Author Topic: Ideal charge/discharge profile for Solar Charged LifePo4 in an RV using MPPT  (Read 2533 times)

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Offline orneaTopic starter

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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 - 14.0V
2. iTechWorld 40Amp MPPT 13.05V - 14.4V (Mode Setting type "4-3.2"). Chart attached

The 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 life
2. 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.
« Last Edit: October 14, 2023, 04:10:35 am by ornea »

Offline Faringdon

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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 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.
« Last Edit: October 14, 2023, 10:38:45 am by Faringdon »
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Offline orneaTopic starter

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Hi Faringdon,

thanks for the reply.

there is no provider in this scenario, just a self contained camper trailer.


Offline NiHaoMike

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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.
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