battery capacity, does it matter??

[JIMR2]

this'll make ya laugh -

just had a new new alternator fitted on my megane (1.5dci tourer) 1 week ago due to it rattleing somewhat, but still doing the job of charging. then..... today the fricking battery died!!!!! so quickly replaced it with a bosch. (oh and yes i multimetered the voltage to make sure the alternator was working)

anyway my point is:

the replacement is slightly larger physically (still fits, infact i think it fits better) and is 70ah where my original was a 60ah. there maybe some more info on the batts but its too dark and cold to go back out and check.

So from what i've written, can anyone say if this could cause any problems or will it perform better?? and just require a bit more pace and time to charge???

thankyou

[AlexB]

The original, I think, gives 750A starting current, which is equivalent to Bosch Silver S4001.

[davelowe]

the replacement is slightly larger physically (still fits, infact i think it fits better) and is 70ah where my original was a 60ah. there maybe some more info on the batts but its too dark and cold to go back out and check.

So from what i've written, can anyone say if this could cause any problems or will it perform better?? and just require a bit more pace and time to charge???

thankyou

The amp/hour rating is a measure of capacity. So a 70A/h battery will provide 70amps for one hour before being discharged, or 140 amps for 30 minutes, 1 amp for 70 hours, etc. It has no bearing on the time taken to charge when used normally. Assuming it was fully charged when you got it, your normal driving habits will keep it topped up without problem if they haven't changed since you replaced the battery.

As AlexB pointed out, another measure of battery quality is the peak current output. Starting the car is almost always when the highest current is drawn, so you should ensure that is meets the original unit's minimum rating. It is simple to test. Wait for a cold day (battery efficiency declines with temperature), switch the ignition and everything electronic you can think of (lights etc) on, then start it. If it fires up without problem, then there is nothing to worry about.

[JIMR2]

thanks guys! i shall give that a go and check out what the model no is

[AlexB]

Dave is right... just, please, don't attempt to run your battery at 140A for half an hour continuously - it will boil!

It is also know that the A*H capacity under high loads is significantly lower than under the 10h discharge cycle. This is because the battery under load is considered to be discharged when its voltage drops below a certain point. Any battery has some internal resistance, which according to the Ohm's law contributes to the voltage drop. This effect is more pronounced under increased currents or the internal resistance increased due to low ambient temperatures.

[davelowe]

Alex is right, the old copper coil battery short circuit testing kit is practically banned - though not unofficially obsolete! Don't do anything stupid - that's why its good practice to always remove the negative terminal of a battery first on negative earth cars - you can't short the positive terminal to the chassis with a wrench. I've done this in the past and it will spot weld itself. You need to be very well prepared to remedy the situation in seconds (a hammer).

The internal resistance is a bit of a weird concept, but in practical terms it means that the battery voltage is lower when it under load (this would not happen with a regulated voltage source). Kirchoff pioneered the physics behind it by some clever deduction based on all the measured currents at any point in a closed circuit summing to 0. Based on the equation V = IR, the R component is split into the load you apply and also the internal resistance of the battery. 'R' in this case being comprised of the battery resistance and the circuit resistance. There is no practical point in this explanation, I just remembered some A-Level physics! That said, with some indirect measurements, and a bit of algebra, you could determine the peak current output from the battery, but I wouldn't bother! Just drive the bloody car!

[JIMR2]

Alex is right, the old copper coil battery short circuit testing kit is practically banned - though not unofficially obsolete! Don't do anything stupid - that's why its good practice to always remove the negative terminal of a battery first on negative earth cars - you can't short the positive terminal to the chassis with a wrench. I've done this in the past and it will spot weld itself. You need to be very well prepared to remedy the situation in seconds (a hammer).

The internal resistance is a bit of a weird concept, but in practical terms it means that the battery voltage is lower when it under load (this would not happen with a regulated voltage source). Kirchoff pioneered the physics behind it by some clever deduction based on all the measured currents at any point in a closed circuit summing to 0. Based on the equation V = IR, the R component is split into the load you apply and also the internal resistance of the battery. 'R' in this case being comprised of the battery resistance and the circuit resistance. There is no practical point in this explanation, I just remembered some A-Level physics! That said, with some indirect measurements, and a bit of algebra, you could determine the peak current output from the battery, but I wouldn't bother! Just drive the bloody car!

thanks again.. (at least the a level knowledge is still in there) now just bloody driving the car cheers!!!!!

while i'm here tho, incase you check up on this post - my n/s front window didn't work last night?? i don't think it was even getting any power to the regulator as i couldn't hear anything or notice any drain on power (it was on way back at midnight tho so no thorough check done so far) - i fitted a new regulator/motor approx a 1yr and a half ago so hoping its not that again??!! just wondered where you might start testing?? ta

[davelowe]

The obvious start point would be the fuses. According to the circuit diagram there are two - one for each front window rated 25A (F1L & F1M). Assuming they are ok, or don't blow immediately after being replaced, you will probably have to get at the regulator. All the other wiring and fuses appear common to both front windows, so if one works, and the other doesn't, you need not concern yourself with that. Pin 4 should be 12V at all times when ignition is on, and pin 3 should have continuity to ground. All the other pins bar one seem ganged together between the passenger door switch and the drivers door switch. Check both buttons fail to open the window. The remaining pin (6) goes to the multiplex module, but again is common to both front windows. So in summary, unless there is a wire loose somewhere or a bad earth connection (the windows do not share the same earthing points), another regulator.

[JIMR2]

cheers davelowe

i shall first check the fuse as easiest option. then probably realise its gonna be another new reg needed. great! this time i'm gonna seal the damn thing up as well as possible as i guess its water thats knackered it.

i've had bit of a poor show so far, its been over 2 weeks and i've still not got around to doing it. lol . nevermind its the wife's motor, it can wait!! hehehe

cheers