MPPT Charge Controllers and LiFePo4

[Natasha Lee]

Hi everyone,

My husband and I have been reading through the forums here and found so much helpful information as we finally set up and connected our system last week.

We have a 9KW solar system, 18 x 500w panels and 2 MPPT charge controllers Which are 150volt and 100 amp, with a max Pv input of 5200 watts and the two of them are wired in parallel. Our system is 48 volts and we currently have a 6000 watt inverter and our battery consists of 16 LiFePo4 300 amp hour battery cells connected in series.

After many searches I am here to ask:

1. which brand of charge controllers (aside from victron) are suitable for 300amp LiFePo4 cells wired in series to 48v.?
2.We have some concerns that our charge controllers are not the best for LiFePo4, as the settings do not seem to allow for them. We have noticed the CC charges the batteries to capacity basset on the parameters we have set, and then goes into Float mode quite quickly. Even when we begin drawing from the batteries and consuming power it remains in float mode, especially if it is a low draw. If we turn the CC off and back on, it resumes charging and displays as 77% full and charges to full.

These are the settings we can program on the CC and we used the document provided in another forum to calculate the settings, I will send through a photo of that too shortly.

Thank you in advance for any help or suggestions you may offer

 

[Supervstech]

Lithium doesn’t need a float setting. I would set the charger to the desired charge point, usually 3.5 or so volts per cell, and set float the same.

 

[Supervstech]

The over voltage recovery setting should allow charging to continue once battery drains to that point. Solar should output as long as there is demand.

 

[reg]

Lithium doesn’t need a float setting. I would set the charger to the desired charge point, usually 3.5 or so volts per cell, and set float the same.

That may be true generally, but Lithium Werks, for example, calls for a float charge of 13.8V (standard charge is 14.6V) if a Valence U1-12RT battery is unused for a month. From the User Manual and Data Sheet:

User Manual:



Data Sheet (see the Specifications box):

 

[Just John]

I would use 3.4v (54.4v) as a float voltage, that's about 95% charged. But to each his own.
3.5 is already well into the knee.

 

[Steve_S]

Hmm different.
Now I use a Midnite Solar Controller and separate Inverter system so that "tech" is different.
I run 24V system. I have the SCC setup to BULK/Boost charge to 28.2V,(3.524vpc) then it floats my battery bank at 27.9V (3.487vpc).
Once the SCC hits 100% (batts concur) and goes to float, the amps being provided by the SCC service the Inverter & any demands on it, while providing low amps to the battery packs (I have Active Balancing so there is always a little trickle going in (<2A for 4 packs) until fully topped & level).
IF my batts are 100% and I turn on coffee maker (60A draw) the SCC will provide ALL of the amps required IF there is enough sun on the panels to produce it. The battery bank will not be used. If the Solar can only provide 50A, then 10A will be drawn from the battery bank. If the Batts drop to 98% after that, then the SCC goes back into Bulk @ 28.2V and once 100% is attained then back to Float.

I run 4 Battery Packs ATM, 2x24V/280AH & 2x24V/175AH in one bank. (910AH total soon to be 1190AH)
I've spent many months testing and cycling systems and scenarios including 6 weeks of Thrash Tests (semi abusive, edge case, hard tests)
I am offgrid and solar only, so the system runs 7/24/365. The Samlex EVO Inverter is only drawing 13W so there is always a bit going out.

These are my Midnite Classic-200 SCC Settings, my Inverter/Charger has an identical profile.
Multiply the values X2 for a 48V system
Absorb: 28.2 for 15 minutes (3.525vpc) (boost for some brands ?)
Equalize: OFF
Float 27.9V (3.4875vpc)
MIn Volts: 22.0 Max Volts: 28.7
Rebulk Voltage: 27.7
End Amps: 14A

This get's the bank charged to full with high amps (Constant Current) and then float (Constant Voltage) tops off so the cells are on average between 3.475-3.500. I am running 7/24/365 so float is used up by the Inverter + provides whatever the packs will take to top off.

Hope it helps, Good Luck.
You likely should download the Voltage Chart linked in my Signatrure. It will help clarify a few things.
.