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battery condition
jabbahutt - 7/8/08 at 07:01 AM

Okay now need help with a battery problem. After wiring up dash couple of nights ago couldn't resist quickly checking out that warning lights etc worked.

Some things using relays weren't which were okay before fitting dash. Turns out battery was run down so connected it up and put it on charge. Tested later and things worked okay.

Last night even though battery had been on charge all night and following day there seemed less charge than the night before.

Multimeter says there is only 1amp charge in the battery!! (should be 85A) Is this recoverable with charging or does the battery need replacing? also if battery is beyond help does trying to charge it normally make it worse? as it seemed to have more oomph the night before.

Any help much appreciated.

britishtrident - 7/8/08 at 07:49 AM

Could be a few things anything from blown alternator diodes to a dud battery charger, but a problem is that modern batteries don't take to being completely discharged often it isn't possible to recharged them afterwards.
The few test you can do on a battery with a multimeter are checking the voltage with everything off, checking the cranking voltage and checking the charging voltage on and off load.


Questions

Were you charging the battery with the terminals still connected to the car ?

(1) What is the voltage across the battery off charge ? 12.8 to 13.2 volts is the norm for battery with a half decent charge.
(2) Will the battery crank the engine ? If so what is the voltage across the battery when cranking but with the igintion disconnected ? should be about 10 volts if the battery has a reasonable charge.
(3) Check the voltage the engine is running at at 2,500 rpm first with the lights and heater blower off and then repeat with the lights on. Both should be 13.4 volts to 14.9 volts.


(4) You also want to check the voltage when the charger is charging to get the output voltage of the charger --- ie it should be about 13+volts

jabbahutt - 7/8/08 at 07:58 AM

Both the postive and negative leads were connected to the battery. I must admit didn't give it a second thought (does it matter?)

When I was having problems with the alternator wiring in the past the battery was trickle charged withe the earth lead disconnected as that prevented the battery going flat whilst I worked out the alternator problem.

So is there a combination of leavbing leads connected/disconnected to a battery whilst charging that can cause problems?

Must admit never thought about it until now.

BenB - 7/8/08 at 08:48 AM

I think BT is onto something with that advice

Check the alternator, check that the battery is actually getting a charge and check when cranking to ensure the battery isn't knackered.

Batteries of a non deep-discharge variety do not like being completely run flat and often won't recover fully (the plates warp and get all sulphated IIRC)...

02GF74 - 7/8/08 at 09:17 AM

quote:
Originally posted by jabbahutt

Multimeter says there is only 1amp charge in the battery!! (should be 85A) Is this recoverable with charging or does the battery need replacing? also if battery is beyond help does trying to charge it normally make it worse? as it seemed to have more oomph the night before.


I don't know what multimeter you have that can tell you that!

Battery capacity is not measured in amp but in Amp/hour.

Basically that is how much current for how long can the battery supply.

Let's say your battery capacity is 60 Ah.

This means fully charged it can supply 1 A for 60 hours or 60 amps for 1 hour or 600 hams for 6 minutes.

In real life it does not work like that for the really high currents due to a number of factors that I won't go into.

So basically I do not know how you came up with those figures.

The open circuit (i.e. nothing connected to battery) voltage across the battery is a reasonable indication of battery condition and how well it retains charge. Also measuring specific gravity (density) of the electoclyte, assuming you have lead acid battery, can give similar info.

Anyways, all of that is neither here nor there.

If you charge the battery overnight and it looses oompf so as to stuggle to turn the engine over or headlamps dim quite quickly, then it is knackered. No amount of charging will help as the charge is not held in the battery.

What voltage do you see across battery once fully changed? What voltage do you see 6 hours later?

This has nothing to do with alternator but is purely a dying/dead battery. (I am talking of case where alternator is not connected to battery - yout post yesterday about the lamp not being lit may indicate the alternator is shot and not the battery - if you have left the battery connected to alternator, it may be discharging the battery have my point about open circuit). You need to isolate problem to one component by disconnecting the other from the circuit.

Was the battery new?

As ^^^, lead acid do not like being in a disharged state.

jabbahutt - 7/8/08 at 09:50 AM

As usual it looks like I have the wrong end of the stick.

