Hi all, just refitting a yacht and want to limit my batteries charge to around 80% while they are not being used much and I don’t require the full capacity (rather than leave them at 100% for Months).
I only have MPPT solar chargers connected currently and I know I can change the charge profiles in them to achieve my goal, however when the system is all linked up through the GX, the DVCC is forced on and it seems like no changes can be made. So without disconnecting the MPPT controllers from the GX/BMS system and running them as stand alone, is there any way I can tell the system that I only want it to charge the batteries to 80% or alternatively to a set adsorption/float voltage?
Hi Trevor, it’s because they are best not stored at 100% charge. As I am not yet using them properly (still doing refit) and I am just running a fridge and lights, they are not cycling and remain at close to 100% all the time.
So basically if you are not using them, they are better off sitting at 80% than 100%.
Probably a good feature for people who are leaving their boats, they should not have the batteries (lithium) at 100% all the time.
From a victron training course, however it is common knowledge with lithium batteries, 100% SOC stresses cells as well ad deep discharge.
Victrons White Paper on DVCC also goes into these details.
This is a direct quote from victron also ‘Storing the battery at partial SoC will not harm it and will in fact extend its lifetime compared to regularly cycling it from 100% to empty’
A common mistake is think storgae means leaving them in the system not used.
Storage means completely disconnected from the system at a certain soc. (Not left in float service) Many batteies require the start soc for this state to be 90% and a recharge every 6 months at least.
You will always have a bit of cycling how you left it as the mppts and small parasitic drains are there.
They will cycle whethere you like it or not. It is now better they remain where the cells are kept balanced.
But yes, if you have your mind set on this this is the way - you can even set them to the same value with victron connect over the vrm if you aren’t local.
Ok thanks, that’s a good point, they are still cycling a few % as running fridge etc…
But as for your last comment, this is what I am seeking advice on, as the DVCC is locked on I can’t make these changes anywhere. I can only set the adsorption/float inside the MPPT controller but it is then overridden by the DVCC and I cannot change the parameters there.
@Kiwirob …As I trying to learn the origins of this advice, could I please have a reference to the Victron training course and the white paper on DVCC? I really am trying to learn where this is coming from but have the following comments for context.
With reference to the direct quote, "storing at a partial SOC will not harm it and in fact extend its lifetime compared to regularly cycling it from100% to empty. " It does not say it is preferable to store at partial SOC, merely that it is preferable compared to regularly subjecting the battery to complete discharge down to 0% SOC and back up to 100% SOC.
I realise that some do speak about this and therefore it may be regarded as “common knowledge” but I fear the myth and legend is gaining traction over practical reality. The theoretical degradation of the cell at 100% may not be worth the trouble of overcoming it.
The Victron battery manual for their Lithium batteries does not mention long term storage best below 100% and in fact advocate a periodic period of at least two hours of absortion level charging. The reason they advocate for the battery being fully charged is to ensure cell balance, an important aspect of Lithium battery health.
Given the batteries have a service life of about 5000 cycles, which on my boat is about 14 years, do I really need to increase their service life by somehow modifying the existing Victron algorithms for charging their own batteries?
If this really was “a thing” don’t we think Victron may have addressed this in at least one of their charging algorithims in at least one of their devices they have developed to specifically charge lithium batteries? They are pretty knowledgeable about battery charging and it seems strange to undertake our own somewhat forced modifications to their charger settings to somehow improve upon the manufacturers wisdom. They may have made the determination that the theoretical improvement is so minor as not to be worth the trouble.
Hi Trevor, I think you are correct in all you are saying. I can’t find the ‘white paper’ or anything else from Victron making these statements, and I had better not reference the training course as it was conducted by a retailer and not Victron themselves and they have fallen into ‘the myth and legend is gaining traction over practical reality’.
I do think that it is correct from a chemistry/physics perspective and common to all lithium batteries but agree more theoretical than practical and as you allude to, trying to avoid this situation by managing the charging yourself will very likely do more harm than good.
Also my situation is common for boats that are left for long periods of time, the battery will always be 100% or close to it and solar still connected, this is a situation Victron would have considered.
Thanks for you time and advice you make a very strong well reasoned case that I agree with. There are so many cases I have seen in many different fields where ‘the myth and legend is gaining traction over practical reality’. It’s a great quote!
The voltage cap is working and I think it’s set at 50v or close.
Can you advise, is this voltage the voltage that is going into the batteries (not the higher voltage upstream of the MPPT). I am sure that it is, but wondered why it’s set so high as in practice it will never be more than say 14.5v. Thanks for your help.
That will be the cap on the voltage max charge voltage sent to the battery.
If there is no shunt in the system or any comms from the battery, that will be what is at the mppt terminals. So uncompensated.
You have a 12v system and it wont let you set it low enough for it?
Are you on GUIv2 or the Classic?
Yes a 12v system. It will let me set it no problem, was just interested in why it was set so high. I have a Lynx smart BMS and comms from the batteries. Think running GUIv2.
Ok so should I choose to lower the SOC of the batteries, I can just set that max voltage to 13.5 or something?
Based on all the discussion I probably wont do this as will do more harm than good.
I have no idea why it started so high on a 12v system. As long as you can adjust it thats ok though.
So Victron batteries?
