# BMS Solution for 600a discharge?



## RyanM923 (Mar 15, 2016)

Just as the title suggested, I'm looking for a BMS solution for a battery pack that will see 600amp continuous (30sec-1min) and 1000amp burst (3-5sec) discharge rates...can't seem to find anything that will cover that kind of power draw.


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## Duncan (Dec 8, 2008)

I think you are getting your wires crossed,
Battery Management Systems operate using battery voltage - the current drawn is irrelevant


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## z_power (Dec 17, 2011)

At these current levels no BMS alone will protect your battery; it must cooperate with charger and controller, "telling" them informations about battery status. It's up to these components to adjust charging/discharging actions properly. 
It's not like in small e-bike packs where BMS alone can break the cicuit if limits are exceeded.


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## Duncan (Dec 8, 2008)

No not true
All your BMS would need to do is to cut the throttle 
As I said above the current is irrelevant


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## RyanM923 (Mar 15, 2016)

I'm confused then, how come all the BMS I look at refer to max charge and discharge rates, with the price going up as power requirements do?


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## Moltenmetal (Mar 20, 2014)

No idea what you're talking about. BMS are just voltage alarm trips on each cell which are used to trigger secondary relays to interrupt charge at high voltage on any cell and alarm or cut back throttle if any cell drops below the low voltage setting. Some have a balance resistor which is put in parallel with the cell at a voltage below the high voltage cutout, to help top balance the pack. That feature isn't absolutely necessary and has some downsides.


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## RyanM923 (Mar 15, 2016)

This is what I'm referring to: http://www.electriccarpartscompany....ilt-Lithium-LiFePO4-LFP-br-PCB-PCM_p_199.html


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## RyanM923 (Mar 15, 2016)

Every BMS I have searched for has an amperage rating on it.


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## RyanM923 (Mar 15, 2016)

But if I can do it for a reasonable price for a 4s2p pack and now have to worry about current then I need to do some more reading I guess.


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## Sunking (Aug 10, 2009)

Huh. What does discharge current have to do with a BMS? 

Answer, Nothing.


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## Sunking (Aug 10, 2009)

RyanM923 said:


> Every BMS I have searched for has an amperage rating on it.


That is Bypass Current while charging. The Balance Boards can bypass a small amount of current at the end of the charge cycle. Typically less than 1 amp. Most are around 300 milli-amps. 

The BMS has nothing to do with Current on the discharge cycle, and not much to do with the charge current except at the end, and then only a very small amount of current.

Now some BMS systems have a high current monitor capability, and some even have Coulomb Counting capability. But no current goes through the BMS to do that. You use a Shunt to derive a 50 milli-volt signal to the BMS, then the BMS converts that voltage signal to a current reading.


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## RyanM923 (Mar 15, 2016)

Makes sense. I'm doing a simple 4s3p pack with some 20ah cells and I'm wanting to keep max charging voltage at 14.2V, and do some balancing on the top end as well to keep cell voltages the same and prolong the life of the pack. Low voltage protections isn't much of an issue for me, don't see a reason to protect it there.

I'm looking for the most cost effective way to do this and just can't seem to find the right info on how to do it the way I want and with the cell layout that I have.


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## Sunking (Aug 10, 2009)

RyanM923 said:


> Makes sense. I'm doing a simple 4s3p pack with some 20ah cells and I'm wanting to keep max charging voltage at 14.2V, and do some balancing on the top end as well to keep cell voltages the same and prolong the life of the pack. Low voltage protections isn't much of an issue for me, don't see a reason to protect it there.


You have all your priorities mixed up. Low voltage protection is the most important thing.

With a 4S system (12-volt) you have absolutely no need for a BMS. Bottom Balance the pack to 2.5 volts per cell. Then limit charge voltage to 14.2 volts (3.55 vpc) which is roughly 90% SOC. A BMS requires you to charge to 14.4 (3.6 vpc) volts before it even turns on. At 14.2 it would never be used or turned on. 

