BMV-712 Smart SOC not accurate with low current consumption

Q&A and troubleshooting
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1

Hi Guys,

I have a 100Ah Victron lithium battery linked to a BMV-712,

I have noticed that on low current consumption the SOC was completely incorrect.
For example the current SOC indicates 87% while the consumed Ah is 38Ah, the battery being a 100Ah the SOC should be 62% not 87%

the current current consumption being 0.05A. The threshold is set to 0A so it should count everything properly.

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I believe this is a firmware issue

Regards,

Philippe

2

Unless the peukerts is set incorrectly… Amongst other things.

Would you mind sharing your system/battery details and set up and settings?

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Sure, here is the current config:

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4

copied from another post

You need to read up on the Peukert effect. A simple overview is a 100Ah battery is specified to deliver 100Ah with a 5A load over 20 hours. With a load above 5A it will deliver less than 100Ah and SOC will fall faster than Ah drawn. When drawn down slowly at less than 5Ayou will get more than 100Ah so SOC falls more slowly than Ah drawn.

The battery monitor allows for this so the SOC and Ah drawn may not necessarily be the same. This is set by the Peukert exponent setting and the effect is small for lithium and the Peukert should be set to around 1.05. The default for lead acid is 1.25. Good AGMs will be in the range of 1.10 to 1.15. Lead acid and AGM are significantly affected by Peukert.

The most common problem is people having a lithium battery with the Peukert set for lead acid.

1 Like
5

Thanks for your reply pwfarnell,

But it doesn’t make sense, how can you have more energy than stored !

6

@pmathot as expected you have got defaults for a lead acid battery. Change Peukert exponent to 1.05 and charge efficiency factor to 99%.

A 100Ah battery holds more than 100Ah. The charge available depends on the rate of discharge and the reaction kinetics as the chemicals in the battery convert from one state to another. If you are discharging at the rated 5A for a 100Ah battery when you get to 100Ah withdrawn after 20hrs there is still remaining capacity, but the reaction rate is too slow to provide 5A so the voltage falls and the battery is called empty. However, it would still keep providing more power at a lower rate. If you look at good lead acid batteries they will quote different capacity at different discharge rates.

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7

Cool thank you very much for the config and the explanation :slight_smile:

very much appreciated

1 Like