Best practices for dumploads when using Lithium Iron Phosphate (LFP) batts?

Started by Luke, June 21, 2022, 12:37:52 PM

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Luke

Hi all,

Looking for best practices with dumploads on the Classic using LFP batt banks. 

I am finding:
1. 1V width for Aux2 PWM is too wide for LFP batts.   Large dump load can pull your batts from 53.0V to 52.0V...that  is ~85% to 30% SOC if the day gets cloudy.
2. 0.1V resolution Hi/Lo volts of Classic  Aux1-Dumpload you will loose ~10% of your battery charge if Sun hides away.  Especially if you are in the flattest part of the LFP discharge curve around 50%.  ...which is exactly where I keep my LFP bank during the summer for improved longevity!

It would be much, much better if Classics had a 0.01 V resolution and option for a much narrower PWM width (0.1 V or so) !  At least for a 48V LFP system this would allow efficient use of built-in Waste-not/Diversion capability.

The only efficient, automatic, solution seems to be custom:
A. Use my Victron BMV-702 to trigger a relay when my SOC reached....activates Arduino.
B. Arduino senses current (use analog inputs, known resistance, measured voltage drop) using my 500W 50mV shunt.
C. Arduino PWM's my Hot water tank element with Solid state relay to ensure battery output stays 0 Amps.

I'd rather a drop in/commercial solution!  Looking forward to hear what others have done and can suggest! 

My system is off-grid, self installed:
- 24 280W panels in 2 strings
- 2 Midnight Solar Classic 200 Charge controllers
- 1 Magnum 4kW inverter
- Replaced my 16 (8x2stirngs) American Made Trojan L16's after 7 years with 32 (16x2 strings) Chinese LFP's (21.5 kWhr).  Breaks my heart.  But DIY LFP packs offer 2x the power/life for same money as lead.
- Each string of LFP cells has its own BMS and balancer.
- Added a Victron BMV-702 + shunt for SOC monitoring.

Thanks in advance,
Luke