Connecting Cell Monitors to OEM Batteries With a Shared Balancing Wires Harness

If you are considering OEM packs like Tesla, Nissan Leaf, Chevy Volt, etc. it is very important to Plan in Advance how the balancing wires will be connected to the cell monitors. 

Why we designed WatchMonPlus

We needed an easier solution for these OEM packs (Tesla, Nissan Leaf, Chevy Volt, etc).  Whilst it was technically possible to pull the battery pack apart and wire them up to WatchMon4 and cell monitors it was labour intensive and needed someone who really understood and knew well what they were doing.  In practice, it was often done incorrectly giving either a false sense of security or in some cases a more dangerous situation than having no BMS.  For this reason we had to find a better way.

WatchMonPlus WM5 was released in July 2019 in its 10s - 15s variant for 48V nominal systems or 24V if using LTO cells.  WM7 our 6s - 10s 24V nominal version of WatchMonPlus was released in November 2019.  It seems to be going well with many systems up and going.

This All-In-One solution makes wiring much simpler and allows you to use the original balancing leads without needing to deconstruct the pack. This system is a lower power version limited to 300 mA, following all other OEM automotive Battery Management Systems in the market.

Now that WatchMonPlus exists we can no longer warranty WatchMon4 if used with the OEM batteries.  We only recommend WatchMonPlus for these batteries.

Why WatchMon4 is not Suitable

Basically, if you have the  Watchmon4 with multiple cell monitors like the LongMon, the short answer is: Avoid using their original balancing harness and wire the cell monitor red/black wires directly to the cells.  

The original OEM balancing wires are usually long and thin designed for much lower balancing power, around 300 mA. Our cell monitors are balancing at  2.2A peak  or 1.2A continuously each which is at least 4X times higher balancing power.

This means that if Two cell monitors are sharing the same wire, it is effectively 4.4 A. When both adjacent cell monitors are bypassing (Balancing) through a wire that was designed for 300 mA, this will cause a Significant Voltage Drop and the cell monitor will not operate properly. 

Shared balancing wires will also cause interference during the cell balancing when neighbouring Cell Monitors are trying to both balance and read through the same wire, at the same time. This will cause voltage drop and misreadings.

Wire Positive (red) and Negative (black) wires directly to each cell terminal and it will work like any other non-OEM battery.

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