[RE-wrenches] LiFePo4

William Miller william at millersolar.com
Mon Sep 15 23:31:00 PDT 2014


Friends:

 

Here are the specific batteries I am looking at:

 

William

 

 

Gradient Cap_mini
Lic 773985
 <http://www.millersolar.com/> millersolar.com
805-438-5600

 

From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On
Behalf Of Jarmo.Venalainen at schneider-electric.com
Sent: Monday, September 15, 2014 7:09 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] LiFePo4

 

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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