Top of charge behaviour - Batrium Watchmon 5 Plus

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batteryshed77

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I'm experiencing some frustrating behaviour at the top of charge when charging my battery with Batrium Watchmon 5 Plus BMS & am wondering if anyone has a solution.

Basically up to about 85% I'm charging at max power, no problems. Above this, voltage limits start to be hit and so power is reduced - perfect.

However.... the BMS isn't smoothly curtailing the power, it's modulating the charge current limit between 0 - max - 0 - max - 0. Not only is this not maximising my opportunity to charge my battery, but can't be good for the lifetime of my charger either.

Here's the resulting profile.
Screenshot 2021-02-22 at 06.48.51.png


I can change this profile by modifying the time it takes the BMS to keep charging when voltage limit exceeded, and the gap until it tries to charge again - but the pulsing behaviour stays the same.

I've got 'Delay Transition - Stop' set to 10s & 'Delay Transition - Restart' set to 20s at the moment.

What I'd like is for the Batrium to slowly reduce the charge current limit until the max voltage isn't exceeded - is that so much to ask? Hopefully the setting exists in some form which I haven't been able to discover... any help appreciated!
 

Roland W

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I know nothing about Batrium, but what you are describing here, isn't this the job of the charger? Not of a BMS! A BMS will only shut down the whole system, when a limit is exceeded, same as you show. But the charger is responsible to change from CC to CV mode and thus take pressure away from the cells when they are becoming full.

What is your cell chemistry, LiFePo4? As LFP will after discontinuing, go on its way to rest thus drop voltage, so it might then release the lock of the BMS again and start that next pulse.
If you do have LFP cells, you must reduce C-rate when near end of charge. That will bring cells back to manageable voltage and stay out of that final spike in the charging curve where every just so small cell imbalance will cause limits to be over crossed.

Maybe you have just set a to low cutout voltage at the top. You need to consider the voltage difference between charging and idling.

So please give us a bit more details of your system.
 
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batteryshed77

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The BMS communicates the current to the charger that it should charge at. After all, the BMS knows the condition and state of the battery and is best placed to determine what this should be.

The charger in this case is just tracking what the BMS is telling it (I should maybe add that the Batrium sends the voltage and current limits over CAN to the charger. When the charger comes up against the voltage limit, the charger is responsible for charging up to this level. However the problem is that the max current signal is cycling between 0-max current)

Cell chemistry is NMC. I'm using 1,440 2170 cells. Pack is being charged/discharged at less than 0.2C.

My voltage cutoff is currently set to 4.2V. I previously had it set lower but this behaviour has forced me higher to utilise a reasonable amount of the pack. If I set it lower, I just get this behaviour earlier. ( I know I know, use a smaller window to conserve life. These cells are having an easy life - I'm happy to charge them to 100%)
 
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Roland W

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Thanks for the detailed info. You are allowed to blame Batrium. So I am out of expertise on this and glad that my old-fashionable approach, which is believe that all this smart communicating things, are just increasing complexity and problems. Hahaha

Sorry to say this, buy yeah, keyboard goes to next one. Hope you can figure out this quickly ;)
 

OffGridInTheCity

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>Batrium Watchmon 5 Plus BMS
Could you expand on details - e.g. what model is the charger? and I gather Batrium is controlling it? I use Batrium but its an advisory BMS and its job is to report data and throw the shunt-trip in an emergency. It has nothing to do with the charger :)
 

daromer

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The bms only sends a signal stating "Lower charge" or "stop charge"

There is no regulation like a PID. That is up to the charge controller to handle and not the BMS. Look into how you have set up the middle point and what it should set it to.

So the bms does exactly what its told to do now. Its told to disable the charge when a voltage is reached. You most likely should change this so the charger starts the limit function first and then if the bms cant balance in time it steps in and shuts it down totally if any cell is above 4.2 volt.

Unfortunately i have not used the function here so cant say exact what paramters to change in your setup but above is how it works. Its ON - Lower - OFF only
 
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slimf

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I also have the same problem. I spent some time on the phone with Batrium, but at the end of the call I was no wiser.

My Victron gear will do as it’s told but the message I got from Batrium was they have no interest in Victron and are aiming for the bottom end of the inverter/charger market - ie dumb inverter/chargers that accept on/off commands.

Having said that, I think Daromer is right - the inverter should be set to ramp down before the BMS, where are I and original OP are getting to the BMS limit before the inverter/charger limit.
 

batteryshed77

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I can see that wasn't clear to explain the issue.

I'm using a Victron Multiplus as the charger, Batrium sends a message over CAN to the Multiplus with a maximum charge voltage (50V) and a charge current (108A).

If I set the charge voltage to 48V, for example - the Multiplus does a beautiful job of controlling the output to 48V. However, I want to charge to 50V.

However in the process of getting to 50V, another voltage limit is breached (e.g. a cell voltage gets to 4.21V) and the Batrium changes its max charge current in the CAN message to 0A & the charger responds by stopping the charge.