I measured the voltage and it read 8v, put it on amps and across the terminals it read 1(this it appears is completely wrong)

The battery allowed me to use indicators and lights but switching on the cooling fan resulted in eiither just the realy clicking or the fan spun once and stopped, the night before it worked fine.

So when I get in tonight with the neg lead disconnected from the battery which was new I'll measure acroos the earth lead to the battery neg and see if there is any voltage/current and the ntry again with the alternator disconnected and see what happens.

thanks for the help

02GF74 - 7/8/08 at 10:16 AM

quote:
Originally posted by jabbahutt

I measured the voltage and it read 8v, put it on amps and across the terminals it read 1(this it appears is completely wrong)

NO!!!!!! DO NOT ATTEMPT TO MEASURE BATTERY AMPS WITH A VOLTMETER!!!!


You will end up damaging the voltmeter or blowing an internal fuse. When in amp reading mode, you are shorthing the battery out so can draw hundreds of amps!! .... well if your battery is good that is.

There are special meters for measuring huge currents. Your voltmeter will have a range, 100 mA, 1 A, 10 A so you should not exceed those - it should be fused internally too.

The battery allowed me to use indicators and lights but switching on the cooling fan resulted in eiither just the realy clicking or the fan spun once and stopped, the night before it worked fine.
battery was charged the night before but come morning, the charge was lost. Either the battery is not holding charge internally or it had something connected, e.g. alternator, that is draining the current out of it. Hence my suggestion below/above of disconnecting the alternator to diagnose the problem.

So when I get in tonight with the neg lead disconnected from the battery which was new I'll measure acroos the earth lead to the battery neg and see if there is any voltage/current and the ntry again with the alternator disconnected and see what happens.
Not quite. Unless you are building a historic vehicle with positive earth, unlikley since you mention alternator, you will find that the neg of the battery is the earth so measuring voltage across those two points should be more or less 0 V and tells you nothing.

You want to put black probe on the battery neg (labelled - ) and red probe on the battery positive (labelled +).

thanks for the help



Here's the plan.

1. disconnect both leads to battery.
2. set DVM on 20 Volt range, measure voltage across battery terminals. (as described above)
3. hook up battery charger - red lead to positive, black to negative.
4. leave on charge for a couple of hours
5. remove charger leads and repeat 2.
6. leave battery alone for a couple of hours and repeat 2.

battery voltage for a charged battery is hitting 13 V.

you may find that a couple of hours after disconnecting the charger, the battery voltage can drop a bit but I wouldn't expect that to be more than 1 V. i.e. it still should read more than 12 V.

In the morning reconnect the battery back into the car and try your fan test. If the same result, your battery is a gonna.

If not, it will appear alternator or something else is draining the battery overnight.

[Edited on 7/8/08 by 02GF74]

Macbeast - 7/8/08 at 10:21 AM

quote:

put it on amps and across the terminals it read 1


NOOOOOOOO !!!!!!!!!!


Do this and you will get the full current that the battery can put out (200A ?)going through your meter and you will be able to find your needle in the next county.

Unless, of course, you are using a proper battery-testing rig which is designed to do the job ?


Ermmm - yes wot he said ^^^^^

[Edited on 7/8/08 by Macbeast]

jabbahutt - 7/8/08 at 12:47 PM

so it looks like I can add not knowing how to use a multimeter to my growing lists of useless skills

I'll try these out tonight as the battery has been on charge since last night with the car +'ve & -'ve leads disconnected.

fingers crossed.

Thanks for all the help
Nigel (electrical fool)

BenB - 7/8/08 at 01:05 PM

Usually a multimeter with an amp setting will have a seperate terminal for the plug when used in amp mode.
As said, amps in series, volts in parallel (usually anyway....)....

britishtrident - 7/8/08 at 05:53 PM

Multimeters usually also have an internal 10amp fuse wired in series with the current socket.

David Jenkins - 7/8/08 at 06:04 PM

If it's disconnected from the car, has a reasonable voltage after charging, and doesn't lose much voltage over a period (e.g. 12 hours) then it's probably worth taking it to a local motor factors so that they can test it for you (most will, in the hope that you'll buy one from them if it's knackered). However, if you've lost a lot of volts after 12 hours it's knackered.

The standard tester works by measuring the voltage when a very high current is being taken - the voltage should drop, but not by too much - the dial should have a green area for a good result, and a red bit to show when it's knackered.