Ideal charge/discharge profile for Solar Charged LifePo4 in an RV using MPPT - Page 1
Forum section: electronics / general

author: ornea
author_rank: Unknown
timestamp: October 14, 2023, 03:50:56 am
content: Three questions:-1. has anyone observed their RV MPPT Solar chargers actually prioritising on using available solar power to drive a load rather than discharging the battery. It has not been my experience.2. Would floating LiFePo4 at a fixed 13.5V be damaging or dangerous consider the warnings against CV charging LiFePo4 and the iTech120X discharge curve attached. At 13.5V I would consider it not actually charging and therefore safe and would ensure that the system would prioritise on solar energy if available and fixed at 13.5V. If damaging/dangerous, could you please point me to the relevant source.3. Would you consider it a bad design to continually discharge/charge a LiFePO4 while there is available solar energy that could be powering the load.For background, read on.Two MPPT chargers have measured cut in/cut out charge voltages as follows:-1. Echo EPP3Mk1 13.2V - 14.0V2. iTechWorld 40Amp MPPT 13.05V - 14.4V (Mode Setting type "4-3.2"). Chart attachedThe low cut in values above highlight the fact that any load connected to the system will deplete the battery to approx 70%before charging commences according to the manufacturers data attached.I find it very frustrating that the chargers will begin to deplete the battery while the sun shines. I have observed the fridge running for hours before the charger cuts in.This has the following consequences:-1. Reduced battery life2. Highly likely reduced charge when the inevitable happens and the sun goes away resulting in a more depleted battery for the night or cloudy times than neccessary.As such I have gone against the manufactures recommendations and decided to use Mode setting type USR1 and set the float level to 13.5V while active loads attached and mode setting type 4-3.2 while stowed.Any thoughts, personal observations and shared experiences welcome.I posted a similar question on the PICLIST and responses tended to agree that constantly dipping into the battery store is not ideal.

author: Faringdon
author_rank: Super Contributor
timestamp: October 14, 2023, 10:36:45 am
content: Please forgive me if ive misread but , yes, i thought it went without saying that whilst there's solar, the loads should be powered by solar, and not from the battery.But maybe they messed your systme up?When i worked in battery storage briefly, there was a rule of "zero-ing" the meter....ie not exporting any power if possible.But maybe your provider is concentrating only on that...and not caring whether its battery or solar that does the powering.

author: ornea
author_rank: Unknown
timestamp: October 14, 2023, 11:22:52 am
content: Hi Faringdon,thanks for the reply.there is no provider in this scenario, just a self contained camper trailer.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 16, 2023, 02:30:55 am
content: 13.5V will be perfectly fine. It will be 3.375V/cell which is probably a bit lower than what you want. I generally go for 3.45V/cell which would be 13.8V.Note that you'll want to slow charge to the full 3.6V/cell (14.4V) occasionally (e.g. once a month) to allow balancing to work better, LiFePO4 is too "flat" of a curve to balance well at lower voltage.