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I had a problem starting my car after not driving it for two weeks. After charging the battery with a CTEK trickle charger via the cigarette lighter socket, the car started fine.

I had my car battery tested at a couple of garages and they said the battery was fine, it was just undercharged.

I charged it again with the trickle charger and then once the charger had indicated that the battery was full, I disconnected the charger and then measured the battery voltage using a multimeter. This showed a voltage of 12.23v, which according to this site means it is only 50% charged.

I have tried charging it again, however it won't go above the indicated 50%. Does this indicate that the battery needs replacing? It is currently five years old, however I am wondering why the tests came back fine unless there's a fault in my method?

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    Batteries should be load-tested IMO. A single voltage reading won't be conclusive – Zaid Jan 16 '16 at 17:26
  • After you take your battery off of the charger it has a 'surface charge'. If you started the car after the test and let it run a moment and test again, you may get a different result with a reduced voltage rating. Also, a 5 year old battery is ready for the scrap heap. If will let you down soon. You don't want to 30 miles away from home when it happens. Probably a good idea to dump it. – DucatiKiller Jan 16 '16 at 17:52
  • @DucatiKiller: Yes, I thought that before my battery was tested. However, both garage tests said there was life in it yet. – SilverlightFox Jan 16 '16 at 18:36
  • May need fluid... If u Pop open the two rectangles on top you can check the battery acid level. If low add "distilled water" only. Distilled water has no charge thus it can be used for positive or negative applications. – Dee Jan 17 '16 at 1:33
  • Was the battery temp at about 80f when you tested or was it cold out? – DucatiKiller Jan 19 '16 at 7:10
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I took it to a main dealer and had the car diagnosed there.

They replaced the battery and so far the car seems fine.

I guess that battery test equipment can only test it to a certain degree - e.g. it can tell if a battery is definitely gone. In my case it was gone but undetectable by such equipment.

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As mentioned in the comments, I think what you need to do is verify that you have obtained reliable data about the status of your battery before you come to any rash conclusions.

I am wondering why the tests came back fine unless there's a fault in my method?

Yes, there probably is.

Does this indicate that the battery needs replacing?

A trickle charger is one of the best ways that you can charge a car battery. It is much better (if you have the time) than the much more common and more convenient jump start. If a trickle charger doesn't charge your battery, likely nothing will. Also, do check the level of your battery acid because it could just be that it is low, hence your problem. Distilled water is good to use to top it up because the charge is neutral.

If you live in an area with a particularly cold climate, the quality of your battery is going to be much more important because a car in a cold environment will put a much larger strain on the battery than it would in a warmer environment. This could be a problem if you can't actually charge your battery fully (only to 50%) and you don't drive your car on a particularly regular basis (every two weeks).

Answer

It depends.

If you think that you will be leaving the car stationery for long time intervals, or you live in a cold climate, also that you have verified the level of the battery acid and verified that you are testing the battery correctly, then I would definitely suggest that you replace your battery. Get a good quality one too (or wait until you can afford one).

If your climate if generally fairly hot, you plan to drive your car very regularly or due to further testing you have found that the battery status is actually fine (I suspect you might, as you said you had been to a couple of garages to get it checked) then I would suggest that you do not need to replace your battery.

Listen Method

One way to tell how healthy your battery is (and this is very easy to do), is just to start your engine and listen. If your engine springs in to life rather quickly - your battery is fine. If it sounds like it is trying to start for a few second and then eventually does, then your battery may need to be replaced.

Note that this sound will become more apparent if it is cold outside.

Personal Experience

When I had my cam belt and water pump done at the VW dealership, they also did a standard check where they identified that my battery was in poor condition. After doing some more work on it myself with the interior lights and boot light on for the majority of the time, my battery became flat enough not to be able to start the car (this was on a really cold day). I just jump started it and so far my car has been fine, although I do still intend to get it replaced as you can hear the poor condition of my battery even in somewhat warmer weather to a lesser extent.

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Tap water in a car battery? Always use distilled water. Distilled water is inexpensive and filters out 99% the minerals and other impurities that are present in 'regular' water (this includes "tap" water provided by your city or town, artesian and point well water, bottled spring & mineral water, etc.). Reverse Osmosis ("RO") water is a good second choice, as it is nearly as pure; but not as readily available and tends to be a bit more costly when found. Unless the water container specifically states it is "distilled" or "RO" it isn't! The minerals and impurities found in any type of non-distilled or RO waters will reduce the battery's performance and life.

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one cheap way to check if you altinator is charging is to disconnect red cable from battery while engine is idling if engine keeps running altinator is charging

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    Welcome to Motor Vehicle Maintenance & Repair! This is very bad advice. It may have been good advice back when cars had generators, but with today's charging systems, this is a good way to damage not only the alternator, but the electronics in the vehicle as well. See this for more information – Pᴀᴜʟsᴛᴇʀ2 Oct 21 '18 at 22:14

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