[RE-wrenches] Lithium Iron Phosphate

[Starlight Solar Power Systems]

Hello Peter,

Panasonic does not make lithium iron (LFP or LiFePO4) batteries, theirs are Lithium cobalt oxide (LiCoO2). Because RE system commonly have multiple charge sources, I think LiCoO2 is not as suitable due to the poor handling of over voltage events. Lithium iron phosphate (LFP) are far more overcharge tolerant and readily available in high capacity cells. Another problem with using LiCoO2 cells is they require 4.2V/cell to charge making it a difficult fit with common RE equipment.

Each manufacturer has recommended charge profiles. It will be difficult to follow these unless the battery is disconnected from loads while charging.

For all Lithium batteries, the discharge rate is many times greater than would ever practically be used in a RE system. Most can be discharged at 3C and surge up to 10C. A 20kWh bank could supply 60kW continuously. Don’t try that with your LA battery!
 
Low voltage disconnect is (or should be) built into any LFP battery system as permanent damage can happen instantly. I recently had to repair a LFMP propulsion battery system (not designed by us) that was over discharged one time. It cost $6000 for me to make the repairs and install the protective solenoids. The Battery manufacturer had warnings in their installation manual against not using proper LVD but the builder skipped that part.

Magnum Energy has a come out with a CC/CV profile that works well with LFP batteries but it’s still not the needed profile. There are many differences and factors to be addressed for the RE industry. One hurdle to overcome in RE applications is that varying loads are applied while charging. A proper LFP charger will vary the power during the charge cycle but has no way of knowing that loads are present. That creates a real challenge for equipment makers but can be resolved by designing CAN interface and software that communicates with chargers and control CPU’s to make the adjustments on the fly.  

Larry Crutcher
Starlight Solar Power Systems


On Aug 18, 2015, at 8:31 AM, Peter Parrish <peter.parrish at calsolareng.com> wrote:

I too am interested in LiFeP batteries. I think we need some more information on charge/discharge cycles. I’ve bits and pieces, but there must be some solid data from Panasonic or one of the other LiFeP manufacturers. Such as,
 
Recommended charge cycle
Maximum discharge rate
Low voltage disconnect
 
Also I expect all of the CC manufacturers are thinking about LiFeP or similar Li technology. Maybe if one of us has a friend at Outback Power, Morningstar, Midnite Solar, we could find out more.
 
-          Peter

 
Peter T. Parrish, Ph.D.
President, SolarGnosis
1107 Fair Oaks Ave.
Suite 351
South Pasadena, CA 91030
(323) 839-6108
petertor at pobox.com
 
From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Daniel Young
Sent: Monday, August 17, 2015 2:29 PM
To: 'RE-wrenches'
Subject: [RE-wrenches] Lithium Iron Phosphate
 
Hello Wrenches,
 
I’ve been trying to think through how I can mate some lower cost charge controllers (prostar 15A 24v PWM units, or the new 30A Midnite solar Brat) to some 8s LiFePo battery banks.
 
As I understand it, lithium batteries meant to be charged to around 3.65V per cell (29.2V for the 8 cell bank) and then the charge is terminated, not put into a float mode.
 
Assuming I use a PCM to protect the batteries, so that each cell has a balancing function and also over/under voltage voltages and start to overcharge/degrade?
 
Again, I’d plan to have a full PCM circuit protecting cells and balancing them, but I’d like to have a good way to chargeJ).
 
With Regards,
 
Daniel Young,
NABCEP Certified PV Installation ProfessionalTM: Cert #031508-90