What is the point of BMV synchronisation?

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markcx asked May 16, '20

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 Monitor smart solar set-up help

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Answer 1 · 2020-05-17T16:14:46Z

Alexandra answered · May 17, 2020 at 04:14 PM

@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 ♦ commented · May 17, 2020 at 04:32 PM

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 Alexandra ♦ commented · May 18, 2020 at 09:10 AM

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 commented · May 18, 2020 at 09:08 AM

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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Answer 2 · 2020-05-17T20:25:07Z

Daniël Boekel (Victron Energy Staff) answered · May 17, 2020 at 08:25 PM

Hi @MarkCX

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

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markcx commented · May 18, 2020 at 09:04 AM

hi Daniël,

These are the settings I am currently using:

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Daniël Boekel (Victron Energy Staff) ♦♦ markcx commented · May 18, 2020 at 10:13 AM

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 Daniël Boekel (Victron Energy Staff) ♦♦ commented · May 18, 2020 at 11:24 AM

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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Answer 3 · 2020-05-18T08:25:44Z

john-hagtharp answered · May 18, 2020 at 08:25 AM

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 commented · May 18, 2020 at 09:16 AM

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 markcx commented · May 18, 2020 at 10:49 AM

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 john-hagtharp commented · May 18, 2020 at 11:19 AM

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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john-hagtharp markcx commented · May 19, 2020 at 01:30 AM

You're assuming the SOC reading is more accurate than the synchronisation trigger point. That is not necessarily true.

There is no relationship between the current SOC reading and the synchronisation trigger point. Those trigger voltage and current conditions should be reached at the same actual battery SOC every time. That's why they're used to keep the monitor accurate. The question is whether they're triggering at an actual SOC significantly lower than 100% SOC.

If you haven't already, I suggest reading up on 3 stage charging (bulk, absorption, float). Then observe battery voltage and current during charge to determine where to set the synchronisation trigger.

As I mentioned previously, the time to start charging your first gadget is not when the SOC reading is 100% but soon after absorption voltage is reached (or float if the charger has stayed in float). If the charge voltage does not drop, you are not affecting the charging of the battery. Remember these voltage points change based on temperature.

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markcx john-hagtharp commented · May 19, 2020 at 07:34 AM

This Is exactly my point. I KNOW the SOC reading is more accurate than the sync point. That is essentially the point of my original question - it IS “triggering at an actual SOC significantly lower than 100% SOC”. I fully understand 3 stage charging. Sync is occurring when there has been ZERO time spent in the absorption stage (let alone float being reached) when the recommended BMV settings have been used. The SmartSolar goes into absorption way after sync occurs. Setting the Sync voltage below the absorption voltage will nearly always result in a premature sync.
The only time I have even come close to auto-syncing at the correct time with solar is to increase the sync voltage to absorption voltage with a long detect time. Even better still, was when I made auto-sync unattainable via the settings, then 100% SOC was reached pretty much at the same time as float was achieved.
The whole point of the BMV is to tell me the State of Charge of my battery - it simply does not do this when relying on auto-Sync!

To expect me to do the job of the BMV by monitoring voltages, charge stages etc., puts me back to the point I was at when I didn’t have the BMV, just the SmartSolar. I have therefore wasted a significant amount of money and time with the BMV.

Just another thought - seeing as the BMV and SmartSolar are talking to each other anyway, would it not make sense for the SmartSolar to say to the BMV ‘Hey, we’ve reached float here, you might want to sync roundabout now’, rather than the BMV essentially guessing what is actually going on with charging.

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john-hagtharp markcx commented · May 19, 2020 at 01:51 PM

btw. i don't work for Victron. just trying to help.

In my case, I have done as you mention and set the sync voltage closer to the absorption voltage. I force a long absorption weekly and that's when it syncs.

The sync voltage is not all that critical, it's really just a choice of whether you want it to sync during float or happy for it to only ever happen if it reaches 100% during absorption. The tail current percentage is the more important figure and it sounds like yours is too high. If you watch the current during charge and see how low it gets before it stops dropping significantly, that will indicate where to set the tail current. You know the battery is full when the current at absorption or float voltage stops dropping (within reason or you'll be waiting days...).

The solar charger doesn't know state of charge. If you look in "expert mode" you will see it just switches to float based on a timer or tail current. The BMV is in a better position to know as it sees everything actually going into and out of the battery. These days with smart communications, it probably feeds that information back to the charge controller too which would otherwise not know if current was going to battery or loads.

Every battery behaves differently. Even with the same battery, the capacity and tail current change with age. Charge efficiency changes depending on where in the SOC range you're operating them. The peukert's constant is a bit easier. If not supplied by the manufacturer, there are calculators online that derive it from other figures on the datasheet.

The end result is that it's relatively easy to get it reading accurately while the battery is under discharge but much harder to have it accurate on the way back up. With some work you can get it very close, especially if the way you cycle the battery doesn't change much.

None of this changes the timing of when to start charging gadgets. If you wait for the batteries to reach 100%, you've not used all the energy your panels were capable of producing.

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markcx john-hagtharp commented · May 19, 2020 at 05:05 PM

Hi John,

I apologise if I came across a little gruff. I realise you don’t work for Victron and are only trying to help. I think I’m more frustrated at myself for not being able to clearly explain myself in the first place, and the fact that I thought I’d bought a magic bullet, an expensive magic bullet, with my limited means! The intention was to protect my (to me) very expensive Victron AGM.

I now realise that that battery monitoring is something more akin to a dark art than science. Too much chemistry/alchemy for my liking, with the additional chaos of weather thrown into the mix.
Once again, thanks for your help, I’ll keep plugging away at it, bearing in mind your suggestions.

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john-hagtharp markcx commented · May 20, 2020 at 12:23 AM

I just checked your peukerts value and it seems about right. Being a new battery, the capacity should be correct too. This means it's probably accurate on discharge so long as it's 100% starting point is correct. The high level steps I would take are:

Get it auto syncing correctly at 100% as discussed previously.
Tweak the charge efficiency until it's reaching 100% about the same time as the battery really reaches 100% and the sync happens.

It should then be quite accurate unless you start operating the battery in a different SOC range, in which case charge efficiency will need tweaking again. If efficiency is set too high, it will indicate 100% too early. Assuming reality eventually catches up, the monitor will still be accurate on the way down again. If efficiency is set too low, it may never reach 100%, indicating lower and lower each charge cycle. Synchronisation is really important in this case as it will bump it up to 100% when the battery really is full.

The reason it syncs at 100% is that the flattening out of the charge current curve at that point is the only time it has a really good indication of reality. In between those times, it's estimating.

It would be more typical to bulk charge that battery somewhere in the 18A to 25A range but you're probably under 5A. This will make your charge current curve look different (flatter). It will also be reaching absorption at a higher SOC than it normally would.

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markcx john-hagtharp commented · May 20, 2020 at 09:56 AM

You’re right. Using solar, my charge current is never over 5 amps. On an average day I’m seeing somewhere between 1-2 amps for most of the day, although it has crept nearer to 4 amps in the early afternoon in some of the good weather recently.
Over the winter I was charging with a 25amp charger about once a week, but I don’t have access to mains electricity, so several hours with a generator running is not much fun.
With the 25amp charger and most of the settings fairly close to default, the BMV was syncing pretty much when the charger reached float. This contributed to my disappointment when charging purely on solar.
i see what you mean about the flatter charge current curve.
Maybe two different configurations, one for Summer & solar, the other for Winter & generator might be useful

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