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Adding distilled water to my car battery, I've noticed only the side cells very short of reaching the meniscus.

The most positive cell, facing the front of my '08 Jetta 2.5L (which has empty space, and probably collects a lot of heat from the engine, especially when the two frontal engine cooling fans are on), required almost 90ml of water, the next cell only some 50ml, the following ones seem to already be at around meniscus (added 20ml each for prophylactics after 6 years of service), and final negative one, closer to the cabin, 40ml.

Is it always like this? If so, why aren't they making cells of varying size and level of meniscus, so that more water could be contained within the cells more likely to be kept at a higher temperature and evaporate and/or gas prematurely?

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cnst
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    In these days of cost conciousness it is usually a case of simply replacing an aged battery (3+ years) if it presents a problem. – Allan Osborne Dec 09 '14 at 21:15
  • @AllanOsborne, and most such replaced batteries are replaced way prematurely -- maintenance-free lead-acid batteries still need water every now and again. – cnst Dec 09 '14 at 21:22
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    When you buy a battery it will come with a warranty, usually three years. This is its life. Lasts longer its a bonus. Having changed thousands of batteries over the years I can only think of your assertion that 'most batteries are changed pre-maturely' is based on supposition and conjecture rather then any real monitoring of battery replacements. If it makes you feel any better, I have just replaced my cars battery for £71. Winter has arrived in the UK. – Allan Osborne Dec 09 '14 at 21:36
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    @AllanOsborne, don't cars themselves come with a 3-year warranty? Are you giving your 3-year-old cars away, too? The fact that batteries are replaced prematurely is a widely known fact -- http://batteryuniversity.com/learn/article/caring_for_your_starter_battery -- "A leading Germany battery maker checked 400 starter batteries that had been returned under warranty and found that 200 of them had no problem. Additionally, prior to replacement, do you actually bother to check water level, and put the battery on the charger overnight? Or, who has the time, right? – cnst Dec 09 '14 at 21:44
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    Your 'battery university' is a company that makes battery testers so it not subjective nor independant in the 400 batteries story. As for a three year old car out of warranty repairs, if the owner does not pay to have a repair then they might as well give the car away as it will simply become a lump of immovable steel. The warranty being for three years means just that. Charging a battery overnight to see if it will revive just does not happen any more with the advent of conductive testers, say Midtronics. – Allan Osborne Dec 09 '14 at 22:01
  • I couldn't agree more with @AllanOsborne (which is somewhat of a change, lol). Spot on brother. It seems the OP has all of the answers anyway, so leaves me wondering. I would add that a lot of batteries in the US come with a 3yr warranty for full replacement, then are pro-rated over the next two years. Even so, if you are getting more than the warranty period, it is bonus. – Pᴀᴜʟsᴛᴇʀ2 Dec 10 '14 at 02:05
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    @AllanOsborne My battery is an Odyssey battery. It comes with a 2 year warranty and is the most expensive battery. I have an Odyssey battery in a motorcycle and it has been working great for the last 12 years. I have used their batteries in many vehicles and they usually outlast the vehicle. I'm glad I don't follow your advice. – Paul Dec 10 '14 at 08:30
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    @AllanOsborne At least cnst went to the trouble of citing a source. – Paul Dec 10 '14 at 09:01
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    Odyssey batteries are not what we are talking about. They are made to stand up to the sort of use that a standard lead acid battery would be unable to handle. A standard battery sheds active material in normal use that an Odyssey just does not do. That is why thier life is so great. My replacement battery cost £71, an Odyssey would set me back around £240. I would specify an Odyssey for critical use vehicles, fire, police, ambulance, military, haulage and the like without hesitation, but baulk at the cost for my Ford Focus. – Allan Osborne Dec 10 '14 at 10:36
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    @AllanOsborne Not only are we not talking about Odyssey batteries, we are talking about a policy of replacing batteries because they are at the end of the warranty. I used Odyssey batteries to demonstrate that this is poor policy. The battery manufacturers make their warranties based on the worst expected operating conditions. This is so they can be sure not to lose money on the warranty. The tester company you point out that runs Battery University is actually trying to save consumers money by letting them know when it is appropriate to replace a battery. You don't, by chance, sell batteries? – Paul Dec 10 '14 at 15:35
  • I do not advocate a policy of changing batteries at the end of thier warranty period, thats you saying that. What I am saying is that if you have a battery that is faulty and it is at the end of its warranty period (ie designated life) then changing the battery for new is what you should do. As batteries lose thier active material during routine service, and as you cannot dismantle and repair a battery, and as charging and testing to pedantically determine its fault incurs an additional cost, the smart money is on simply replacing the battery. – Allan Osborne Dec 11 '14 at 11:01
  • @AllanOsborne, Smart money is peeling the label of the maintenance-free battery, topping up each cell with distilled water for meniscus (if your maintenance-free battery is simply flooded lead-acid anyways), and charging over the weekend at a stationary charger. Warranty period and designated life are not the same thing. – cnst Dec 11 '14 at 16:23
  • Could be. You can spend the smart money you 'saved' on a hire car or cab fares over the weekend. – Allan Osborne Dec 11 '14 at 17:21

