so that's probably a tiny load on even a 25 A-H cell. Good for 10,000 hours.










If this would be useful maybe I could work on it. It would cost less than a dollar to make. Biggest cost will be packaging and reliable connection to the cells. I really think the manufacturer should put such a device in their cells so you can just look into a little window, like the "green eye" batteries.


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## dougingraham (Jul 26, 2011)

Everyone thinks about doing their BMS this way at first. The big problem is that you are powering the PIC and LED's from the cell it is monitoring and every cell has a little different load on it. This will imbalance the pack.


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## PStechPaul (May 1, 2012)

dougingraham said:


> Everyone thinks about doing their BMS this way at first. The big problem is that you are powering the PIC and LED's from the cell it is monitoring and every cell has a little different load on it. This will imbalance the pack.


I admit that I do not have any direct experience with this, but I am used to dealing with precision instrumentation and measurement technology, and one of the main principles is the effect of the measurement on the system being measured. While in theory a current draw of 5 mA will affect the voltage of a cell, and there might be tiny differences in the current draw based on tolerances of components, there is no way that it will cause any perceptible imbalance on the cells of the pack. 

In fact, if the battery monitor circuit draws more current from a cell with a higher level of charge, it would tend to balance the cells. I don't know the circuitry of a commercial BMS, but I would venture to guess that its current draw would be on the same order as the circuit I showed. And since 2.5mA is 0.01% of the one hour discharge rate of a small 25 A-H cell, it is probably well below the internal self-discharge current. In any case, it would take 10,000 hours, or over one year, to deplete the cell if it were just stored and unused and not on a float charge. 

Moreover, the circuit I would use for a commercial version (or even for my own purposes) would use a microcontroller such as the PIC10F320, which has an operating current of just 25uA at 1.8V, and can also operate in a standby mode with just 500nA for the watchdog timer which can awaken it every 10 seconds or so to perform its measurement and display functions, such as flashing a status LED, and then resume sleep. If the status LED flashes for 0.25 seconds every 10 seconds, and uses 10 mA, the average current is 0.25 mA, which would deplete the cell after about 11 years of unused storage, at which time it's probably dead anyway. 

I don't want to start an argument, but I'd just like to hear an explanation as to why the ideas I presented would cause problems. Thanks!


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## dougingraham (Jul 26, 2011)

PStechPaul said:


> And since 2.5mA is 0.01% of the one hour discharge rate of a small 25 A-H cell, it is probably well below the internal self-discharge current. In any case, it would take 10,000 hours, or over one year, to deplete the cell if it were just stored and unused and not on a float charge.


The problem isn't so much the 25 microamps the PIC draws. And it isn't so much the parasitic draw bringing down the batterys after a few months. The problem is the variation in the LED's and the resistor dividers. If the board on one cell draws 3ma and the one on the next cell over draws 2ma that is a difference in state of charge of 0.72 AH in a one month period. This may be a severe example but if it was 1/10th of that it would be an amp hour per year of drift. Since I expect these batteries will last more than 10 years and they dont seem to go out of balance on their own without some outside agent causing the imbalance you have to ask how much imbalance is allowable? Well if you are bottom balancing in order to avoid bricking cells on an overdischarge event a 1% imbalance might be too much. In a top balanced situation a 1% variation is certainly too much.

I haven't worked the numbers but I am guessing that you could get away with 0.1% resisters in the dividers. Since the LED's will only be lit when there is some sort of event so at that point I expect you could rebalance the cell that has had the LED turned on. And this could be on for days since you are not bringing back the error to a central display point and you will get tired of popping the hood or trunk to look at the LED's after a few weeks of seeing nothing.

My own view is that it it were free I would have a millivolt accurate reading on every cell and a temp sensor that could do a 0.1 degree reading on that cell with no load on the individual cells. This would be displayed on LCD screen on the dashboard. It would be cool stuff right? Actually no. Engineers and scientists are the only ones who want to see this stuff. And I know that I would watch it for a few weeks and then wonder why I thought it was a good idea to spend all that time and effort on something that is useless to the normal operation of a car. Something that inherently imbalances the pack over time, maybe not enough to cause a problem in days or months but in years. Differences in microamps matter over a 10 year period.


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## PStechPaul (May 1, 2012)

Thanks for the detailed explanation. I found more information on BMS ICs such as the http://cds.linear.com/docs/Datasheet/68021fa.pdf, and I see that it has provisions for discharging individual cells to balance the charge, and a means of daisy-chaining to achieve communication with a controller for display purposes. It's a bit costly, at about $10/1 and $5/100, but since it can monitor 12 cells at a time the per-cell cost is minimal. 

I agree that this is a better solution than the individual simple cell monitors, although I think it would still be useful and I doubt there is any way that it could affect the balance of charge as you claim. But since the cells are generally concealed in the vehicle, such a system would not be useful, although it would be highly desirable for cells that are in storage outside the car. It would also be a great help for batteries that are used only for backup purposes or for occasional starting of an ICE. And it may be ideal for my own intended use for 12V lead-acid batteries used in electric tractors, where there may be only 3 or 4 batteries and they are generally exposed for inspection.


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## dougingraham (Jul 26, 2011)

PStechPaul said:


> Thanks for the detailed explanation. I found more information on BMS ICs such as the http://cds.linear.com/docs/Datasheet/68021fa.pdf, and I see that it has provisions for discharging individual cells to balance the charge, and a means of daisy-chaining to achieve communication with a controller for display purposes. It's a bit costly, at about $10/1 and $5/100, but since it can monitor 12 cells at a time the per-cell cost is minimal.
> 
> I agree that this is a better solution than the individual simple cell monitors, although I think it would still be useful and I doubt there is any way that it could affect the balance of charge as you claim. But since the cells are generally concealed in the vehicle, such a system would not be useful, although it would be highly desirable for cells that are in storage outside the car. It would also be a great help for batteries that are used only for backup purposes or for occasional starting of an ICE. And it may be ideal for my own intended use for 12V lead-acid batteries used in electric tractors, where there may be only 3 or 4 batteries and they are generally exposed for inspection.


The only reason for the discharger on the cells is to fix the inbalance caused by the device itself (at least in the case of the LiFePo4 cells).

There are documented cases of LiFePo4 cells in storage sealed in their original boxes for over 4 years that when opened are still all at the original 3.30 volts that the ones put in service 4 years before were. There is no self discharge of these cells. Now they might have lost some capacity just sitting there in boxes but the state of charge didn't change. I dont know of anyone who performed a capacity test and then put the cells back to 3.30 volts and stored for many years and then tested capacity again. I've only done 6 months and could not see any degradation. If there was any it was below my measurement threshold. I also didn't see any degradation in a cell stored fully charged for 6 months. I did see degradation of about 1/2% of a cell stored for 6 months at a resting voltage of 2.7 volts. So don't store them discharged for any length of time.

Since LA batteries have high self discharge characteristics there could be some merit to a monitoring system. I am not certain it would help the life of the batteries but it would let you know when one went bad. And since they seem to last only a couple of years if you take really good care of them this seems like it would get used.

Note: My comments apply only to LiFePo4 type cells. This includes ones with a pinch of Manganese and those with a pinch of Yttrium in the mix. Other varieties of lithium cells almost certainly behave differently.


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