Minus batt

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Minus batt

Postby RS-4 » Sat Mar 19, 2016 3:09 pm

Minus sa anlasera na bakteriju.jpg
IMAG3401.jpg


The voltage difference between the engine block and battery minus is 160 mV but all consumers is ON and it is about 110A ... How much is the maximum voltage difference when he spends so much ampers?

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Re: Minus batt

Postby FioranoCars » Sun Mar 20, 2016 6:54 pm

Sorry, not clear about a couple of things ...
1. where do you have your probes on the DMM connected (several leads and not clear which is which)
2. Is the engine running?
3. what are the other probes going to?
4. don't understand the "spends amps" .. unless engine running with alternator output or starter engage during crank, please explain?
5. Can you clamp the negative battery terminal with amp measurement?

but if engine running then check alternator output (can measure at negative battery lead {as this is your battery reference for volt drop} or even the earth strap if only one), so Alt output of say as little as 20amps could cause this size volt drop if a bad earth strap or bad battery strap.

If engine not running then knowing the amps or what else would be critical, more info would be needed.

If you have a starter cranking at 180amps then a volt drop on negative of 250mv would be as much as I'd like to see, less much less ideally.

While an Alternator outputting 40amps should be circa 100mv negative volt drop. yours is more, try cleaning the earth straps and earth cable (visual inspection of the connectors and cable) etc.

Try a thick high quality "jump lead" from engine to battery terminal and see the bias of amps on that verse the existing earth path/earth cable, that will tell you a lot (if the earth straps are hard to reach, it's a quick and easy way to prove the issue).

HTH
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Re: Minus batt

Postby STC » Mon Mar 21, 2016 4:54 am

RS4

I agree with Richard in that MINUS 160mV is probably not excessive. It is MINUS because you have the Red Lead on the Battery Negative.

The Fluke 88 a great Multimeter, has few advantages over the Picoscope. One being that the Pico cannot directly measure and confirm 60 ohms across a High Speed pair of CAN wires.

You don't say if you a have a problem with this car, or what the problem is. Perhaps you are just playing and experimenting. Kool and the Gang !

I would like to see the same test conducted with a scope,
Ch 1. One Lead on Batt - the other on Engine Block.
Ch 2 Amp Clamp on Batt Neg. All consumers ON, from Ign Off, Ign ON, Cranking, Idle and fast Idle (1500rpm) then Switched Off.
Ch 3. Across the battery, monitoring Battery Voltage.
Ch 4 on CKP as a Checksum. A Massive amount of facts can be gained there

Your Question
How much is the maximum voltage difference when he spends so much ampers?

Can we first see, through the eyes of the scope, what drop you have before we pass judgement ??
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Re: Minus batt

Postby FioranoCars » Mon Mar 21, 2016 11:32 am

Sorry, STC
I agree with Richard in that MINUS 160mV is probably not excessive

Not quite what I said. BUT I missed the 110amps comment first time (Am I going blind too :!: :oops: or was that an edit :?: )

To give any answer then we'd Need to know :
- if the engine is running or not
- if the 110amps is alternator output or actual load (so precisely where the amp clamp is)
- what's battery voltage (charge)? (affect alternator output)
- What fault symptoms are you trying to diagnose, might help understand why you did this test (engine to battery negative), which is a very valid test, but not for an "engine not running" scenario!

If it's purely user created load and engine is not running, why measure voltdrop (VD) to engine, should be chassis?

110amps on the earth lead of the battery, giving 160mv drop, I think is high, but a weedy cable and it might be acceptable. What is the normal load the cable is supposed to handle, that would tell you if someone has swapped to an inferior cable ?

Yes "Engine Running", then we're looking at alternator output, so the volt drop will be reversed, from say a cranking capture. VD goes both ways, depending on what's consuming or generating!

I could create 40 or more scenario's from the description, so it's pointless to guess.

Setting up a scope I'd do this:
ChA Battery + and earth to - as our base line for all VoltDrop
ChB Amp clamp to BATTERY earth lead (will show what the battery is doing) (clamping all leads to Positive will show the NETT replenishment of the battery - even if the Alternator lead is shared)
- Test 2 - Move to Alternator Lead (if a separate cable on the battery, there would be good)
- Test 3 - Move to Earth Strap of Engine (each in turn if more than 1)
ChC Chassis (near the earth strap but not on it)
ChD Engine Block

The Total Volt drop for an Alfa 147 during cranking (so engine strap and weedy battery lead, with identified need for a clean of all terminals/bolts) at 110amps was 180mv negative, but only 120mv Positive (hence clear need for clean and recheck), negative reduced to about 100mv, could have benefited from new engine earth strap, but wife's car, so we'll monitor!

If this was just the battery lead then definitely would clean terminals / connections and if no improvement replace the lead, but I'm fussy about volt drop!

What you can calculate from amps and Volt drop is RESISTANCE (Ohm's law) as a maths channel, that will tell you a huge amount, rather than guess as to good/bad...

That's all a bit excessive though, this can be done on a DMM far quicker, but not as much fun I agree!! :lol: :lol:

I don't have a graph/chart/table of Amps to volt drop, but this topic sparks the ideas so I might create one ... just so our tech's can have a guide as to good, questionable, excessive per range of amp loads. :idea:

While my proposed "table" will be "IMHO", I'll consider sharing it and getting comments from you guys, as the more robust it is the better. Don't hold your breath, but any upfront ideas welcome. See a draft at the end of the message (it's off the top of my head, so don't shoot, but please comment!) ... cable length/diameter clearly are a factor, as are the number of joints ... otherwise we could just use a default wire resistance table ...

In fact no draft, I'll think some more, but any comments on the idea welcome! While some might say it's pointless, this thread illustrates the broad range of interpretation possible, and need for advice/comparison. :D

HTH a little bit?!

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Re: Minus batt

Postby RS-4 » Mon Mar 21, 2016 4:53 pm

Plus batt - alternator B+.jpg
plus batt
130A Bosch.psdata
PS data
(9.28 MiB) Downloaded 22 times







Thank you all for your answers. I'm glad that I read the answers of experts. I apologize because my English is not good.

To answer these few questions:


1. First DMM probe is connect to minus battery and engine block and get 160mV, second on battery plus and alternator B+ and get 130mV... with this test I wanted to check the status of the main power cables.

2. During the test the engine at idle and all consumers are involved (see PS data)


Customer complains that he had three times to change the alternator for 120000 km.

I noticed that heats minus terminals and because of this I started to measure.


After Easter, he will come to fix heating minus cable and then I will do more measurements.


Regards
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Minus batt - engine block.jpg
minus batt
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Re: Minus batt

Postby busjockey » Wed Mar 30, 2016 4:57 pm

What are the conditions for your test? Are you at idle no loads on or at 2500 rpm with loads on? Or are you using a carbon pile to load the system? 12.64 Volts at the battery isn't going to cut the mustard on this. Did you load test the battery first before beginning any charge testing? Did you check the recharge current of the battery? Here is a pic i did for you. :)
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