[RE-wrenches] LiFePo4

boB at midnitesolar.com boB at midnitesolar.com
Thu Sep 18 11:26:42 PDT 2014


Yes, well of course there are these details.
   If the LFP surge is a limitation, but not its amount of overall
energy storage, then you may have to add a very large amount of LFP 
storage to overcome that which
could be overkill and cost a lot of extra $.

It's all a compromise.

boB


On 9/18/2014 10:55 AM, Starlight Solar Power Systems wrote:
> 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 
> <mailto: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 
>> <mailto: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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