The battery voltage is displayed on the voltmeter. The ammeter is connected such that all the current going in and out of the battery passes through the meter. Observing the ammeter will tell the user ‘how heavy’ the load is that is powered by the battery or how much current the charging system is providing.
State of Charge Related to Battery Voltage with No Load
Voltages shown are for a 12-volt battery system at 25 degrees C (77 degrees F) . These voltages are for batteries that have been at rest for 3 hours or more and not being charged. It is important to realize that voltage measurements are only approximate. The best determination is to measure the specific gravity with a hydrometer, but in some battery installations, this is difficult or impossible.
| Approximate State of Charge | 12 Volt lead acid battery | Volts per Cell |
|---|---|---|
| 100% | 12.7 | 2.12 |
| 90% | 12.5 | 2.08 |
| 80% | 12.4 | 2.07 |
| 70% | 12.3 | 2.05 |
| 60% | 12.2 | 2.03 |
| 50% | 12.1 | 2.01 |
| 40% | 11.9 | 1.98 |
| 30% | 11.8 | 1.96 |
| 20% | 11.6 | 1.93 |
| 10% | 11.3 | 1.89 |
| 0 | 10.5 | 1.75 |
table showing approximate state of charge for an unloaded lead acid 12V battery.
It is very important to remember that the voltages shown in this table represent a 12-volt deep-cycle battery that is at rest - there is no load and there is no charger connected to the battery.
For longest life, batteries should stay in the green zone. I recommend that the battery state of charge not be allowed to drop below 50%. Occasional dips into the yellow are not harmful, but continual discharges to those levels will shorten battery life. Lead acid batteries should be fully charged at least every 7 days.
If the user has time and records battery and current readings regularly, a profile of the battery may be used to keep the battery state of charge in the recommended zone. Most people however greatly benefit from a quality battery monitor that is designed to do this work for you!