The Bottom Balance will eliminate any possibility of an Over Discharge. As insurance set your low voltage disconnect to 12 volts (3.0 vpc) . 

Doing that you will never over charge or over discharge any cell.

Having said that, if you do use a BMS, you had damn well better monitor cell voltages and use the BMS to disconnect the pack if any cell falls to 2.5 volts.


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## RyanM923 (Mar 15, 2016)

The reason why I wanted to control charging but not discharge voltage is because this is going to be used in a car with an alternator charging it. If the person using it does not have a voltage regulator then overcharging is definitely an issue, and with the battery always being charged while in use, hitting 10v on the low end will likely never happen.

And isn't overcharging just more dangerous as well?


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## Sunking (Aug 10, 2009)

RyanM923 said:


> And isn't overcharging just more dangerous as well?


No. Who told you that? The exact opposite is true. Now if you are talking about Lithium Cobalt, then yes over charge is critical. 

LFP batteries are fairly tolerant to over charging unlike the rest of the Lithium family. They can withstand up to 4.2 volts or 16.8 volts. Limit the charger output to 14.2 volts, and you have no use for a BMS. On th eotherhand LFP like all Lithium battery types are extremely sensitive to over discharge. 

Keep in mind a BMS only bypasses a very small amount of current. For a pack your size (AH capacity) 100 milli-amp is the right amount. Now here is the catch When your first battery hits 3.6 volts and triggers bypass, unless your charger can limit charge current to 100 ma, and say the charge is stil pumping 10 amps. You bypass .1 amp and still have 9.9 amps flowing n the fully charger battery.

With Lithium batteries you must have a very stable and accurate voltage regulator on the charger. Makes no difference who is at the wheel.


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## RyanM923 (Mar 15, 2016)

Charging will be done via alternator, likely in the 100-300 amp range. Aftermarket high output alternators usually have an external voltage regulator, so over charging isn't an issue. But if a stock alternator is used, and especially in winter time when the voltage will be higher than normal, thays more so what I am worried about.


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## Sunking (Aug 10, 2009)

RyanM923 said:


> Charging will be done via alternator, likely in the 100-300 amp range. .


Say goodbye to your batteries.


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## RyanM923 (Mar 15, 2016)

They're LiNMC chemistry GS Yuasa cells (rebadged Blue Energy cells I believe) if that makes any difference. I've been putting them through some pretty tough load tests, recharging them at a 40 amp rate and just keeping an eye on voltage, then topping off with a 4 amp charge to 14.4V for the 4s2p pack I have.


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## RyanM923 (Mar 15, 2016)

And I'm not trying to be difficult or argumentative, just trying to find out what I need to know since I've been getting confused trying to figure all of this out from previous threads and such. 

What do you guys recommend (looking for specific models) for a BMS that can cut off charging and discharging at say 14.4V and 10V, and balance cells as well? I know you said discharge rate won't affect the BMS but what about higher charging rates?

I really do appreciate the help.


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## Sunking (Aug 10, 2009)

RyanM923 said:


> And I'm not trying to be difficult or argumentative, just trying to find out what I need to know since I've been getting confused trying to figure all of this out from previous threads and such.


Ryan we know you are not trying to be argumentative. The issue is you do not understand what you are being told and how batteries charge and discharge. For lack of a better word, Ignorance is what is going to get you into trouble.

Batteries are very simple to grasp, but you have to thoroughly understand basic DC and AC circuit principles and theory. Most notable Ohm's Law. I can show you Ohm's Law but until you have worked several hundred calculations, you are not going to grasp the relationship of Power, Voltage, Current, and Resistance. Most notable is *Voltage = Current x Resistance*

Right now you still are not clear what a BMS is or does. Don't feel bad as not many here really do. Not a single person can define what a BMS is or does. A BMS can be as simple as a Plain Jane volt meter. I can tell you what they are not. They are not battery chargers. 

