[RE-wrenches] LiFePo4

Jarmo.Venalainen at schneider-electric.com Jarmo.Venalainen at schneider-electric.com
Wed Sep 17 09:15:49 PDT 2014


Hi:

I understand from your e-mail that the EV DIY industry has been 
successfully using a Lion pack with minimal monitoring/battery management 
and perhaps the packs will continue to work out.  However, a relatively 
small number of systems do not validate the safety and reliability of a 
solution.

Over the last decade or so, the biggest application of series connected 
Lion cells has been in laptops.  From that it has been learned that 
battery management is needed for reliable, long life and safe operation. 
Indeed, to meet that need all the major semiconductor manufacturers have 
designed a multitude of dedicated battery management chips.  Here for 
example are links to some of the Texas Instruments offer and its easy to 
find much more from any number of manufacturers.

http://focus.ti.com/download/trng/docs/seminar/Topic%202%20-%20Battery%20Cell%20Balancing%20-%20What%20to%20Balance%20and%20How.pdf

http://www.ti.com/lit/an/slaa478/slaa478.pdf
http://www.ti.com/lit/an/slyt322/slyt322.pdf
http://www.ti.com/lit/an/slua463/slua463.pdf

http://liionbms.com/php/about_us.php


The specifics of how much and how complex the monitoring should be could 
be debated, but as rule the key elements are precise voltage monitoring 
and regulation, cell balancing, temperature sensing and for large banks 
over current protection.  Indeed, we have found these functions in all the 
Lion battery packs which we have so far evaluated.

JARMO

_____________________________________________________________________________________ 


Jarmo Venalainen  |   Schneider Electric   |  Solar Business  |   CANADA | 
  Training & Development Specialist - Senior 
Phone: +604-422-2528  |   Fax: +604-420-1591  |   Mobile: +604-505-0291 
Email: jarmo.venalainen at schneider-electric.com  |   Site: www.SEsolar.com 
|   Address: 3700 Gilmore Way, Burnaby, BC V5G4M1 


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From:
Starlight Solar Power Systems <larry at starlightsolar.com>
To:
RE-wrenches <re-wrenches at lists.re-wrenches.org>, 
Date:
09/16/2014 10:14 AM
Subject:
Re: [RE-wrenches] LiFePo4
Sent by:
"RE-wrenches" <re-wrenches-bounces at lists.re-wrenches.org>



Jarmo,

I must say I disagree with most of what you have said. In the EV DIY 
industry, there are a growing number of conversions using LFP battery 
banks without any BMS or EMS at all. These pioneers are risking there 
multi-thousand banks because they have learned something about these 
cells. They will stay in balance and perform perfectly as long as they 
prevent over and under voltage or charging a frozen battery with high 
current. 

My reply numbers correlate to yours:
1.      I agree for most customers a BMS is necessary.
Disagree: The battery systems (with BMS) that I sell do not regulate cell 
voltages during charge or discharge. They only adjust voltage after the 
cells reach 3.55 Vpc. However, at that 3.55 Vpc, I have terminated charge 
and entered a float/maintenance charge. On my personal battery bank, 24 
cells, I have charged and discharged daily since May. My cell voltages 
still only vary by 0.02 volts per cell.
Disagree: There is no temperature adjustment for voltage during charge or 
discharge. All temperature compensation circuits are removed from the RE 
equipment per manufacturers specifications.
Disagree: We provide a continuous float voltage per manufacturer 
specifications. A float voltage can keep the battery at any SoC. I program 
for 90%.
Our CPU provides protection for over current BUT that will likely never 
happen because the cells can be charged and discharged up to 10C and 
continuously at 3C!
2.      Disagree: No need to limit inverter current as the battery can 
produce 10C.
3.      Disagree: Ditto above.
4.      Disagree: No need to limit inverter charge current because of the 
10C capability.
5.      I agree that for most consumers a BMS is required. So far we 
install the BMS provided by the manufacturer on every system. I will be 
installing systems without BMS in the future but openly for select 
customers.

I have not found that LFP batteries are “amazingly sensitive”. I have 
discharged cells past 100% DoD, in fact as low as 0.35 Vpc, and recharged 
without any harm. I have started very complex loads like air conditioners 
with repeatable success from small battery banks. While I do not recommend 
this, I believe there is a robustness with modern LFP cells. 

The benefits of LFP batteries are many. Each year I recycle between 30,000 
and 40,000 pounds of lead acid batteries. MOST of them are damaged by the 
consumer deficit charging them. I consider this to be a very unfavorable 
aspect of lead acid when compared to LFP batteries that never need to be 
fully charged or equalized or watered.

One thing that will damage LFP batteries is if a cell in a series string 
is ever discharged below 0 volts, it will take on reverse polarity and is 
usually not recoverable. But, you can simple replace it and balance it 
with the other cells. 

I welcome your comments.

Respectfully yours,

Larry Crutcher


On Sep 15, 2014, at 8:08 PM, Jarmo.Venalainen at schneider-electric.com 
wrote:

Hi: 

We've done some testing and operation of Lion battery banks in the 10 kWh 
range with our inverter chargers and so far the most important findings 
are: 

1. The battery pack must have its own battery management system to: 
a. regulate the individual cell voltages during charge and discharge 
b. balance the cell voltages during charge and discharge 
c. adjust cell charging voltages due to temperature variations 
d. not provide a continuous float voltage to the battery bank 
d. provide a built in safety cutoff disconnect, which turns off the 
current if for any reason either the charge or discharge current is too 
high. 

A series string of Lion cells, without the battery management system 
functions above is very likely to result in damaged cells or worse.  The 
most likely mechanism which ultimately causes damage is individual cell 
temperature or cell to cell voltage imbalance. 

2. The inverter system must be designed so that the inverter never draws 
so much current that the current limit circuit in the battery bank is 
triggered. 

3. It may be necessary to adjust the current limit circuit as they 
typically respond within milli-seconds and may be triggered on simple 
power up as the inverter cap bank charges up. 

4. When working with Lion, the inverter/charger should be configured so 
that it is a simple current limited voltage source when in charge mode. 
The built in battery management system should take care of the required 
charge cycle operation. 

5. Whether the pack is LiFePO4, Lion polymer or other does not appear to 
be as important as a having a reputable brand and more importantly a well 
designed built in battery management system. 

The short version is that lead acid cells are amazingly forgiving with 
regards charging and discharging voltages, currents and temperatures. Lion 
cells on the other hand are amazingly sensitive and going out of bounds on 
any of these can and will cause irreparable damage.   

Given the sensitivity, it doesn't make sense to take on responsibility for 
Lion battery management. 

Management of that sensitivity and responsibility is best left to the 
battery bank manufacturer. 

JARMO









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