What you want to do CAN be done.
Will it be lab grade precision? No. But it can be done and can give you reliable results if you bear a few things in mind.
First, a $20 or even $80 multimeter usually has a PRECISION of four digits, but an ACCURACY of only 2+. The rated accuracy on the DC voltage scales may be 1/2%, which on 12.00 volts mean it may be off by 00.12 volts. And, there is usually a "float" of 2 or 3 "LSD" least significant digits, so 12.00 may mean 12.03 or 11.97. Add in your 1/2% error, and 12.00 volts could be 12.45 or 11.85 volts on the display. You don't and won't know that for sure, although most cheap
meters tend to display plus-or-minus 0.20V within reality.
Back to the practical side: Put the meter in 20VDC mode, stick the test probes on the battery. If the battery has been sitting isolated and out of circuit for 24 hours, the readings will be valid. If the battery has been given a charge, the readings will be INVALID, because of something called a "surface charge". This is, literally, the effect of the electrolyte acid not evenly transferring a charge in the battery, and it means that your battery will read deceptively HIGH after a charge.
To compensate for this, you put a reasonable load (like all the running lights and cabin
lights) on the battery for 2-5 minutes. then shut it, and test the voltage. That short drain pulls off the surface charge from the plates, and the voltage you read now is pretty much valid. (And accurate, if the meter is.<G>)
Depending on what they are made from, lead acid batteries measure roughly 12.7 volts when fully charged and brand new, dropping about 1/10th of a voltt to 11.6 or 11.7 volts when they no longer hold a useful charge. You might get one engine start out of a lower battery--but you won't be able to run lights, etc. for more than a few minutes from it. Call it discharged and dead.
Now, when you fire up the alternator, that should put out about 14.4 volts, and you should see 14.4 volts at the battery. 13.8 volts from an older cheaper regulator, or if the batteries are fully charged and the system is just loafing. Shut the alternator, and you'll read 13.6 volts at the battery--that's the surface charge, and you can watch it go away over the next few hours.
If the alternator voltage is over 14.4 volts, you have a bad regulator. They can hit 16-17V and at that point, things start to burn out.
All of the above is 'good enough' most of the time. If you really want to measure amps and see what the battery is doing, you can use a SHUNT, which is just a very low caefully calibrated resistance wire, in line with the batteries. For instance, you can buy a 100A shunt (it will look like a metal bar with screw terminals at each end) and install it in series with a battery cable. Then, you take your cheap
multimeter and put it on the 2000 MILLIVOLT scale, and connect it to each side of the shunt. You are now reading the voltage DROP in the resistance wire (the shunt) which increases as the amperage through the wire increases. If you use a foot a 4AWG cable as a shunt (or, tap into a 4AWG battery cable at two spots exactly 1 foot apart) you might measure a one millivolt (0.001 volt) drop per amp of current
drawn. So your cheap multimeter, punched into a 4AWG battery cable, could measure 2000millivolts, or a two thousand amp draw. (Don't worry, the cable would be on fire by then.)
Whatever battery cables
you have--you can look up their resistance online in charts
, and use them as shunts the same way. Again, this is not LAB ACCURACY, but it gives you a relatively good guideline. I'd stick to the voltmeter, by the way, and not punch holes in the battery cables
as a matter of course.
If your batteries read "12.5" when fully charged and allowed to set for a few hours this week, and "12.3" next week...you know you've lost
20% of the battery capacity or drawn a bigger load or just not recharged them as much, along the way. And that's the primary value of using a multimeter, you CAN reliably get an indication of those 10% CHANGES in battery state. The meter may not be accurate--but it will usually show THE SAME numbers each time, giving you something repeatable to base decisions on.
If you want a quick check on the meter's numbers, buy a fresh silver-oxide watch or calculator battery, a physically big one. They have a shelf life of years, so the voltage they display on your meter should be unchanged from one month to the next. Is that lab-grade meter calibration? Hell no, but again, it gives you a reasonably good way to tell if your meter is staying constant (in its display) from one day to the next.
How's that for a not-too-technical nuts and bolts way to get you started?<G>