[RE-wrenches] LiFePo4

[Starlight Solar Power Systems]

boB,

When charging a LFP battery, the  charge terminates when absorb voltage is reached unless you have an enormous charger in which case you will need to terminate based on AH returned. You would also need to isolate the LA battery to finish charging it. Plus, LA needs higher voltages than LFP. I don’t think would solve Williams problem. A better solution would be another LI-ion bank that is not current limited. Just a 50AH bank could provide an additional 500amps.

Larry

On Sep 18, 2014, at 11:09 AM, boB at midnitesolar.com wrote:


One thing that could possibly be done is to have a smaller lead acid battery in parallel with the LiFePo bank
so the LiFePo contains most of the "energy" storage, but the LA can supply the surges and low impedance
source voltage.

boB



On 9/18/2014 8:53 AM, Starlight Solar Power Systems wrote:
> William,
> 
> This is one reason I choose to avoid the "black box" approach to Li-ion batteries for our customers. There are several companies offering enclosed battery/controls like the OES. Their approach does not match well to the off-grid and mobile markets that I serve. 
> 
> Since most LFP batteries can handle 10C for up to 30 seconds (Winston LFP can surge to 20C!) and 2C-3C continuous discharge, it makes sense to me to design a protection system that can't utilize that capability. Our LFMP battery has an external CPU that I can program. We also use external high current relays for the protective circuits so the full load of the inverter can be put to use.
> 
> With a LFP battery bank of 48 volts and 400AH, somewhat small for off grid, the battery can operate continuously at 1200 amps or about 60kW output and surge to 4000 amps/600kW. This obviously is way beyond the demand capability of the inverter(s). A quick look at the SI6048 data sheet shows maximum AC current is 11kW for 3 seconds.
> 
> I am glad you noticed the extremely low voltage sag under load. This feature means lower current which should translate to longer life for inverters. It also means starting loads that you can’t with a similar sized lead acid battery. A 1C load typically has only 0.2 volt sag. For 48 volt systems, this means maintaining 51.2 volts throughout the full battery use. In fact, a 1C load will still only have 0.2 volt drop until about 90% discharged. Simply amazing.
> 
> Larry Crutcher
> 
> 
> On Sep 17, 2014, at 10:15 PM, William Miller <william at millersolar.com> wrote:
> 
> Larry:
>  
> …...The internal battery management system will allow them to disconnect under surge loads, just when you need your batteries to stay connected.  I will be conversing with the manufacturer to learn more about them.
>  
> William
>  
>  
> 
> 
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