Victron charging and BMV settings review

K8MEJ

Solar Addict
Joined
Nov 23, 2020
Messages
265
Location
Central Ohio
I have two banks of EVE 280Ah in 4S (12v) configuration and each with an OverKill 4S BMS. To start with, I'd like to cycle the system between 20%-80% SOC. On my BMS I have full cell voltage=14.00v, 80%=13.28v, and 20%=12.88v. On my MultiPlus and both MPPT controllers I have absorption voltage=14.00v, float voltage=13.40v, repeated absorption time=1Hr, repeated absorption interval=7days, and absorption time=2Hr.

Charger Questions:
  1. Should I change absorption voltage to 13.28v instead of 14.00v? or a different value?
  2. Should I change float voltage to something below 13.28v?

BMV-712 questions:
  1. Should Battery Capacity be set to 560Ah or 80% of that?
  2. Should charged voltage=14.00v or the 80% voltage of 13.28v?
  3. What would I set tail current to be?
  4. Given my desire to run the system between 20%-80%, should I set the BMV settings so that the BMV shows 100% capacity when the batteries hit 80% real SOC or should I program the BMV so that it shows 80% SOC when the batteries are really at 80% SOC?

Thank you in advance for any suggestions. Configuring the BMS, chargers, and BMV for consistency is still not fully clear to me.
 

ohthetrees

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Joined
Jun 28, 2021
Messages
66
Regarding #4, this is probably a personal choice, but my choice would be to have the displayed value match the actual true SOC of the battery. It seems silly to be doing otherwise, and then do the math in your head. OK... display says 75%, so that really means 75% of the way between 20 and 80%, which means... *gets pencil* (80-20)x0.75+20=65% so batt is *actually* at 65% when display reads 75%. Too confusing, and what if SOC somehow gets outside the 20-80 range, what would be on display? Too confusing in my opinion.
 

mikefitz

Solar Addict
Joined
May 28, 2020
Messages
859
Should I change absorption voltage to 13.28v instead of 14.00v? or a different value?
13.8 or 14 volts as a target charging voltage with no absorption period seems in my tests to be a suitable compromise in charging the battery quickly is a low stress manner to over 90% charge capacity. If you charge to 13.28 target, the current falls to a low value at around 60% and needs a very long absorption period to get to a higher capacity.

Not using a charge target over 13.6 volts may cause issues with battery balance , as most BMS are programmed to operate over 3.4 volts per cell.
Should I change float voltage to something below 13.28v?
This voltage in my tests holds the battery around 60%, Selecting the float value depends on the application. If you need to use solar power for the load and to prevent a charged battery discharging, it will have to be higher , say 13.5 volts.

The low limit of charge is easier to determine by voltage, I use 12 volts. Loads are disabled below this
BMV-712 questions:
Set capacity to actual real capacity, it may be useful to set within the actual capacity , say 540Ah.
To use the automatic sync of the BMV the following parameters need setting in relationship to the charger settings,
voltage, just below the charge voltage value to allow for device calibration variation, with a 14.0 charge voltage , use 13.90 volts
tail current
, % of capacity, with 14.0 charge you need below 2.5 amps per 100Ah, say 3% .The current must fall below this value for a sync to take place.
Time to go, this is the period that both the voltage and current settings must be met before sync takes place, typically 4 minutes.
Efficiency 99%.
Peukert Exponent: 1.05

Depending on the charge rate the values of voltage and tail current may need slight adjustment.

Small errors creep in over time, I prefer to manually sync to 100% from time to time when I know for sure the battery is fully charged and disable auto sync by setting parameters that will never be reached.

If you wish to use the BMV internal relay in some way to indicate or control charge it can be set to operate at a charge or voltage level.

repeated absorption time=1Hr, repeated absorption interval=7days, and absorption time=2Hr.
Not too sure what this refers to . Is it the terminology used by Epever for Equilisation? If so is not needed for lithium.

I'd like to cycle the system between 20%-80% SOC.
Whilst it is practical to determine the low limit and control charging and loads to meet that requirement, keeping within the upper limit is somewhat impractical. My technique is to charge to a moderate target voltage, stop charging and use the battery. Setting float and recharge voltages below 13.4 volts

Mike
 
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