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Testing battery under load

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bumblebee
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PostPosted: Wed Sep 11, 2019 5:58 pm    Post subject: Testing battery under load Reply to specific post Reply with quote

If I check the voltage on my GM Pickup with a 5.3 engine at the same time that I am cranking it, what should the voltage be? How low can the voltage get before the battery is probably needing to be replaced. This is on a 2017 Silverado. Thanks.
 
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SVcummins
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PostPosted: Wed Sep 11, 2019 6:05 pm    Post subject: Re: Testing battery under load Reply to specific post Reply with quote

12 volts
 
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55 50 Ron
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PostPosted: Wed Sep 11, 2019 6:09 pm    Post subject: Re: Testing battery under load Reply to specific post Reply with quote

I've never made such a measurement, but would think it could have a fairly wide variance depending on several factors such as condition of alternator, corrosion of battery cables and posts, the quality of your voltmeter, weather conditions, condition of starter, etc. (whatever else the gurus can think of)
 
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Janicholson
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PostPosted: Wed Sep 11, 2019 6:10 pm    Post subject: Re: Testing battery under load Reply to specific post Reply with quote

VS is correct, a fully charged good battery should crank at or above 12v. Its rest voltage, fully charged should be 12.6. Jim
 
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Bob
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PostPosted: Wed Sep 11, 2019 6:18 pm    Post subject: Re: Testing battery under load Reply to specific post Reply with quote

CVPost-bumblebee wrote:
(quoted from post at 20:58:23 09/11/19) If I check the voltage on my GM Pickup with a 5.3 engine at the same time that I am cranking it, what should the voltage be? How low can the voltage get before the battery is probably needing to be replaced. This is on a 2017 Silverado. Thanks.


The "old standard" was 9.5 Volts, NOT sure what's needed to keep the 'puter happy in a modern vehicle during cranking.

But you'd have to be awfully quick to measure the cranking voltage on a vehicle that new, your truck should crank and start in 3 or 4 seconds.

SHORT version is if you are questioning this, you probably need a new battery.

New batteries aren't cheap, but neither is getting a tow or a jumpstart, or having to replace a starter wounded by piddling around with a weak battery
 
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john in la
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PostPosted: Wed Sep 11, 2019 6:31 pm    Post subject: Re: Testing battery under load Reply to specific post Reply with quote

Under a 30 second load test a battery should show at least 9.5 volts.
 
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jeffcat
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PostPosted: Wed Sep 11, 2019 7:17 pm    Post subject: Re: Testing battery under load Reply to specific post Reply with quote

Make it simple.....stop at your local auto parts store. They should be able to test your battery in a couple of minutes. You might want to go on Ebay and buy one of these. They can anilize your whole electrical system in a couple of .minutes. Cold crank, charge, ripple if your altinator is going bad, detect bad cell, etc. Have one and they are handy and nifty.

 
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Geo-TH,In
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PostPosted: Thu Sep 12, 2019 2:37 am    Post subject: Re: Testing battery under load Reply to specific post Reply with quote

But you'd have to be awfully quick to measure the cranking voltage on a
vehicle that new, your truck should crank and start in 3 or 4 seconds.

And good luck measuring the voltage with a digital voltmeter.
 
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Geo-TH,In
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PostPosted: Thu Sep 12, 2019 2:54 am    Post subject: Re: Testing battery under load Reply to specific post Reply with quote

There is no one test that will tell you the true condition of any battery.

Don't think there is a test for self discharge rate.

The voltage test only tells you condition at that time. So voltage will be higher is you have driven truck and battery was charged while driving. A voltage test works best if battery has not been used for days.

It would be best to use an analog meter to measure cranking voltage. Digitals will jump all over the place. Not really a good test if you are only cranking for a second or two. So carbon load testing for 30 seconds is better than a few seconds of cranking.

Hydrometers can't be used on AGM's.

I like digital conductivity testers. However no tester will tell you everything.

I like to replace the battery in the vehicles that I take of vacation before the battery leaves me stranded. That battery will be used in a tractor or dump trailer.

So if you think you are having battery issues, replace it and reuse it.
 
