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Sarah avatar image
Sarah asked

Changing Lithium Battery State of Charge threshold on Lynx Smart BMS

Hi all looking for your thoughts on increasing the State of Charge threshold on my lithium batteries. I have just upgraded my boat 12 volt system with 3 x 200 AH Smart Lithium batteries and a Lynx Smart BMS to replace my old 1180 AH AGM batteries. I also have a Muliplus II, Cerbo, and two MPPT controllers with 1400 watts of solar. I also have a generator installed and no shore power. I found with the new install, when not on the boat, I consume about 8% of the batteries overnight (mostly fridges). During the day the solar panels supply the load in float and a small amount of charge. This continues for 5-6 days until the discharge eventually reaches the default State of Charge threshold of 70% and a new charging cycle commences with the batteries recharged from the panels in bulk in around three to four hours. While this is fine and exactly how the system is intended to work we have been having a very wet winter with many cloudy days in a row with minimal solar output. I have considered increasing the State of Charge threshold to 85% so that I start a new charging cycle every 2-3 days instead of every 5-6 or so days. This increases the likelihood of getting on the boat with a full charge and minimises the impact of poor weather. This will also result in more frequent, shallower discharge/charge cycles (which is obviously what we are used to with our older AGM batteries). I am aware of the different charging characteristics for lithium but unfortunately can’t find any good literature to suggest whether this change is a good or bad idea - hence my question to the group. Any thoughts or similar experiences fine tuning State of Charge greatly appreciated.

Thanks

Sarah

Lithium BatteryBMS
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Stefanie (Victron Energy Staff) avatar image
Stefanie (Victron Energy Staff) answered ·

@Sarah

From my experience over the last two winters, it made no difference whether I set the threshold to 70% or increased it to 90%. Discharge rate of like 25Wh. Every few weeks I had to power up the AC charger since PV (510Wp) could never complete a full charge cycle while SoC continued to decrease. In addition, the low temperatures, which also severely reduce the capacity of the lithiums, also adds to the problem. I had to constantly add a heater (controlled by temperature) to keep the batteries above 5°C. Even doubling PV would not have been enough to come up for the losses.
I'm almost at 55° North. Depends where your boat is located during winter, but it's not easy.

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Sarah avatar image Sarah commented ·
Hi @Stefanie thanks for your feedback - we are on a catamaran in Sydney so 33 degrees South and our winter minimums are only 7-10 degrees so a lot milder than your conditions. It’s the incessant cloudy weather and rain that has been our problem - periods of 2-3 days without the panels producing any output. Also we are yet to stay on board with the new batteries which will be different again. The more I think about it I prefer the idea of returning the SoC to 70% and reducing the recharge interval to three days.


Next job is to install two 30 A Orions to the alternators (one per engine - they are only small 80 A temp regulated alternators and I did not want to push them too hard) for when we are underway. Then we will have finished the upgrade.


Many thanks

Sarah.

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Stefanie (Victron Energy Staff) avatar image Stefanie (Victron Energy Staff) ♦♦ Sarah commented ·

>>The more I think about it I prefer the idea of returning the SoC to 70% and reducing the recharge interval to three days.<<

At such a rather low discharge rate it is absolutely not necessary to start a new charge cycle every 3 days. I just checked my installation and the last full charge cycle started automatically a few days ago, after 30 days. Between the repeated absorption interval the battery sits at around 80-100% SoC throughout the summer, sometimes lower when we have a prolonged bad weather period. Only with heavy daily discharge rates (maybe down to 25% SoC, not when you run the hair dryer for a couple of minutes) it is necessary to perform a full charge cycle every other day. Lithium, and this includes our Victron Lithium Smart Batteries, are designed to stay at a low SoC for very long. Victrons recommendation to balance batteries in systems with low discharge rates is once a month or every 30 days.

And just in case you didn't know, the full charge cycle is not to bring the battery back to 100% but to balance the cells. Not recommended, but I know of ppl who balance their cells only once in a year.

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Sarah avatar image Sarah Stefanie (Victron Energy Staff) ♦♦ commented ·
@Stefanie thanks again for your input it is very much appreciated. It takes a while to get out of the mindset that the batteries should be at 100% all the time.
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Sarah avatar image
Sarah answered ·

My other option is to leave the State of Charge at the default 70% and reduce the repeated absorption interval to from 30 to three days. Is this a better alternative?

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klim8skeptic avatar image klim8skeptic ♦ commented ·

repeated absorption

This looks like a multi setting? As apposed to a mppt setting?

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Sarah avatar image Sarah klim8skeptic ♦ commented ·

hi @klim8skeptic you are correct this is a multiplus setting but the ‘repeated absorption interval’ is also configurable on the Smart BMS which is managing my DVCC parameters and hence control of the MPPT’s. See the Smart BMS manual on DVCC https://www.victronenergy.com/media/pg/Lynx_Smart_BMS/en/configuration-and-settings.html#UUID-2a3623df-8422-a501-03ef-345bd8f0982a

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