Originally Posted by NJKris
I was looking at some of the China special monitors on Amazon, pretty good for 30 bucks, but I noticed from comments that current in and current out are totaled, no way to see either by themselves. In that case, a simple voltmeter/percentage is about the same. I think it's worth a few extra bucks to see current in, current out.
The issue with these meters is that it's not as simple as current in, current out, with a lead acid battery. The faster you draw current out or put current in (charge) the more inaccurate they become for determining the actual SOC of the battery. It's all due to the "Lag" in the chemical reaction in the battery and thus the need for a monitor that can utilize the Peukert Factor in it's calculations. THAT is what adds to the cost.
Also, Lead Acid batteries are slaves to temperature. Temperature correction becomes a factor as well and ideally the monitor has a temperature sensor as well.
If one wants to do all the calculations in their head (good luck) or on paper (I'd rather do a crossword puzzle) then a simple meter is probably OK. That said, if it's essential that one know how much power they are using and how long it will last, then spring for the better meter. You only have to buy it once and it may even allow your battery bank to last a few years longer.