How to maintain 50% SOC when away from the boat?

[curiouscarbon]

Coulombic Efficiency = Watt Hours Out / Watt Hours In

(1-Coulombic Efficiency)/(Time of Cycle) vs (Time) at different temperatures.

Higher on graph means higher degradation per unit time, if I understand correctly.

LiFePO4 is the third from left (picture is kind of low res)

Basically, the thickening layer aspect of cell degradation starts out fast, and then slows down in general.
This seems to hold generally for LiFePO4.



Link to specific timestamp 1465 seconds in

"So it's telling you that time of exposure is the bad actor here in the failure of these cells at elevated temperature"

 

[wholybee]

Ok, wow huge range of opinions on this one…
Here is mine!

This is for LiFePO4…
First, there is no memory effect…
Second, if you want an easy way to allow the system to stay connected, set the bulk to 13.2/12V set, and set the float to 12.8 done.
If you leave, the cells never hit 100%, solar can maintain phantom and self discharge, and the bank will float around 50% perfect for months of storage, and when you return, change the settings back where ya like.

Marine lithium batteries in operation | Nordkyn Design

This article details the relations between voltage, state of charge, current and temperature for lithium iron phosphate (LFP or LiFePO4) battery banks.

nordkyndesign.com

Quoteemphasis mine)

For a memory effect to appear, an incomplete charge cycle followed by a rest period and a discharge must have taken place earlier (memory-writing cycle). A partial charge followed by an immediate discharge is not sufficient to record a memory of the incomplete cycle [7]; this is important because the practical consequence is that a charge-and-hold strategy is particularly harmful when full charge was not achieved. It is not uncommon for DIY lithium battery systems to implement deficient charging strategies which in fact result in this scenario taking place and it is detrimental to the long-term performance of the battery bank.

Nordkyndesign is often quoted as a reliable source of information on LifePO4. If there information is correct, partially charging a LifePO4 battery, without performing a deep discharge or full charge afterwards, will indeed induce a memory effect. Is it significant? IDK. I opted to change my regimen to avoid that situation. It is simple enough to just use the battery and not worry about it, instead of having a charger hold the battery at 50%.

 

[MurphyGuy]

This video is great. Thanks for posting it. Really appreciate it. Seriously.

Active Thermal Management all the way. It's more complicated, but if I insulate my LiFePO4 battery cell mass, and be careful about failure modes, my hope is that keeping them consistently below 25 deg C will reduce calendar aging, extending usable operating lifetime. Trying to include a per cell lifetime histogram of temperature vs time spent at that temperature, voltage vs time spent at that voltage, etc. in a logging module to additionally assist with cell life analysis going forward. Just have a loop every one second, add one second to the bin of the histogram that represents current temperature and voltage.

Counting cycles is also a topic. I would like to additionally be able to monitor how many Ah charged and discharged at a given voltage and given temperature. These are the critical aging bits that I want to see the data from my batteries in a year or more.

From what I have read, there is a memory effect, but that the magnitude of the memory effect is so small (<2% if my hazy memory is operating nominally) that I feel sort of pedantic pointing it out, and feel that you're colloquially accurate in assessing it as no memory effect. 50+2 or 50-2 is still only 48-52% which does not meaningfully affect the critical mission outcome of maintaining middle SOC to encourage longer battery life.

The thermal issue is why we see so many Nissan Leaf cells on the market.. Leaf cells are very high quality batteries, but the reason we see so many being sold everywhere is because Nissan screwed up and tried to naturally cool the battery.. which resulted in a whole bunch of crispy cells with barely half their original capacity remaining.
This is why I won't buy them.... its going to be a while before we know the difference between a battery worn due to cycling, and a battery worn due to BBQ'ing.

My next project is going to be either an Amber Kinetics flywheel, or a 100kWh battery in my barn... If I go with the battery, I'm going to be installing an air conditioner to keep it cool when not in use.. even if it means installing extra solar panels.

 

[Sabre602]

To address the windlass connection. If you bypass the BMS, then the BMS will be unable to track SOC. You will draw the battery down with windlass use, and the BMS will still report it as full (or whatever it was before windlass use) As long as you know that, it isn't a huge concern. A 200A BMS should support a typical windlass. I know of at least one boat using a 100A BMS with a windlass.

I'm glad that you brought this up. I had been concerned about burning up the MOSFETs on the BMS and was picturing using a switch to bypass the BMS but which would leave the shunt in the circuit when using the windlass. I really can't afford a proper marine BMS with robust mechanical contacters!

 

[BobR]

For long term storage its easy. Just disconnect everything, including the BMS. Tougher is where there is stuff powered on and working, and you will be away for a long time.

The salvage company will love you if you disconnect everything (bilge pumps).

 

[mitiempo]

The salvage company will love you if you disconnect everything (bilge pumps).

If, like many boats with a LiFePo4 bank, there is a separate start battery that is one form or another of lead/acid the bilge pumps can be wired to that battery.

As far as burning Fets one option is the Electrodacus BMS. With the Electrodacus it is also very easy to change the set voltages for ending charging to a lower level, say 13 volts.