[RE-wrenches] LiFePo4

[boB at midnitesolar.com]

Oh, OK then.  I thought that somebody had noticed a voltage drop when 
surging.

When people ask me if they can use LFP or other Lithium chemistry 
batteries, I tell them yes because
when they buy a LFP battery system, (or one of their Lithium Ion 
relatives)  it has all the protection
and balancing built into it.  It's simple to use with an off the shelf 
charge controller.

We were asked  to add an input so that the LFP BMS could  tell the CC 
when to stop charging.
That was a special case and I don't remember who's system that was.  
Normally, I see what you just
mentioned I think where it charges up to a voltage (Absorb) and then 
immediately goes to Float
which would be some very low voltage, enough to be called "off". 
Couldn't be easier.

Now we're just talking price compared to a tried and true LA battery ?

Some may remember Boeing's little problem recently though and that may 
turn them off.

boB



On 9/18/2014 11:33 AM, Starlight Solar Power Systems wrote:
> Truth is there is likely more than enough capacity in the LFP battery 
> the customer has but the manufacturer built a limit into the "box". 
> The limit of our battery is set to 10C for 30 seconds. WAY more than 
> any inverter can draw with even a small bank.
>
> Larry
>
> On Sep 18, 2014, at 12:26 PM, boB at midnitesolar.com 
> <mailto:boB at midnitesolar.com> wrote:
>
>
> 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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