I just completed an installation of four 200NG lithium batteries, Lynx BMS and distributors, CerboGX with display, and Quattro inverter/charger. All is well except charge voltage from the Quattro never drops below 13.7v. The display never shows “float”, it just hangs in “absorption” with the voltage never dropping below 13.7. In my previous installations (Lithium Smart batteries, non-NG), the voltage would eventually drop to 13.5 in float. I understand that the BMS is controlling it. My only concern is whether 13.7 is a level that will cause damage.
The Quattro will never show float, it is now under external control from the Lynx BMS.
I noticed the 13.7V with my NG install and have raised it with Victron. The Lynx BMS is conyrolling this. The manual states 13.5V. It is possible to limit this in the DVCC settings, limit charge voltage, but it will not allow absorption if you do this.
Thank you. I’m not bothered by it not showing “float”. Just didn’t want to be overcharging over a long period of time. Maybe they will make a correction/update in the future.
Thinking further about this, maybe the bms is stipulating a charge voltage that satisfies what it sees as correct for cell voltage rather than entire battery voltage, including any external connection deficiencies.
No, the battery is sending a CVL of 13.7V even when the battery is balanced and there is no current flow so no voltage drop.
Good point, no voltage drop involved.
I have just finished an install on a boat using 24v 300NG batteries, Lynx NG BMS, Cerbo GX and Multiplus 24/3000 inverter/charger. Everything seems to be working extremely well and I’m VERY impressed with the performance of these batteries so far!
My question is exactly the same. While the boat is at the slip, plugged in to shorepower and being charged/maintained under DVCC the charger never shows ‘Float’, always ‘Absorbtion’ (or idle) and holds the batteries at 27.4v. Just like you I am concerned that this voltage is too high for weeks-long storage and my understanding form the documentation is that the DVCC was supposed to allow the batteries to come down to, and stay at a float voltage of 27v after they were fully charged.
I currently just manually limit the voltage to 27v using the ‘limit charge voltage’ function, but I shouldn’t have to do this.
Peter, did you ever hear anything back from Victron on this issue?
Mine is a 12V install so trying to run at 13.7V, but I have added a Node-RED code so it runs at 14.0 / 13.5V. I will give them a prompt.
Thank you again!
Hi @schaefersc @Keith7WA @pwfarnell
The good news is that there is no unintended behaviour here from Victron’s perspective. Both 13.5V & 13.7V (and their 24/48 nominal equivilents) are acceptable and normal for the NG series.
The documentation is inconsistent with this, and is a bit confusing, but it’s going to stay like this for now.
We have ongoing refinement tests and research, and the current default operational settings of 13.7V for float with 0Amps charge, without drawing down on the batteries to hit a lower (like 13.5V) float voltage seem to be giving the best results.
We may return with more information, updated documentation and adjustments around these thresholds through the testing process.
If you override this some how with node red or otherwise to force 13.5V, also fine, but then please return to the defaults before seeking any kind of support for other issues.
Right now, both 13.7V and 13.5V are valid float voltage levels.
The float/absorption labelling is an outstanding issue, and hopefully resolved, but has no operational difference. The systems are charging as expected.
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Thank you Guy, that is surprising and important, but at least good news to know the system is functioning as intended!
On a related note, there is talk that for LiFePO batteries that are not being used for a long-ish period of time, like when a boat is in port for a month and connected to shore power, that floating them at a lower SOC, say 80% may have benefits for longevity of the batteries. Is this the case for the NG batteries and would you recommend this? Assuming of course you bring them to a proper 100% and equalized SOC before using them again.
As long as the cells are rebalanced before cycling, then I would say it does no harm and possibly some good to leave the cells ~80% for longer term storage.
Also, as an aside it’s common to discharge to 50% or less for transport so there is less stored energy in case of an accident.
But both of those are very generalised, as far as I know Victron hasn’t released any specific advice for these alternative situations and the NG battery. Perhaps over time there will be more guidance.
For now I would be comfortable using the batteries, settings and software as the engineers are intending.
Perfect - thank you Guy.
Thank you for all this information.
I have removed my 13.5V limit per the response from Victron, performed an 14.0V charge with the 2 hours absorption 3 days ago. I have been watching the recharges closely since then.
“Floating” at 13.7V allows faster recharge so this makes the batteries more usable in a marine / RV situation, 13.5V led to slow charging and thoughts of more frequent 14.0V charges. 13.7V eliminates that need.
Secondly, and key here is that the batteries do not return to 100%, they stop just short. Attached graph shows the batteries held at 13.7V for 9 hours and the used Ah does not return to 0, it stays static at a slightly negative number.
The data shows that the battery stops charging just short of full and does not continue charging at a low rate so it will not overcharge, the usual concern. Based on this, I am happy with the 13.7V for normal daily cycling.
