Charge Controller Bulk/Float Set Points

nate7058

Hi everyone I am working a solar charging station for electric bikes and am in the process of wiring/setting up each component. I am using a Xantrex C35 Charge controller to regulate the charging of four LiFePO4, 70Ah 3.2v batteries wired in series (12.8V) from two 180 Schuco Solar Panels in Parallel (24Volt). To address the voltage issue there is a 24V DC to 12VDC converter right after the solar panels.

I know that I need to set the bulk/float voltage values in the charge controller, but have no idea how to decide on these values... I assume the max charge/discharge voltage are needed for this:

NOMINAL CAPACITY: 70Ah
STANDARD DISCHARGE CURRENT/AMPERAGE: 0.3C-21
MAX DISCHARGE CURRENT/AMPERAGE (10 SEC) 10C-700
STANDARD CHARGE CURRENT/AMPERAGE: 0.3C-21
MAX CHARGE CURRENT/AMPERAGE: 3C-210
MAX CHARGE VOLTAGE: 3.6
DISCHARGE CUT-OFF VOLTAGE: 2.5

A little more background on this system: the four LiFePO4 batteries are then ran to an inverter with plugins for the electric bike charger. Because this station will be outdoors the batteries, inverter, charge controller, DC-DC converter will be held in a box with the temperatures being regulated via two 12V DC fans and a 12V DC heater that are also powered off of the battery bank.

Thank you for any help/suggestions

westbranch

Re: Charge Controller Bulk/Float Set Points

If you used a different ie MPPT like a MidNite KID Charge controller http://www.midnitesolar.com/pages/kid/index.php, you could do away with the down converter and the XC-35 PWM charge controller. The Kid is much more versatile in the custom setting department...

Ethan Brush

Re: Charge Controller Bulk/Float Set Points

nate7058 wrote: »

Hi everyone I am working a solar charging station for electric bikes and am in the process of wiring/setting up each component. I am using a Xantrex C35 Charge controller to regulate the charging of four LiFePO4, 70Ah 3.2v batteries wired in series (12.8V) from two 180 Schuco Solar Panels in Parallel (24Volt). To address the voltage issue there is a 24V DC to 12VDC converter right after the solar panels.

I know that I need to set the bulk/float voltage values in the charge controller, but have no idea how to decide on these values... I assume the max charge/discharge voltage are needed for this:

NOMINAL CAPACITY: 70Ah
STANDARD DISCHARGE CURRENT/AMPERAGE: 0.3C-21
MAX DISCHARGE CURRENT/AMPERAGE (10 SEC) 10C-700
STANDARD CHARGE CURRENT/AMPERAGE: 0.3C-21
MAX CHARGE CURRENT/AMPERAGE: 3C-210
MAX CHARGE VOLTAGE: 3.6
DISCHARGE CUT-OFF VOLTAGE: 2.5

A little more background on this system: the four LiFePO4 batteries are then ran to an inverter with plugins for the electric bike charger. Because this station will be outdoors the batteries, inverter, charge controller, DC-DC converter will be held in a box with the temperatures being regulated via two 12V DC fans and a 12V DC heater that are also powered off of the battery bank.

Thank you for any help/suggestions

I dont know much about charging specifics of Lithium ion batteries, but do know that it is more complicated than FLA so you have to know what you are doing and/or have the right equipment. There may be more to it than just a voltage setpoint.

westbranch

Re: Charge Controller Bulk/Float Set Points

Nate there are a raft of threads on LiFePo4 batteries here on the forum, 150 in all if you search LiFePo4...

LOTS of info in the latest ones as there is a lot of dialogue on pros and cons.

hth

karrak

Re: Charge Controller Bulk/Float Set Points

To answer your question the maximum bulk charge voltage should be 3.6*4 (14.4 volts). To increase the lifespan of the LiFePO4 battery and reduce balancing issues I would reduce this to 3.5*4 (14 volts) or even better 3.45*4 (13.8 volts). Charging to 13.8 volts will charge the LiFePO4 battery to more than 95%. I see the Xantrex controller has a fixed one hour 'absorb' period which is not ideal for LiFePO4 batteries. Again to maximise battery life I would set the float voltage to 3.3*4 (13.2 volts) which is the resting voltage for a nearly full LiFePO4 battery.

Are you planning to have any individual cell monitoring and balancing for the LiFePO4 battery?

Are you planning to use the Low Voltage Disconnect option on the Xantrex?

Can you give more details of the 24 volt to 12 volt converter.

I charge our electric bikes which have 10Ah 48 volt LIPO battery packs directly from our 24 LiFePO4 volt battery using this http://www.hobbyking.com/hobbyking/store/__44112__Turnigy_Reaktor_300W_20A_6S_Balance_Charger.html. The only problem is that I have to break the 48 volt bike battery down to three 12 volt batteries to charge. The other option which I an currently implementing is to use a 24 to 48 volt power supply. I will still use the Turnigy supply to balance the LIPO batteries from time to time.

Simon

PNjunction

Re: Charge Controller Bulk/Float Set Points

nate7058 wrote: »

Hi everyone I am working a solar charging station for electric bikes and am in the process of wiring/setting up each component. I am using a Xantrex C35 Charge controller to regulate the charging of four LiFePO4, 70Ah 3.2v batteries wired in series (12.8V) from two 180 Schuco Solar Panels in Parallel (24Volt). To address the voltage issue there is a 24V DC to 12VDC converter right after the solar panels.

Do NOT ENABLE equalization!
Do NOT use temperature compensation probe
Use a voltmeter, and not the silkscreen label and set to 13.9 to 14.0v. Use something like a Fluke, and not a junk multimeter! Better to be a tad conservative here.
For float, set to 13.7v, although float is not really necessary. It will serve as a minor parasitic load catch, and also in case the absorb timeout occurs too fast during solar conditions.

The Xantrex units will work in a pinch, although they are not ideal for lifepo4. I've found the 1-hour absorb timeout too limiting for solar, but what you have may get the job done in a hour or so of perfect conditions.

Monitor your cells to make sure none go below 3.1v (12.4v total). Easy to remember and conservative. Don't be tempted to push it further, even if the battery seems willing.

Use of a hobby charger like Karrak suggested will get your cells in shape initially. I have personally used an iCharger 306B for this purpose but you may also consider a Tecmate-Optimate TM-291 5ah lifepo4 (4S) specific charger. It is two-terminal, and works well without having to go the balance-lead route. (it is not your average CC/CV charger - different algorithm at the top, along with IR and lifepo4 charge curve testing / monitoring)