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REC BMS initial wiring and fuses question

geolboy

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Feb 24, 2021
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Hello folks,
Another beginner set of questions while wiring Rec ABMS with a single 12V/560ah pack in 4S2P configuration.
Inline Fuses: The reseller I bought the Rec ABMS from (@the_colorist ) was suggesting to fuse battery connections Cell+ 1-4 for safety. I also saw a suggestion from someone to do same for the BMS main relay DCH (RELAY DCH F) that attaches to the battery Pos site. I understand that the last one already fused internally , but it’s just a matter of convenience to replace the inline fuse first, instead of inside the BMS. My contact is away and I am looking for the recommendation on the proper fuse rating for the above connections.

Because I had to extend the length of the Main Relay DCH NO wire to reach the pre-charge unit, I also used a wire with the inline fuse(holder). Same I had to do for the Cell1- wire. Would these extensions work, and if so, again, what fuse rating I can safely use for both connections?

With my Rec BMS order I got the Contactor and 500Amp-50Mlv Shunt. What I can’t locate is information on the direction of the flow for those. Does it matter which side of the Contactor unit I will attach the POS from the battery and which post on the shunt from the NEG battery cell 1 will go?

Last question I have about the lithium cells I have and the initial charging. I bought 8 of 280Ah EVE cells from Ali in Feb 2021, as I thought my RV build will progress much faster. But due to different delays I am just now getting to the electrical part. Those cells were shipped at 3.29V each, and still shows the same voltage now. They were not charged over a year and four month now. Neither I top-balanced them, since my reseller said the ABMS will balance them (eventually). I have been reading in these forums that ABMS may not trigger charging/balancing if the cells are too low. Is there any BMS settings recommendations you can suggest to ensure that the balancing/charging process will commence when I power the unit on? I got at least couple of month before I can start using my Van. Just fyi, my battery will be connected to Victron Cerbo GX, Orion-Tr Dc-to-DC charger and Mutltiplus 3000 12v inverter. But for now only Multiplus will be supplying shore power from the household 15A plug.

I would greatly appreciate the helpful comments, I am trying to do my part in research and reading but sometimes the very basic info still hard to find. Thanks a lot in advance.
 
1: I don’t fuse the sense wires, i use 22awg silicon insulated wire. If shorted it will self-fuse without catching fire. Fuses introduce extra failure points and opportunities for inaccuracies.

2: the internal fuse in the REC is 2A, an external 1A fuse will be fine.

3: the shunt is directional, it will read negative if you put it the wrong way but will cause no damage - if you want i’ll take a photo of mine tomorrow.

4: the contactor is also probably directional, positive should go to positive of battery.

Good luck with your install.
 
Thanks for the info @toms ! I would appreciate the picture, as besides voltages on one side I don't see any other markers on the shunt. I found A1+ A2- markings on the contactor, so I'll go with A1 to the battery end. As for the fuses, I already wired the connectors, maybe I should try them first. Do you think 2A on sense cables will be adequate to try? Given that Rec uses 2A on RELAY DCH F... Thx again.
 
Thanks for the info @toms ! I would appreciate the picture, as besides voltages on one side I don't see any other markers on the shunt. I found A1+ A2- markings on the contactor, so I'll go with A1 to the battery end. As for the fuses, I already wired the connectors, maybe I should try them first. Do you think 2A on sense cables will be adequate to try? Given that Rec uses 2A on RELAY DCH F... Thx again.

I think there isn’t much point trying to protect a 2A fuse with a 2A fuse.

If the sense leads are already fused, just make sure to check the voltages of all cells with a meter when you first connect to make sure the REC is getting accurate readings.

I double checked my shunt - the end stamped 50mv is connected to the battery. Not much help with the BMS wires as they are the same color. I’m certain i have connected them backwards before and there is no damage - just a negative current reading.
 

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Just got this info from REC folks as well, had to wait for the answer so I asked here as well. Anyway, that's what they replied to my questions above, I quote:

..Cell wires should be connected directly to the battery, without the fuses. They are already internally fused, additional fuses can cause error when startup (because of voltage drop on additional fuses).
You can add an additional fuse for the DCH relay which is attached to the pos battery site. Fuse inside the BMS is 3.15A slow blow. So you can add an additional fuse rating 3A. Fuse should be slow blow as the contactor can run up to 7A when changing state (for a few milliseconds).
Extension for DCH NO relay wire will work. Cell -1 wire should not be extended, it can cause the error when starting up. All cell wires should be the same length and not extended if possible.

Contactor and shunt do not have any direction, both sides are the same. Shunt should be connected directly to the battery -.

All cells should be the same voltage when connecting the pack together. First few charges can last longer so the BMS can top balance the cells. For the first charging I suggest using a low current charger (around 10A) (or set the multiplus to low current) so the BMS has time to balance all cells. BMS will start balancing at 3.45 (default setting) and if current is bigger then 1A. It can take a few hours to do the job...

I guess its some redo work for me for now.
Thanks again folks.
 
I gather the shunt can be in either direction, if you connect the BMS to shunt wires the wrong way you will get a negative current reading.

It’s good they upgraded the internal optocoupler fuses.

Exactly which contactor do you have. Some are directional - they have arc suppression that only works one way. Make sure you check.

I would lower the balance start voltage to 3.40V.

Good that REC replied to your queries.
 
Late to the game but yes, the shunt is bi-directional and the newer version of the REC Active BMS has internal fusing for each cell and the relay. Also, the slowblow fuse on the relay works well with Tyco type contactors that have a high inrush current for a few hundred milliseconds.

And thats great feedback from REC regarding the wiring and fusing.
 
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