It is mostly a myth that lithium batteries, in this application, should not be at 100%.
This is something the manufacturer has already taken into account, and failing to keep the battery regularly at full charge will limit its ability to balance and ultimately its longevity. Any perceived benefit is lost.
Ignore the internet and myths and follow the manual, which your warranty is based on.
The Lynx BMS you should not hold the batteries permanently at 100% if you have typical daily living loads. I believe you need a GX device in the system for this to work via DVCC. The BMS only calls for a full absorption charge if the SOC falls below 70% (SOC threshold) or after 30 days (repeat absorption interval) if the SOC threshold is not reached. These are user configurable and are fully detailed in the manual.
From the docs, and nowhere does it suggest intentionally keeping a battery at a lower SOC. Nor does the battery manual.
Once the Lynx Smart BMS is in operation, it is important to take care of the batteries.
These are the basic guidelines:
Avoid total discharge of the battery at all times and use the discharge floor setting to prevent this.
Familiarize yourself with the pre-alarm feature and act when pre-alarm is active to prevent a DC system shutdown.
Charge your batteries as soon as possible when the pre-alarm is active or the BMS has deactivated loads.
Minimize the time the batteries spend in a deeply discharged state as much as possible.
The batteries need to spend at least 2 hours in absorption charge mode each month to ensure sufficient time in balancing mode.
When leaving the system unattended for some time, make sure to either keep the batteries charged during that time or make sure the batteries are (almost) full and then disconnect the DC system from the battery. Do this by disconnecting the positive battery pole.
If the batteries needed to be maintained beneath 100%, manufacturers would state it for longevity. SOC is a calculated metric, voltage is what actually matters.
This subject, broadly, when it pops up on the forum is based on hearsay that has been removed from its context.
Typically this guidance relates to the charging regime for a lithium cell.
A battery is not a lithium cell, it is a collection of cells under management.
A cell has an upper and lower voltage limit, if you operate a cell there continuously, its life will likely reduce.
Battery manufacturers do not operate to these limits, they use a narrower, optimised set of limits that keeps cells where they need to be for longevity, this is what the pack capacity is based on.
So doing it twice is unproductive.
The battery will also not spend its time at full SOC at these voltages, more so under typical use.
The biggest variables on lifespan is time, temperature and usage - things you can do little about.
Most of us can attest, with older systems that batteries do last, when appropriately sized and kept within specs, and you don’t need to follow the myths and hearsay that does the rounds, mostly from people who don’t fully understand what is involved.
Following the manufacturer’s recommendations (Winston 700Ah 16S1P), my battery bank charges up to 54V (3.375V/cell, about 99% SoC) every day from solar and drops to 3.32V/cell (about 97% SoC) each night. Once per week it’s taken up to 100%SoC (57.3V, 3.58V/cell) for a couple of hours (to reset the SoC reading and do any cell balancing needed) but then charging stops and it drops back to 3.375V/cell or below over the next few hours.
As pointed out above, it’s the voltage that really matters, the usual recommendation is not to spend extended periods of time above 3.4V/cell. The voltage/capacity curve is very steep near 100% SoC, that last 1% raises the voltage per cell by 0.2V – so you’re not going to 100% for any capacity reasons, just for cell balancing and to reset the SoC measurement – without this (if you stay at 3.4V/cell or below) the SoC reading can gradually drift away.
The voltage slope in normal use is very flat, by 70% SoC it’s dropped to 3.33V/cell and at 30% SoC it’s 3.28V/cell, so only about 1mV/%. That’s why it’s difficult to use voltage to derive SoC for LFP batteries, the voltage changes with load current are bigger than with SoC.
Yes, there’s no positive side to doing this, and it could reduce battery lifetime. Normal/regular charging (e.g. from solar) stops at 54V (3.375V/cell), you can think of this as the “float” voltage – and it’s very close to 100% SoC, it usually reads as 99% (VRM only has 1% resolution).
@iand iirc Winston makes cells, so are you referring to a DIY battery or which specific battery pack are you referring to?
This discussion is about commercial packs on the supported list, and in particular, Victron’s own brand.
Most of which keep the cells in a perfectly acceptable 3.45V range, at top of charge, and slightly lower at float.
I’m referring to the battery bank in my boat (Winston cells, external BMS) put together by my boatbuilder, because I have a lot of visibility about what’s going on with them. The Victron batteries have very similar recommendations but at slightly different voltages – charge up to 100% SoC (or absorption, or whatever the BMS labels this as) for cell balancing for a couple of hours occasionally (at least once a month, as opposed to once a week for mine), don’t spend long periods at such a high voltage, “float” at a lower voltage (3.45V?).
If the battery BMS is the master (external control of the Multiplus and any MPPT controllers) all this should be dealt with automatically, there’s no need to do anything special once the system is correctly set up – which it should be with all-Victron gear
That pretty much sums it up. Between a commercial, or application specific, BMS that is designed to work in the victron environment, coordinating charge sources. Just let it do its job - it is better at it than any human trying to tinker with the system for perceived benefits.
There is little anyone can do to improve tried and tested configurations, other than make it less efficient.
Agreed 100% – as long as the Multiplus is configured for external control, meaning the BMS is the master. This should be done by any reputable installer, or anyone doing a DIY install…
