Traditional glass and rubber Hydrometer. Not so easy to come by these days, but useful if your battery is not the sealed type. Could save you the price of a new battery and help diagnose where the problem lays.
Open the battery caps, dip in and squeeze the bulb to suck some of the electrolyte into the tube. The float will tell you the condition of the Electrolyte so you can top it up, add reconditioning tablets, etc. Use in a ventilated space.