On Connecting Two discrete BMS/Pack Assemblies in Paralell

[rbenash]

Hello:

Apologies if this is a duplicate, but I've looked through previous posts here and couldn't fine one that talked specifically to my question.

I have two Li-ion packs/systems. Each one has discrete BMS/SBS PCB. Both are built and rated exactly the same.

Composed of 40 Panasonic CGR18650CG cells
29.6V/11.25Ah/320Wh

I would like to modify one of the system units and simply leave the BMS attached and parallel that pack to the other leaving the second unit's BMS/SBS intact.

I would separate and charge separately as needed.

Seems to me that if I leave BMS attached to the "slave unit" and parallel to the "master" unit and charge separately I should be OK in this case?

Don't know where else to go with this question. I'm simply trying to increase overall runtime when these two systems are connected, then charge each system
seperately.

 

[amberwolf]

If you're just paralleling at the packs' main outputs, I'd leave them both completely intact, so that the BMSs can do their job and protect each pack individually as needed for both discharge and charge.

It's much simpler than messing around with the packs internally.

If left intact, you can also charge them in parallel.

 

[rbenash]

Thanks amberwolf - yes each packs chemistry and number of cells are exactly the same as well as the electronics.
I simply want to be able to link them together when needed to double run time.

I would still be able to carry as a disconnected spare as needed and didn't want to carry both batteries.

I use these on a motorized kayak for "single put ins" and need to do long upstream/downstream runs.

Sometimes is not that easy to get out of kayak and swap batteries.

Wasn't sure how a single charger would work with both chained, good to know.

I can keep the same cases, just add waterproof connections to run a waterproof cable between
the two jacks.

Ray

 

[chvidgov.bc.ca]

I do this with two identical Sunthing 48v 10Ahr packs, unchanged from "as new", both with their intact functioning BMSes. I discharge and charge both packs simultaneously through a "Y" cable. After many months like this, they are working fine together, each BMS protecting its pack with no issues. It's worth ensuring they are truly connected together for discharge, so they stay at roughly the same voltage and don't "intercharge" each other, one way or the other, if the pack voltages get out of whack, which could happen if a connector came apart on one of them un-noticed while riding.

 

[d8veh]

You can connect two batteries in parallel and leave them together for charging and disharging. It'll work OK, but it's not without risk.

The battery with the charger attached will charge the other one through the discharge wires, so you've by-passed the charging safeguards on the 2nd battery. You should, therefore, disconnect the power wires while charging. You also need to make sure that the batteries are the same voltage when connecting them in parallel.

The above only applies to batteries with a separate charging mosfet, which is most BMSs. Some BMSs use the discharge mosfets to switch off charging, i.e. they charge through the discharge terminals on the BMS. You can leave that type in parallel for charging and discharging.

Some people use blocking diodes on the discharge wires to stop cross-charging, but that can bring other problems, especially if you've joined the batteries for extra current rather than range.

 

[chvidgov.bc.ca]

Good point d8veh. The Sunthing packs I'm using both charge and discharge through the discharge lines (no separate charger input). I also do the same Y type charging on my Pings and Headways, but I use a diode in those two situations - I'm not sure if I need the diodes though.

 

[rbenash]

All great info - thanks for all the feedback and guidance.

The batteries I have are the Torqeedo units for the UL403 kit.

They have a separate charging port and power out ports. The power out port also has data pins to monitor and control the motor/pylon.

I was told by a friendly Torqeedo tech that the charging port path has several paths taken depending on the voltage levels applied. He
implied that if applied an input say that was over the typical levels of the chargers it would be directed to the motor. I trust him as I've worked with
him over a few years as I was customizing my install, etc.

So - he suggested that I should be able to connect output power from the "slave" battery in this case right into the charging port. Although
that sounded like a simpler solution I was concerned by the gauge of the internal wiring from the charging port to the PCB board. Seemed
way light. I also could not find heavy capacity rating for the required 5.5/2.1 connectors.

So to be safe I figured best solution that should not affect the PCB (Controller/SBS/BMS) and burn things up would be to implement the
solution above.

In my case I'll go with direct parallel connection from a given slave to the master in this scenario. I will simply add shunts from the battery outputs
on the units to this connection on all of my 3 batteries so each can act as a slave or master. Only the master will have the controller and motor
connections attached.

I'm still inclined to charge separately as I'm expecting that the time need to charge both batteries to 100% should double? Or are folks suggesting
that if I leave the shunt/parallel connection on between both batteries charge time would be the same a if I was charging a single battery.

Seems to me it would take about twice as long given same discharge rate on both batteries.

I'm thinking using two chargers of the same rating would keep charge time reasonable.

Looks like I should have posted this on the electric boat tech forum - my apologies. Let me know if I should ask the monitor to move it??

Thanks again

Ray

 

[d8veh]

What your tech friend says doesn't make sense to me. Theimportant thing is whether there's a separate charge mosfet. If there is, you need to understand the implications.