I have a solar electric system from many years ago. The setup runs on 400 to 480VDC from a string of solar panels.

I have a unit that converts 250 to 600VDC to 48VDC for charging lead acid batteries.

I was thinking of stringing a bunch of the inexpensive 12.8VDC $30 LiFePo4 batteries into a 4 kWh energy store.

Everyone including the manufacturer Miady tells me not to parallel the batteries. So series it is.

Could some one point me in the right direction for building a charger for this setup? To maximize battery life Miady says to stay under 0.2C charge rate. This would be around 960watts in the setup I am envisioning. So 400 or so VDC at 2 Amps.

I have around 400VDC at 7 Amps to work with. What would be appropriate to regulate the current so the battery bank charge is limited to a particular current? I am thinking along the lines of a switch mode regulator.

In addition to regulating the current, I would probably need a voltage cut off as LiFePo4 do not need a float voltage.

More or less the process would be: cut out, wait till VDC around 400 and then charge batteries at up to 0.2C (around 2amps) and then cut out later when the voltage reaches a certain level on the battery side.

Some people may feel I need to shunt the excess power somewhere. With solar electric panels you can just not draw off the power and nothing bad happens.

  • \$\begingroup\$ Q!: For non CV mode charging (CC only) what cutoff voltage do you use for "12 Volt) LiFePO4. (14.4V or ...?) Q2: Can you provide a link to the batteries? \$\endgroup\$ – Russell McMahon Feb 12 at 7:46
  • \$\begingroup\$ Non float LiFePO4 CC is usually at 3.6 to 3.65V./cell. You may choose different. 400/(4x 3.65) = 27 x "12 Volt" LifePo4. Vmax = 27 x 14.4 = 389V. Enough headroom with care. Slightly fewer would make life easier. Nominal terminal voltage charged at 3.2V/cell = 3.2 x 4 x 27 345V. So while your cutout voltage is 389V, you re-cut in is rather lower. Maybe not as low as 345 but well under 389. || WARNING: DO NOT choose a re-cut-in voltage that is rapidly reached by settling, self discharge or BMS loads. With no real load a battery should last say 1 day + before cutting back in. Or longer. Rapid ... \$\endgroup\$ – Russell McMahon Feb 12 at 9:16
  • \$\begingroup\$ ... short low capacity charge cycles eat away at your battery capacity for no purpose. (Ask me how I know :-(). \$\endgroup\$ – Russell McMahon Feb 12 at 9:17
  • \$\begingroup\$ Recently came across an excellent practical article on this. marinehowto.com/lifepo4-batteries-on-boats/… \$\endgroup\$ – Brian Drummond Feb 12 at 12:24

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