Additionally a BMS cannot Balance your batteries. At least not the way you think they can. All they can do is maintain an already Balanced pack. The initial or bulk balanced is not done with a BMS, it is done by you and some special equipment. Whether you Top or Bottom Balance, you start either method the same exact way. You connect all the cells in Parallel, and walk away. After several hours all cell will be at the exact same voltage. The voltage will not be 100% or 0% Somewhere in the middle. Where you do not know. At that point you choose Top or Bottom Balance. To Bottom Balance, you connec ta load and discharge the pack until you get a resting voltage of of 0% SOC. On LFP that is 2.5 volts. Other lithium type will be higher. If you Top Balance you need a heavy duty 3.6 volt charger. You charge the cells in parallel until they saturate at 3.6 volts. 

Once you have them Balanced either Top or Bottom, you are now ready to install the cells in series. Once the Cells are Balanced, they do not go out of Balance unless you have parasitic loads on the cells. What are parasitic loads? Your BMS. The BMS of whatever form you use connect to each individual cell with a dedicated circuit to monitor the cell. Each of those dedicated circuits do not draw the same amount of current. Thus over time the cells become unbalanced. 

So how does a BMS make small corrections. Well there are a few ways. But for simplicity only one way you can afford to implement. I call it Vampire Balance aka Bleeder or Shunt or Bypass Balance. Regardless of what you call it, you have a circuit across each cell. When the voltage reaches roughly 3.6 volts for LFP, the circuit turns on and puts a Resistor across the cell and bleeds it. The current is small ranging from a few milli-amps to an amp or two. 

But there is something that has to happen between the BMS and Charger. The BMS has to signal the charger when the first Vampire circuit turns on, and when the last Vampire Board Turns on. When the first Board Turns on, the charrger has to limit current to what the Vampire board can shunt. Example if that current is .1 amps, the charger needs to reduce current to .1 amp. If it does not and is pumping 10 amps at the time, when that first cell turns on, you still have 9.9 amps flowing into a fully charged cell. 

When the last cell board turns on, the BMS sends a signal to the Charger to turn off. 

So here is the first point you need to get and why a BMS cannot Balance a out of Balance pack. If the highest to lowest cell are 10 AH apart, how long does it take to Balance at .1 amp? Real simple answer. 10 AH / 0.1 amps = 100 Hours or just over 4 days. Thus a BMS cannot Balance an out of Balance pack. It can only keep a balanced pack balanced. 

How about doubling you battery cycle life? Know how to do that? Real simple, do not fully charge your cells. Charge them to no more than 90%. If you do that, then your BMS never turns on.

Ever wonder how commercial EV manufactures can off long warranties on their batteries? You should because the batteries they use are not known for many cycles. About 500 to 1000 cycles is all they can get with the types they use. How do they do it? They do not allow you the customer to fully charge the batteries. 

OK to charge a lithium battery or any battery for that matter there are two things you must be able to do. Control Voltage and Current. With Lithium battery types you have to be able to control Voltage precisely, less than 1% tolerance. On a 12 volt system 1/8 of a volt or .125 volts. A vehicle alternator voltage regulator cannot do that. There is one Marine Manufacture I know of that makes voltage regulators for boat alternators. I know of none for automobiles. To control the current is pretty simple on alternator. You just select an alternator where the maximum current is equal or less than the battery maximum charge rate. So if you have a 300-amp alternator, and your batteries can only handle 50 amps, you are in trouble. You can work around it with the voltage regulator by lowering the voltage. But nothing exist for an alternator that I know of.


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## Wiilrc (Jun 11, 2016)

I was looking to buy a salvaged tesla battery but evidently they dont come with a BMS. Anyone know where I could get one? or what a good brand is?


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## akseminole (Jan 5, 2014)

Wiilrc 
Check out http://www.zeva.com.au
The prices are in Australian dollars, which is lower than American dollars at the moment.


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