After the pole shift many will be involved with measuring batteries and concerned about the state of charge. If discharged too much one could damage one or more cells. As a bare bones minimum low cost way of measuring state of charge of Lead-Acid batteries. Make a copy of page 67 and 68 cut out the graph, put it back to back and lamented it. Store this with your digital voltmeter near your batteries. If you need to purchase an inexpensive Digital Meter check out Harbor Freight Tools which has one for $6.99. The graphs will tell you your state of charge based on rest state (no charging or charging) or if charging or discharging at a given rate. I think, as a simplistic general rule of thumb, if a 12-volt battery gets below 12 volts in a rest state it needs charging. If it measures 12 volts then it has about 24% charge. Full charge in a rest state is about 12.6 volts. Half charge in a rest state is about 12.3 volts. If one is charging at the same time then you will measure a higher voltage. If one is discharging at the same time of the measurement then it is lower than the above. Consult the graph in the above article.
If you need to make a DC Amp meter to measure charging and discharging rates, then, purchase a second digital voltmeter or use the same one used above. Measure the voltage drop across a known length of one of your battery lead wires. If you get the distance right you can match this with 1 amp = 1 mili-volt (using ohms law .001 ohms), Then you can read this off the 200 mili-volt scale (the one above measures down to .1 mili-volt) on your voltmeter directly as amps. Each different size wire has a different amount of voltage drop per unit length. If your wire is not long enough then build a reference graph that you can translate from volt to amps. The values are published in tables, for example.
Gauge(AWGorB&S) Diameter(inches) Distance in feet for 1 mili-volt drop 0000 .4600 20.40 000 .4096 18.18 00 .3684 12.83 0 .3249 10.18 1 .2893 8.070 2 .2576 6.400 3 .2294 5.075 4 .2043 4.025 5 .1819 3.102 6 .1620 2.531 7 .1443 2.007 8 .1272 1.592 9 .1144 1.262 10 .1019 1.001 11 .0907 .794 12 .0808 .630 14 .0641 .396 16 .0508 .249
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