Question: Now that many automobile batteries are...

Question: Now that many automobile batteries are sealed and cannot be opened to check the condition of the charge with a hydrometer, how does a person check one?E.A.

Answer: Hydrometers have long been a cheap and reliable way of determining the condition of an automobile battery, but their days are numbered. In general, you can test a sealed battery only with a voltage tester because you cannot insert a hydrometer into the electrolyte. The exceptions are those batteries with built-in hydrometers.

Delco batteries, for example, have built-in hydrometers in which a single cell on the battery has an indicator that checks the specific gravity of the electrolyte. If the battery is 100% charged, the indicator will show a blue light. A yellow indication means the there has been a loss of electrolyte. Black indicates a low charge, which typically is 65% of a full charge.

This is a nice feature, but it tells you only whether the battery is good or not good. If your car will not start and you aren’t sure whether the battery is dead, this feature will give you important information. But if you already know your battery charge is low, it is not as good as the old-fashioned hydrometers because it only reports on a single cell and it provides rather imprecise information about the charge.

A voltage tester can achieve the same results as a hydrometer, but it also cannot tell you whether a single cell has died or whether all the cells have degraded. A 12-volt battery charged to 100% of its capacity should produce 12.6 volts. If the battery has no capacity left, it would register 11.6 volts.

Many maintenance-free batteries can be pried open, but manufacturers discourage the practice. Delco, for instance, asserts that it seals its batteries to protect consumers from explosions and burns.

Nonetheless, many weekend mechanics defeat the seals on batteries so they can add water as the electrolyte evaporates. Maintenance-free batteries are supposed to lose less electrolyte than older unsealed batteries because they use calcium alloy in their grids, which results in less gasing of the electrolyte. But over time, the electrolyte in many sealed batteries will go down and the battery will fail prematurely.

Q: I have a 1988 Buick Century with the 2.8-liter engine. The owner’s manual specifies using the AC PF-47 oil filter, which is a short filter. But the General Motors service manual for my car specifies using the AC PF-51 oil filter, which is a longer filter. Which is the correct oil filter?J.H.

A: General Motors likes to advertise the superiority of its genuine GM parts. Too bad it can’t decide which is the right one.

You should probably stick with the recommendation contained in your owner’s manual because it is based on your specific model and is the official source of all information pertaining to your car.

The PF-51 is the same oil filter as the PF-47, except it is longer. That means it contains more filtering medium and should operate for more miles before becoming clogged with dirt.

But the PF-51 may be positioned down too far, risking foreign object damage from road. If you change your filter every 4,000 miles, the PF-47 should work fine.

Vartabedian cannot answer mail personally but will attempt to respond in this column to automotive questions of general interest. Do not telephone. Write to Your Wheels, The Times, Times Mirror Square, Los Angeles, Calif. 90053.