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Battery/Starter/Alternator Testing Procedures

 
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Old 09-24-2010, 04:15 PM
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Battery/Starter/Alternator Testing Procedures

Took this down awhile ago, forgot to put it back up.




A fully charged battery is 12.6v
A 75% charged battery is 12.4v
A 50% charged battery is 12.2v
A 25% charged battery is 12v
A dead battery is 11.9v or less


To properly check the battery on the car, you need to remove the surface charge. If the car has ran recently, or has been recently charged, it will have a surface charge. The easiest way to remove this and accuratly test the battery is to turn on the headlights for about 2 minutes.

How to do a battery test:

A proper battery test has 3 components. Unless you have a vat machine, its very likely you cannot do all of them, but you can improvise to some extent. First and foremost, just because the battery says it has 12.6v, does not necessarily mean the battery is good. The battery could still be sulfated, which means the plates in the battery have transformed from sponge lead and lead dioxide to lead sulfate. This process can be somewhat reversed by charging, but its degenerative. Thats why you should never run the battery completely dead, then charge it, and so on. Automotive batteries are not meant to be deep cycled like this and it will kill them. However, its unlikely you will encounter this on a car, as long as your alternator is running properly. Anywho, a sulfated battery is basically a dead battery, and they typically read good, but they won't hold a charge and it can be dangerous to try and charge them excessively, as they can heat up fast.

That said, onto testing.

Before we even go onto the actual testing, do a visual inspection. This is important and often overlooked. Make sure your posts are clean (corrosion on terminals usually indicative of a sulfated battery) make your your connections are clean and tight and your cables are solid and not rusted or frayed. Also make sure your battery is clean. A dirty battery can cause voltage to travel across the surface of the battery from one post to another, which will slowly discharge it. Also make sure your alternator belt is on and the alternator output wire (covered later) is hooked up.

Step 1: State of charge testing.

Pretty self explanatory. You're just checking to see how many volts the battery has. Remove the surface charge using the above method then hook up your DMM to the posts. 12.6v is a good battery. You need a minimum of 12.4v to proceed with testing, if you don't, then charge it. *Note that you should always slow charge a battery on the lowest amp setting possible, although for the first 10-15 minutes you can have it on a higher setting, for it to get an initial charge. Always slow charge to avoid overheating. It can warp the plates inside and do damage to the battery, not to mention cause it to blow up. Google batteries blowing up, watch some youtube videos of it. Its pretty intense. **Also note that premium cause of shortened battery life is overcharging, never charge at a higher rate than 15.5v. ***Also, those charge indicator "eyes" are worthless. Don't even look at them. ****Also note that CA (cranking amps) is basically a worthless rating. CCA (cold cranking amps) is actually important. Get the battery with CCA that your car recomends. Without enough CCA, the battery will have a hard time powering the starter.

Step 2: Capacity (load) test.

This is one that pretty much requires a vat machine. Basically you need an amp clamp, and you need to hook it around either machine lead. Then apply 1/2 the CCA rating for 15 seconds. Needs a minimum of 9.6v to pass.
Needless to say, you prolly can't do this at home. Best bet for this, get in the car, and with the engine off turn on the headlights, radio, rear defrosters, a/c, every electrical component you got. Then place your DMM meter leads on the battery and see if its got over 9.6v. If not, the battery can hold a charge, just not under a load. Which is bad. And you need a new battery.

Step 3: 3min. Charge test.

This is to test if the battery is sulfated. Like I said before, a sulfated battery can pass the first two tests, but fail this one. A sulfated battery can be like a snake in the grass. You don't know its a problem til you step on it. But its still a ****ing snake, and it'll bite you.
Hook up a battery charger to your battery. If you have a maintenance free battery (you do, unless your like 90 and still driving a 50's F-150) then you charge that sucker at 20-25amps for 3min. while observing the voltage. (because while your charging, your DMM is hooked up to the terminals and your watching it, right? No? Then do it, do it now.) If at anytime in that three minutes the voltage exceeds 15.5 volts then your battery is sulfated and needs to be replaced. If it stays under 15.5, then charge it at a lower rate and retest. Or just charge it at a lower rate and throw it back in the car, cuz a good trickle charge is prolly all it needs. *If the battery exceeds 15.5v, stop the test. Even if its only 5 seconds in. You risk overheating and explosion. A sulfated battery will heat up, very very fast. And either pop or explode. Neither is very good.


