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Hmm - thought so! I'll take your advice!
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The ammeter on a battery charger will indicate how much current is passing through the battery. This won't actually tell you if it's any good or not but think of it like a bucket of water. The more full the bucket is the less water it needs. If the bucket is empty it requires more water. If the bucket has a hole in it then it won't fill up no matter what you do.
An amp-clamp is a very useful tool. Simply place it over the main battery cable (positive or negative it doesn't matter, just make sure you observe the correct polarity or your reading will be wrong), and it will measure charge current and discharge current at the same time. The reading in this case is the amount of current that the battery itself is accepting, as what is being shown is the difference between charge and discharge. There is no magic figure here, but a reading of 10-20 amps will most likely indicate a good battery. A lower reading of 5 amps or less will almost certainly indicate a battery which is unserviceable. Start switching on loads and the figure should not alter too much.
A volt-meter is also very useful. Charging voltage which is around 13.5V - 14.5V and doesn't fluctuate also more often than not indicates a good battery. Charging voltage which is low or one that varies considerably is usually an indication of a faulty battery.
If you stick to the above you will be right far more often than not. Battery diagnosis's can catch you out as some do seem to go against the grain, and I have been caught out many times, but considering how many battery problems I've attended over the years it's not a bad set of rules to follow......
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There is no magic figure here but a reading of 10-20 amps will most likely indicate a good battery. A lower reading of 5 amps or less will almost certainly indicate a battery which is unserviceable. Start switching on loads and the figure should not alter too much. A volt-meter is also very useful. Charging voltage which is around 13.5V - 14.5V and doesn't fluctuate also more often than not indicates a good battery. Charging voltage which is low or one that varies considerably is usually an indication of a faulty battery.
I have to say that I cannot agree with any of that. My experience is quite different.
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I have to say that I cannot agree with any of that. My experience isquite different.
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That's interesting, but it may also be part of the reason why the motor trade has got such a bad name for itself. Someone takes their car to a garage and is told this, someone else tells them that, and so on.
Take it from me, I must have without exaggeration attended somewhere around a couple of thousand battery related faults over the years, and on the vast majority of those I have tested using the procedures listed above, and with a considerable amount of success too. So I stick with it, but I would be very interested to hear your diagnosis methods, as I don't claim to know everything and I'm certainly not infallible.......
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Batteries and their charging have been a bit of a hobby interest of mine for many years - alongside my day job as a diagnostic engineer and autoelectrical specialist. [Try 35,000 electrical breakdowns.] A car battery is a fascinatingly complex system that still isn't fully understood and is still untestable in under about three days. [Carbon pile load test.]
Currently available "inductance" and "capacitance" testers are little more than a bad joke. I've already got an order in for the very interesting £1200 Cadex Spectro as soon as the first one lands..... [18 months and waiting...]
The current flow rate into a battery tells you precisely nothing about the charge state or the overall condition of the battery. There are so many variables of things like temperature, electrolyte strength and connectivity present; that all that can be reliably extrapolated is that the alternator is charging - at least a bit.
The potential across the terminals when charging, is exclusively the result of the alternator's regulator setting and output; a low reading does not mean a failed battery and a stable one at max regulated voltage would be the same if the battery was completely dead. As with current, nothing worthwhile can be deduced from it. Adding load only tests the alternator, cabling and drivebelt - not the battery.
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Our charger ammeter only goes up to 5A and rarely reads above 3A with a fully discharged battery. Settles to around 2A when fully charged.
I've needed it once per year for the last 5 years on the same battery.
Edited by mfarrow on 07/01/2008 at 00:32
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In addition to Screwloose's thread.
A battery is merely a box that holds electricity. To test it, it has to be charged, and then see how well it holds it, and delivers it. No other way, other than a good guide!
Alternators are the electric provider once the engine is started and their testing is quite dfferent.
A lot of battery "faults" are not! and vice versa!
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OK, eventually got my local garage to look at this, since it kept going flat rather quicker than usual ... anyway, they diagnosed a new battery, as the old one had a faulty cell - the cause though baffles me. The garage 'reckon' that a remote boot release switch, which to my knowledge hasn't worked for a year, is causing a drain on the battery? I thought switches were on, or off? How can I check to see if there is really a drain - ammeter, and disconnect the fuses in turn?
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The cause is that you let it run and stay flat. It was probably on its last legs anyway. Car batteries are constructed of a lot of thin plates, which give it the ability to provide a lot of current for starting, but they are not designed to be 'deep cycled' i.e. run flat and charged up again, about 5-10% discharge is the norm, any deeper than this and the battery will considerably shorten its life. The active material falls out of the plate grids, causing lost capacity, and leaving it in a discharged condition causes both sets of plates to turn to lead sulphate.
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A trick to tell where your current drain could be, is to connect a bulb between the negative pole and the car body and it should light up. If its pretty bright, then pull the fuses one by one until the light dims right down or goes out. (this assumes the alarm system is not running and the radio is not on). Look on the fuse diagram which tells you which appliances are on that circuit, then look for the short. Aston Martins used to flatten their batteries because of a faulty boot light switch.
I know there is loads of expertise on batteries here, but if its more than 2 or 3 years old, leaving the car for weeks at a time will probably kill it. You can revive them when they are flat if you trickle charge, but the prognosis is usually not great. If a car is run every day, a battery can last up to 12 years apparently. The older they get, the less well they hold the charge. I have a couple of spare batteries and the newer one loses very little current in 3 months, whilst the 10-year old one loses about 40% (roughly).
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