# The Future of BMS?



## ruckus (Apr 15, 2009)

Hello,
I know the pros and cons of BMS are hotly debated. Please do not do that here. 

I am starting this thread in hopes of avoiding WHETHER there should be a BMS and instead wish to discuss WHAT TYPE of BMS would work best, what are necessary features, and what are optional features. (I am assuming Lifepo4 as the current standard).

First, a little background to give a common starting point:

If EV's are to be mainstream, they need ABSOLUTE reliability. I am talking 10-20 years and 200,000-300,000 miles as is common with internal combustion engines.

My grandmother is NOT going to check cell voltages and "bring up" low cells. This is a ridiculous concept. Rickard (EVTV.ME) has proven the need for some type of BMS. After a mere 3,400 miles his pack already varied by ~.6 volts. Way too much to meet the long-term reliability function described above. This is worse than points ignition and carburetors. Unacceptable. 

Here are the 3 things I think are absolutely necessary in a BMS:

1. High voltage cutoff (HVC). Right now, shunts seems to be the most popular and effective way to do this. They pass the charging current past a "full" cell to those still in need of charging.

2. Low voltage cutoff (LVC). Could be an alarm or total system shutdown.

3. Capacity balancing. (bleeding energy from the strong cells to the weak). A pack is limited by it's weakest cell. Capacity balancing seems like the only way to utilize the extra energy from the stronger cells and not be limited by the weakest cell. This could be a resistor system or a capacitor system not unlike the failed evassemble junk. This seems especially important as packs age and the capacity difference becomes greater. 

I am building an EV using the EVpower bms. It only has #1 and #2. Is there a simple/inexpensive/compatible way to achieve #3? It seems like the key to getting maximum range AND max life from the pack. Any input/experiences on the evpower bms appreciated.

Cheers!


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## Anaerin (Feb 4, 2009)

I don't think "Capacity balancing" is really going to be a problem. With all cells connected, the parallel strings (if left alone) will average themselves out, with weaker (lower voltage) strings being brought up and stronger (higher voltage) strings being depleted as the pack attempts to reach equilibrium. In thoery, you could connect every cell in parallel and let the entire pack balance itself out, but that would require a lot of switching, and definitely can't be done while the pack is in use.

As I talked about in another thread, I think each cell should be individually managed, with a smart controller. Then each cell can be charged to "Fully charged voltage" (Call it CVC) and switched out, easing the load on the charger and speeding up the charging rate. It would also top-balance all the cells (At CVC, rather than at HVC level, which would be above CVC). And it would be able to proactively switch out cells when they reach their LVC level. Along with thermal monitoring of the cells, to enable heating when necessary, and many other potential monitoring points, this would give the best protection for all cells, and the longest life.


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## Jimdear2 (Oct 12, 2008)

ruckus said:


> Hello,
> I know the pros and cons of BMS are hotly debated. Please do not do that here.
> 
> I am starting this thread in hopes of avoiding WHETHER there should be a BMS and instead wish to discuss WHAT TYPE of BMS would work best, what are necessary features, and what are optional features. (I am assuming Lifepo4 as the current standard).
> ...


ruckus,

I agree in principle, if not in process. I think the best way to accomplish #3 is to combine it with #1.

If each cell has its own dedicated charging system and all the chargeing systems are set to a common HVC then each charge will not only charge the battery to the selected percentage but balance the battery as well.

We are in process of building individul charging boards for our 3P 50S Headway 38120S battery. Each 3 cell parallel buddy stack will have a dedicated charger attached. The heart of the charging boards is an isolated DC to DC converter board that is capible of very accurate voltage control, In our crude system we will have to manually monitor the output voltage. A properly designed commecial system all the manual monitoring would be automated.

The process of charging works as follows. 
A 48 volt buss supplies the charging boards. 
Each board will charge it's 3 cell stack (this could be a single prismatic cell as well) until the preset HVC point is reached. Once the voltage tops out the board just sits there witha trickle charge to the battery 
During charging the 48 volt circuit is monitored for current. 
When all cells reach the common preset voltage and current flow in the 48 volt buss drops to a maintainence point, the battery can be considered charged and balanced.

There are a couple of very nice positives to this system as well. The input voltage of all of the DC to DC boards I investigated was quite broad. The boards I chose have a 35 to 75 volt input tolerance. Another nice feature is the potential for fast charging. The boards I chose will output our chosen voltage (3.55 volts) at up to 25 amps if supplied with 48 volts at 1.8 amps so potentially I could supply each of 50 3 cell stacks at 3.55 volts and 25 amps with a 48 volt 100 amp supply. Reasonably easy to do with a decient charger and and PBA battery pack. That means I could recharge our 170 volt 24 amp hour battery from a 50% SOC in about 30 minutes.

I know some sloppy math in there but you get the idea.

Our pack is small because it is going into a small vehicle. I'm sure that a good EE could scale this up to any size.


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## cyclops2 (Feb 12, 2011)

I am new to your forum.

All the ideal charging features have been in use for many years in the electric powered airplanes. 
Our LIFEPO4 chargers do every possible check before starting a charge. They check for out of average cells, balance, ramp up to a fast charge switchover to ramp down at 90%, then do a final peak charge to cutoff. Full shut off.

The motor controllers have a management system that is able to reduce power if 1 cell drops to much compared to a preset level.

The BMS chips are out there. Just have to web search several hours or be very lucky to find them quickly. Most are already in production by the Chinese electronic houses.


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## karlos (Jun 30, 2008)

You guys are making a lot of sense; I've long thought individual cell chargers are the way to go and it's good to hear they are out there. Now to find a source....


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## ruckus (Apr 15, 2009)

Thanks for the replies.

I love those blue Headway cells compared to the plastic-yellow of Thundersky. 

I will lay out why I think "capacity balancing" is necessary (I made that term up). In any given group of cells, the capacity is never equal. Over time the difference can become greater. There are other causes of inequality such as replacement cells.

Top balancing is good because it fills the tank brim full, so to speak. But at the end of day, your range is still determined by the weakest cell. You might have 10% of your pack left, but one weak cell will trigger the low voltage alarm/shutoff. 