Starter Testing

This one doesn't involve a vat machine, a DMM will do.
Some interesting cliff notes for you...
To help diagnose the starter, crank the engine with headlights on and observe headlights....
headlights go out = poor battery connection
headlights dim = discharged battery (low volts) or engine mechanical issue
headlights brightly lit (more than normal, like really bright) = open circuit.

***Also, if your headlights ever flicker really fast, you have a short and the circuit breaker is opening and closing the circuit to protect it.

****Also, if your starter is cranking really slow, its probably a discharged battery. If the starter whines, its either improper starter to flywheel clearance or not enough battery power to even turn the flywheel. like at all.

Starter Testing Step 1: Starter Draw test

First you gotta disable the fuel. This prevents flooding as everytime you crank the engine the injectors will squirt fuel in, and if you crank it enough...
To disable the fuel on a 5th gen, pull the fuel pump fuse. Its down behind the drivers side kick panel, located directly left and slightly in front of your clutch pedal. A knob unlocks it. The fuel pump fuse is a light blue 15a fuse located near the top. Pull it. Fuel disabled
Next you gotta attach an amp clamp around all battery positive cables, with the arrow on the clamp facing away from the battery. Don't have an amp clamp? That sucks. Place meter leads on posts. Crank engine for 15 seconds MAX. Doesn't have to be 15 seconds. I don't recommend it. Just enough to get a reading. While its cranking you need a minimum recorded voltage of 9.6v. Your amperage needs to be at car specs. I dunno the specs for preludes, I don't have mitchell or alldata at my disposal at the moment.

Step 2: Insulated voltage drop test (positive)

Hook up your DMM from battery positive to the "M" terminal. The terminal is one of the bolts on the starter, I think it has a wire attached to it, maybe not. There's like three bolts on the starter solonoid. How to tell which is "M"? Well when you have your one lead touching battery positive and touch the "M" terminal, you should be reading potential on your meter, that is 12.6 volts. Now with the meter leads still on it, crank the engine for 15 seconds MAX and record results. MAX reading should be .6v (vd) (you need two people to do this, btw)

Step 3: Ground voltage drop (negative)

Place on DMM lead on battery negative, the other on the starter case. Needs good metal contact, scratch it with your DMM lead if you must. Crank engine for 15 seconds MAX and record. Max voltage should be .3v (vd)

If your numbers are higher than MIN (first test) and lower than MAX (second/third) then your starter is just peachy. And hopefully not fuzzy.


Alternator Testing


STEP 1: Alternator output test.

Engine needs to not be disabled, so put the fuel pump fuse back in.
Hook up meter leads to battery posts, with an amp clamp around the alternator output wire. There's really only one wire coming from the alternator, its big and goes to your battery positive terminal. Thats the ticket.
Run engine at 2000rpm and record voltage. Should be between 13.5-14.5v
(VAT MACHINE ONLY PART: maintain 2k rpm and apply load to bring volts down to 12.5, record highest amps. This will give you max output. See specs. Needs to be within 10% of spec. *Note, do this quick, or you'll burn up your alternator)

STEP 2: Insulated VD (voltage drop)

DMM from battery positive to alternator output terminal (bolt where alternator output wire is hooked to) and run at 2k rpm with the HEADLIGHTS ON. Max voltage drop should be .5v

STEP 3: Ground VD

DMM from battery negative to alternator case, once again, needs solid metal contact. Run engine at 2000rpm with headlights friggin on dammit, Max voltage drop should be .2v





And thats just about it for Battery, Starter and Alternator testing.
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Battery, Starter and Alternator Testing Procedures
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Old 09-24-2010, 04:16 PM
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Re: Battery/Starter/Alternator Testing Procedures

If you have question, ask away.
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Battery, Starter and Alternator Testing Procedures
Problems starting engine? | 5th gen resource

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Last edited by havikprelude; 09-24-2010 at 04:19 PM.
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Old 05-07-2015, 12:14 PM
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Re: Battery/Starter/Alternator Testing Procedures

Great write up! I'm sure it'll help the few who put in the effort to RESEARCH.
Quick question I'm trying to test my starter and when I hook up the battery to the starter and cross the bolts on the alternator, the Bendix doesn't push out. It will spin but that's about it.
I've brought her to a shop and they said it was my starter. Without even looking at it. Do I took it out and brought it to auto zone, they said it was fine.
I'm going to try some of the tests your talking about but currently I only have a copy meter -_-.
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