Battery tender for agm battery via jump posts advised?

[omodos]

Originally Posted by andrewwynn

I plan to get the reference measurement of voltage drop during start which will make is very easy to measure your starter current draw. My wife will go a week driving less then a mile twice a day with no problem of the battery getting low. Not diesel but when temp not low you should not need to use a battery tender to get a reliable start. You need to determine your start current so you won't kill a brand new battery

@Andrewwynn and wpoll,hello gents, right got round to probing, one probe into the car lighter socket and the other probe went to B+ in engine bay, I set multimeter set to 20V (ps when probe was dead center in the lighter socket got 0v then when touching inner side of lighter socket got the V of the battery at rest)


Started engine and the volts dropped to 8.98V(used phone to record it) please advise how I can ascertain what the starter draw is via this?maths/physics i was pants at in school...

[wpoll]

Originally Posted by omodos

@Andrewwynn and wpoll,hello gents, right got round to probing, one probe into the car lighter socket and the other probe went to B+ in engine bay, I set multimeter set to 20V (ps when probe was dead center in the lighter socket got 0v then when touching inner side of lighter socket got the V of the battery at rest)


Started engine and the volts dropped to 8.98V(used phone to record it) please advise how I can ascertain what the starter draw is via this?maths/physics i was pants at in school...

I think you've misunderstood what you are testing here... you are trying to measure the voltage drop in the main cable due to the starter current. Once you know this voltage drop, you can calculate the starter current draw (using ohms law).

This voltage should be less than 1v but may be a bit more in your case.

To measure this, put the black lead from the DVM on the B+ terminal in the engine bay.

Put the red lead from the DVM on the centre contact in the cigarette lighter socket in the dash.

You should see a voltage very close to zero.

Now start the car and note the voltage that appears during cranking. It could be between 0.5 and 1.5v so set your meter to a low voltage range before testing.

[omodos]

Originally Posted by wpoll

I think you've misunderstood what you are testing here... you are trying to measure the voltage drop in the main cable due to the starter current. Once you know this voltage drop, you can calculate the starter current draw (using ohms law).

This voltage should be less than 1v but may be a bit more in your case.

To measure this, put the black lead from the DVM on the B+ terminal in the engine bay.

Put the red lead from the DVM on the centre contact in the cigarette lighter socket in the dash.

You should see a voltage very close to zero.

Now start the car and note the voltage that appears during cranking. It could be between 0.5 and 1.5v so set your meter to a low voltage range before testing.

Morning I must be doing something wrong or my multimeter ain't up to it or both, this a.m before work I connected as per your notes, and indeed was showing 0V before crank. However only change I got on crank was the display showing 0.01V can't be right (I know the connection is good as when I probe the inner sides of the lighter socket I got the voltage of the battery) I also tried it another 3 times in pretty quick succession and got no reading...???


I will give it another shot but note, same thing as last time, after repeated successive starts or start attempts the starter then sounds much much quicker.

[andrewwynn]

Repeat starts will take much less current.

You measure from the B+ to the center bottom not side of cig lighter. Also make sure you have power on the socket or it won't work.

You want the positive lead to be in the cig socket or the reading will be negative. If you have meter on auto range mode It won't work as it will take too long to make the measurement.

The expected result will be about 1 to 1.5v.

[wpoll]

I suspect the DMM is too slow to display the reading - it all happens pretty quickly. I know even my Fluke (with no peak hold) struggles to display the voltage variation in a way I can read - I can see the "spike" on the bar graph but the numbers aren't readable.

I still have to dig out the 'scope meter and get a reading myself but I haven't even driven the X5 in over 10 days - been riding the BMW bike to work instead.

[omodos]

Originally Posted by andrewwynn

Repeat starts will take much less current.

You measure from the B+ to the center bottom not side of cig lighter. Also make sure you have power on the socket or it won't work.

You want the positive lead to be in the cig socket or the reading will be negative. If you have meter on auto range mode It won't work as it will take too long to make the measurement.

The expected result will be about 1 to 1.5v.

thanks for feedback, there is power to the socket for sure cos i can use tyre inflator contraption using it, my meter is crappy that's probably why readings are pants, pretty sure i had it connected up ok this a.m....gonna reply to Wpoll now

[omodos]

Originally Posted by wpoll

I suspect the DMM is too slow to display the reading - it all happens pretty quickly. I know even my Fluke (with no peak hold) struggles to display the voltage variation in a way I can read - I can see the "spike" on the bar graph but the numbers aren't readable.

I still have to dig out the 'scope meter and get a reading myself but I haven't even driven the X5 in over 10 days - been riding the BMW bike to work instead.

Yup temps out here are mild 20C-25C, we are heading rapidly towards desertification, right yup probably my cheapo DVM I got via secret santa ain't up to speed....am seriously considering getting one of the cheaper starters installed rather than spend 2x the amount on another denso and see how it goes...IF I see a noticed starter speed increase result...if not am stuffed would have wasted hmmmm upwards of about 200euros

Already changed battery but that was 5 years old and maybe a good idea to do so.


https://www.motor-doctor.co.uk/products/7560726-starter

[omodos]

Originally Posted by wpoll

I suspect the DMM is too slow to display the reading - it all happens pretty quickly. I know even my Fluke (with no peak hold) struggles to display the voltage variation in a way I can read - I can see the "spike" on the bar graph but the numbers aren't readable.

I still have to dig out the 'scope meter and get a reading myself but I haven't even driven the X5 in over 10 days - been riding the BMW bike to work instead.

Right, spoke to the 'chief' at a second garage, same garage that swapped out my air filter in 20mins, only said a few words to him: "starting problem every year for last few years at this time of year ", he immediately said starter...sooooo if he is so convinced looks like I will go for a new one waiting for him to get back to me with pricing and seems he is fixated on it being a bosch...(no bad thing but the reseller in cyprus totally fleeces public) anyway will post back with results when i get it swapped....

[wpoll]

It may be that the original unit can be re-built but I guess time and labour costs may preclude that option.

*edit* Depends on the failure mode too...

https://www.youtube.com/watch?v=PdNQuPS0U6w

[andrewwynn]

Looking at the motor it makes perfect sense how the starter current spikes as it wears out: the commutator bars get shorted so more than one field will be energised at once!


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