To actually get the potential range out of the pack, the weaker cells need help from the strong cells. Otherwise, the extra capacity of the strong cells will never be used.

I realize batteries in parallel naturally do this, but mine are in series.

If the cell equalizing function was strong enough, it could replace the shunts normally used in top balancing.

It should be mentioned that neither top or bottom balancing allow the full capacity of the battery pack to be used. Only some kind of energy transfer will allow this.

Suggestions?


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## Anaerin (Feb 4, 2009)

ruckus said:


> Top balancing is good because it fills the tank brim full, so to speak. But at the end of day, your range is still determined by the weakest cell. You might have 10% of your pack left, but one weak cell will trigger the low voltage alarm/shutoff.


Which is something else with my proposed BMS. An LVC won't disable the whole system, just drop the failing cell out.


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## cyclops2 (Feb 12, 2011)

Below are the pages of a VERY accurate & safe LIFEPO4 charger. Reason I am posting it is so everyone can read all the different checks and safety steps it AUTOMATICLY does. 

The BMS chip...charging in this case....is old. But proven very reliable. Updates are available. I have never needed one. The unit is a brick. Have connected it to the power source in reversed polarity more than once.

This is for electric powered RC planes.


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## cyclops2 (Feb 12, 2011)

Below are the pages of a RC plane speed controller. Small. Yes. ONLY 6 amps.
The selectable features are very good for $15 . Load failure to start immediately, features are very good. Overheat protection is excellent. 
The battery chips & processors are out there.
The " GUARD SERIES " does have Low Voltage Sensing of the battery & EACH CELL. 1 cell goes low. Reduced power to limp home.
This controller ranges up to 6 LIFEPO4 cells or 18 NICAD / NIMH cells.

The DIY electronic guys could daisey chain the chips to any # of cells.


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## EVfun (Mar 14, 2010)

It should also be mentioned that you don't want to use the full capacity of the pack. Leaving some capacity on the table at both ends of the capacity will extend pack life. However, Lee Hart has already built such a system for his AGM lead acid battery pack. The short of it is to have an isolated charger than accepts DC input and outputs the voltage required for one cell. Then charge the lowest cell while driving, while parked, or while charging. Every X number of seconds stop, wait a few seconds, and then check for the lowest cell. If the cells are all close enough do nothing. If they are to far apart charge the lowest cell for X seconds.

I would suggest looking at cars out there using Lithium packs. See what kind of information is available on the the BMS in the Tesla or Leaf.


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## tomofreno (Mar 3, 2009)

> High voltage cutoff (HVC). Right now, shunts seems to be the most popular and effective way to do this. They pass the charging current past a "full" cell to those still in need of charging.


 Shunts don't give an HVC signal. They are there to shunt current when cell voltage is above some threshold voltage until the HVC voltage is reached at which point the charger is shut down by the bms. You don't need shunts for HVC, it will work quite well without them - at least on the minbms I use. Same for evpower I believe, since I think they both use comparators to trigger a relay for HVC.



> Low voltage cutoff (LVC). Could be an alarm or total system shutdown.


 It can also be an alarm with a significant reduction in throttle to get your attention, but leave enough power to get out of the way of that truck you just pulled out in front of and floored your throttle to get out of the way because it is going faster than you thought, which caused your pack to sag and trigger LVC.


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## GizmoEV (Nov 28, 2009)

tomofreno said:


> It can also be an alarm with a significant reduction in throttle to get your attention, but leave enough power to get out of the way of that truck you just pulled out in front of and floored your throttle to get out of the way because it is going faster than you thought, which caused your pack to sag and trigger LVC.


This reminds me of another thing a BMS should do. It should keep track of the Ah in and out of the pack. I can get a LVC with cold batteries. The system should see that there is plenty of energy in the pack and delay the alarm. Maybe watch the individual cell voltage and if it continues to drop then give an alarm otherwise no alarm. If the voltage bounces back when the load is reduced then all is ok. Also, it could compare the voltage of one cell with the rest. If it is low compared to the others maybe alarm otherwise stay silent.


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## ruckus (Apr 15, 2009)

Anaerin said:


> Which is something else with my proposed BMS. An LVC won't disable the whole system, just drop the failing cell out.


I see problems with the switching system:

1. you are talking about buying 30+ contactor-sized switches. This is more cash, space and complexity than desired. This also means WAY more connections, lugs, nuts, etc. Plus, there is the added resistance of all those switches. Bus bars are bad enough.

2. Lack of balancing GUARANTEES that some cells will become deviant from the pack, causing more and more switching as the pack ages.


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## ruckus (Apr 15, 2009)

EVfun said:


> It should also be mentioned that you don't want to use the full capacity of the pack. Leaving some capacity on the table at both ends of the capacity will extend pack life.


Right, but even if you are aiming for 70% dod, 1 low cell will only allow you to get, say, 65% out of the pack. You are still missing that 5% unless you want to push that one cell harder which means it will age faster and the problem will only get worse and worse. 




EVfun said:


> However, Lee Hart has already built such a system for his AGM lead acid battery pack. The short of it is to have an isolated charger than accepts DC input and outputs the voltage required for one cell. Then charge the lowest cell while driving, while parked, or while charging. Every X number of seconds stop, wait a few seconds, and then check for the lowest cell. If the cells are all close enough do nothing. If they are to far apart charge the lowest cell for X seconds.


Now THIS is starting to sound good. Maybe not perfect, but good. I would only want it to do evaluation and charge while driving since they will start the drive equal from the shunt charging. It seems like the charger could be pretty small since the amount of energy needed is only the difference in capacity between the strongest and weakest. I would not want that on all the time as an errant cell could cause pack drainage if the system was constantly trying to "fix" the low cell.

Has anybody used a trickle resistor system in parallel for a series pack? I am not an electrician, so this may be a really stupid idea.


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## Anaerin (Feb 4, 2009)

ruckus said:


> I see problems with the switching system:
> 
> 1. you are talking about buying 30+ contactor-sized switches. This is more cash, space and complexity than desired. This also means WAY more connections, lugs, nuts, etc. Plus, there is the added resistance of all those switches. Bus bars are bad enough.


The way I saw it, you could use something like a pair of FETs to do the switching.


ruckus said:


> 2. Lack of balancing GUARANTEES that some cells will become deviant from the pack, causing more and more switching as the pack ages.


