Re: drop LiFePo4 batteries vs LiFePo4 cells in a bundled pack #solution


Scott SV Tengah
 

Michael,

 

On a Victron charger / BMS / battery system, additional balancing devices are not needed, but you do need to take the battery up to absorption voltages for a few hours now and then. I try to do it at least once every two weeks since we get carried away and find ourselves using two induction plates, boiler and toaster at the same time, certainly qualifying as a system that has high discharge currents.

 

 

How to ensure that the battery remains balanced

A 2-hour fixed absorption period is recommended for lithium batteries, so that there is enough time for cell balancing to take place. It is important to regularly fully charge the battery. This so that the battery spends enough time in the absorption stage. A full charge once a month should be sufficient. However, there are some applications where the battery cells will become quicker unbalanced than usual. This is the case when the system is intensively used, or if the battery bank consists of multiple batteries in series. To ensure a well balanced battery a weekly full charge is required for:

  • Systems with a battery bank that contains batteries that are connected in series.
  • Systems that are charged/discharged every day or a few times per week.
  • Systems that have high discharge currents.
  • Systems that have short charge periods or low charge voltages.

 

 

https://www.victronenergy.com/media/pg/Lithium_Battery_Smart/en/operation.html

 

If you don’t balance, all that happens is that one cell hits 4v earlier than others and the BMS tells the chargers to stop charging. That causes a TEMPORARY loss of capacity, fixed once you balance them.

 

Hope this clarifies things, at least with respect to how Victron does it.

 

From: "main@AmelYachtOwners.groups.io" <main@AmelYachtOwners.groups.io> on behalf of Michael Fuchs <fuchsmi@...>
Reply-To: "main@AmelYachtOwners.groups.io" <main@AmelYachtOwners.groups.io>
Date: Thursday, May 5, 2022 at 10:28 AM
To: "main@AmelYachtOwners.groups.io Group Moderators" <main@AmelYachtOwners.groups.io>
Subject: Re: [AmelYachtOwners] drop LiFePo4 batteries vs LiFePo4 cells in a bundled pack #solution

 

Scott,

I appreciate your knowledge and based on your documentation I'm currently planning an Amel 54 Victron setup.

 

I'm confused regarding the need of balancing the Victron Smart Lithium Batteries. In the datasheet it states:

" Our LFP batteries have integrated cell balancing and cell monitoring. Up to 5 batteries can be paralleled and up to four 12V batteries or two 24V batteries can be series connected, so that a 48V battery bank of up to 1500Ah can be assembled. The cell balancing/monitoring cables can be daisy-chained and must be connected to a Battery Management System (BMS)."

 

So the daisy-chained cables from the Smart Batteries should eliminate any additional balancing device or manual balancing as you discribe.

 

Michael

 

On Thu, 5 May 2022 at 20:12, Scott SV Tengah <Scott.nguyen@...> wrote:

Dean, small detail, but I misspoke - high voltage disconnect occurs at 4v per cell. Further, I only charge to 28.4v, so 3.55V per cell. Victron seems to believe the additional capacity above that voltage level is not worth the stress on the batteries.

I do not use the DVCC but rather simply a VeBUS BMS with the Victron Smart batteries. I installed my system in 2018 so I'm not sure DVCC was available then? 

In any event, here's what I think happens. Please correct me if I'm wrong. The lifepo4 batteries draw as much current as necessary to charge themselves. So as the batteries approach 100% SOC, the batteries naturally take less current. In other words, even when my 200amp chargers are connected to a nearly fully charged Victron Smart lithium battery that is going into top balance mode, the batteries only demand 1-3 amps and I can see, through bluetooth, that the batteries are being balanced. This can continue for hours. But usually if I want to top balance, I'll run the generator for a bit to take the batteries to 90-95% and let the Victron MPPT do the rest. No use in running in the generator to provide 25-50watts of power!

I don't know what is going on behind the scenes, but my ve-bus BMS is connected to my Quattro charger via ethernet. With respect to my other charging sources (Skylla-i 80amp, Victron MPPT and Mastervolt 110amp alternator), the ve-bus BMS, through an on-off signal, turns off charging in a safe manner. I reiterate, for others, that it does not disconnect the batteries from the charging source, but rather turns off the chargers safely.

As aside, for my MV alternator, I have it set at a very low absorption voltage. This is for two reasons (1) prevent holding the batteries at high SOC if I am motoring for a long time (2) let my solar do its work as a I have an aversion to "wasting" solar energy. To be honest, more often than not, I turn off my alternator because we simply don't need that charging source. This is done through an additional manual switch installed inline with the "reg-on" wire.


On Thu, May 5, 2022 at 03:53 AM, Dean Gillies wrote:



Scott,
You mentioned about charging at 200A causing your overvoltage protection to trigger at 3.7V. Does your system use Victron's DVCC?  This allows your BMS to control the charging voltage and current directly, and is particularly important in the situation you mention.  By way of example, my BMS disables my alternator and Mastervolt charger when the SOC reaches 95%. That leaves my Victron MP and MPPT as the only charging sources when the system is above 95%.  The BMS then uses DVCC to exert fine control over the charging voltage and current at the top-end.  When the first cell reaches 3.45V, the total charge current is reduced significantly by the BMS which starts to balance the cells in a controlled manner using DVCC to control the MP and MPPT.  As the cell voltages increase and come into balance then the charging current is ramped down and reaches zero when the cells are all at 3,58V.  Balancing currents are small, in the order of 1A, I don't believe you can effectively balance a LFP battery with a free-running 200A charger.  Maybe your Victron BMS can do more for you?

On the subject of alternator protection, my system turns-off the alternator (Using the RegON signal) whenever the SOC exceeds 95% and turns it on again when it drops below 90%. My Mastervolt charger is also controlled in this way. So my normal operation is to cycle the charging.  If all that fails then my AGM batteries will act as a fail-safe sink as you mentioned.

Dean
SV Stella
A54-154
(Trapani, Sicily)


 
--
Scott 
2007 A54 #69
SV Tengah
http://www.svtengah.com


--
Scott 
2007 A54 #69
SV Tengah
http://www.svtengah.com

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