Smartshunt and Peukert coefficent?? Doubts...

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Fra asked Jan 15, '21

Smartshunt and Peukert coefficent?? Doubts...

Goodmorning everyone.

I bought victronenergy Smartshunt.

I must say that I am extremely satisfied with the accuracy of all the measurements.

Comparing the readings from TracerAn 3210 Epever and Smartshunt, they are surprisingly nearly identical, in both current and voltage.

They differ only by 0.02A and 0.02V. That is practically zero.

However, I am not convinced that the Peukert coefficient works.

In theory, by increasing the coefficient, the remaining runtime should shorten as the discharge current increases.

But it's the other way around:

for example, with a discharge current of 3 amperes, if I put coefficient 1.5 there is more time remaining, compared to coefficient 1.0.

The percentage of SOC also increases as the Peukert coefficient increases. I expected the opposite.

In my case, the statistics with default coefficient 1.25, seem too optimistic and less realistic than coefficient 1.0 or 1.1.

Can anyone clarify this for me?

BMV Battery Monitor SmartShunt SOC

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Answer 1 · 2021-01-15T20:09:13Z

lehrling answered · Jan 15, 2021 at 08:09 PM

What kind of batteries do you have?

Please note what manual says about setting coefficient to 1.0:

Peukert exponent

When unknown it is recommended to keep this value at 1.25 (default) for lead acid batteries and change to 1.05 for Li-ion batteries. A value of 1.00 disables the Peukert compensation.

In the BMV Peukert’s exponent can be adjusted from 1.00 to 1.50. The higher the Peukert exponent the faster the effective capacity ‘shrinks’ with increasing discharge rate. An ideal (theoretical) battery has a Peukert Exponent of 1.00 and has a fixed capacity; regardless of the size of the discharge current. The default setting for the Peukert exponent is 1.25.

This is an acceptable average value for most lead acid batteries.

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Answer 2 · 2021-01-16T03:44:44Z

JohnC answered · Jan 16, 2021 at 03:44 AM

Hi @Francesco

You have a preconceived view of this, but what you've observed will be correct if your "3A" is less than your battery C20 rating (ie > 60Ah.

From an (ex) Victron competitor (forgive me), this article explains it very well: http://www.smartgauge.co.uk/peukert2.html

Mathematicians rejoice. And it should all be aboard your (and my) Smartshunt. I can't vouch for that, as I'm not privy to the code, but I expect so.

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Answer 3 · 2021-01-16T09:45:57Z

Fra answered · Jan 16, 2021 at 09:45 AM

they are lead acid batteries.

Thanks, I have already read very carefully what you have copied ... I have also done the peukert equation.

What is not clear to me is WHY BY INCREASING THE COEFFICIENT, THE CAPACITY INCREASES, INSTEAD OF REDUCING, AS THE MANUAL SAYS.

But I'm probably missing something mathematically.

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JohnC ♦ commented · Jan 16, 2021 at 11:04 AM

@Francesco

What you observe is because you're testing with a current below C20 rate. And if you look at your batt specs (if available) you'll see the C100 capacity is far higher.

Try testing with a higher discharge rate closer to C5. You'll then see the capacity (or time-to-go) reduce as you expect. Maybe even more than you expect.. :)

Fortunately most of us with sufficient batt capacity are on the 'upper' side of C20, and gain somewhat of a bonus with slower discharge. Those fast-discharging with undersized batts, cop a penalty with lower discharge efficiency.

Running my own batt specs through the Victron calculator gave me a Peukert of 1.16. With my relatively low loads the default of 1.25 would overestimate my SOC.

You can trust it, well as much as a calculated figure can be trusted with the 'empirical' application of data.

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Fra JohnC ♦ commented · Jan 16, 2021 at 02:51 PM

thank you John ...

I understood and I did not understand at the same time.

So, with my low loads (on average 2-4 Amper ... I live in a camper), would it be more appropriate to set a coefficient around 1.15?

From my experience of many observations and records over the years, I have seen that the estimate of Soc with 1.15 and low loads, becomes very close to reality.

Besides my batteries have 2 years and 7 months of continuous daily use ... with daily discharges at 12.5v.

So about 1000 light cycles. And they've certainly lost some capacity.

In fact, with the same use, compared to 2 and a half years ago, they have 0.15v less voltage. In any condition.

For example, after 8 hours of tv on inverter, lights etc(about 25 ah) it was 12.65v from new, now it is 12.50v (oper circuit).

And with loads of 15 amps, the deterioration is even more evident.

If when new, the 100ah agm battery gave me back 80ah with a load of 15 amperes, today it gives me 50/55 ah with the same load.

(this would indicate an increase in the Peukert coefficient, right ??)

Sorry for my english..i help me with Google translate......

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JohnC ♦ Fra commented · Jan 17, 2021 at 12:54 PM

@Francesco

I'd keep using the Peukert you decide upon, and if you're absolutely sure you've lost capacity, then change the capacity Ah in the settings.

But don't rush into this, as there are many factors that can throw off your 'feelings' about your batts. Not the least of which is temperature, not often mentioned, but your Pb batts will perform differently when they're cold vs warm. And using just V to assess them isn't good enough.

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Fra JohnC ♦ commented · Jan 17, 2021 at 01:51 PM

Thanks for your suggestions.

Yes, I know that the temperature affects various factors ...

In fact, decent quality mppt controllers have temperature compensation.

So you are right about the voltage, which is not the only parameter.

But I can tell you that I did a test with my "cyclic use" Agm.

I applied a load of 15 Amperes (trivalent refrigerator on inverter and tv on inverter).

In 3 hours and 45 minutes it reached 10.5v under load.

So it was almost exhausted.

It means it returned about 55ah.

That battery (100 ah cyclic AGM) should give around 75 ah in those conditions.

As I told you, it has 2 years and 7/8 months of charge and discharge (light) daily.

It is loaded almost exclusively by the Epever TracerAn 3210 mppt regulator with these parameters:

-Maximum current

- absorption of 120 or 90 minutes at 14.5 v.

-floating at 13.75 v

- temperature compensation of 3mV / C / 2V (about 20mV per degree.)

- operating temperature always between 17 and 30 degrees (Canary Islands).

The battery manufacturer says to charge it at 14.7 / 14.9v if in cyclic use. 13.8v if in buffer use.

But mine is a light cyclic use, almost a buffer use .... and I have carefully monitored that at the end of the absorption at 14.5v it always loads correctly and arrives below the tail current.

In summary, at the end of the absorption phase at 14.5v it draws less than 1 amp, and increasing it to 14.9v absolutely nothing changes.

So 14.7 / 14.9v would be useless. Indeed, it could cause a harmful overcharge.

Only once every 30 days the mppt controller does a slight equalization of 140 minutes at 14.9 v.

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