What lack of balancing? I'm proposing that all the cells be charged to their individual "Fully Charged" voltage, rather than charging until only one cell trips HVC and stopping (Which will leave all the cells in an indeterminate state. Hopefully that state will be "close to full", but it's not guaranteed). As the cells reach "Full", they are switched out, and charging continues on the rest of the not-quite-charged cells until the entire pack has reached "Charged" status, fully and completely top-balancing the entire pack.
With the LVC isolation as well, it means that the pack can effectively be both top- and bottom- balanced, and if you put some Ah counting in there, you could even middle- balance the entire pack too.


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## GerhardRP (Nov 17, 2009)

EVfun said:


> The short of it is to have an isolated charger than accepts DC input and outputs the voltage required for one cell.


I was thinking of using these VTM modules. 
http://www.vicr.com/cms/home/products/vi-chip/vichip_VTM_transformers
They are actually bi-directional dc transformers. 
I would have one on each cell with an intermediate bus at 24-48 volts. The only problem is that the operating voltage difference limit is not high enough, even though spec'd isolation is > 2kV. They could be disabled except near the top or bottom state-of-charge.
Gerhard.


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## ruckus (Apr 15, 2009)

Anaerin said:


> What lack of balancing? I'm proposing that all the cells be charged to their individual "Fully Charged" voltage... As the cells reach "Full", they are switched out, and charging continues on the rest of the not-quite-charged cells until the entire pack has reached "Charged" status, fully and completely top-balancing the entire pack.
> With the LVC isolation as well, it means that the pack can effectively be both top- and bottom- balanced, and if you put some Ah counting in there, you could even middle- balance the entire pack too.


Ok, I see, sorry. Yes, top balanced through switching instead of shunting to produce the same effect. I wouldn't say the bottom is balanced, but protected from under-voltage. 

How would pulling cells out affect other components like the dc-dc? If the nominal voltage is 96v, then when the cells reach 2.8v the total is already down to 84v. Switching out a couple of cells at that point could drop it into the 60-70v range and cause the DC-DC to shut down.

Also, is there currently available a cheap switch which can handle the 1000A?


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## Jimdear2 (Oct 12, 2008)

ruckus said:


> Ok, I see, sorry. Yes, top balanced through switching instead of shunting to produce the same effect. I wouldn't say the bottom is balanced, but protected from under-voltage.


Ruckus, GerhardRP, Anaerin,

Please reread my post, #3 in this thread. Help me out here. What did I not include in my post. I thought I had described exactually what you guys are discussing here.

In our system we have dedicated battery chargers mounted to each parallel stack of cells. We are using this configuration instead of individual prismatic cells because we need to draw a large current for a short period.

We have a an isolated DC to DC converter (a SynQor PQ48033QNA25NKS )that takes in a 48 volt nominal (35 to 75 volts) input from a charging buss. This board when modified allow adjustible output of 3.4 to 3.6 volts and up to 25 amps. These are mounted to a circuit board that allows us to adjust the output of the DC to DC unit to voltages with with 0.xxx digit accuracy. 

Depending on the SOC of the cell(s) we can pass up to 25 amps of current into each battery stack until the output voltage reaches reaches the desired cell voltage, in our case 3.550 volts, at that point the individual charger can no longer push current into the battery so it stops charging that battery stack and floats.

Meanwhile each individual charger board is working to reach the same status. By monitoring the cell voltage of each battery and the input current of the 48 volt power supply buss we can see when all batteries have reached fully charged status.

This is a prototype system that is built with off the shelf components and requires manual monitoring. A purpose built system could have a lot of bells and whistles added that would eliminate need for manual monitoring.

One thing we will check during the next summer is, if as we beleive, we can just leave it plugged in because when the battery cells and charger boards reach equalibrium nothing will happen. The chargers should just float.

We chose 3.550 volts to start out because that is just below the knee of the charge curve. We may go lower. Whatever end voltage we choose the final result will be all cells charged equally.

So we have a system that will safely charge a LiPo battery without worry of over volting and damaging a cell and each charge ends with all cells charged to the same voltage i.e. a balanced pack.

Our 50 cell system can be probably be duplicated for under a $1000 dollars and does include in that price cell voltage monitoring and LVC that is part of the system (Cellog 8Ms with the low voltage alarm for each monitored cell set to trigger a throttle cut back). So we have a safe LiPo battery charger with a Battery Monitoring and Management system for around a $1000.00. 

Jim


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## DIYguy (Sep 18, 2008)

It really is a nice system Jim. If I had not already purchased a charger for almost $2,000.... I would have likely gone this way. I think you can get these bricks that pass a lot more than 25 amps for a fairly low price also... yes/no?


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## Jimdear2 (Oct 12, 2008)

DIYguy said:


> It really is a nice system Jim. If I had not already purchased a charger for almost $2,000.... I would have likely gone this way. I think you can get these bricks that pass a lot more than 25 amps for a fairly low price also... yes/no?


I guess it is an idea that has kind of been around for a while but no one, at least no one willing to spread the word, has woked with it. It does seem to make sense to limit the voltage right at the battery with individual monitored chargers that cannot damage a cell then with a very expensive charger and and after the fact shunts or disconnects.

Two areas that seems to need further investigation by someone with more knowledge then me. 

One is ths systems ability to deliver large current right at the battery, current produced by the DC to DC brick from a higher voltage relitivly low amprage feed buss. Seems like a good way to deliver 1 hour recharge without a dangerous voltage/current, produced by a very complicated and expensive charger in a cable handled by the end user. 

The second is monitoring using coded information packets piggybacked on the charging buss or main current cableing. If each board has a unique ID, that information could be processed in a central unit and a simple user friendly interface developed. There is also the RF that is assigned to systems like tire pressure monitoring. 

I still think it's the battery cell OEMs responsibility to design an intigrated modular system that at least can be purchased with the battery for a few dollars more per cell.

When I first though of trying this type of recharge set up and posted our thoughts. RWaudio was the only person who respondid had been activlly persuing it. He was very helpful as well.

