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Does anyone have any information on how much current does a car starter motor draw at no load? It's from a BMW E46 .The motor specs:

Power max: 1.4kW Voltage: 12VDC Armature resistance: 0.61 ohm Brand: Bosch Model No.: 1740374

A little history into why I'm trying to find this information.

I'm trying to make a DIY bench power supply to test car and bike motors after rebuilding it. Usually the smaller motors are pretty easy to test as long as their current draw is under 17 Amps.So, a modified ATX supply and Lead acid battery does the job. But testing starter motors is not that easy due to their larger current requirements as the ATX supply can never handle them even at no load. Until recently I used to use a spare car battery to test outside without no loads. But since the climate I'm living is quite hot during summer (like 113F) the battery goes out pretty quickly in a year and half. So, this power supply would be a great replacement for the battery.

So, for this project I used a transformer from an old center tapped 700VA Tripplite UPS. The output wire of the transformer is a 10AWG which means it should be under 35 Amps I guess. So, I connected the transformer via a KBPC3510 bridge rectifier. It can handle 35A at 1000V. So connecting the transformer leads to the rectifier and taking the output across the rectifier + and the center tap through a 15000uF 25V cap I was able to obtain 13.21 VDC. So, I tried connecting the starter motor mentioned above without the solenoid and no load to the setup and after 3 seconds the bridge rectifier gave its magic smoke and went out. The transformer wire was also little warm. I have another transformer from a 2000VA UPS and its massive. I think it can handle 55Amps but again currently I have only a KBPC5010 rectifier and it can take only 50 Amps almost. Currently I can only test motorbike starters almost without any worries.

UPS Transformer Starter Motor

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    \$\begingroup\$ just use an automotive battery and a charger \$\endgroup\$
    – jsotola
    Apr 18 '20 at 0:12
  • \$\begingroup\$ "taking the output across the rectifier + and the center tap through a 15000uF 25V cap I was able to obtain 13.21 VDC" - I suggest not using a capacitor. It increases peak current and puts more stress on the rectifier. A car battery won't deliver 13.2V @ 50A anyway, so the lower voltage without the capacitor should not matter (remember the rms voltage is higher than the average voltage shown on a meter). \$\endgroup\$ Apr 18 '20 at 2:12
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    \$\begingroup\$ A microwave oven transformer with EHT secondary removed and a new LV secondary wound (not many turns needed) may suit well. These have a sagging response to load due to the magnetic shunt so would load down on start current then rise in voltage as load backed off somewhat. The 12 V 1.4 kW suggests about 120 A full load but presumkablyt rater less unloaded once turning. || Rectifiers? -> what Austistic said. \$\endgroup\$
    – Russell McMahon
    Apr 18 '20 at 2:39
  • \$\begingroup\$ @jsotola Well I was using that method. Except as I mentioned above the hot summers kill the battery in a year. Been through like three batteries already, they last only for a year and half mostly. :( \$\endgroup\$ Apr 19 '20 at 11:04
  • \$\begingroup\$ @BruceAbbott I added the filter cap only to smoothen it since I planned to use it also for charging lead acid batteries. The rectifier as per the data sheet should be capable of handling 400Amps in a instantaneous peak and then in normal operation only 50Amps max. \$\endgroup\$ Apr 19 '20 at 11:05
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Car Starter motors on large cars do draw a lot of current even at no load .Old pre internet figure is 70 Amps at 12 VDC revving at 7800 rpm .This is from Joseph LUCAS Prince of darkness .The more modern permanant magnet motors will draw less and rev less under no load .Delco units like the lucas units are series wound and behave pretty much the same .Your bridge rectifier is not man enough to do the job .Your transformer will be OK for intermittant use under no load .We did this for a saw bench with rectifier diode aggregate rated at 400 amps .Remember that the start current of the starter motor is many times the run current at no load .I used lots of 100 Volt TO247 shottkies to rectify the DC .You could parallel your bridges to stop them blowing up at start up .You could also use big stud mount diodes .These series wound motors are not happy at 7800 rpm long term,There are stories about solder in the commutator softening in the heat and the copper bars flying apart that I have not seen .If your transformer setup makes less than 12VDC when running the starter motor then it will not turn as fast making your set up more idiot proof .I would check your Armature resistance figure .It seems far too high ,

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    \$\begingroup\$ "Joe Lucas says 'Don't go out at night'" \$\endgroup\$
    – Russell McMahon
    Apr 18 '20 at 2:36
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    \$\begingroup\$ Why do the British drink beer at room temperature? Because Lucas builds refrigerators too. (I miss my Norton Commando though...) \$\endgroup\$ Apr 18 '20 at 3:22
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    \$\begingroup\$ Many of the older starters need 600A or more - in fact some tractor units were 1000A at 24v. Which is why the battery cables were thicker then than they are now. \$\endgroup\$
    – Solar Mike
    Apr 18 '20 at 5:29
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    \$\begingroup\$ @Autistic I performed a small experiment yesterday. Added a 60Amps gauge and tested it with the bench supply with the new 50Amps rectifier. I never thought the bike starter would draw that much current, it spiked to like 35Amps and then quickly settled to 19-20Amps. Tried with the car starter motor and good lord the current jumped past 60amps and then settled to close to 60amps there for the next 2-3 seconds so I shut it off immediately to prevent damaging the new 50Amps rectifier. The transformer output is 8.1VDC without the cap and with cap 13.1VDC. \$\endgroup\$ Apr 19 '20 at 14:25
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    \$\begingroup\$ @Autistic I do have an old 60s English LUCAS heavy duty car battery charger (the only problem is its 230VAC model and I don't have many 220V outlets here, mostly 115VAC) that has a 100A boost setting, but the person who gave it to me had warned that I never use that function standalone and should only be used in conjunction with a lead acid battery or else I might risk damaging the transformer and the precious selenium rectifier in it. That thing would cost a lot if it got damaged and its very rare to find. \$\endgroup\$ Apr 19 '20 at 14:27

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