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markcx avatar image
markcx asked

What is the point of BMV synchronisation?

Ok, so I am having problems getting synchronisation to to occur at the correct point in charging with my solar panels.

I have 80w of solar (2x40w in series), a SmartSolar 75/10 and a BMV 712 connected to Victron 125Ah AGM battery. No matter what settings I try, sync always occurs before the battery is anywhere near 100% based on ah out/ah in, especially if I have charged voltage set to just over float. My solar amps are often below the tail current for a significant amount of time, purely due to the weather conditions where I am.

Does synchronisation actually serve any purpose? If so what am I missing. If I set the parameters so that sync could never be achieved, relying solely on ah out/ah in to indicate 100%, what difference would that make.

I feel as if I’m spending time and effort chasing something that might be unimportant. Please could someone explain if otherwise.

Thanks

Mark

BMV Battery Monitorsmart solar set-up help
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3 Answers
Alexandra avatar image
Alexandra answered ·

@MarkCX Have you tried Smart networking?

https://www.victronenergy.com/live/victronconnect:ve-smart-networking.

All of the elements in the system would work better if they could communicate with one another.

It is slightly more complicated charging an AGM than AH in and out as they are a little more inefficient than that. So you do need to make sure your Peukert setting and absorption time are set correctly.

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Alexandra avatar image Alexandra ♦ commented ·

Since you mentioned you have poor solar, have you considered adding another two panels to you system? you could add them in parallel. In fact you can possibly go with another two strings in parallel of 40W panels.

https://www.victronenergy.com/mppt-calculator. Here is the online calculator so you can have a look at improving the charging a bit, or here if you prefer to download: -

https://www.victronenergy.com/support-and-downloads/software#mppt-calculator-excel-sheet

I think it will be syncing before it is ready as your solar amps are under what it requires. And this would be bad as you will start to have an inaccurate SOC reading and end up having a poorly charged battery.

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markcx avatar image markcx Alexandra ♦ commented ·

Unfortunately this setup is on a 7 metre sailboat. There is no more room for any more solar, I struggled to find a space for the second panel. I need a bigger boat!

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markcx avatar image markcx commented ·

Hi Alexandra,

I have the battery voltage and temperature sensor connected to the BMV, which is connected by Bluetooth to the SmartSolar

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Daniël Boekel (Victron Energy Staff) avatar image
Daniël Boekel (Victron Energy Staff) answered ·

Hi @MarkCX

what settings are you using in the BMV, and what charging voltages in the MPPT's?

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markcx avatar image markcx commented ·

hi Daniël,

These are the settings I am currently using:


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Daniël Boekel (Victron Energy Staff) avatar image Daniël Boekel (Victron Energy Staff) ♦♦ markcx commented ·

So you're charging with 14.4V, but have the BMV set to sync at 13.2V, this can / will lead to early syncing.

you'll have to set the BMV 'charged' voltage closer to the float voltage.

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markcx avatar image markcx Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

It is set at this, as this is what the documentation suggests and this works fine when on an AC charger, syncing when expected. It just doesn’t work for a relatively low powered solar installation where charging current is frequently lower than the tail current due to cloud cover.
What voltage would you suggest? I’ve tried various combinations of settings.

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john-hagtharp avatar image
john-hagtharp answered ·

Hi @MarkCX,

Short of drawing some fluid from a flooded battery and measuring the specific gravity, all methods for monitoring the SOC of a lead acid battery are approximations.

The worst method is measuring it's voltage, although better if it's been sitting disconnected for many hours.

A battery monitor comparing AH in to AH out does a decent job, especially in a controlled situation where you get a steady discharge followed by a steady charge back up to 100%. It's still not perfect though and most real world situations are far from this ideal.

The battery's actual capacity reduces with age. Also, the total power you can draw from it is less if you draw it out quickly. The peukert's constant you program into the monitor, if correct, does a good job of acccounting for this. A well configured battery monitor tends to be most accurate during discharge.

During charge, the charge efficiency factor comes into play. You can program it into the battery monitor but, in reality, it is not a single number. If you're battery is at 50% SOC, it will take charge with almost 100% efficiency but this drops off to very low percentages as you work through the top 15% SOC.

The direct answer to your question is that the purpose of automatic synchronisation is to keep your SOC measurement as accurate as possible. Without synchronisation, the SOC measurement gets less acurate over time. The idea is to define a a set of conditions under which the battery must be fully charged so the monitor can synchronise it's SOC measurement to reality.

Victron's BMV monitors are highly respected so probably have one of the best algorithm's attempting to take all these factors into account but no monitor is perfect.

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markcx avatar image markcx commented ·

Hi John,

My main concern is with the BMV syncing early. When I see 100%, I usually use the day’s ‘excess’ solar to charge various devices, phone, tablet, tools etc, that I might not use otherwise. If this happens, several days in a row, won’t the actual state of charge gradually creep downwards?

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john-hagtharp avatar image john-hagtharp markcx commented ·

Hi Mark,

For using "excess" solar, you could just monitor the charging voltage. If the voltage does not drop when the load is applied then it's using "excess". The potential for this starts when the voltage has reached your temperature compensated absorption or float voltage depending on which charge state it's in. This could be a long time before the battery reaches 100% as the battery just won't accept charge quickly as it gets closer to full.

If you're cycling an AGM battery daily and taking any significant power from it, you may have trouble getting it fully charged. Unless you're near the North Pole now, there are just not enough sun hours in the day to slowly trickle in those last few percent. If the boat is just a weekender and can sit there charging all week, this will be ok.

I've only managed to get my BMV settings close to correct by observing the way the charge current tapers off during absorption and float. The curve should flatten out but if you wait for the current to stop dropping completely, you will be waiting for a very long time.

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markcx avatar image markcx john-hagtharp commented ·

I’m cycling the battery daily, only by a dozen amps or so - my needs are minimal, but even when away from the boat my alarm and CCTV draw 8Ah a day (⅓ Ah).
Using the settings I have posted, the BMV has just synced (noon). Immediately prior to this the SOC was reported as 96.3% with 6.4Ah consumed and voltage at 13.24V. I realise it’s only a small percentage, but if I now start dumping the 2 or 3 amps of charge from the panels into charging gadgets assuming my battery is at 100%, and I do this over several days, after a week my SOC could be showing 100% when it’s actually only 80% - the BMV will still see that the sync criteria has been met, even though the SmartSolar hasn’t reached absorption.
Maybe I’m asking a lot for a small system, but it’s the best I can install on a small boat.

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