The bricks can be a bit pricey, shopping helps. The types we are using have been on eBay priced at from $5.00 to $20.00. I believe they are obsolete but there seem to be replacement units with the same specs available I got lucky and found someone who had 63 new ones for just under $5.00 each shipped.

Oh Heck, sorry I've gone and done it again . . . Run on and on and on.
Jim


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## cyclops2 (Feb 12, 2011)

Depending on the battery chemistery, time can be saved by a charger.
It should do all the safety checks of each cell first.
Then check each cell voltage. IF IF.. All cells are CLOSE ENOUGH. It goes directly to a total, series current & voltage charge of the entire battery. Why not ?

It then does a voltage check of each cell on a regular time basis. All cells still close ? Keep charging in series at full current. 

If it detects a high or low cell.........It stops the series charging & corrects the cell to be the same as the average of the others. When that occurs. Full current series charging resumes. 

That type of charger is what a FMA Cell Pro does for small electric powered toys.

It also adjust full charge to the present air temperature. 
Lastly it tops off to final voltage of each cell. This step is really not needed if charging stops at the 99 % of full charge voltage level. Longer life of cells is far better than 1 % .
It should also sense a WEAK cell & end the charge for the pack based on that cell. Allows the weak cell to be changed before battery failure while driving.
Weak cells are deadly when they are in 2 or 3 in parallel. Weak cell shifts more & more of the current to the other one. Charging looks fine. But the good cell is being cooked to death when full power is used. It now is passing 2 X the current it should be......So 2 cells go up as maximum power drops under a full load. The problem continues at lower & lower levels. However. Unless you are cells in parallel trained, you will find nothing wrong when voltage testing. You may see 1 cell " bumped up a little " as the only indication.


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## ruckus (Apr 15, 2009)

Jimdear2 said:


> Ruckus, GerhardRP, Anaerin,
> 
> Please reread my post, #3 in this thread. Help me out here. What did I not include in my post. I thought I had described exactually what you guys are discussing here.


Hello,

I appreciated your response, but it seems a variation of top-balancing while charging. This is good, since top balancing means the pack is totally full, however, at the bottom, the pack will not be in balance.

I am seeking a way to balance WHILE DRIVING so the pack is balanced at the bottom and all cells reach LVC at the same time. Maybe that is inherent in your design and I just missed it. 

Individual chargers like you suggest might be a good way to create such a system since you could charge any cell which was dropping below the rest of the pack. It would need just a trickle to keep the low cells up. Zapping an already weak cell with high charge rates would just shorten it's life even further. 

EVfun in post #10 describes Lee Hart's system which accomplishes the task I desire. But I also want to hear other schemes existing or potential. Obviously, the energy required by the "balancing while driving" system must actually increase the range (if cells vary in capacity), but not cause reduced range if cells have very similar capacity (like with a new pack). 

I am thinking the need for this will increase over the life of the system as the cells age, especially if one or two cells are replaced for whatever reason. In the end, a weak cell might be charged the whole drive to keep it from stopping the system. 

Cheers


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## cyclops2 (Feb 12, 2011)

ruckus

Your last sentence is really requiring the the battery / cell charger to bridge the dying cell with the same capacity as the cell. It will be running on the charger. 
Range will be reduced by the amount of power the charger needs. I agree with your logic of keep the battery ONLINE until repairs can be made somewhere.


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## EVfun (Mar 14, 2010)

ruckus said:


> Hello,
> I am thinking the need for this will increase over the life of the system as the cells age, especially if one or two cells are replaced for whatever reason. In the end, a weak cell might be charged the whole drive to keep it from stopping the system.


What is the chance this delux BMS system will be smaller or lighter or cheaper than enough extra cells to eliminate the need for it?


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## ruckus (Apr 15, 2009)

EVfun said:


> What is the chance this delux BMS system will be smaller or lighter or cheaper than enough extra cells to eliminate the need for it?


Have you seen a 260ah cell? They are BIG. We have just enough room to upgrade to 120v in the future. Maybe. It's tight. Right now we have ~25KW.

The "delux" energy distributing system could be small and cheap. Imagine each cell with a small dedicated charger running off the dc-dc (like car cell phone charger). Cells a certain amount below "average" get charging current. Just one example.

Obviously, you are losing 10-20% in the conversion/charging process, but otherwise that capacity goes completely unused. With 260ah cells, a 10% difference is a whopping 26ah. If one cell is 90% capacity and the others are good, then the range drops about 10 miles (from 100). Assuming 80% efficiency of the charge/redistribution system, you would get 8 of those miles back.  

If round trip is 91 miles, you have failure without redistribution, but arrive nicely home with 7 miles to spare using redistribution. (round trip to city is about 75 miles in real world)


If the batts stayed nicely together, than the system would just sit and wait. Some may see this as a waste, but again, I am looking for a 300,000 mile system (or at least 100k) that your GRANDMOTHER can drive. Eventually, late in life, the system would see more and more use as the batts became old and their variance greater.


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## cyclops2 (Feb 12, 2011)

I can positivily state that with A123 chemistery, LIFEPO4, unbalanced cells are VERY RARE.
Besides. What I pay for them is very high dollars. They get top dollar.
I want & expect PERFECTLY MATCHED cells. Just like my money was perfect.

We should NOT have to put up with this baloney of 1 or 2 cells not being the same.

DAMM IT. They are ALL SUPPOSED TO COME FROM THE SAME assembly line batch !! How difficult is it for everyone to understand that BASIC fact. That means they AGE TOGETHER. 
10 years later with LITTLE maintanance ageing is expected.
At that point the BMS Charging system is needed.

Why do I get upset with companies taking advantage of people paying the full asking price for a product ?

I am off the topic. Sorry guys.


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## GizmoEV (Nov 28, 2009)

cyclops2 said:


> I can positivily state that with A123 chemistery, LIFEPO4, unbalanced cells are VERY RARE.
> Besides. What I pay for them is very high dollars. They get top dollar.
> I want & expect PERFECTLY MATCHED cells. Just like my money was perfect.


Are you saying that you have data to show this? Would this be true for TS or CALB prismatic cells or just the higher priced cells?

