Even the cheapest hydrometer is fine. Rinse it after use, and keep it clean. For sealed batteries, it is the ONLY definitive method. See next paragraph. It is necessary to help you to read the status of the sytem.
Many charge controllers have indicator lights and readouts built-in.
They also record more detailed information that can be useful for troubleshooting. The monitor may be mounted in another room or building, for handy viewing. Hydrometer A hydrometer will help you to determine whether the battery bank is getting fully charged, and whether any individual cells are falling behind. You should be aware that a hydrometer will give you false readings under the following conditions. A hydrometer will show a greatly reduced reading until the fluid mixes.