Did anyone just run battery pack without a BMS ??? - DIY Electric Car Forums
Go Back  

DIY Electric Car Forums > EV Conversions and Builds > Batteries and Charging

Register Blogs FAQ Members List Social Groups Calendar Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Display Modes
  #1  
Old 08-16-2012, 01:11 AM
batterytang batterytang is offline
Member
 
Join Date: Nov 2011
Posts: 30
batterytang is an unknown quantity at this point
Send a message via MSN to batterytang Send a message via Skype™ to batterytang
Default Did anyone just run battery pack without a BMS ???

Just heard from other people, saying BMS is a waste of money, only need to use a charger charging battery packs and do balancing, no need for a BMS? Anybody have similar experience?
Reply With Quote
Sponsored Links
Advertisement
 
  #2  
Old 08-16-2012, 01:55 AM
WarpedOne's Avatar
WarpedOne WarpedOne is offline
Senior Member
 
Join Date: Jun 2009
Location: Slovenia, Europe
Posts: 107
WarpedOne is on a distinguished road
Default Re: Did anyone just run battery pack without a BMS ???

Real BMS is also an automatic top balancer.
If you love to manually balance your cells after every few charges, you are welcome. Others like to have it done automatically.
Reply With Quote
  #3  
Old 08-16-2012, 02:09 AM
batterytang batterytang is offline
Member
 
Join Date: Nov 2011
Posts: 30
batterytang is an unknown quantity at this point
Send a message via MSN to batterytang Send a message via Skype™ to batterytang
Default Re: Did anyone just run battery pack without a BMS ???

Thank you...
I just wonder if here is any safety problem.
Reply With Quote
  #4  
Old 08-16-2012, 05:24 AM
Siwastaja's Avatar
Siwastaja Siwastaja is offline
Senior Member
 
Join Date: Jul 2012
Location: Finland
Posts: 566
Siwastaja is on a distinguished road
Default Re: Did anyone just run battery pack without a BMS ???

a BMS is not optional. It is mandatory.

For LiFePO4, it is not strictly 100% mandatory for safety as they don't catch fire so easily.

A pack without a BMS, however, will last something around 1...100 cycles. With a proper BMS, it can last for 5000 cycles. Do the math for the price.

However, there are (or have been) people out there who decide to be a BMS themselves. Some of them have changed their mind after a long way, some are still happy. If you make a lot of effort to manually balance and monitor your battery pack all the time, it really IS possible! You may get more than 100 cycles if you are good on that and really understand your battery. You can make it a lot easier by deciding to give up a part of your range; don't charge full and especially don't discharge fully, and you may get out of the problems. But from this viewpoint, you can as well buy smaller/fewer cells and get a BMS for the money.

A real passive BMS will give you about 20-50% more range than "human BMS". An active (charge transfer) BMS may give you a further 10% on the top of that of a passive BMS. This is for a NEW battery pack. After 1000 cycles, a "human BMS" pack is probably dead while proper BMS pack still has almost full capacity left.

However, the BMS critique may be justified, and the BMS opposition is understandable. Apparently, there have been some faulty and totally unusable BMS products available. Some have drawn too much current when idle, destroying the cells. Some have overheated from wrong shunt design. And most are a bit expensive so I understand why people want to avoid them if possible.

So, to choose a BMS, it MUST include the following two features as an absolute minimum:

- A cell-level monitoring for every cell that COMMUNICATES WITH THE MOTOR CONTROLLER. If ANY of the cells drop above minimum voltage, it instantly limits or cuts power. If the motor controller does not support this, an alarm sound is acceptable.

- A balancer that shunts off or redistributes extra voltage on any cell when charging.

Without those features and without a proper connection to the motor controller (or alarm), it is not a BMS.

Without a BMS, you typically just leave enough leeway on the both sides. This means you limit the charger voltage much BELOW numberOfCells * 3.65 V, and when the pack is nearing this point, go around with a volt meter to check random cells and stop if any of them is nearing 3.65V. This way you may have a SoC of 90% when you stop charging and you are probably fine.

Similarly, when you are driving, you must stop BEFORE you have any signs of heavy voltage sag. Without simultaneous monitoring of all cells during accelerations, it is impossible to know when your pack is near empty; when you start seeing the symptoms, you already are on the very dangerous region for the cell life. If you know what you are doing, you may be able to stop at around 30-40% SoC. With a BMS, you can stop at 10-20% SoC. This is why a BMS gives you more range; you can run your battery more flat without risking destructing it.

Simply put the usable SoC range is:
No BMS: 90 ... 30 % of the weakest cell
Passive BMS: 100 ... 10 % of the weakest cell
Active BMS: 100 ... 10 % of the average cell (becomes important after the pack starts getting older)

So just use a BMS. For me, it's a natural choice because I can do my own BMS without relying on products I know nothing about. So I don't need to take a risk, I can just do one that works -- internally, a BMS is very simple electronics. I'm going the active way to get every single drop of juice out of the expensive pack, but OTOH, I have access to some free components such as small ferrite cores to make the charge-balancing transformers.

Also, the lower the cell uniformity (even good quality cells can have variation if the production involves a lot of manual steps or they are from different batches), the more important the BMS becomes.

Hope this helps.

Last edited by Siwastaja; 08-16-2012 at 05:33 AM.
Reply With Quote
  #5  
Old 08-16-2012, 05:35 AM
Duncan Duncan is offline
Administrator
 
Join Date: Dec 2008
Location: Southland New Zealand
Posts: 2,805
Duncan will become famous soon enough
Default Re: Did anyone just run battery pack without a BMS ???

