Getting a Battery Monitor Dead On

williaty
williaty Solar Expert Posts: 60 ✭✭
I have a Victron Energy BMV-600S battery monitor that's a pretty spiffy little thing. When I first set it up and programmed it, I contacted the manufacturer of my batteries, who recommended using 1.16 as the Peukert's Constant and 76% as the charging efficiency. The monitor is capable of being sync'd to 100% SoC both manually and automatically. The criteria for the automatic syncing are currently configured such that the voltage must be over 28.3VDC (within half a volt of the Absorb setpoint) and the current must be below 1.5A (1.5% of bank capacity) at the same time for a full minute. Reviewing the logs from the battery monitor, the monitor is correctly detecting when to sync itself to 100% SoC. However, the estimate SoC counter has not returned to 100% SoC by this point. The estimated SoC was only at 94.3% when the automatic resync was triggered.

Since the estimated SoC during charge is lower than the real SoC (implied by current during charging), was this error caused by the programmed charging efficiency being slightly too low? Instead of 76%, should I use something a point or two higher?

Comments

  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Getting a Battery Monitor Dead On
    williaty wrote: »
    The criteria for the automatic syncing are currently configured such that the voltage must be over 28.3VDC (within half a volt of the Absorb setpoint) and the current must be below 1.5A (1.5% of bank capacity) at the same time for a full minute.
    <snip>
    The estimated SoC was only at 94.3% when the automatic resync was triggered.

    Adjusting the charge efficiency now is jumping the gun. First you must be sure that your criteria for syncing are correct. You are using 'end amps' as a criteria. How sure are you that 1.5 amps at 28.3 volts really represents 100% SOC?

    What is the SG of your batteries at the alleged 100% SOC?

    You may determine your battery's actual end amps by monitoring the current during absorb. Set absorb time to 'forever' and watch the battery current with your monitor. The current will decrease with time. When it stops decreasing you have found the end amps for your batteries at a particular absorb voltage.

    Next you must figure out if your batteries are fully charged when you achieve end amps. An hydrometer is the best way. If your SG indicates that you are not at 100% SOC, you must raise your absorb voltage a bit and start over (the next day) to determine your end amps at the higher voltage. (The end amps value will increase if you increase your absorb voltage).

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Getting a Battery Monitor Dead On

    74% battery efficiency is pretty low (even assuming flooded cell batteries)... If this was a battery that is only cycled between 100% and 90% SOC--Then it is probably appropriate. If, however you cycle it to 80% or less SOC fairly often, you could certainly up the efficiency by quite a bit (80%-90% would a good range to try).

    But, as vtMaps says--Make sure your battery is getting appropriately charged (mostly, you should be charging to >90% several times a week). Nobody should try for 100% SOC every day. That is pretty hard on a battery bank anyway (some batteries are difficult to get to 100% SOC without a "near equalization" type charge--which can be a bit hard on a battery bank--excessive equalization causing plate erosion, positive grid corrosion, heating batteries, excessive water usage, eroding catalysts/venting in AGM, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Getting a Battery Monitor Dead On

    battery efficiency percentages are pretty much guesswork. you can roughly put in a number, but even if you got it exact then as that battery ages the efficiency will change. occasionally verify by other means if you can for the approximate soc of a battery as depending on a monitor to be exacting can be detrimental.
  • williaty
    williaty Solar Expert Posts: 60 ✭✭
    Re: Getting a Battery Monitor Dead On

    Batteries are AGM, no way to check SG.

    During the first absorb that cooked it too long, current decayed to 0.003C (300mA on a 100Ah bank) and stopped dropping. However, I see figures of around 1% C used on this forum as suggestions for ending amps all the time.

    Also, I can't tell the system to exit absorb and go to float, it does it only when it wants to.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Getting a Battery Monitor Dead On

    the thread is about a battery monitor accuracy which i generally have addressed here and not about your charging the agms with a particular charger as that is a separate issue for you and one that research into a different purchase may have solved. once in a great while battery load testing can be done as per the manufacturer's recommendations, but don't expect great lifespans from this particular agm anyway.
  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
    Re: Getting a Battery Monitor Dead On
    niel wrote: »
    battery efficiency percentages are pretty much guesswork. you can roughly put in a number, but even if you got it exact then as that battery ages the efficiency will change. occasionally verify by other means if you can for the approximate soc of a battery as depending on a monitor to be exacting can be detrimental.

    Niel is 100% correct, you can never get it perfect. I have the same monitor and FLA batteries and although most of the time, the SOC and the meter are pretty close, there are situations that make the meter incorrect. I have found that with FLA batteries, during the winter when the charge current can get low. The meter (just counting amps and volts) will say that the SOC is 100% when in reality, the SG is quite low. In the summer, when the getting is good, then the meter and the SG tend to agree a bit more closely. I realize that with AGM's, you don't have the luxury of measuring the SG so your job will be even more difficult but the moral of the story is there is no sure way (to my knowledge) to determine actual SOC without measuring SG.