There seems to be two vocal camps and a quiet camp on this issue. There is the BMS camp which says they have to be balanced on every charge otherwise you will be in trouble. There is the non-BMS camp which says they don't unbalance since they age the same. Then there is a very quiet group which says that you should balance once in a while only if you need it and only at low currents (<500mA) but you probably will want some sort of a warning/protection system.

I just want to see the data and so far I can't seem to find it. I'm collecting my own but by the time I have all of it my pack will have died because it will be at the end of its life. I hope that will be at least 10 years.


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## cyclops2 (Feb 12, 2011)

GizmoEV

I run and recharge every day with all my cordless tools & appliances. I converted everything in our house & vacation place. All are either A123 1100 mahr or 2300 mahr cells. The Black & Decker 1100s get run at between 10 & 20 C all the time in the cordless vacuums at both places. At least 7 years.

The Dewalt 2300s get used on all the Briggs & Straton ICE starter duty. 5 to 15 hp motors. 
Winter. They start the 5,500KW generator when needed. They are equal to a Lead mower battery on cranking speed any day.

That is all I can tell you........They DO like to be dropped on concrete from about 1' or 2' & keep on working.
Must have Timex genes in them.

Almost forgot ....They hold full power for a year. I always forget to recharge the generator inbetween starts.


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## BMI/LiFeTech (Aug 12, 2009)

cyclops2 said:


> I can positivily state that with A123 chemistery, LIFEPO4, unbalanced cells are VERY RARE.
> Besides. What I pay for them is very high dollars. They get top dollar.
> I want & expect PERFECTLY MATCHED cells. Just like my money was perfect.
> 
> ...


But you only get what you pay for...nothing more...nothing less!

"We should NOT have to put up with this baloney of 1 or 2 cells not being the same"- but that is the reality of all the cells targeted at the DIY market eg, TS, CALB, Headway, etc. where people look first at the price rather than the quality/reliability of the product. The DIY product comes straight off the production line with either minimal or zero testing and grading so the result is you get to buy cheap cells which will vary considerably in terms of both capacity and internal impeadence.
It costs money in both time and cell and battery testing equipment which is what you are paying for when you buy cells made for the professional EV industry such as LiFeTech Energy, A123 or Valence (as typical examples). 
For example did you know that approx. 25% of cells manufactured by Headway are not within specification? 
When I talk to the professional EV companies and ask them if they have considered using cells like TS or CALB in their EV designs for mass production they just laugh at me. So if you can show me any large scale manufacturer such as GM, Toyota, Ford, etc. using TS or CALB please let me know so I can laugh back at them.

So everyone has two choices they can make-
1) Cheap DIY grade cells which require a good BMS to closely monitor and manage the cells since they are far more likely to get out of balance due to greater variation between cells in a pack or
2) Professional grade cells which initially are more expensive to purchase but have been extensively graded and matched which results in less stringent BMS/balancing requirements due to the natural tendency of the cells to stay in balance much better by themselves.


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## ElectrifiedMonteSS (Feb 27, 2011)

Hello.. Being new to EV and I must say I'm totally bitten by the concept.. So much so that I want to ditch my last 5 years of planning and parts collecting to build my ICE hot rod and now make it an EV! So my question is this: Has anyone considered or does anyone know of the SoC (system on a chip) being used for BMS functions? I ask this because the SoC's can be programed to preform way more functions (if not most all the functions) mentioned in this thread.. If I'm way off base here.. Just tell me so and I'll go back to reading up on things more..


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## ruckus (Apr 15, 2009)

Yes, there are BMS that will make coffee and get the paper for you in the morning. 

But you have to fork over several grand, they may or may not be reliable, may or may not set your car on fire, and you may or may not get them to actually put sugar in the coffee as requested.

Less BMS seems to be the growing paradigm. Minimum protection at minimum cost. Over and under voltage protection seems to be a reasonable place to start (imo) since that is what damages the cells. 

If you go BMS-less then you get to wrangle with the top vs. bottom balance crowd. My assessment is that these are EQUALLY fraught with danger. Top balancing is sketchy at the bottom and bottom balancing is sketchy at the top. 

Think Lilliputian. I don't know if you've read Gulliver's Travels, but it is much like the argument of whether you are less likely to cut your finger opening the big or little end of an egg. Similarly, you can argue whether it is more problematic to run an ICE without oil or to over-rev it. The correct answer is to do neither. 

A decent lithium pack is a LOT of money and resources to just let it go "poof" through one driving incident or a careless night on the charger as demonstrated by those such as Jack Riccard who at least had the guts to tell everybody when he proved that humans are NOT reliable battery protection devices.

I am putting my feelers out for a device that balances WHILE YOU DRIVE so you are not limited to the range of your weakest cell. So far, there has been one suggestion of a system to analyze and charge the weakest cell, but I am holding out that there could be some sort of more direct transfer device that would do nothing if the cells were equal.


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## icec0o1 (Sep 3, 2009)

ruckus said:


> I am putting my feelers out for a device that balances WHILE YOU DRIVE so you are not limited to the range of your weakest cell.


I never get that argument. Why not swap out your weakest cell then instead of spending 20x more on a BMS?


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## ruckus (Apr 15, 2009)

icec0o1 said:


> I never get that argument. Why not swap out your weakest cell then instead of spending 20x more on a BMS?


There is ALWAYS a weakest cell, or several of them actually.

Just look at Riccard's pack:
http://4.bp.blogspot.com/_i_c2BM_uB...0/Electric+Spyder+550+bottombalance+graph.jpg
The first ten cells are lower because of being used as a power supply, but you can still see quite a bit of variation in the other cells. 17% actually. And this is only after a few months. Imagine what 10 years would do. 

Please note: I am NOT suggesting a bms that costs "20x more" than a cell. (That would be $6000!)  In fact, I wouldn't even want to call it a "bms" which has become a 4-letter word. Maybe a Cell Balancer or cell equalizer. 

I am looking for a CHEAP, SIMPLE way to drain energy from the stronger cells and give it to the weaker cells to get the actual range you paid good money for. The larger the battery capacity, the longer the range of the vehicle, and the longer the lifespan of the batteries, the MORE this becomes important. You might only be losing 10% now, but down the road this will turn into 20% or even 50%. It only takes one weak cell.

Bottom or top balance does not matter, it just shifts where you are losing the 10-20% of your range and where your cells are potentially being damaged.

