I agree, you wouldn't want to leave a Battery Medic connected all the time, especially if you have it in the balance, mode, which brings down the level of all higher cells to the level of the lowest. Sometimes, a channel will overshoot, so that cell now becomes the lowest. This can keep happening, which would drain the cells down. I use the discharge mode, which brings all the cells down a programmable set point, and then it stops. I still wouldn't leave one connected for an extended period, but I've yet to see the discharge mode drain any cells past the set point.
The point I'm trying to make is that with LiPos, balancing just isn't required with every charge, so whatever balancer-type that is used, doesn't have to be left connected all the time, like with a regular BMS. For basic charging, the LVC/HVC part of the BMS, and the charger controller card can be used by themselves. An example of this is shown below. This is an 18s2p 66V/10Ah pack made from six Zippy 6s-5000 packs.
Buried inside the pack are three 6s LVC/HVC boards, like the one shown earlier in this thread. The opto outputs from all three boards are ganged together, and brought out as a single two-wire JST-BEC plug, which plugs into a matching connector coming from the charger controller. I also brought out a separate set of 12-gauge leads for charging. These have standard 45A Andersons, which also plug into mates, coming from the charge controller. The picture below shows the charge controller plugged into the pack. Ahead of the charge controller unit is a Turnigy Watt Meter.
For balancing, each LVC/HVC board has a set of 18-gauge wires, brought out of the pack, into a Molex Microfit 3.0mm 7-pin connector. As I said, most of the time, these are simply not connected to anything, but if balancing is required, they can be used to connect external balancers, such as the ones shown below. These are basically working prototypes that I put together, using modified shunt boards and end plates, from earlier versions of the 4.0 BMS unit. Shown are both a 6s and a 12s version. For my 18s pack, both will be used.
The balancer units have fans, and are completely self contained. They simply plug into the MicroFit connectors coming from the pack, whenever balancing is needed. When used while charging, the combination acts just like the full BMS, but they will also work standalone, after a charge. In this case, cells above about 4.14V will be brought down to that level.