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bilgebay avatar image
bilgebay asked

Throttling the MPPT down and keeping the SOC between 60% - 75%

Hello,

I have a caravan trailer where I have the following equipment installed:

2x 12V 100Ah Lifepo4 batteries, in 2P configuration

2x 145Watt flexible solar panels

1x Victron Cerbo GX

1x SmartSolar MPPT 100/20

1x SmartShunt 500A

My average daily consumption is between 30-35 Ah.

Thanks to the SmartShunt, I am able to closely follow the Ah values put in to and taken out of the battery, rather than guestimating the SOC and remaining capacity by looking at the voltage reading which is almost meaningless with Lifepo4 batteries.

To keep the SOC between 75-25 %range, I have limited the "Max charge current" to "2A". My plan is to change this to max value once a week so that the battery comes to 75-80% SOC level and then starts gradually going down to 50-60% SOC level again. I will also top up the batteries once a month both for synchronising the SmartShunt and for balancing the cells in the batteries,

My questions are:

1) Do you see any problems with limiting the charging capacity of the MPPT to a small fraction of its capacity, with regards to MPPT ot PV health ?

2) Do you see any problems with keeping the battery at almost constant level, continually charging and discharging within a 1-2% SOC bandwidth ?

Many thanks in advance for your kind contributions.

Best wishes

Sedat Bilgebay


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2 Answers
snoobler avatar image
snoobler answered ·
  1. No. the MPPT won't care.
  2. not really, but there are better/easier, handsfree ways.


Your efforts to maintain 25-75% SoC will not bear meaningful fruit if your goal is to prolong the life of the battery.


If your goal is improved cycle life, here's my recommendation:


  1. Charge to 13.8V, 4 hour absorption.
  2. Float at 13.6V
  3. Every 14 days, equalize to 14.2V for 2 hours AFTER a full charge.
  4. Set discharge cut-off to 20%
  5. Set shunt charged voltage to 14.0V and 7A.


#1 will slowly take the battery to very nearly full charge - 95-98% SoC and substantially reduce cell stress vs. a full charge to 14.4V

#2 will Float at about 95% SoC

#3 will force all cells into the upper leg of the voltage curve and activate passive balancing to ensure all 4 cell groups are at true 100%.

#4 will protect the battery from excessive discharge.

#5 will force the shunt to re-sync twice a month during the equalization cycle.

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bilgebay avatar image
bilgebay answered ·

Excellent answer, thank you!

I was inspired by an article where it is shown that the highest number of cycles can be achieved by keeping the SoC between 75%-65% levels. But maybe all this is pointless in my case since 2000-3000 cycles are more than enough as either myself or my caravan trailer will wear out much before the battery :)

Best wishes

Sedat

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klim8skeptic avatar image klim8skeptic ♦ commented ·

Dont believe half of the nonsense you read on the net.

Andy has some toys and this video might be helpful.

How you interpret such information, and implement a charging regime is up to you.


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