The correct answer is to START with a balanced pack and END with a balanced pack. The only way to do this is to balance while driving, charging, sitting, etc.

In fact, the "Cell Balancer" could actually ELIMINATE the need for a BMS as we now know them. 

Since it would keep the cells perfectly together, the charger could be used as the HVC (high voltage cutoff) without worrying about the weak cells going "pop". Also, since the cells are kept together at the bottom, a simple cell or pack voltage meter could be used effectively without worrying about a few cells being less than "average" and being driven into the ground.

I am sure this is possible, the solution will surface with time...

Cheers


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## Ektus (Feb 15, 2011)

This online cell balancing scheme sounds very promising. But...

Preamble:
I'm just about starting into this whole EV business. I'm trying to determine my needs and if and when I will build or buy an EV is completely open. I've been reading here for some time, though. My daily commute is 2x 11km, but on the weekends there are more like 130km one-way and sometimes 180km. I drive approx. 20000km a year.

From the above requirement, I would need a very big pack for the long distance job, or a range extender like in the Volt.

In case I go the "big pack" route, I would also go high voltage. Industrial AC systems are designed for 600V on their DC side. High voltage means low amperage, thinner wires, less weight. The high capacity I would need would result in continuous discharge rate of 1/2 C.

To reach that voltage, I would need some 165 cells. And to reach the capacity, I would have to use 60Ah cells. This would give me approx. 25kWh at 3.2V per cell (80% DOD) and should be good for 200+ km.

Sorry for the long preamble, but now comes my point:
To monitor these 165 cells with a centralized system, I would have to put a lot of wires into the pack. I would have 594V total pack voltage at 3.6V per cell. 594V on a PCB is doable, but I'd rather not. A lot of wires carrying high voltage sounds hazardous, too.

For the online cell balancing to work, you would likewise have to carry the high voltage to the individual chargers. 

As has been stated before in this thread, you want to monitor your pack on cell level in order not to damage it and to achieve reliability. The more so with a high voltage pack, where individual cell failures cannot be detected from the pack voltage.

To that end, I'd envision a distributed monitoring system with shunt capacity, that is strictly local to one cell or a small group of cells. This BMS system should be able to trigger both LVC and HVC alarms as well as top-balance during charging. It would work autonomously to engage the shunt and issue warnings.

In order to keep voltages down and reduce the risk of shorts and fire, those BMS systems should communicate with a centralized management and information computer through an optical bus system. Thus, the high voltage is confined to where it is absolutely needed: the battery pack, controller and motor.

The distributed boards should be extremely low power devices, as they cannot be disconnected easily. A wrist watch runs for years on a tiny lithium cell, that kind of power consumption. This may include a local LCD for cell voltage and error messages. They may have two operating modes, one "sleep" mode with ultra-low energy consumption that is active whenever the car is not in use, and a working mode for driving and charging.

The cost would have to be reasonable also. Those 165 cells can be had for less than 60€ apiece, excluding VAT, and the monitoring system shouldn't exceed 10% to 15% of the actual battery system cost. This should be doable, though, as e.g. cheap digital multimeters can be had for 5€. Those include a LCD and case. A BMS designed around a small PIC should be possible in this price range if produced in quantity or as a kit.

I'm afraid this price tag as well as the high currents inherent to a EV system will rule out more sophisticated functions like taking low cells out of the loop. The online balancing system might be more cost effective in this area as it allows for a very simple charging device.

Reliability has got to be designed into the system, of course. Fail-safe design, fuses, centralized monitoring system with failure diagnostics capability, failure override to move the car out of the traffic, whatever.

The centralized monitoring system would allow for gathering data from each cell (voltage and temperature), thus providing a means of detecting any change in cell health.

I've been looking on http://liionbms.com/php/bms_options.php but haven't seen anything even close to what I've been sketching above. The non-distributed systems may deliver the features, but I don't like the hairball needed.

A distributed system like e.g. the miniBMS is a capable yet very basic implementation of what I would consider, but lacks the data gathering capability and IIRC won't work for this pack voltage.


I'm just brainstorming here, so any comments are welcome.


Ektus.


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## Jimdear2 (Oct 12, 2008)

I'm not an electronics guy so there are most likely many problems in this presentation below. But it sure sounds to me like it would work. We know the charging voltage limiting works because we have tested it, the low SOC battery recharging is speculation.

Right now we are building indivdual charger boards that will go into a 50 cell 170 volt battery. Each charger is powered by a 48 volt buss. Each charger board contains an isolated DC to DC 1/4 brick board. These DC to DC boards will take 36 to 75 volts in and produce 3.X volts, adjustible from 3.3 to 3.65 volts with the addition of a variable pot. Some newer versions of these boards can pass 50 or more amps to a battery. Ours will do 25 amps and maintain the set voltage. You want to find a DC to DC board that wont overpower the charge rate for your battery. We had to modify ours to cut back on the amps when charging a battery with a low SOC.

Each of the boards can be turned on and off if desired. So each battery cell gets its own charger that can be turned on and off at a command.

So combine Lee Hart's or some other SOC monitoring system along with a DC to DC converter to convert vehicle pack voltage to 48 volts to feed the charger buss. You would use the cell SOC monitoring system to turn the individual chargers on and off as needed to charge individual batteries in the total pack.

There are going to be losses here, but as I see it you are taking capacity from the batteries with more amp hours then the pack average and putting them back into the batteries with less then the average.

You also have the added advantage that these individual chargers are also the toal pack charger when pluged into the mains. These chargers when properly adjusted would never charge a cell above yor set voltage, so overchargin is not a problem and each charge is a balancing charge. 

So you have protection to prevent overcharging and protection to prevent over DIScharging.

Hope this gives some EE an idea.

Jim


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## ruckus (Apr 15, 2009)

There are pros and cons to higher voltages. 

It means less current, sure, but it also means more connections, more bolts, washers, busbars, and wire lugs. It also means more cells to monitor, more bms boards, and more spaghetti. This will more than counteract any weight savings in wire gauge. (the difference between 2/0 and 4/0 for the EV I am building is a pack of beer, not really that much)
Also consider that high AC voltages produce a lot of Electro-magnetic radiation.

