[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
*** Please consider the environment before printing this e-mail
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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