Thanks for that update
I also have a similar setup with a Lynx BMS and two 24V 200AH NG Lithium batteries. The Charge Voltage Limit, a value set in the Lynx BMS is set at 28.0V (this is a setting for the batteries). This value is usedby the BMS to control the charge sources, if the SOC falls below the SOC threshold. This seting is the BMS under DVCC settings. The default setting here is 70%. So if the batteries are discharged below 70% and then charging occurs, they will charge at 28V until 100% SOC is achieved. It is what happens next which is a bit odd. After the batteries achieve 100% SOC, then charge voltage limit appears to drop to 27.4V. Note this is not a setting in the BMS or Cerbo. Over time by having a lower charge voltage limit of 27.4V, also results in the battery’s SOC dropping below 100%. This is due to 28.0V being the required voltage for 100% SOC. So over a number of days, the SOC will typically fall and will settle at about 98.4% which I assume is the SOC based on a voltage of 27.4V. So why does this occur and how is it possible to maintain 100% SOC, if this is what is desired? Further what is the basis for the 70% SOC level for the DVCC threshold. Is a higher value ok? Say 90%?
I am afraid that you are missing the key factor of lithium batteries, you MUST NOT keep them on charge at 100% for long periods. The whole purpose of charging lithium batteries is to get them to 100% and hold them there for a short time to allow the cells to balance to the same voltage of 3.5V each (14.0V /4 or 28.0 / 8), but once they are full and balanced the voltage backs off to a voltage just under 100% full. Occasionally going to 100% also resets the SOC to 100% to match the full state of the battery as it can drift.
A voltage of 14.0 / 28.0V is not equivalent to 100%, it is equivalent to more than 100%, if you kept them at 14.0V indefinitely, they would slowly keep charging, in fact it would be overcharging which would damage them. This is why backing the voltage down is much safer, it means the batteries are less stressed. Do not bring lead acid mentality along to lithium where you need to keep getting back to 100% or they get damaged, lithium is opposite, you want to avoid staying at 100%.
This thread arose because many people consider 13.5 / 27.0V to be the safe “float” voltage and Victron use 13.7 / 27.4V. My long post 2 above yours goes into detail of why the higher voltage is OK saying how pleased I am that this DOES NOT get back to 100% to avoid over charging.
Your batteries are working fine being held at close to 100% but not getting there.
Charging in general is discussed in the battery manual, link below. In Section 5.2.1 it states
We recommend a minimum absorption time of 2 hours per month for lightly cycled systems, such as backup or UPS applications, and 4 to 8 hours per month for more heavily cycled (off-grid or ESS) systems. This allows the balancer enough time to balance the cells properly.
So at a 70% threshold you could consider the batteries a lightly loaded system so if they never go below 70% they will get 2 hours absorption per month. This applies to mine, but I occasionally have a heavier discharge so I have reduced my SOC to 50% so if I have a deep discharge I get 14.0V and faster recharge, but this may only happen once or twice a month, so I now get more absorption time. If say you cycle to 50% every day then you may want to set your SOC threshold to say 40% and the time to 7 days so you now get 4 cycles per week and 8 hours absorption. Or you could leave the threshold at 70% and reduce the absorption time to 10 minutes so you get a cycle every day, but shorter absorption every day but totalling 5 hours per month.
The choice of SOC threshold, absorption time and interval duration need to be set to match your use and the guideline absorption period per month. This then limits the period exposed to 100% charge to maximise battery life.
If you have daily cycles DO NOT allow a 2 hour absorption time every day.
Hope this is useful.
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Thank you for your detailed explanation. Very helpful. My system is located on a boat, so typically has three different operating profiles. 1. when at a marina and connected to shore power, the batteries typically get limited use. 2. when the boat is actively being used, the batteries will get much more heavily used and may be discharged to at least 50% on a daily basis. They will then be recharged by up to three sources (alternator, generator and to a small extent by solar). Daily recharging by the generator is the typical approach and charge levels of up to 0.5C can be achieved ; and 3. the boat maybe left on a mooring and would in this case be off grid and with limited loads on. The lithium bank maintains the charge on two other AGM banks via DC-DC chargers so if the solar charger is unable to maintain these requirements indirectly through the lithium batteries, then they may slowly discharge. In the event of a deep discharge, the generator would automatically turn on and recharge the batteries at up to 0.5C. Given these different profiles would you recommend also using different settings for each circumstance? If so what would you recommend. One last question the manual suggests a recommended charge rate of 0.5C (unless below 15 degrees celcius - then is should be 0.3C). Do you have a view on these values.
Thank you in advance.
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As the batteries cool down, ideally charge rate should be reduced, the values listed by Victron are in line with most cell and battery suppliers.
You are right about the different boat scenarios, my NG batteries are on a boat, but I have more batteries vs typical use and a lower charging rate so one set is ideal. Yours is less easily covered.
One point I will note is that you will need to be at 14.0V to get your high recharge rates from the genset, the 13.7V setting only lets me charge at 0.2C max, which is fine because that is all I can do, but I rarely go below 70%.
I think for your number 1 and 2 you need something like 70% SOC threshold, 30 day interval and 2 hr absorption. When on shorepower you should never drop below 70% and you get once a month full charge. The same if moored off grid with good solar. If moored off grid with poor solar the 70% will kick in 14.0V charging ready for the generator kicking in. The 70% could be varied as long as the SOC threshold triggers before the generator starts.
Then in use, use the 70% threshold, 30 day interval and swap to 10 minute absorption. Then you get good charging rates most days but avoid too much absorption.
There will be some trial and error to get the best out of them.
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