multimeter (silly question)

kid · Post by **kid** » 26 Dec 2004, 01:22

This is a stupid question for most, but after a week of lying idle the car decided not to start
So charged battery and all ok.
I want to check the battery and i have a digital multimeter, however i unsure which setting it should be on, i.e. dcv or dca, also which setting i.e. 20 or 200?

I have a feeling its the a battery as its 8 yrs old.
Once I know what im doing (!) to check the battery I will do the following
When switched off tomorrow morning - should be around 14 v
Start car- test take for a run and test voltage across battery when idling - should be nearer 12.5 volts
Am I on the right lines re. check of battery?

All help appreciated
Cheers

beezer · Post by **beezer** » 26 Dec 2004, 01:24

You should set it to DCV and 20 volts. I would expect a better idling reading. Digital can even have the probes the wrong way round. It will just show a minus reading instead. Don't try this with an analogue meter though.

Stuart McB · Post by **Stuart McB** » 26 Dec 2004, 01:28

Leave the car standing engine off for at least 20 mins before testing the battery. Once checked take the car for a run then with the engine running check again the voltage should be a little higher than when off because of the alternator shoving some juice in.

kid · Post by **kid** » 26 Dec 2004, 01:40

thanks for v quick responses.
with re correct voltages i take i have figures wrong way round, ie when cold should be 12.5 v, when warm (after drive) should be approx 14?

Dave Bamber · Post by **Dave Bamber** » 26 Dec 2004, 01:54

Voltage should be about 12v at idle and up to 14.5v at about 2000rpm. Switch the lights, HRW, and the heater fan on, you should read no lower than 12v at idle and again about 14.5v at 2000rpm. If you are getting alot lower readings, the altenator is suspect. To test the battery, pull the HT lead to stop the engine starting, and crank the starter for 3 lots of 10 seconds about 30 seconds apart, each time the battery should return to 12v. If lower, your 8 year old battery is goosed. At 8 years old though, I think you have had your money's worth!

Stuart McB · Post by **Stuart McB** » 26 Dec 2004, 02:49

Not too sure about the voltage on this. Is the vehicle petrol or diesel? Just been reading some old manuals and they all say that the battery should be reading abprox 13.5 volts befor start up as you need at least 12v to turn over the engine. The alternator will then recharge to about 14.5v. Or is this the same as what Dave Bamber just said?

beezer · Post by **beezer** » 26 Dec 2004, 04:03

13.5 is the usual idle voltage with lights off. Anything less than 12 is not good. That is the sign of a good battery and charging system.

jeremy · Post by **jeremy** » 26 Dec 2004, 12:37

In the ZX manual Haynes states that the battery voltage should be:
poor condition 12.5 volts
normal 12.6 volts
good 12.7 volts
The test shpuld be carried out when the battery has not been subjected to any form of charge for 6 hours - otherwise they suggest turning on the headlights for 30 seconds then testing again after 4 or 5 minutes.
In fact a lead acid battery produces a very steady voltage which makes ascertaini the state of charge using a meter very difficult as with no load when almost discharged it will show something like 12.2 volts.
Do the tests as outlined above by putting it under load. If you find a reading of 10 or 11 volts then the alternator has probably failed in one phase. I've had cars that will actually start quite well at this voltage! but generaly the warning light flashes slowly.
Whatever you do DO NOT USE the amps settings on your meter. The 10 amp setting is usually not protected against overload and you starting current is probably 300 amps or more dropping back to 100 plus when the engine is cranking.
I wouldn't expect any difference between petrol and diesel in voltages.
jeremy

kid · Post by **kid** » 26 Dec 2004, 18:17

an update
tested this morning (from cold, infact very cold!)
reading 12.4v
with engine turned on rose to 14.5v
1)so the alternator seems ok, doesnt it?
connections all ok so which me to lead to either starter motor or the battery.
if its any help when not started for a few days, when i turn key, the interior dash lights are dim and the starter motor turns slower than when turning from hot.
2)this is a typical symptom that the battery is on the decline-right?
3)so is there any way of checking if battery is holding charge (no HT leads BTW)
regards

beezer · Post by **beezer** » 26 Dec 2004, 18:26

After a good run do a voltage test with the engine off and (if your radio coding and alarm system are taken care of) the battery disconnected. Test again after a few hours. If there is a significant drop you need a new battery. There might be something draining it so the disconnection eliminates that possibility.

kid · Post by **kid** » 26 Dec 2004, 18:36

Ok thanks I will give it a go
btw i did a voltage test (battery not disconnected) straight after turning engine off from warm and its showed 12.7v
so i presume if after disconnection and retest anything below 12.5v (as per jeremy’s post) means battery not holding charge, thus draining slowly and cant start.

beezer · Post by **beezer** » 26 Dec 2004, 20:01

That's about it.

CBR600FV · Post by **CBR600FV** » 28 Dec 2004, 01:07

As a confirmation of voltages. After a long dwell period of no activity, a good battery should read between 12.5-12.7volts. If a load is applied, i.e. interior light on or bigger then the voltage will drop but should recover quite quickly. Engine running should show voltages between 13.8-14.2 volts depending on revs/load. A 12v battery has 6 cells of nominal voltage between 2.1-2.3voltsso agood battery should show 6*2.1 volts for healthy. Your battery at 12.4 volts is coming to the end of its useful life. If you have a diesel I'd change it ASAP. A petrol might soldier on for a while, but its time for a chamge sorry....

kid · Post by **kid** » 17 Jan 2005, 02:26

Just a final update on this,
Finally got round to testing the battery and was losing around 0.1 voltage in around a few hours, so battery was most likely losing ability to keep charge.
Went to ha*****s and the battery check showed 163 (presume this is A), and so replaced the battery with a 'heavy duty' 3 year guarantee type.
Now the reading on the multi meter is does not drop after a period of inactivity. So it appears it was a battery. Also the starter motor is now quick to turn over and start the car from cold unlike before.
Also the new battery has an A of 590, and a reserve capacity of 105minutes, is this a good type?
Thanks to all that helped.

kid · Post by **kid** » 25 Jan 2005, 00:59

Just another post on this
I tested the terminals today and was showing at 12.4, however when i checked my second car it was 12.7, so maybe there is a problem small somewhere.
When opened door (and courtesy lights) the reading went to 12.3, and when closed door when back up to 12.4
Switched on engine (from cold) and went up to 14.5, switched on all electrics tested immediately and showed 14.3, so appears alternator is ok, all connection look ok too.
The car started immediately and starter motor was positive only one/two turns. So i think ill leave it at that as the new battery has helped and car started immediately despite being standing for three days.
Maybe something very minor wrong, but not really worth bothering with.
maybe the battery is not fully charged, as since fitting ist not been on a long run, just stop/start driving which may account for a slight variation, or maybe not!