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07-10-2020, 23:59
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#1
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Registered User
Join Date: Sep 2017
Location: Kiel, Germany
Boat: TRT 1200 GT
Posts: 176
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Victron BMV 700 SOC Calculation
Hi,
I noticed that the battery monitor on my boat gives strange values. Is there any secret to the SOC calculation on a BMV 700?
See the attached pictures. Currently I have
Battery capacity configured to 800Ah.
297.5 Ah used.
SOC 74.4%
However 297.5Ah / 800Ah = 37.18% -> To me this means SOC should be 62.82%. What am I missing?
Paul
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08-10-2020, 00:23
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#2
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Registered User
Join Date: Nov 2013
Location: Nice, France
Boat: Hunter Marine 38
Posts: 1,342
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Re: Victron BMV 700 SOC Calculation
Did you started the day at 100% capacity? I am aware that cumulatif calculations show an error after several days of not reaching 100% full.
This is not illogical. To get a battery back to 100% you need to put in more AH then the AH consumed and the Peukert coefficient is an approximation.
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08-10-2020, 00:32
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#3
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Registered User
Join Date: Nov 2015
Location: Ireland
Posts: 467
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Re: Victron BMV 700 SOC Calculation
The percentage used displayed depends on how fast it was discharged. The slower you discharge a battery the more energy you can extract from it. If you read the section of the battery monitor manual about Pukerts (sp?) Law it will explain it.
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08-10-2020, 00:33
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#4
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Registered User
Join Date: Mar 2019
Location: Live aboard in Simonstown (Cape Town) - now Grenada having crossed the Atlantic.
Boat: 1986 Bruce Roberts 45 raised saloon
Posts: 374
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Re: Victron BMV 700 SOC Calculation
The BMV uses Peurkert formula to calculate discharge rate which is not 1 to 1. At a low rate of discharge 1ah may have a value less than 1ah, at high discharge rate the opposite.
The calculation is in the manual but from memory I think 0.2C discharge rate is the break even point - but read the manual for confirmation.
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08-10-2020, 00:33
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#5
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Registered User
Join Date: Mar 2019
Location: Live aboard in Simonstown (Cape Town) - now Grenada having crossed the Atlantic.
Boat: 1986 Bruce Roberts 45 raised saloon
Posts: 374
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Re: Victron BMV 700 SOC Calculation
Answered while typing...
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08-10-2020, 01:03
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#6
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Registered User
Join Date: Sep 2017
Location: Kiel, Germany
Boat: TRT 1200 GT
Posts: 176
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Re: Victron BMV 700 SOC Calculation
From the manual: "Consumed Amp-hours - The amount of Ah consumed from the battery.. Example: If a current of 12 A is drawn from a fully charged battery for a period of 3 hours, this readout will show -36.0 Ah. (-12 x 3 = -36) "
So I have taken 297.5Ah from the 800Ah battery. If anything the Peukert factor (configured at default of 1.25) could explain, why the displayed SOC is less than (800-297.5)/800 = 62.82%. But it displayed more remaining capacity than that.
Paul
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08-10-2020, 01:23
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#7
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Registered User
Join Date: Mar 2019
Location: Live aboard in Simonstown (Cape Town) - now Grenada having crossed the Atlantic.
Boat: 1986 Bruce Roberts 45 raised saloon
Posts: 374
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Re: Victron BMV 700 SOC Calculation
That is the simplified explanation, as answered above the BMV modifies that result using Peurkert formula.
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08-10-2020, 01:28
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#8
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Registered User
Join Date: Mar 2019
Location: Live aboard in Simonstown (Cape Town) - now Grenada having crossed the Atlantic.
Boat: 1986 Bruce Roberts 45 raised saloon
Posts: 374
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Re: Victron BMV 700 SOC Calculation
Also consumed Ah is actual value (as you described) - SOC is calculated using Peurkert formula so you cant use your simple crosscheck unless you also use rate of charge/discharge and apply Peurkert formula.
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08-10-2020, 01:41
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#9
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Registered User
Join Date: Mar 2019
Location: Live aboard in Simonstown (Cape Town) - now Grenada having crossed the Atlantic.
Boat: 1986 Bruce Roberts 45 raised saloon
Posts: 374
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Re: Victron BMV 700 SOC Calculation
Also note that charge rate (not discharge) is modified by efficiency factor which is a number that you set. Lead acid could be 80% (or worse depending on SOH - state of health) whereas LiFePO4 would normally be pretty close to 100%.
So at 80% efficiency 10Ah of charge would provide the equivalent of 8Ah increase in SOC.
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08-10-2020, 08:46
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#10
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Registered User
Join Date: Nov 2015
Location: Ireland
Posts: 467
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Re: Victron BMV 700 SOC Calculation
As well as applying Peurkert's Law you have to bear in mind that the BMV does have to be synchronised quite regularly. Generally you do this by fully charging the battery with the engine or a shore charger. Do you know when your batteries were last at 100%?
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08-10-2020, 13:51
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#11
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Registered User
Join Date: Jan 2011
Location: Cowichan Bay, BC (Maple Bay Marina)
Posts: 9,705
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Re: Victron BMV 700 SOC Calculation
For everyone installing a battery monitor: The " Gotcha Algorithm" thread, a "MUST READ"
http://c34.org/bbs/index.php/topic,4922.0.html
DEFAULTS are factory settings that are made to be modified to suit your setup.
Also read this one:
Programming a Battery Monitor (by Maine Sail)
https://marinehowto.com/programming-a-battery-monitor/
*******************************8
Don't know if you've seen these.
I never use the SOC on my Link 2000, all I need is V and ah used. I recharge when ah used = 50% of house bank. If voltage sags it's time for new batteries. Simpler than trying to figure out what the
SOC is telling you, and really all you need to know, I think. Been doing this for 20 years.
__________________
Stu Jackson
Catalina 34 #224 (1986) C34IA Secretary
Cowichan Bay, BC, SR/FK, M25, Rocna 10 (22#) (NZ model)
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08-10-2020, 14:13
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#12
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Registered User
Join Date: Aug 2011
Location: Petersburg, AK
Boat: Outremer 50S
Posts: 4,229
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Re: Victron BMV 700 SOC Calculation
The key point here is your batteries are 800Ah at the 20-hour rate (that't the standard metric for deep cycle). In a nutshell you can pull 800/20 = 40A for 20 hours to go from 100%SoC to 0% SoC. If you only withdraw at 10A then you're using the 80-hour rate to calculate capacity, and your "800Ah" batteries may be 1200Ah. Peukert rears his head. Note that these values are also temperature dependent.
If you look at the datasheet for the T-105, for instance, the battery capacity is 185Ah at the the 5-hour rate, 225Ah at the 20-hour rate, and 250Ah at the 100-hour rate. You enter the 20-hour value and the estimated Peukert value into the BMV, it then calculates the capacity based on the load you put on the batteries.
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13-10-2020, 04:35
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#13
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Registered User
Join Date: Nov 2013
Location: Nice, France
Boat: Hunter Marine 38
Posts: 1,342
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Re: Victron BMV 700 SOC Calculation
[QUOTE=Stu Jackson
.........
.........
I never use the SOC on my Link 2000, all I need is V and ah used. I recharge when ah used = 50% of house bank. If voltage sags it's time for new batteries. Simpler than trying to figure out what the
SOC is telling you, and really all you need to know, I think. Been doing this for 20 years.[/QUOTE]
That's what I do. Ah used and voltage is all you need to manage your batteries.
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