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I think my battery is on the way out - on cold mornings, it turns the engine over very slowly to start with, and takes a number of turns before the engine finally starts. This has been the case for a few weeks, but the car has always started eventually. (Note it's not the glow plugs as I had them changed a couple of months ago.)

I never use the car for anything I couldn't use train, bus or bike for, so if one day it doesn't start at all then I'm happy to sort it out at my leisure. However, I don't wish to do any damage to other parts of the car - so my question is, is a battery of this description likely to be shortening the life of any other components?

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You've really picked the wrong answer here. –  kahbou Nov 27 '13 at 15:59
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4 Answers

up vote 3 down vote accepted

Replace, the battery; your alternator and starter will both be subjected to higher stress if the battery is low. If you can't replace it immediately put a trickle charger on it when the vehicle is not in use. (The rare-use scenario you outlined would justify a trickle charger whether the battery is dying or not. But not all vehicles can be garaged, so that may not be an option.)

Mike's answer isn't complete. Lower voltage does require higher current draw in the case of a variable resistance load, and cable gauge, etc. dictates the higher the current going across a wire, the greater the voltage drop will be before even reaching the starter motor. You have a diesel engine, so the higher compression ratio creates a higher load from the get go. Other components that have a fixed resistance will actually have a lower current draw as a result of the decreased voltage which can cause a different set of problems in electronic components, but those are less likely than the increased wear and tear on your starter and alternator.

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-1, I don't buy this. For the alternator any increase in charging current would be marginal, or negative. Sure the open circuit voltage is lower with a weak battery, but remember that the increased series resistance for a weak battery applies when charging as well as discharging. At any rate, the draw will be less than charging current of a fresh but discharged battery, so it'll be well within the design limits of the alternator. No reason for concern. –  kahbou Nov 27 '13 at 15:09
    
Also: I have no idea what's going on in the last paragraph. Stall current is simply V/R. After that it's useful to know that current draw is proportional to torque, so unless you think that output torque is higher when starting on a weak battery, the current draw will necessarily be lower. It's true that you can kill a motor by giving it low voltage by keeping it close to stall for longer that it was designed. Whether it's a problem depends on how long the OP means by 'a few turns'. Failure would be from overheated windings, so the fact that this only occurs on cold days helps. –  kahbou Nov 27 '13 at 15:44
    
@kahbou I guess that depends on how dead or weak the battery actually is. If it is a little weak, you're right, there's probably not a large difference, but with a damaged battery that has been completely discharged, you will see increased wear on both components. I did suggest an alternative with a trickle charger on a rarely used car. –  Nathan L Dec 2 '13 at 14:57
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The battery is there for starting the engine. Once the engine is running everything is powered by the alternator/generator. The efficiency of the battery is reduced in cold weather but should still start the vehicle. 1.Check the voltage acoss the terminals of your battery, you want to see somewhere around 12.4V. 2.Then start the vehicle and check the voltage again, you want to see around 13-14.4V. If 1. is lower than around 12.2V your battery is discharged. If 2. is lower than 13V your charging system is not charging the battery properly. If your battery is old, say three years or more and your charging system at 2. is producing a good voltage then it is time for a new battery. You can trickle charge the battery at around 2Amps and then discharge it fully with a 5W side light bulb and then recharge at 2A again. Repeating this several times will reduce the sulphation in the battery and improve its performance. It is not usually not worth while against a simple battery replacement. A battery in a low state of charge cannot hurt your vehicle in any way because when the volts drop off so does its output. A low battery charge will have no effect because of the alternator powering everything. A last check is a heavy discharge test, normally a repair shop test, where a load is put on the battery for ten or fifthteen seconds and its voltage must not drop below 9V. This tests the physical condition of the battery.

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Low battery voltage can cause the starter to overheat along with the cables and terminals. The lower the voltage the higher the current draw, the higher the current draw the more heat is generated, which in turn raises the current higher in a vicious cycle. This not a one time effect but cumulative. The more times it overheats the worse the problem will get, until something fails completely.

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-1 "The lower the voltage the higher the current draw" This is just wrong. Unfortunately, a lot of people have used 'P=V*I' and got it into their heads that there's some higher authority that keeps power fixed. It's only true for a motor when it's coupled to a load that increases at low rpm. –  kahbou Nov 27 '13 at 15:51
    
Also: resistance increases with temperature. –  kahbou Nov 27 '13 at 15:52
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What ? Increased wear in starter and alternator because of weaker battery ? Wow... you should read yourself a few times and see what have you wrote there :) There is no way in the world that this could happen. Lower battery power = LOWER stress and lower wear in those components. Period. There is eventually tiny chance of problems with electronics while beeing powered by weaker battery but it's actually negligible issue. Summing up there is nothing that could brake because of weak battery and I have never heard of such a case.

Make a test. Take some weak battery and connect a car bulb to it. But watch out, less voltage more current. It may explode eventually lol :)

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