# What do LiPo bms units do?



## Woodsmith (Jun 5, 2008)

I'm getting interested in the LoFePo4 BMS now too. When I have a car to use LiFePo4 I don't want to destroy the most expensive component by mis charging them.


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## Qer (May 7, 2008)

BMS is a very loose term and seems to come in flavours that can include any combination of:


Monitoring pack voltage
Monitoring individual battery voltage
Shunting current over the individual cells to maintain pack balance
Protecting from pack under voltage
Protecting from individual battery under voltage
Monitoring temperatures
Protecting from over temperatures
Feedbacks status back to the charger
and probably a few more variables. When it comes to LiFePO4 what you want is a BMS that at least handles 2 and 3 and probably 8, but the more the better. It's of course a question of what you plan to do too, if you're going to stress the batteries with brutal charges/discharges you probably want to keep a good eye on temperature as well.

The smarter the BMS, the less you risk to stress the pack and the longer it'll live. This, of course, goes for any chemistry.


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## Woodsmith (Jun 5, 2008)

I guess all this comes at a price too.

I notice that the individual 3.2v Thundersky cells are popular but Lifebatt do 12v batteries with built in monitoring at more cost.

Is it a complicated process to monitor, say, a pack of 72 Thundersky cells or is the built in monitoring in the Lifebatt style of battery worthwhile?


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## dimitri (May 16, 2008)

Woodsmith said:


> I guess all this comes at a price too.
> 
> I notice that the individual 3.2v Thundersky cells are popular but Lifebatt do 12v batteries with built in monitoring at more cost.
> 
> Is it a complicated process to monitor, say, a pack of 72 Thundersky cells or is the built in monitoring in the Lifebatt style of battery worthwhile?


There is no way to give a single answer to this question. It all depends on your skill / budget / comfort level, etc. 

If you know how to hold a soldering iron, you can make DIY BMS by getting some VoltBlochers or TPPack kit. Those will take care of most critical BMS functions to minimize the risks, but not completely bulletproof.

You can spend more $$$ to buy peace of mind, but at some point ask yourself if its worth spending $$$ on fancy BMS or just spend few $$$ to replace one or two cells over the years. Its all up to you.

I personally would not pay more than 15% of battery cost for BMS, no matter how fancy it is.


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## Woodsmith (Jun 5, 2008)

dimitri said:


> I personally would not pay more than 15% of battery cost for BMS, no matter how fancy it is.


Given a detailed circuit diagram I can make up something with no problems. Testing is another matter. I'll worry about that later on.

I guess that 15% is a good ratio of costs to factor in especially for those who can put in more time then cash.

Thanks.


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## the1cyrus (Aug 6, 2009)

speaking of bms.. is there a list somewhere of the top bms kits/systems for LiFePO4? Ive really been having a hard time finding anything.. I would love just simple bms units that you connect to each of the batteries to protect against over/under charge. I dont really need anything else.


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## brainzel (Jun 15, 2009)

my "short" personal DIY-BMS list:

http://www.voltblocher.com
- one per battery
- "balances" during each charge
- adjustable shunt voltage
- only upper voltage, no lower voltage protection (I think)

http://www.tppacks.com/products.asp?cat=26
- boards for 4-8-12-16-20-24 cells and up
- "balances" during each charge
- upper voltage and lower voltage protection

(rectified me, if I'm wrong)


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## hardym (Apr 2, 2008)

Here is a good comparison of the latest in BMS functionality for a 48 cell Highway capable EV on LiFeP04: 

http://convertthefuture.com/shop/index.php?main_page=page_4

(follow the link to Lithiium_BMS_Tutorial.ppt

Most of the non-home built units are $1500-$2000.

The PPT is basically a product marketing position for a new BMS on the website. Roughtly $950 for a centralized architecture BMS with display, balancing, charge cutoff etc. You can also get data logging, and current monitoring.

This is the first BMS based on the LTC6802 chip, that was introduced last year by Linear.

There's also a users manual posted, that has all the technical info:

http://convertthefuture.com/shop/index.php?main_page=page_2


Mark


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## the1cyrus (Aug 6, 2009)

I like the BMS that is distributed. Each cell has its own circuit. Anyone happen to know of a schematic of a system like this? I think id rather build my own to keep costs down.

However so far the cheapest bms ive found is http://www.evpower.com.au/-Thundersky-Battery-Balancing-System-.html


and http://www.voltblocher.com/Buy.html is nice too


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