So now you're consulting for EV West?
If EVwest isn't individually monitoring each cell in each module with a BMS module to prevent over/under voltage, I will be sadly disappointed in their EV knowledge, especially considering that on the surface they have built many successful EV conversions. Maybe I shouldn't be surprised - they don't appear to have any BMS equipment on their online shop.
Step #1: Install BMS on each individual cell - you need this for safety and so one doesn't need to babysit a pack to make sure it's properly balanced when charging.
Step #2: You'll want to monitor pack voltage as this will most easily show overall SOC. Ideally you would also see the average cell voltage along with min/max cell voltages since your pack will be empty when the weakest cell gets low.
Step #3: You need to monitor energy going in/out of the pack and start coulomb counting. Won't be perfect but you'll get an idea of how much energy you've used which one can use to correlate with voltage data.
There's quite a few BMS solutions out there for Lithium packs. I've never used any of them so I can't recommend any.
I've heard of MiniBMS
being used with success by the DIY crowd. There are also more cabpable solutions such as the ones here: http://www.evolveelectrics.com/BMS.html
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IMO BMS is the single most important piece of equipment for a multi-cell battery pack. This is not an area to get lazy or cheap out - especially when you have a "$40k" battery pack to protect and use.