Originally Posted by PNW_Steve
Not trying to be argumentative. Just pointing out that a battery monitor by itself is not going to give you the best performance unless you "tune" or calibrate it with the help of a hydrometer.
No worries, I generally agree. Only a hydrometer will be completely accurate.
However; I think it is important to understand that using an hydrometer every time one would like to know SOC is simply NOT going to happen (in the vast majority of cases). For example; I probably look at my battery bank SOC 3-4 times per day. I am now lithium but in my last RV, the notion that I would retract one of the slides, walk outside, open the battery compartment, pull the battery tray slides out, remove all the caps (multiple 6V batteries) and use the hydrometer on each cell EVERY time I wanted this information is simply NOT realistic. The reasons to skip this process are nearly infinite - I'm in my pajamas, it's raining, I'm only curious, I don't want to get battery acid on this shirt, it's too hot, it's to cold, etc...
In my opinion, expecting someone to ONLY use a hydrometer will guarantee that he/she will ignore their battery bank and probably end up abusing it. That is certainly not best performance.
I do think it is realistic to properly tune a battery monitor and then validate its information at some interval (using a hydrometer) - maybe once a month. In this way, the user can easily access pretty good information about their battery bank and treat them appropriately.
We may be coming at this from two different angles. I prefer to think that people should and will learn a bit about their electrical system and how to use it. It is possible that another angle is that people generally don't care about this stuff and no matter what they have, are going to abuse it (due to lack of knowledge). Using this latter perspective, I can see that giving them a tool that requires some understanding and tuning and expecting them to use it correctly might be a mistake. The erroneous information might be worse than telling them to use a hydrometer and them just ignoring their batteries. At least then they cannot say "my monitor told me all was well".