The charge of the battery can be estimated using an inexpensive device with the engine off. How to do it?
Is it possible to assess the charge of the battery without using a load plug?
Indirectly, you can – by measuring the voltage at the battery terminals with the engine off with a multimeter. If the battery is fully charged, it should be 12.6-12.7 V.
The measurement must be carried out a few hours after the engine is stopped – otherwise the readings may be too high. A voltage of 12.3 V indicates that the battery is undercharged.
It makes sense to make sure that the charge circuit is working by measuring the voltage with the engine running – it should be about 14.0-14.4 V. If it is lower, the cause may be in the generator or in bad contacts.
Almost any device that has a scale or a digital indicator allows you to measure voltage. Probes with light bulbs do not count: they can only work in the “Yes / No” mode (they will tell you if there is some kind of voltage or nothing at all).