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ASEguy
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PostPosted: Thu Sep 12, 2019 3:13 am    Post subject: Re: Testing battery under load Reply to specific post Reply with quote

I was taught 10.2 volts so there is enough voltage left over so the coil doesn't starve and 9.6 for a battery load test. Remember that the voltage readings assume bright and tight connections. I did a no crank last weekend on my farmer friends tractor, a 4600 Ford. He claimed the battery was charged for a long time. It was. It had 12.78 volts when I got there but virtually nothing at the cable. I ended up replacing the positive cable end, and for good measure, I cleaned both battery posts. I also don't trust the auto parts store battery testers. I've seen them pass too many 6 year old batteries.
 
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JDEM
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PostPosted: Thu Sep 12, 2019 4:37 am    Post subject: Re: Testing battery under load Reply to specific post Reply with quote

Minimum cranking voltage is 9 volts. That is the low threshold the starter is designed to work at.
 
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JDEM
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PostPosted: Thu Sep 12, 2019 4:44 am    Post subject: Re: Testing battery under load Reply to specific post Reply with quote

When a battery is rated in CCA (cold cranking amps) the test is at 7.2 volts. Starter on your truck as well as the ignition system is designed to work as designed at a minimum of 9 volts. You are getting some nonsense answers on this thread.

If you have, let's say a battery rated for 700 CCA, then a proper load test is done at room temperature at 350 amps and voltage should not drop below 9 volts.

Your starter likely cannot draw more then 175 amps so it does not load the battery very much. That is part of the design since the battery loses half its power in extreme cold and is still supposed to be adequate.
 
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MARK ROBKE
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PostPosted: Thu Sep 12, 2019 4:56 am    Post subject: Re: Testing battery under load Reply to specific post Reply with quote

I am a retired truck mechanic. When I used to check a truck for cranking issues, I would disable it not to start, and would crank it over for maybe 15-20 seconds. Usually the voltage was above 11.5v, but it take several seconds of cranking for it to level out to a steady reading. After cranking like this, feel all the connections-any heat/warmth is a bad connection/resistance. Mark.
 
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John T
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PostPosted: Thu Sep 12, 2019 5:24 am    Post subject: Re: Testing battery under load Reply to specific post Reply with quote

Bumble, actually the battery voltage on YOUR truck when cranking depends on several factors including but not limited to:

Cranking time
Temperature
Condition, State of Charge, Reserve and CCA capacity of your battery.
Obviously a big honking 1000 CCA battery is going to maintain more voltage for longer time then a small 500 CCA
Compression
Oil Viscosity
Battery Cables and connections and resistances
Condition of starter motor

That being said, there remain certain baseline and minimum voltages for adequate starter motor performance MATCHING TO YOUR TRUCK AND ENGINE AND OTHER CONDITIONS. But if voltage drops down to the 9/10 Volt range (subject to ALL the conditions above) I would be looking at the state of charge, reserve capacity, and CCA of your battery. IE HAVE THE BATTERY TESTED, if its good maybe check the starter and cables and connections...??.

Sooooooooooo theres no one size fits all exact answer for YOUR particular truck and battery (many variables affect battery voltage under load) but still baseline figures and minimums I liked to see were voltage not dropping too much under 10 volts.

A good battery load tester along with a volt meter and a hydrometer are the tools I use in my arsenal to test flooded lead acid batteries and when I was a used tractor dealer and farmer believe me I had to test hundreds of them grrrrrrrrrrrrrr

Many shops will test your battery including a load test for free

NOTE even with a good battery a faulty starter motor can draw excess current and be a problem you know !!!!

Hope this helps

John T
 
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JDEM
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PostPosted: Thu Sep 12, 2019 5:35 am    Post subject: Re: Testing battery under load Reply to specific post Reply with quote

Here are the specs from the GM factory tech manual. I tend to take the word of the GM manual over some of the other things I hear or read

Battery 770 CCA
Load test at 70 degrees F or higher, 9.6 volts at 300 amp load
Load test at 0 degrees F - 9.1 volts at 300 amp load
Load test at 0 F - 8.5 volts at 300 amp load
Load test at 10 below 0 F - 8 volts at 300 amp load

Starter - 1400 watt, 155 amps draw at 9 volts
 
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