The voltmeter's distance from the batteries, and the current passing through the circuit between battery, fuse and then the fuse panel, will determine how well it represents voltage at the battery terminals.
Light loads and the voltage will be fairly close to actual battery terminal voltage
Heavy loads and there will be a lot of voltage drop and the voltmeter will read much lower than actual battery voltage
Voltage at the battery terminals is what one wants to display, not the voltage further downstream, but if the wire is thick and the load light there will be little difference.
How voltage represents battery state of charge is of limited usefullness.
If a simple display is wanted, I recommend knowinng how many amps are flowing at that voltage,
12.2v at the battery terminals under no load is much different than 12.2v at 10 amps of load. With out knowing the load on the battery, Then knowing the voltage only, is misleading in the extreme.
This is a simple voltmeter/Ammeter One simply slides a single battery cable through the ring terminal. I have one and its voltage was 0.2v off and not able to be calibrated. however the Ammeter reads very closely with my other ammeters and I only want it to display amps anyway. Other owners report the voltmeter to be accurate.
https://www.amazon.com/dp/B01DDQM6Z4/ref...il_4?psc=1&pd_rd_i=B01DDQM6Z4&pd_rd_wg=zU1ix&pd_rd_r=MXH8MAZJ3ZGB15KYKB6X&pd_rd_w=l9Tlu
it only has a 0.2amp resolution though.
If one really wants to always see voltage and watch it rise when charging and fall under discharge, then go for more than tenths of a volt, go for two decimal places, hundredths of a volt.
This one has 3 decimal places. These are very simple to install cleanly.
https://www.amazon.com/DROK-Digital-Volt...industrial&ie=UTF8&qid=1512196623&sr=1-1&keywords=drok+3+decimal+voltmeter
E bay might have them cheaper.
The shunted amp hour counting battery monitors are not required. Watching how many amps are flowing and how much voltage the battery is maintaining, One can develop a knowledge of where the battery is.
Watching how many amps a charging battery is accepting at absorption voltage, is very indicative of state of charge, the less the amps the mroe charged.
Watch a few cycles, see how long it takes to get a 12.2v rested reading, or lowest morning voltage before solar or other charging sources are applied.
When you turn on any load, watch how much the voltmeter drops when it is fully charged, vs depleted to some unknown degree.
One can develop a good idea of where the battery is at by noticing trends and tendencies of voltage. this is most easily learned by watching voltage fall under loads during discharge, and how many amps the battery is accepting at what voltage when charging.
KNowing when the battery is true full is also kind of important. With flooded batteries the hydrometer is the battery polygraph. With AGM one needs an ammeter, and one needs to be able to hold the battery at absorption voltage, and hold it at absorption voltage until amps taper to 0,5% of the capacity of the battery, 0.5 amps for a 100AH battery.
The Shunted battery monitors are great, when they are wired up properly, set up properly, and rezeroed occassionally. The person who can do this well, can also make an accurate guess as to state of charge, if they are regularly looking at just a voltmeter and an ammeter.
The shunted AH counters are nice when one simply does not want to bother, but too many people put 100% faith in them and the hydrometer will often prove that although the trimetric says 100%, Specific gravity is not in teh 1.270 range on all cells.
I use 45 amp anderson powerpoles on just about everything that will not pass more than 40 amps.
I install 45 amp powerpoles in this device or one of its many clones:
https://www.amazon.com/Tenergy-Precision...oding=UTF8&pd_rd_i=B017YCTRKK&pd_rd_r=VT1211MC2Z9MN8KFPQ1Q&pd_rd_w=Ct3ua&pd_rd_wg=AGrrC&psc=1&refRID=VT1211MC2Z9MN8KFPQ1Q.
While these come with 12awg wire, i have replaced it to the shunt and circuit board with 8awg to handle 40 continuous amps. I would not pass more than 25 amps continuous through this wattmeter with the 12awg, unless the short wires have forced ventilation over them.
8 awg gets warm passing 40 amps for an hour, 12awg gets really hot.
Gary, if you have a square file and a drill, you can make your own powerpole fuse panel in some plastic/wood/acrylic/metal.
I actually like having some loose wire with a powerpole on it, as when they stick straight out of a panel, with 10AWG, there can be a lot of stress on the plastic housings. I have broken the housings, and it is not always obvious when they are compromised, but not completely broken.
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