It has been several weeks since I hit full charge. My battery has ranged between 45% and 65% according to my Victron during this time. So neither the Victron or the BMS has had a chance to re-sync. As I write this, the Victron reads 56%, and the BMS 74%. That is a really big difference. I believe the Victron to be closer, but understand neither is going to be correct.I have cycled my cells several times over several days and the ah's hardly changes. It's always within a couple of ah's but this is with low C rates. The built in coulomb counter might not be the most accurate but it is consistent in my case.
Based on my testing it might be off by a few ah's and this was compared to a DROK I have which uses a hall effect sensor. I don't have a Victron or anything that would be more accurate and I don't need it. The BMS is good enough for me and comparing the voltage to the ah's used I believe it is fairly accurate.
As was pointed out, changing any of the parameters will throw off the SOC and coulomb counting and the battery must be fully cycled in order to get accurate readings.
During a single cycle, if I discharge to a relatively low SOC and then fully charge, the Victron and BMS will be within about 2% of each other.
I agree, the BMS is probably fine for most people.