Hi there,

I have a bit of a problem with understanding measured Amps in/out from my batteries. 

A coupIe of weeks ago I replaced my GEL batteries with some new LiFePo4 packs in my Off-Grid PV System.

My setup:
2 x Axpert MKS 5KVA parallel
16 x TrinaSolar 275W (4.4KW)
2 x YIY LiFePo4 battery pack 51.2V/150AH (15KWH)
Victron BMV-702
ICC on Raspberry 3

The new LiFePo4 packs work just great so far, but I am a bit confused when comparing data from the BMV-702 with data from the LiFePo4 BMS...
There is quite a difference in measured Amps going in/out to/from my batteries between the BMV and the internal BMS. Accordingly, other info like AH consumed and SOC are also different.
See attached screenshots of ICC and BMS in different situations...

My connections and wiring should be ok. The two battery packs ar connected to a -/+ busbar. The BMV-702 is connected via original shunt to the - busbar and via original USB cable to the Pi3.
See attached picture...

Anyone have an idea what could be the reason for these big differences in measured current?

Thanks for any help or advice

Felix

 

There are two issues here. The one is energy (amp hours), the other is current (ampere).

For the energy issue, you must check that the Peukert factor and the efficiency on the BMV is configured correctly for LiFePO4. Charge efficiency is 95% typically, and the peukert factor is 1.05. If you're running lead-acid values, the BMV will be pessimistic. Although in your case the BMV appears to be optimistic compared to the BMS.

Secondly, the BMS usually has the ability to determine the usable capacity available before the LOWEST cell is empty, while the BMV does not know that.

Third, the BMS might not use the same way of calculating the SOC. It uses measurements through shunts, a number of voltage measurements inside the pack, or some combination thereof. So a slight discrepancy between the BMV and the BMS should be expected. Yours does not seem too out of wack.

For the current issue, two things has to be noted. The first is latency. Many BMSes only measure the current every so many seconds (or more specifically, they only report an updated value every few seconds). For many batteries this delay is as much as 2 seconds. The BMV has a similar reporting delay of somewhere between 500mS and 1 second (I forgot which), and then there is also the delays introduced in the software itself. So you are essentially looking at two slightly outdated values taken at slightly different times in the past. There will always be a small discrepancy.

Secondly, BMSes are often not very accurate at lower current levels. A BYD battery, for example, is somewhat inaccurate below 5 amps. So the BMS is probably the "wrong" one here.

1 hour ago, plonkster said:

There are two issues here. The one is energy (amp hours), the other is current (ampere).

For the energy issue, you must check that the Peukert factor and the efficiency on the BMV is configured correctly for LiFePO4. Charge efficiency is 95% typically, and the peukert factor is 1.05. If you're running lead-acid values, the BMV will be pessimistic. Although in your case the BMV appears to be optimistic compared to the BMS.

Secondly, the BMS usually has the ability to determine the usable capacity available before the LOWEST cell is empty, while the BMV does not know that.

Third, the BMS might not use the same way of calculating the SOC. It uses measurements through shunts, a number of voltage measurements inside the pack, or some combination thereof. So a slight discrepancy between the BMV and the BMS should be expected. Yours does not seem too out of wack.

For the current issue, two things has to be noted. The first is latency. Many BMSes only measure the current every so many seconds (or more specifically, they only report an updated value every few seconds). For many batteries this delay is as much as 2 seconds. The BMV has a similar reporting delay of somewhere between 500mS and 1 second (I forgot which), and then there is also the delays introduced in the software itself. So you are essentially looking at two slightly outdated values taken at slightly different times in the past. There will always be a small discrepancy.

Secondly, BMSes are often not very accurate at lower current levels. A BYD battery, for example, is somewhat inaccurate below 5 amps. So the BMS is probably the "wrong" one here.

Hi Plonkster,

Thank you very much for your quick and detailed answer. 

For the energy issue... I did update my BMV settings after changing to LiFePo4. Current settings are:
Peukert-Exponent: 1.05
Charge Efficiency: 99%
Maybe I should lower the Charge Efficiency to 95%, to get BMV AH and SOC closer to the BMS values ...!? 

Other question:
Is the "Charge Efficiency" related to the "Battery Efficiency" which I can see in the ICC -> BMV-Values?
I guess, ICC is calculating the Battery Efficiency by Battery Discharge Energy / Battery Charge Energy. In my case the Efficiency is always around 80%. Is this not too low for LiFePo4?
See screenshot...

Actually... with my current settings, ICC gives me quite good and probably accurate SOC readings. But it's optimistic compared to BMS values, i.e. When BMV SOC is around 50%, BMS SOC is around 40%... 
See screenshot... 

Thanks for any comments
Felix

2 hours ago, Felix Merz said:

Is the "Charge Efficiency" related to the "Battery Efficiency" which I can see in the ICC -> BMV-Values?

I have no idea how the software does it, sorry 🙂