Hi Guys

I've had a flat battery issue for a while.

It seemed that the battery wasnt getting enough volts (particularly under load) from the alternator. (and had it checked at a garage)

I've changed the alternator, tested the voltage and was pretty pleased with myself as under load it was still showing 12+volts at idle increasing with a few revs. with the engine off there was also showing no battery drain.

Job done i thought and a big [] on me face.

However......

driving for a hour with just the lights on. switched car off, then went to restart 5 mins later and not enough power from the battery to kick the engine into life []....

I've just been chatting to a chap at a radio / alarm install place and he put an amp meter across the battery and it was hardly registering at all, so i've got volts but no / few amps.

I dont think it can be the voltage regulator as isnt it built into the alternator so if i've replaced the alternator i've also replaced the regulator?

Any ideas what can be causing this guys?

Thanks for any help

Aaron

p.s. its a new battery & i've had it tested again just to be sure and its an '86 targa.

 

Well, first, an ampmeter "across" a battery isn't going to work, the mostly likely result is a blown fuse in the ampmeter and thus no reading. The only way to really test your battery is with a load cell that includes an ampmeter.

But were I in your shoes I would first check all battery connections for tightness, maybe even remove, clean and burnish them, especially at the battery posts, and the nearby negative body grounding point.

And...

New batteries DO sometimes come to us with dead cells.

 

What does "12+ volts" mean exactly? If under 13.5v or so at idle, your alternator isn't all that healthy; and I would personally consider anything under 13v marginal.

BTW, how old is the battery? Anything over 2 years and it may be marginal, as well, and not accepting a full charge, especially if the car is not driven regularly.

Edward

 

thanks so far guys.

battery is < 6 months old and i got it re checked about a month ago and apparently all is fine with it. btw what is a load cell test ?

with the car running at idle and no electrical devices on the volt meter shows over 13.5 volts, the above 12+ volts comment was with everything on i.e. full beams, fog lights, heater, wipers etc..and the car at idle.

Will check and clean the battery connections and the neg grouunding point

is there anything else i could look at?

 

Most people forget that "something" has to connect the alternator to the battery,
and that "something" may cause a voltage drop that reduces the voltage at the
battery.

Yes, in some cases a low battery voltage maybe the result of a weak/bad alternator,
but in many cases it's because of a voltage drop from the alternator to the battery:

V(battery) = V(alternator) - V(connections)

V(connections) = I(alternator/loads) X R(connections)

Therefore, bad connections (loose or crimped without solder) may cause large
voltage drops reducing the battery voltage, e.g. alternator to the startor connection.

 

Thanks Loren, I dont seem to be having a Voltage problem just amps which is what is puzzling me.

Could a bad connection between the altenator / battery or alternator / other connection or battery / other connection casue a lack of amps to the battery even though the volts are ok?

 

"Could a bad connection between the altenator / battery or alternator / other connection or battery / other connection casue a lack of amps to the battery even though the volts are ok?"

Amps comes from VOLTS. Without volts NO amps.

I (amps) = V (volts) divided by R (resistance)

The greater the resistance (voltage drops - bad connections) the less the amps will be.

The answer to your question is YES.

 

To expand on Loren's advice, the classic analogy is to a water hose:
amps = current = flow of water thru the hose
volts = elec. potential = water pressure
resistance = [ohms] = say the size of the hose.

so, if the hose is way too small for your job, you will still get pressure, but not enough flow. A partial kink in the hose would be the same. Only a complete kink (infinite resistance) will "stop" the voltage/pressure from being measured, i.e. no current/water flow at all.

That help?

 

"with the car running at idle and no electrical devices on the volt meter shows over 13.5 volts, the above 12+ volts comment was with everything on i.e. full beams, fog lights, heater, wipers etc..and the car at idle."

I'd say the alternator/regulator is no good. Yes, even the new one. Either that or you've left some wire connections loose. Have the system checked by an automotive electrical shop. The stereo guys are good for ..... stereos. If they knew that much about diagnosiing electrical problems, they'd be at the aforementioned electrical shop making big bucks.

Sherwood