Quote:
Originally Posted by Siwastaja View Post
a BMS is not optional. It is mandatory.
That's NOT what a lot of other people say

There seems to be no reason for cells to go out of balance and people without BMS's run for years without problems

The main reason for cells to go out of balance is ...... the BMS!

Personally I am going for a split pack check circuit and no BMS
Reply With Quote
  #6  
Old 08-16-2012, 06:15 AM
Siwastaja's Avatar
Siwastaja Siwastaja is offline
Senior Member
 
Join Date: Jul 2012
Location: Finland
Posts: 566
Siwastaja is on a distinguished road
Default Re: Did anyone just run battery pack without a BMS ???

Quote:
Originally Posted by Duncan View Post
That's NOT what a lot of other people say
Hah, I was quite sure that someone will quote that out of context by stopping reading my post after the very first sentence.

Later you can see that I propose a solution of a "human BMS", what is what you are going to do. I just count it as a BMS, just an inferior one but indeed with zero initial cost. There is a reason why I count it as a BMS. Simply, it does not come completely without a cost -- at least a cost of your time. It will work, as I say, but you cannot just ditch a BMS concept completely and forget about it; you have to be the BMS, at least to some level. You MUST understand this, or you can ruin your $10 000 pack on your few first test drives by driving it flat or charging it full without knowing what you are doing. But it's of course up to you. I am poor and can overcome my monetary situation by using my brain. There are people in opposite situation, too. Most are somewhere in-between. Nothing wrong in any of this.

Quote:
The main reason for cells to go out of balance is ...... the BMS!
I know there are broken and dangerous BMS products out there. At least the rumors say so. Instead of spreading the misinformation about the BMS concept, we should create a list of those exact products. Say the names. We need to know what to avoid! We need to do some BMS testing. Which is a bit sad because the BMS should protect our battery, not destroy it, but the world is cruel.

One more professional view on the matter: http://www.metricmind.com/education/ (see 2. Do I need a BMS)

Last edited by Siwastaja; 08-16-2012 at 06:30 AM.
Reply With Quote
  #7  
Old 08-16-2012, 06:32 AM
DIYguy's Avatar
DIYguy DIYguy is offline
Senior Member
 
Join Date: Sep 2008
Location: Kitchener, Ontario, Canada
Posts: 2,480
DIYguy will become famous soon enoughDIYguy will become famous soon enough
Send a message via Yahoo to DIYguy
Default Re: Did anyone just run battery pack without a BMS ???

This topic has been beaten to death. Do some searches and read about it.

There are many examples of BMS-less lifepo4 (chemistry is important here) running for years, including my own.
Reply With Quote
  #8  
Old 08-16-2012, 06:37 AM
DIYguy's Avatar
DIYguy DIYguy is offline
Senior Member
 
Join Date: Sep 2008
Location: Kitchener, Ontario, Canada
Posts: 2,480
DIYguy will become famous soon enoughDIYguy will become famous soon enough
Send a message via Yahoo to DIYguy
Default Re: Did anyone just run battery pack without a BMS ???

Quote:
Originally Posted by WarpedOne View Post
Real BMS is also an automatic top balancer.
If you love to manually balance your cells after every few charges, you are welcome. Others like to have it done automaticaly.
There is no need to balance manually if you have no parasitic, unbalance loads. Have you tried? There is no mechanism for cell drift other than perhaps extreme temperature differences within the same pack. I have never re-balanced my pack after 15000 km and going on two years. Others have longer/more miles than me.
Reply With Quote
  #9  
Old 08-16-2012, 06:49 AM
Jan's Avatar
Jan Jan is offline
Senior Member
 
Join Date: Oct 2009
Location: Gouda, NL
Posts: 898
Jan is on a distinguished road
Default Re: Did anyone just run battery pack without a BMS ???

Quote:
Originally Posted by DIYguy View Post
...never re-balanced my pack after 15000 km and going on two years.
It seems a little that people with no problems with their choice of components only share their positive experience when provoked.
Reply With Quote
  #10  
Old 08-16-2012, 08:25 AM
jeremyjs jeremyjs is offline
Senior Member
 
Join Date: Sep 2010
Posts: 370
jeremyjs is on a distinguished road
Default Re: Did anyone just run battery pack without a BMS ???

Quote:
Originally Posted by Jan View Post
It seems a little that people with no problems with their choice of components only share their positive experience when provoked.
That works both ways. Many a BMS has failed and killed cells/or burned buildings, boats, cars, etc. We're only talking lifepo4 here obviously. There is little evidence of any cell drift in this chemistry without unbalanced loads on the pack and several years worth of evidence with certain people bottom balancing, using a reliable/accurate charger, slightly under charging the pack and never having a problem, even with instances of running the pack nearly flat. There are conditions for going BMS-less, but in my eyes it's been well proven to work very well and reliably. I might say even more reliably than running with a cell level BMS. I'm not saying you shouldn't check up on the pack once a year or so, but balancing should only be necessary once unless you did something wrong from the start. There's more than a little evidence showing this to be the case.

Last edited by jeremyjs; 08-16-2012 at 08:40 AM.
Reply With Quote
Reply

Share or Bookmark this

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

 
Support DIY Electric Car
Sponsors

All times are GMT -6. The time now is 07:09 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.

Ad Management by RedTyger