Thanks Steve, a great and detailed overview of the battery test.
A couple of things ... first from your example
Would for a 3.3amp output from the alternator, not render 373mv of ripple as a fail, or at least and bright shade of yellow?
Second, from the initial voltage and output of alternator (and load between) could a traffic light be added to current output? (In this example with a 12.5v starting voltage, clearly alternator is not needing output much, but other times a simple formula(yes, with compromises!) might via colour highlight a suspect issue.
Providing a zoom into the waveform for advanced user interpretation
Icing time, for the ripple, allow us to see the A/C capture you took (assuming you do switch the scope to A/C mode from DC during the test?
You did not for your example use an extended test, and that's where I have a few extra issues, or requests and I'm sure I've written more detailed requests, but from memory, here are the highlights ...
1. Traffic lights for client visibility - they like colour! The extended have no traffic lights at the mo
2. More the extended results to be with the Starter not at end
3. add option for alternator extended (and show results under that heading if selected)
4. Show peak figures, but also the mid cranking averages, as that's where the actual VD is more qualifyable as being good or bad, so rather than at 500+amps, the figures from typically 150-250 amps of the post initial inertia inrush ? Both are valid for different reasons, so putting both on the report (2 columns - Peak and "normalised") the resistance figures need only 1 set, but voltage at starter, amps, and pos/neg volt drops need 2 sets please (IMHO)
5. From memory I think the starter resistance remains the same on the standard and exteneded tests, which is wrong, as the cable/ground paths can be removed. on the standard test, maybe stating "Starter and wiring" or avoid confusion?
Anyway, thanks for the explanation, and look forward to hearing if these and some of the historic feature requests might make a future edition of the PicoDiagnostics software.
I know the battery in my Van is duff and I need a new one. Have had to jump it with a boost pack every morning for the last 10 days or so.
Saturday evening I took the Negative Lead of the battery and substituted my Jump pack for the Battery. Sunday morning it burst into life, Monday Morning I could have driven it to the corner shop in first gear on the starter with no fuel whilst scoping 4 things of your choice.
Steve, i have a question regarding the calculation of the internal resistance of a battery.
You state to calculate it from VD and peak in-rush current. This does not consider internal starter resistance nor the wiring resistance. Are these judged to be negligible even with such low resistance levels? Thanks for replying.
Internal battery resistance is indeed calculated using the volt drop value acquired during cranking divided by the inrush current
Given the current flow through a series circuit is the same throughout:
Battery + > Positive lead > Starter motor > Ground lead > Battery - where we choose to measure the voltage drop will allow for a resistance calculation across that component.
In our Pico Diagnostics Battery, Starting and Charging system test, Channel A of the scope is connected across the battery and captures the voltage drop at this point within the circuit, so deriving only battery internal resistance.
The resistance of the circuit, connections, contacts and starter motor assembly, whilst present, are excluded from the calculation of battery resistance
In order to calculate the resistance at various points around the starter motor circuit (and ultimately total circuit resistance) we need to carry out an Extended Drop Test (See the image below)
Image 1
Here we can accurately calculate the resistance of the cabling using channels C and D of PicoScope
As you mentioned, the resistance of these cables is tiny and it reminded me of this experiment here viewtopic.php?p=99455#p99455 where we conclude that the tiny resistance values of such cabling (without faults) proved challenging when trying to measure parasitic drain
Meu ponto de vista que a bateria é um componente químico e estar sujeito a variantes diferentes, precisamos ter em mente a data de fabricação para junto ao teste imaginar algumas possibilidades. Eu uso outro equipamento e nele me mostra a Resistencia, corrente do motor de partida baseado no CCA da bateria. Um teste muito eficaz !
Muito bom esta tela do Pico testando a Bateria !!!
thank you for answering my question. Now that I see how the internal resistance of the battery is calculated by voltage drop and current flow this raises my next question: How then is the starter motor's resistance determined as there is no voltage drop measured across him during the test?
Thank you for the feedback and sorry for the delayed reply
With reference to the image in this forum post viewtopic.php?p=77701#p77701 we have an explanation of how the Test results from the standard PD battery test are derived
Putting the explanation to the test (using image 1 from our October 26th post) we can calculate starter motor resistance as follows:
Assuming we are not carrying out the extended drop test:
Starter motor resistance = Lowest Voltage 9.82 V / Maximum current 399.2 A = 0.0245 Ω (24.6 mΩ)
You are correct, “how can we truly measure starter motor resistance without measuring across the starter motor?”
The answer is, we can’t! (Or can we if we use maths?)
The value above should be referred to as “the total remaining resistance of the entire cranking circuit” (when not using the extended drop test method) of which the majority of the resistance will be the starter motor
This of course assumes there are no cranking lead/grounding connection or cabling errors!
Putting this assumption to the test and once again referring to the extended volt “Drop Test” results above..
Starter motor resistance equals:
Total remaining resistance of the entire cranking circuit – (Resistance positive line + Resistance negative line)
24.6 mΩ - (0.787 mΩ + 1.8 mΩ)
24.6 mΩ - 2.587 mΩ = 22.013 mΩ
Starter motor actual resistance = 22.013 mΩ
Starter motor circuit resistance (excluding starter motor) = 2.587 mΩ
Total remaining resistance of the entire cranking circuit using extended volt drop test = 22.013 mΩ + 2.587 mΩ = 24 .6 mΩ
I hope this helps clarify how we calculate starter motor resistance and the most accurate way is to use the extended volt drop test
In the scenarios presented above both methods have delivered accurate results