An Ammeter would be a wise investment.
Voltage alone while under a load, or immediately after, or after charging can be extremely misleading, especially without knowing how many amps are flowing in or out.
Your 80 watt stereo, likely can only draw that much, at full volume, under a bass thump.
There are a few Ammeters, some of them can count watt hours and Amp hours as well.
https://www.amazon.com/bayite-6-5-100V-Display-Multimeter-Voltmeter/dp/B013PKYILS
https://www.amazon.com/gp/product/B01DDQM6Z4/ref=s9_dcacsd_bhz_bw_c_x_2_w
https://www.amazon.com/Uni-T-UT210E...rd_wg=sLAp7&psc=1&refRID=BFVWDMMEC2ABPR3DM91B
https://www.amazon.com/Tenergy-Prec...&qid=1484188439&sr=8-1&keywords=rc+watt+meter
Without knowing how much amperage is flowing at the voltage( pressure) it is very hard to guess at the actual state of charge accurately, without a lot of experience with your specific batteries in your specific system. Generally if the loads are light overnight, and you are in the 12.2v range, it is time to lay off the loads and time to charge.
Of the two products above I do have the Bayite with the ring sensor. Its resolution is only to 0.2 amps. it can be put over a (+) or a (-) cable.
I do have several of the wattmeters, They only count current flowing in one direction.
The BAyite power meter with the shunt, I hear they are not extremely accurate at light loads. no personal experience with that specific model. I had another ammeter with similar shunt it would not display loads under 0.7 amps but was accurate in the 1.2 to 45 amp range. The Shunt has to go on a Ground cable Inbetween ALL charging sources/loads, and battery (-).
The clamp on Ammeter will be the most useful. Clamp it over any single wire and read amperage flow at that specific moment. Check alternator amperage..... amperage into battery.....amperage to fuse block, any single wire, + or - .
I have a sears craftsman version, not the one I linked. No personal experience with that specific clampmeter.
The battery Polygraph, as to state of charge, is the Hydrometer.
https://www.amazon.com/OTC-4619-Professional-Battery-Hydrometer/dp/B0050SFVHO
Use it the next time your smart charger declares them fully charged.
If All the cells do not read 1.275 or higher, your smart charger is Lying to you.
Restart charger after applying loads to reduce battery voltage below 12.6v.
After charger stops/ drops to float/maintenance voltage, recheck Specific gravity with hydrometer, add or subtract 0.00X. Is is 1.275 or higher?
If not , apply load to reduce voltage, restart charger.
When your batteries get older, the required number of these 'lather rinse repeats cycles' required to trick your smart charger into completing the task of a true full charge, will increase. Achieving the true 100% recharge is important for achieving very good to great, to perhaps excellent longevity.
Likewise, if you can control absorption voltage and its duration with your solar controller, your hydrometer will tell you if it is dropping into float too prematurely. Most do. Most people are not aware of this and believe that a solar controller going into float means the batteries are indeed full.
It does not. Not until proven with a hydrometer in a specific discharge regimen, and it will change as the batteries age and the seasons change, along with your usages.
No need to stress this, just be aware of it, and if you want to extract the most life possible from the battery, the hydrometer is the tool for your golf cart batteries which will allow you to maximize your charging source's potential.
And going below a rested 12.2v is not the end of the world for your lead acid batteries, but th emore time they spend down there, or lower, is much harder on them and makes it much harder to recharge them to a true 100% full. It will take much longer. If they do not get this 'longer' then they get more petulant and lose capacity faster than they would. Again the hydrometer will tell you for sure.
But the hydrometer is also kind of a pain to use regularly. With an Ammeter, when your amps taper to 2% or less of the battery capacity( 4.8 amps for 240AH) At absorption voltage(14.4 to 14.8v) then you are in the range of full charge.
If your smart charger stops when your batteries are still taking 8 amps( disinclude DC loads), Skoff, and restart charger after skimming off the surface charge by applying loads until it falls to 12.6v or under. Kind of handy to have a big load to do this quickly.
