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


Peter Luke
 

Scott,

Small detail, but I think you misspoke. :-)

"So as the batteries approach 100% SOC, the batteries naturally take less current."

I think you meant "So as the batteries approach absorption charge voltage, the batteries naturally take less current."

Without DVCC or some other SOC current limiting device/software, charging is based on a voltage level generated by a charger.

BTW - love your work.

Regards,
Peter

On 2022-05-06 4:12 am, Scott SV Tengah 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

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