I have tested the theory of keeping batteries voltage limited. It doesn’t go well. It is ok for a short period such as for troubleshooting cell high voltages, but extended time no. You end up with greater issues.
The thought on keeping the cells between 20% and 80% stems from a misunderstanding of cell voltage range use. The epitomy of the saying ‘a little bit of knowlege is dangerous.’
The bms already give a range in the cells that is the useable range. So not need tonthink about it.
Energy in and out of a battery ages it.
Calendar time aging ages it (so use it or lose it anyway)
You shouldn’t keep them at one voltage either (which is why there is a bit of a difference between float and absorption) on many. And the manufacturers who use one value for both want you to cycle them.
Not balancing the pack causes problems in the battery cells. I have a friend who has been doing evil things to a battery and this is definitely the worst one to do to a pack. (It is in rehab right now but it is not looking good for recovery).
If anything set the reabsorption times longer. But i wouldn’t personally cap voltage for ‘storage purposes’. And really what the manufacturer recommends is the vest course of action. At the end of the day if you do end up with a problem, it is their problem since you followed recommended charging not because of something you may have done.
Yes Victron everything aside from the Alternator regulator.
Chat GTP has the following comments, but in practice may be very different.
Why Charging to 100% Can Reduce Lifespan
Increased Cell Stress
At higher voltages (near 100% SoC), lithium-ion cells experience higher internal stress. This can accelerate cell aging or degradation over time.
Elevated Rate of Side Reactions
When a cell is held at 100% SoC—especially if it’s also exposed to higher temperatures—side reactions inside the cell become more pronounced. These reactions can permanently reduce capacity and increase internal resistance.
Long Dwell Times at 100%
It’s not just reaching 100% that matters; how long the battery remains at or near 100% also impacts degradation. Frequent, brief excursions up to 100% (e.g., daily cycles) are less harmful than storing the battery at 100% for days or weeks on end.
Still Benefits from Partial SoC
Even though LiFePO₄ is more forgiving, you’ll still see increased cycle life and reduced capacity fade if you operate it in a partial SoC window (e.g., 10–90% or 20–80%) rather than fully charging to 100% on every cycle.
Chat GPT is still in baby stages and definitely great for code, but i disagree with its ‘reasoning’ based on experiential use.
Yet on a system 100% SOC is not the cells full range of voltage use. Also on a victron system it is not held at the higher voltage. It drops to float (still considered 100% SOC) so using soc is not valid.
Cycle life is not increased or reduced. Lifepo4 is energy in and energy out.
Whether you do that end to end or sipping up and down, it is energy in and out.
The bms already hovers the cells between percentages of its range. So yes correct but also not correct.
Discussion with the Pylontech technical department…
Question:
Is there any problem if, after they reach 100% SOC, I will keep them constantly at 52.5V (3.5V/cell) ? Response:
This is not a problem and will not cause battery damage.
Cycling near the top of charge (75%–100% SOC) is detrimental to LFP/graphite cells. Our results show a correlation between the average SOC of battery operation and capacity fade rate, meaning that the lower the average SOC, the longer the lifetime, in these 2500 h of testing. The average SOC was found to be the most critical factor influencing capacity fade for LFP cells, over the factors of temperature, depth of discharge, electrolyte salt choice or graphite choice. Cells cycled in the conventional 0%–100% SOC window showed capacity fade rates intermediate to 0%–25% and 75%–100%. Therefore, the time spent cycling at high states of charge is critical to minimize.
They are talking about the fact that 0-25% is better than 75%-100%.
True, because 75%-100% will be at a higher temperatures. And temperature is detrimental.
They are saying: “At higher SOC, there is more exothermic heat flow produced from faster SEI growth and electrolyte reduction”.
So higher wear on 75%-100% because higher temperature. They are talking about 40-60 deg.C temperatures.
Lets see…
All LiFePO4 manufacturers say that in order for the warranted cycles to be honored, the following must be respected.
I am picking below the most restricting ones:
Less than 80% DOD, so no less than 20% SOC.
Less than 0.2C (C5) for charging / discharging.
Battery temperature around 25 deg.C +/- 2 deg.C
From the start, their choices:
0-25% SOC - out of the SOC range
0-100% SOC - out of the SOC and temperature range
75%-100% SOC - out of the temperature range (over 40 deg. C)
Not to mention that their 100% SOC is 3.65V/cell.
The 3.45V-3.5V/cell like many manufacturers are keeping / limiting the cells are another matter.
In normal usage (my case) didn’t see temperatures more than 28-30 deg.C, SOC less than 20%, currents more than 0.2C and voltages more than 3.5V/cell.
So, I am still waiting a test for normal life usage, not extreme tests like theirs.
And another thing…
Their entire study is based on a study of SEI (solid electrolyte interphase) growth for the NMC cells. That study is stating that “The experiments show that time and temperature, not cycle count, are the dominant contributors to the growth of the SEI.”
In this new current study we are talking here, the authors admit that “To ensure that SEI growth is the primary degradation mechanism, it would be beneficial to have similar studies on LFP cells, modelling the effects of SOC and temperature on SEI growth”.
So, in a way, is kinda of “on LFP is probably the same” type of study…
But, in the end, each to his/her own.
Only time will tell…