The voltage on that cell reduces to ~4.1V & Batrium changes the max charge current back to 108A.

This cycle then repeats.

What would be great if Batrium could say, reduce current by half if this limit is hit - if PID control were asking too much. There is not a lot the charger could do to manage the situation when the BMS is sending limits in this way.
 

batteryshed77

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I also have the same problem. I spent some time on the phone with Batrium, but at the end of the call I was no wiser.

My Victron gear will do as it’s told but the message I got from Batrium was they have no interest in Victron and are aiming for the bottom end of the inverter/charger market - ie dumb inverter/chargers that accept on/off commands.

Having said that, I think Daromer is right - the inverter should be set to ramp down before the BMS, where are I and original OP are getting to the BMS limit before the inverter/charger limit.

That's a pity. Victron do rock solid hardware at a reasonable cost & are exceptionally open with external integration.
 

batteryshed77

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The bms only sends a signal stating "Lower charge" or "stop charge"

There is no regulation like a PID. That is up to the charge controller to handle and not the BMS. Look into how you have set up the middle point and what it should set it to.

So the bms does exactly what its told to do now. Its told to disable the charge when a voltage is reached. You most likely should change this so the charger starts the limit function first and then if the bms cant balance in time it steps in and shuts it down totally if any cell is above 4.2 volt.

Unfortunately i have not used the function here so cant say exact what paramters to change in your setup but above is how it works. Its ON - Lower - OFF only
As per my updated post, Batrium sends a little more info than this (max voltage and a charge current limit - which in my case is cycling between 0 & 108A)
 

daromer

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As i said it sends ON - throttle(lower) - OFF.

The BMS is not to PID control the charger thats for the charger to do. The "throttle" level in the middle is to tell the inverter "Hey we have one cell that isnt balanced so total voltage is wrong and i cant keep up balance. Please can you reduce so i can keep up"

So the throttle is something you define in middle of max and 0. And if you havent set up your inverter to match those settings you will get it to oscillate instead :)

You can for instance read about it here: https://support.batrium.com/article/111-how-to-control-victron-venusgx-with-watchmon

Example:
  • Low power current should suit the bypass capability of CellMon.
    1614029995751.png
Of course it then depends on how the charger also intepretates it i guess.
 

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daromer

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I think i added wrong image since it was discharge but charge is same:
1614030375422.png
 

batteryshed77

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I think i added wrong image since it was discharge but charge is same:
View attachment 24003
This setting is actually to set a lower charging current up to a certain voltage, before it transitions to the non-limited current.

For example if you set the 'limited power cell voltage' to 4V, it will charge at the limited current up to 4V and then de-limit up to the cell Hi cutout.

This is precisely the opposite of the behaviour I want, if this feature exists anywhere to limit the charging current ABOVE a voltage, it would be perfect.
 

daromer

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Please Read My post again. Both exist but its only full - throttle - offand IF its not set Up properly you get theoscilations.
 

batteryshed77

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As i said it sends ON - throttle(lower) - OFF.

The BMS is not to PID control the charger thats for the charger to do. The "throttle" level in the middle is to tell the inverter "Hey we have one cell that isnt balanced so total voltage is wrong and i cant keep up balance. Please can you reduce so i can keep up"

So the throttle is something you define in middle of max and 0. And if you havent set up your inverter to match those settings you will get it to oscillate instead :)

You can for instance read about it here: https://support.batrium.com/article/111-how-to-control-victron-venusgx-with-watchmon

Example:
Of course it then depends on how the charger also intepretates it i guess.
But it doesn't send ON - throttle - OFF.

All it sends is ON - OFF. There is no throttling - I'm rechecking everything to be sure as I'd love it if you're right...
 

daromer

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It should be. But as Said throttle is for balancing and should only happen when it need to balance. I think you have wrong settings as Said :)
 

batteryshed77

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OK, so it seems that the 'allow limited' has two functions. Limit power when voltage below defined threshold AND when in bypass mode.
Screenshot 2021-02-23 at 08.33.48.png

Great, so this could work - but I can't actually modify this limited power figure anywhere. When I tested it, it limits current to 1A - which might as well be 0A in my application.

I suppose it's limited inline with the bypass capabilities, but when the limit is active my cells aren't above the bypass threshold, because the charging current is so low.

So it seems there is no easy way round this. Thanks @daromer for your input. My charging settings below in case you spot anything glaringly obvious. (actuals are zero as I'm not currently connected...)





1614069207735.png
 

daromer

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Note that the charging settings should firstly be set om your Charger ;) so look into Them so they are callibrated against the bms. Preferable below by 0.1v or so.
 

slimf

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Batrium has just released a new version with Remote RAMPED targets - However, the UI is buggy and the documentation isn't great. I've emailed them for more details and to report a bug in the new version, But when this works it will be fantastic.

Here is the new documentation:
 
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