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S&S Swan General - Battery management system
29 February 2016 - 18:49
Join Date: 17 March 2010
Posts: 48

Hi Lars,

Yes, but don't you think this is only that these units work out the difference between the capacity which has been entered in the setup, and what has been consumed?

Nice to talk to you again, after so many years!

SAILTEC / Germany

29 February 2016 - 20:09
Join Date: 02 January 2008
Posts: 1547

Dear Christian
I think the better ones take the battery deterioration into consideration, but how they do it is their own secret. It is this effect Philippe asks about.

Yes, I remember. It appears our last contact was in 2005, about the cove strip.
Best regards

29 February 2016 - 21:03
Join Date: 17 March 2010
Posts: 48

I just read about this in the manual of the Mastervolt unit Masterlink BTM III. They say this system can - only for the battery bank connected as bank 1 - find out by itself how the storage capacity of the batteries goes down over time. And it can adjust the "time remaining" - this is the time to "batteries down" at constant consumption - automatically. Pretty difficult to understand, I think the only way to find out whether and how this works in reality, is to buy one of these and test it out on board, in daily life.

Maybe this is a reason for me to go for a Mastervolt system, and not buy a simpler system as - for example - Votronic. All Votronic say is that the battery capacity goes down over time, and the Ah capacity put in in the units' setup should be manually reduced by 5% every year (depending on the batteris ... ??).

Lars, good memory! That is a decade ago!

Christian 431/003

01 March 2016 - 07:46
Join Date: 06 January 2012
Posts: 67

Hi, I suppose the most common way to calculate the remaining battery capasity with these "Smart battery management systems" is applying the Peukert's law in their alghoritms inside. The mathematics behind those alghoritms is not exactly an easy reading and there are many variables causing different small errors which accumulate at the final readings at the device.

It is too much simplified to think that the batteries are similar as watertanks; you take some warer out and put some in and calculate the difference. Oh no. To do it properly You should think the battery is kind of a watertank with variable volume depending on the flow rate. The lesser the flowrate the bigger the volume --> Peukert's equations needed. (An easy explanation can be found here:

What these devices are doing is one kind of dead recockning and the charging state of the battery and should not be thought as absolutely accurate. 10 to 20% accuracy is something I wait to achieve. Every now and then the devices find a fixed reference point and are accurate for a while. That reference point is when the batteries are fully charged, an easy to find stage of the battery.

IHe 431-12 CAID

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