For a given pack volume, few large cells have more electrolite weight and less plastic than the same volume of many smaller cells.

This is somewhat counteracted by smaller cells being easier to package in awkward spaces.

If you look at the cycle ratings, larger cells (200+ah) are rated for 3-5000 cycles, while smaller cells are only rated for 1500-3000 cycles.

Just imagine you want to check your cell voltages. With a 600v system you would have to check 165 cells!  I'd way rather check 30-75 cells.

Even if you go with a fairly cheap bms, at $15 per board you would spend an extra $1500 over a 220v system and $2000 more than a 120v system. Just imagine the bill for the individual chargers + all that extra weight.

I think you will find a 120-240v system can do what you want for a lot less money and hassle. Just getting into the 300's tends to fry stuff like dc-dc converters.

Just some food for thought.


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## MN Driver (Sep 29, 2009)

"If you look at the cycle ratings, larger cells (200+ah) are rated for 3-5000 cycles, while smaller cells are only rated for 1500-3000 cycles."

Who provided that specification? I'm not aware of any battery company that has ratings reduced for a smaller size. In fact I'm seeing the opposite, especially for peak amp ratings considering that smaller cells have less internal impedance.


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## T1 Terry (Jan 29, 2011)

At the amp draw required for EV use thinking of cycle life in the thousands is not realistic. The 3,000 and 5,000 figures are for 0.3C discharge rates, 30 amps on a 100ah cell. When lower voltages are used much higher amps are required for the same number of kilowatts, we all know that but a lot seem to overlook it. TS and CALB cells have a max discharge of 3C but they don't quote a cycle life at that rating, just that it can do it. No one is admitting yet that they got a lot fewer cycles than they expected, some probably feel they've done something wrong and it's their fault they died an early death.
If you want to run lower voltage you need bigger capacity cells or high $$ cells that handle heavy discharge rates. It's all an area and $$ juggling match to get the right balance of voltage and capacity to supply.

The future of BMS:
As far as I see it Jim has the answer, single cell level pack charging, monitoring of the cell pack to catch dead cells early, an LVC/HVC BMS with the single pack charger linked to the LVC section to boot it back to life to share the pack charge into the low cell to stop it being dragged down to destruction. A method of recording which cell packs went low and if it happens often then that cell pack needs testing and repairing/replacing as required. A continual ongoing maintenance thing but properly set up a complete spare cell pack with an assembly system that allows for quick plug and play the damaged cell section can be examined at a convenient time. Either big formate cells in a slide out rack system with Anderson plugs to link them into the chain or small format cells built into a pack that can be slotted in or out with the same Anderson plug linking system. I know it's not the method used by most DIY builders but with enough fore thought it could be included in the build making future maintenance much easier.

T1 Terry


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## ruckus (Apr 15, 2009)

MN Driver said:


> "If you look at the cycle ratings, larger cells (200+ah) are rated for 3-5000 cycles, while smaller cells are only rated for 1500-3000 cycles."
> 
> Who provided that specification? I'm not aware of any battery company that has ratings reduced for a smaller size. In fact I'm seeing the opposite, especially for peak amp ratings considering that smaller cells have less internal impedance.


Rather than scan my 260ah doc, I'll link Riccard's 400ah 
http://2.bp.blogspot.com/-QcxsfLdd_Mg/TW0KoATK81I/AAAAAAAACGY/Wc49bwSIfHQ/s1600/wb-lyp400aha.jpg

I'll let you verify that Thundersky (now Winston) rates cells 200ah and under for only 1500-3000 cycles.

I am not saying the doc is correct, only that is what it says. Almost double life when using larger cells.


Terry, I am still on the fence whether many small chargers or 1 big one is better. For on-the-fly charging, only 1 small charger is needed if it has access to all the cells (get out the spaghetti).

As for the amp vs volt thing, there seems to be a pretty large range where things work out in terms of discharge rate. Using this calculator to get watts at cruising speed:
http://ecomodder.com/forum/tool-aero-rolling-resistance.php
I get about 12,000watts @ 60mph. Using various voltages, the cell size can be varied to keep it at about .5c. So:

volts, amps, cell size needed for .5c
96v = 125a = 250ah batt
110v = 109a = 218ah batt
220v = 55a = 110ah batt
330v = 36a = 72ah batt

It may be more fruitful to start with the available batt sizes and work backwards:
batt ah = .5c in A = system voltage = number of batts (3.2v nominal)
60ah = 30a = 400v = 125 batts
90ah = 45a = 266v = 83 batts
100ah = 50a = 240v = 75 batts
160ah = 80a = 150v = 47 batts
200ah = 100a = 120v = 38 batts
260ah = 130a = 92v = 29 batts

All of these should work fine, but the higher voltages have more wires, more bolts, more busbars, more battery boards, more spaghetti, more plastic casing, more volume etc. Those bolts are HEAVY and they come in sets of 2.
The upside is the higher voltages can go steady high speeds without heating up the motor and controller.

I think the $$ math argues for using the lowest voltage that meets your cruising speed and acceleration objectives.

My client went with lower voltage/larger cells because of their longer rated cycle life, cheaper and simpler implementation (less boards and busbars), and because the big batts definitely have a LOT more visual appeal. The downside is: you blow one up, you are out $300 (if you can find a replacement).


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## jddcircuit (Mar 18, 2010)

I have a battery monitoring/balancing circuit that I am working on.

My design goals are to be able display the voltage of each series cell at real time and to balance the cells using the switched capacitor method in the event of a high or low cell condition detected. I don’t plan on my cell balancing to happen while driving but could while sitting. I am fairly new to EVs and I see quite a bit of dispute over BMS. This is a learning hobby for me. From what I read so far it appears to me that if it is cheap enough and easy to wire in then knowing the voltage of each cell in series could have value for several reasons. The cell balancing capability also seems to have value to increase the total pack energy stored and used without violating a single weakest cell limit.

