REC Q BMS w/ Cerbo GX & MultiPlus 24/3000/70 - wont stop charging LiFePo4 batteries!

Ssybert

New Member
Joined
Aug 8, 2021
Messages
11
Hi All, I'm hoping there are some REC guru's who can help. My configuration is a REC Q BMS connected via CANBus to a Cerbo GX which is connected to a Victron MultiPlus 24/3000/70-120 via VE.bus. Everything is configed per REC specifications, as best I can tell. What I'm observing is when the inverter is connected to the Cerbo, the REC appears to be controlling the charge rate, voltage and profile; essentially overriding the user-defined fields I set, which is fine... I believe this is what I want.

The issue is once the REC switches the MultiPlus from bulk to absorb, it stops increasing the voltage when it reaches 28.3v. The current in to the battery drops to almost nothing and the Multi looks to be floating voltage at 28.3v while indicating it's still in absorb mode. If I remove the VE.bus cable from the Cerbo GX, it will stop charging after a few minutes.

I've tried a number of things but by testing has me currently at these values:

MultiPlus absorb voltage: 29v
Float voltage: 27v
Charge curve: adaptive

REC Q BMS balance V END: 3.58
Balance start V: 3.45
End of charge: 3.58
Current highest cell: 3.522v
current lowest cell: 3.516v

Right now there is only a couple watts indicated going in to this pack. This is a new setup and my first time working with BMS and LiFePo4. it's a 2p8s setup @ 400ah. Should this initial balance take a long time? It's already been over 24hrs like this and I don't want to damage my batteries. Any help would be appreciated.

One other question I sent in to REC as well is, does anyone know where to wire the contactor on the REC? All documentation indicates it is wired to the pre-charge unit but I did not buy one of these. There are two pins on the IO connector the pre-charge unit wires to but the reference to transistor legs on these terminals (1 & 2 - emitter and collector) has be concerned about I should be wiring this w/o the pre-charge unit.

Thanks,
Scott
 

nonoceb

Solar Enthusiast
Joined
Jun 10, 2021
Messages
56
Hi All, I'm hoping there are some REC guru's who can help. My configuration is a REC Q BMS connected via CANBus to a Cerbo GX which is connected to a Victron MultiPlus 24/3000/70-120 via VE.bus. Everything is configed per REC specifications, as best I can tell. What I'm observing is when the inverter is connected to the Cerbo, the REC appears to be controlling the charge rate, voltage and profile; essentially overriding the user-defined fields I set, which is fine... I believe this is what I want.

The issue is once the REC switches the MultiPlus from bulk to absorb, it stops increasing the voltage when it reaches 28.3v. The current in to the battery drops to almost nothing and the Multi looks to be floating voltage at 28.3v while indicating it's still in absorb mode. If I remove the VE.bus cable from the Cerbo GX, it will stop charging after a few minutes.

I've tried a number of things but by testing has me currently at these values:

MultiPlus absorb voltage: 29v
Float voltage: 27v
Charge curve: adaptive

REC Q BMS balance V END: 3.58
Balance start V: 3.45
End of charge: 3.58
Current highest cell: 3.522v
current lowest cell: 3.516v

Right now there is only a couple watts indicated going in to this pack. This is a new setup and my first time working with BMS and LiFePo4. it's a 2p8s setup @ 400ah. Should this initial balance take a long time? It's already been over 24hrs like this and I don't want to damage my batteries. Any help would be appreciated.

One other question I sent in to REC as well is, does anyone know where to wire the contactor on the REC? All documentation indicates it is wired to the pre-charge unit but I did not buy one of these. There are two pins on the IO connector the pre-charge unit wires to but the reference to transistor legs on these terminals (1 & 2 - emitter and collector) has be concerned about I should be wiring this w/o the pre-charge unit.

Thanks,
Scott
You can use collector output for driving contactor from positive battery or emiter output for driving contactor from negative battery.
Verify the max current capability of this output and your contactor current requirement.
28.3v is very high for floating mode.
I never set more than 28V for bulk, it's avoid major damage in not very accurate balanced battery.

Did you use the DVCC in RECBMS to "controlling the charge rate, voltage and profile; essentially overriding the user-defined fields I set"?
I installed one rec bms last month and i don't use this because im not sure about firmware compatibility and skylla charger was'nt stop charging before rec bms open the security contactor, even if can bus was ok, remote start stop charger working and Vmin cell and Vmax cell is known by cerbogx...
I think victron product are real shit at this level of price, i can't recommend it!
Cerbo gx cannot setup alarm like user want they decide for you, very happy to know each time my charger lose it ac supply when i shutdown generator lol
Only mppt solar charger are a real good value in my opinion
 

Ssybert

New Member
Joined
Aug 8, 2021
Messages
11
Hi All, sorry for the late reply... I didn't get alerted there were replies.

I did opt to use DVCC but what I don't like is REC puts my Multiplus in to Absorbtion once it hits charge voltage and holds the batteries there.