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In a word, no. Any of the cells might need water. All of them would need water (or electrolyte) if it were tipped over. Usually when one needs it they all need it, but no rhyme or reason to it.

Pᴀᴜʟsᴛᴇʀ2
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  • It is completely normal for wet flooded cell batteries to require more frequent refills when exposed to heat, even on a cell by cell basis. It's just more rapid evaporation. – Paul Dec 10 '14 at 08:37
  • @Paul ... If anything, the inner cells will get warmer due to charging and not having the ability to dissipate heat because of lack of surface area (only the two end cells have more). The inner cells would suffer worse than the outer cells. I would expect, if anything, the inner cells would have more evaporation than the outer. – Pᴀᴜʟsᴛᴇʀ2 Dec 10 '14 at 11:48
  • Heat generated by normal charging is minimal compared with heat generated by the engine, which is the problem described in the question. – Paul Dec 10 '14 at 15:30
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Battery manufacturers make batteries to international standards, one of which was selected by VW as the battery for your vehicle. The problem you describe is specific to your vehicle, so a battery manufacturer would not be able to solve the problem as you are suggesting without violating the standard or creating a problem for the owner of some other vehicle that uses the same battery type.

The solution that battery manufacturers have come up with for the problem you describe is a maintenance free battery.

Paul
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    Maintenance free batteries have an excess of electrolyte and most have a pressure valve arrangement to minimise electrolyte loss when charging/gassing. This is intended so that the battery will be trouble free for its designated life/warranty. – Allan Osborne Dec 10 '14 at 10:41
  • @AllanOsborne It does not matter how the manufacturers arrived at the solution of maintenance free, just that they arrived at it. Note that VLRA batteries are not maintenance free because of the reason you state. – Paul Dec 10 '14 at 15:31
  • Maintenance free batteries are not maintenance free is literally what you are saying. How about, A maintenance free battery that requires maintenance during its service life is a faulty maintenance free battery, or it is the wrong battery for its implementation. – Allan Osborne Dec 11 '14 at 11:11
  • @AllanOsborne I have no idea how you come to that conclusion. I have owned a great many maintenance free batteries and not one has required maintenance, and I've never heard anyone ever say that maintenance free batteries are not maintenance free, until now. – Paul Dec 12 '14 at 23:16
  • My battery IS maintenance free. Maintenance free batteries, sadly, still require for the water to be added. – cnst Feb 16 '15 at 18:36
  • "Maintenance Free" is not a classification of batteries, but a feature. There are several ways to achieve it using various different technologies. For example, Enersys supplies the DOD with 99.7% starved electrolyte AGM batteries for use in applications like tanks in the desert. If water were ever added to these batteries, it would cause premature failure, yet the heat they regularly undergo while in service is substantially greater than will ever be seen in your car. – Paul Feb 20 '15 at 22:10