My current design plan is using distributed cell boards that are connected in a 3 wire daisy chain. Each cell board is also connected to the +/- terminals of its cell. The opposite ends of the daisy chain are optically coupled to a central monitoring/control circuit. I think I have a fairly low part count and low cost design on paper so far. I haven’t seen this method used yet so maybe I am missing something in my design. One of the wires in the daisy chain is used to pass the cell voltage information through the chain, one wire is used to clock the flying capacitor network during cell balancing, and the other wire is the flying capacitor terminals. If you only wanted to monitor the voltage of each cell then it would only require a single jumper wire between cell boards. The cell to cell jumper wires are not optically isolated from each other and exploit the fact that they share a common potential point and low common mode voltage difference between them.

I am not sure if 3 wire daisy chain jumpers between cells would be considered a lot of wiring or not. I am also not sure what would be considered cheap for such a circuit. I hope to be protyping and testing at least the voltage monitoring portion very soon.


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## icec0o1 (Sep 3, 2009)

You're making this way too overcomplicated. If you need to maintain 12KW at 60mph and keep it at .5C, then simply your pack needs to be at least 24KWh. The voltage should be determined by the highest your components can handle (controller/motor) so losses are decreased. Then the required AH of the cells can be determined.


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## icec0o1 (Sep 3, 2009)

jddcircuit said:


> I don’t plan on my cell balancing to happen while driving but could while sitting.


So you want an (expensive) BMS system to increase your range by a couple of miles every time you pull over at a mall or coffee shop? And I guarantee you're not even going to need/use those extra few miles 99.9% of the time. 

The downside of that is that you'll balance the pack at the middle to bottom and then when you charge it, rebalance it again at the top. All of that is hugely inefficient. 

And for anyone who wants to balance a pack while driving, what do you do when you have cells with varying internal resistance? When you hit the gas and one cell drops in voltage more than the rest, do you start pumping energy into it? Then when you pull off the pedal, it'll rebound to a higher voltage and you'll pump the energy back out. Pack balancing only works in contrant energy usage environments. 

Spending even just $1000 on a BMS will never help you more than investing that in newer/additional batteries, if you know what you're doing of course. BMS might be required if you have no idea of how to (bottom) balance and set up a good charging algorithm for your pack.


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## nimblemotors (Oct 1, 2010)

Yep, but the real issue is you don't just cruise at a steady state (and BTW, around here 60mph will get you honked at, its 65-70)
You accelerate and then coast, speed up and slowdown, even on the freeway, city driving is much worse.
It is the acceleration part that can kill the batteries.



icec0o1 said:


> You're making this way too overcomplicated. If you need to maintain 12KW at 60mph and keep it at .5C, then simply your pack needs to be at least 24KWh. The voltage should be determined by the highest your components can handle (controller/motor) so losses are decreased. Then the required AH of the cells can be determined.


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## jddcircuit (Mar 18, 2010)

icec0o1 said:


> So you want an (expensive) BMS system to increase your range by a couple of miles every time you pull over at a mall or coffee shop? And I guarantee you're not even going to need/use those extra few miles 99.9% of the time.
> 
> The downside of that is that you'll balance the pack at the middle to bottom and then when you charge it, rebalance it again at the top. All of that is hugely inefficient.
> 
> ...


The primary purpose of the circuit is to cheaply monitor the voltage of each cell.

The cell balancing feature is potentially optional. The cell balancing would only be initiated when there is a significant voltage difference between cells. High side during charging and Low side when out of gas. I guess if it allowed you to pull over with a low cell voltage detected and then wait for the balancing to charge the weak cell back up it might get you home without damaging the cell.

I am not sold on the balancing part yet but I am considering it if it is simple to implement. If periodic rebalancing is required then it would be more convienent if it is wired in once.

Thanks for the feedback


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## icec0o1 (Sep 3, 2009)

nimblemotors said:


> Yep, but the real issue is you don't just cruise at a steady state (and BTW, around here 60mph will get you honked at, its 65-70)
> You accelerate and then coast, speed up and slowdown, even on the freeway, city driving is much worse.
> It is the acceleration part that can kill the batteries.


Sure, but the method of choosing a battery pack is still valid. And 
I was just replying to Ruckus. Of course, you should get the largest pack you can afford and can fit in your car for longevity purposes. Of course, that'll decrease your efficiency because you'll carry more weight around if you don't need the extra range. And people believe the batteries would outlast the car anyways so I feel range would be the determining factor for a battery pack.


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## nimblemotors (Oct 1, 2010)

You are right, a simple calculation in place of ruckus calcs can be done.
However, it still doesn't apply to sizes a pack, because the amp draw is not a simple linear draw. A 25Kw pack can deliver 25Kw, but you would need a 50kw pack to handle the 2x draw during acceleration (assuming only 2x is needed) if you expect the battery life at the average draw.
Just how much x is needed we don't know because nobody has test it.



icec0o1 said:


> Sure, but the method of choosing a battery pack is still valid. And
> I was just replying to Ruckus. Of course, you should get the largest pack you can afford and can fit in your car for longevity purposes. Of course, that'll decrease your efficiency because you'll carry more weight around if you don't need the extra range. And people believe the batteries would outlast the car anyways so I feel range would be the determining factor for a battery pack.


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## MN Driver (Sep 29, 2009)

ruckus said:


> Rather than scan my 260ah doc, I'll link Riccard's 400ah
> http://2.bp.blogspot.com/-QcxsfLdd_Mg/TW0KoATK81I/AAAAAAAACGY/Wc49bwSIfHQ/s1600/wb-lyp400aha.jpg
> 
> I'll let you verify that Thundersky (now Winston) rates cells 200ah and under for only 1500-3000 cycles.
> ...


I just went back to the Winston site, brought up a 40Ah cell and it shows 3000 and 5000 cycles for the 80% and 70% DOD. That's why I asked, I already checked before I posted to verify, which is why I asked about the manufacturer. I -just- went to the site to verify again and brought up the specsheets for 40Ah, 90Ah, 160Ah, and 200Ah and they ALL say 3000 and 5000. I'm not seeing 1500-3000 anywhere on these smaller cells.


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## ruckus (Apr 15, 2009)

Then they changed their literature. 

It is entirely possible at the time I was looking up the specs that the 260 lit had the new specs and the the others had the old specs.

Fair enough. This makes it very simple.

The advantage of fewer cells is less cost, weight, and complexity.

And the advantage of more cells is better performance.

Yes?


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