This is madness. Why would ANYONE want to hold their LiFePo4 pack at 3.58v (REC default full charge target)!?!

So my alternative is to change the max charge voltage in REC to whatever I really want and let it float there.

Further, the resting voltage of these batteries at 100% charged is well below 3.65v. If you don't use the batteries regularly like me (RV), you can trickle charge your pack to death even if you lower the voltage in REC to, say, 3.4.

I keep mine at 3.2v when not in use but there must be a better way. Precision calibrated shunt and AH in and out tracking with occasional top off calibration seems like the way to go.

REC was VERY expensive and I'm just not impressed with the lack of understanding of LiFePo4 displayed by the features and product design.
 

GMS

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Joined
Dec 4, 2019
Messages
4
I have a similar setup (but 12v) that has been in operation for about 1 1/2 years (cruising boat). I have also been seeking a method allowing the batteries to rest at a much lower voltage - REC has not been helpful when I attempted to address this issue with them...

Watching this thread with interest, as I love the REC integration with Victron and wakespeed alternator regulator - but I don't love the lack of some sort of "storage" option.
 

Ssybert

New Member
Joined
Aug 8, 2021
Messages
11
I'm going to share a portion of an email I sent to REC yesterday in hopes they will add a few features to the product. These suggestions, in my opinion, are simple to implement and will set them apart from any other BMS. This would be game changing and would be welcomed with incredible demand:


What would be ideal would be two optional profiles:

Profile 1- A feature where we can set an 'End of charge voltage' and a 'recharge voltage'. Once end of charge is achieved, say 3.5 or lower for best battery life, the charger turns off or drops voltage to 0. When the pack drops to a minimum set point, say 3.2 or 3.1, it turns charger back on. This feature should also have a manual 'Recharge' button to kick this off manually.


Profile 2 - Allow the user to not only define the pack capacity but to specify what PERCENTAGE they wish to charge to. Since LiFePo4 voltages under rest are different than under load (both charge and discharge), it's difficult to accurately determine capacity based on voltage, specifically during charging. This feature should have a calibrate option where the user can define a given state that is considered 100%, begin discharging and have REC start tracking AH out. Once a defined minimum voltage is reached or AH consumption is reached, the pack will be considered 0% and the consumption used is the new 'pack capacity'. Now, we can charge to 100% of our custom CALIBRATED capacity with periodic calibration routines manually triggered to assess overall pack health and true (or customized) capacity and charge to CAPACITY, rather than resistance. Not everyone wants to charge to 100% where resistance begins. Charging at anything below that is hard to do based on voltage... it needs to be done based on AH.

Are these features I could request custom design for? I know personally this would be a game changer and make REC the very best and most comprehensive BMS available. It would truely allow the user to maximize the life of their LiFePo4 pack, something I think many advanced REC owners are disappointed in. We need better control of how the BMS manages charging, not just balancing.


Regards,
Scott
 

Pyrofx

Flux capacitor builder
Joined
Jul 27, 2020
Messages
524
Location
Oatman Az
Good luck. They are very condescending to deal with. It has taken 4 month to get them to write a program for me. I wanted the inverters to stay working if one of the 6 modes had an issue. Shutting down the whole system because one of 6 battery packs was off is stupid.
The other issue they cant fix is the battery meter display. Each time you add another module , it adds the voltage as if it were series , not parallel. Very annoying.
 

Ssybert

New Member
Joined
Aug 8, 2021
Messages
11
Good luck. They are very condescending to deal with. It has taken 4 month to get them to write a program for me. I wanted the inverters to stay working if one of the 6 modes had an issue. Shutting down the whole system because one of 6 battery packs was off is stupid.
The other issue they cant fix is the battery meter display. Each time you add another module , it adds the voltage as if it were series , not parallel. Very annoying.
It's too bad. They have such a great foundation... the device is built right to displace heat and not just burn up prematurely, it has native Victron integration, they have a WiFi module, remote display panel... They've obviously put some time and money into this product, just get it over the goal line and they'll have something no one else has.

How much did they charge you for the customization? I'm not a software developer but could this be done independent of REC using the RS485 port and a Raspberry PI? They have a standard command set... maybe someone needs to start writing applications that just use existing I/O rather than trying to get REC to do it?
 

toms

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Jul 24, 2020
Messages
737
Don’t like to burst your bubble - but to be fair it’s not worth the effort when you manufacture a product to spend time on edge cases.

If they receive enough feedback on something they will implement it, but it took a long time for them to release a 16s BMS when i first contacted them asking for one.

I know a lot of REC users, not one of them would make use of a “storage” mode.

You cannot use SOC long term, the cells have different charging efficiency for different temp/C rate, so the SOC becomes inaccurate over time.

If you really want a storage mode, just set your float voltage to 3.3V
 

Pyrofx

Flux capacitor builder
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Jul 27, 2020
Messages
524
Location
Oatman Az
The system is solid . I really like it. They are hardware guys.
My issues were minor , and they did fix the bugs. No charge since they were minor tweaks.
 
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