Lithium house bank on long term shore power

[Nightsky]

Originally Posted by donradcliffe

Following. What is harder on the batteries, a series of daily discharges of 5-10% SOC for six months, or being floated at 13.4v for 6 months?

Good question. If I knew for certain that maintaining them at 100% was bad, I would consider no shore power and rely on solar only to keep the dehumidifiers going and the batteries topped up enough to maybe get a few hours of bilge pumps working in the event of a lot of water flooding in. Will mitigate the flooding risk as much as possible by closing all seacocks before we leave, but there is always the unknown to stress about when I am too far away to do much about it.

[Nightsky]

Originally Posted by HBrew

+1 - I have more or less the same setup and I too have left mine on shore power (100% SOC) for long periods without any apparent issues.

That sounds reassuring.

[HBrew]

Originally Posted by Nightsky

That sounds reassuring.

I have the mother of all tests of this theory when I get back to my boat in French Polynesia next year. It has been on the hard since July 2019, plugged in, dehumidifier running

[wholybee]

I have both float and absorption set to 13.3v. The battery SOC fluctuates some, but it stays near 60%. I don't see any reason it needs to stay exactly at a given SOC, just as long as it is kept from being held near full or empty. I bump absorb up to a normal value the night before I set sail. YMMV.

[Philtao]

Originally Posted by sailjumanji

where does it say holding LiFePO4 battery at 13.5 volts, 100% SOC for long period is bad?

There are some studies that have been made, that show that storing a lithium battery at high SOC and high temperature leads to capacity loss. Here is one of them (2017): Calendar Aging and Lifetimes of LiFePO4 Batteries... by John Catton from the University of Waterloo, Ontario, Canada.

Unfortunately the tests do not match our usage conditions but it shows, that at a given temperature, low SOC is much better than high SOC (need to read the 197 pages study to understand conditions, testing procedures... to replace the results in their context)
What seems relevant to the question.
Battery stored at:
1) 100% SOC / 35°C = about 10% capacity loss in 28 months
2) 100% SOC / 60°C = about 80% capacity loss in 4 months
3) 0% SOC / 60°C = about 25% capacity loss in 7 months

Point 1 is probably relevant in the tropics
Points 2 and 3: of course we never use our batteries at 60°C, and the degradation process is much slower... but it happens (just need to change the time scale). My interpretation is that at 100% SOC the loss of capacity is much (5 times in test conditions) faster than at 0% SOC.

The capacity loss may not be large at 25°C, and 5 times something small may still not be important. But I expect to keep my battery healthy for 15 (maybe 20?) years, so every little bit counts.

As a side note, this study shows that temperature has a very negative effect on battery life.

If in addition to keeping the battery at 100% SOC, it is powered at 13.5V, some lithium ions are forced to migrate from the 'lithium' cathode to the 'graphite' anode... that has great difficulties accepting them. This contributes to two effects that are well documented in studies:
1) lithium plating (lithium ions turn into metallic lithium and can no longer contribute to the battery capacity)
2) faster Solid Electrolyte Interface (SEI) layer growth (electrolyte decomposition leading to reduced capacity, and other negative impact)
The outcome over time is reduced battery capacity (but how long???)

I am always willing to learn more and if anyone has seen other studies about LiFePo4 cell ageing as a function of SOC and temperature, I would love to have those references.

[tanglewood]

Originally Posted by sailjumanji

Ok, shoot me. But where does it say holding LiFePO4 battery at 13.5 volts, 100% SOC for long period is bad? Am asking because I have a Mastervolt system with their MassCombi, SolarChargemaster, a couple of Mac Plus DC-to-DC chargers, and their MLI Ultra battery. All of these chargers have MLI-specific settings, which include float.

13.5 isn't bad. Not great, but not bad. 3.35vpc is better which is 13.4, and around 60% SOC, give or take.


As for maintaining them at higher SOC, pretty much 100% of the research on LFP life cycles says it reduces battery life.

[Anders]

Originally Posted by Philtao

I use the TAO BMS (www.taobms.com)

Where did you buy it and what did you pay for it?
I can't find anyone that sells it.

[Nightsky]

Originally Posted by wholybee

I have both float and absorption set to 13.3v. The battery SOC fluctuates some, but it stays near 60%. I don't see any reason it needs to stay exactly at a given SOC, just as long as it is kept from being held near full or empty. I bump absorb up to a normal value the night before I set sail. YMMV.

I have float set to 13.4v

[Nightsky]

Originally Posted by tanglewood

13.5 isn't bad. Not great, but not bad. 3.35vpc is better which is 13.4, and around 60% SOC, give or take.


As for maintaining them at higher SOC, pretty much 100% of the research on LFP life cycles says it reduces battery life.

Thanks for that. When I had the parameters on the Victron inverter/charger set up by the electrician than installed them, I had him set the bulk at 13.9v (IIRC) as per the battery manufactures recommendation in order to extend battery life, float set at 13.4v. I'm wondering if the SOC I see displayed is somewhat arbitrary but that the important number is the float voltage. Am I assuming too much? I am in Luperon for hurricane season and trying to find expert technical help locally is difficult here. So I think I will keep my fingers and toes crossed and hope for the best.

[Nightsky]

Originally Posted by HBrew

I have the mother of all tests of this theory when I get back to my boat in French Polynesia next year. It has been on the hard since July 2019, plugged in, dehumidifier running

I hope you find all is well with your batteries.

[Hesti]

How about a plug in switching clock?

[Franziska]

Originally Posted by Hesti

How about a plug in switching clock?

I like this!
Out of the box thinking and low cost :-)

[Dirk01]

Originally Posted by Anders

Where did you buy it and what did you pay for it?
I can't find anyone that sells it.

Just read the name: Philtao --> Philippe TAO

He use his own system and you can buy it in his shop. Just jump to his website...

[JKMilligan]

My monitor is a Victron 712 with relay. Installed a BP100 (also Victron) on the shore power feed and it is controlled (on/off) by the relay based on SoC which the 712 tracks. Kind of a partial BMS and does what you’re trying to do. I have gel batteries and they are easily cooked.

[Nightsky]

Originally Posted by JKMilligan

My monitor is a Victron 712 with relay. Installed a BP100 (also Victron) on the shore power feed and it is controlled (on/off) by the relay based on SoC which the 712 tracks. Kind of a partial BMS and does what you’re trying to do. I have gel batteries and they are easily cooked.

Not something I am familiar with but will look it up. Unfortunately, not possible to get it here to Luperon before we leave the boat, but may be useful in future.
thanks