DIY bench supply failed when testing starter motors at no load?

Thread Starter

Rahulk70

Joined Dec 16, 2016
521
I used to rebuild starters and alternators for a living and as @Wolframore said, you are looking in the area of 115-200 amps depending on the starter. We had a test bench that we picked up fairly cheap which had a 850CCA deep cycle battery in it and because we were doing alternators as well, it was always charged. If you don' t want to go that way, a battery booster pack would be the best way to go. It charges quickly and can do several cycles of starter tests before needing a recharge. We use them at school for starter rebuilding. Supercaps will not work as they discharge rapidly and need to be backed up by a battery unless you have several supercaps in a bank which can be fairly costly. Good to see people still rebuilding.
Today I ran some tests with the bench supply. This time I added an automotive ammeter with range from -60 to +60A. I was pretty surprised to see just the current drawn by the bike starter motor itself. When powered it spiked to close to 35Amps initially and then quickly settled at 19-20Amps. Next, I tried the Bosch starter from the BMW E46 I mentioned in the original post. The moment I powered it, the ammeter gauge needle jumped past the 60Amp mark and for the next 2 seconds I ran the needle didn't come below 60Amps so I quickly shut it off. No, wonder the initial 35Amp bridge rectifier gave its magic smoke!:D..This time I replaced it with a 50Amp bridge rectifier. I supposedly as per data sheet should be able to handle a peak instantaneous current of 400Amps very briefly. I have already ordered a 150Amp rectifier and its on the way to pair up with another bigger transformer.
 

Wolframore

Joined Jan 21, 2019
2,373
Car jump starters need a car battery in conjunction to boost the current or the jump pack could get damaged. I do have an old massive bench top 1960s Lucas Heavy Duty car battery which has a 100Amp boost function. It was initially used at a industrial facility to charge their Detroit Diesel generator batteries that very very large. The person who gave it to me said that the boost function should never be used alone without a battery to start a car. I don't want to risk damaging the selenium rectifier in it as its very difficult to replace and it cost a lot too.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
521
You are correct @shortbus. If it is a PMGR, they will pull 60-100 amps on a bench. I prefer to test with the solenoid to get the whole picture but to each his own. Either way I think he is going to a battery or battery pack. Power supplies with that much punch arent cheap.
The solenoid is fully functional, its able to make the B+ to starter terminal connection and also make the pinion gear move as expected.. I tested it separately as it draws a sudden current of around 30 Amps and then settles to 17-19Amps. So, I didn't want to overload my supply by adding this too with the motor. I was quite surprised why the solenoid was able to draw such a huge current making me suspect it was faulty until I came across this great article.

Starter solenoid working
 

Tonyr1084

Joined Sep 24, 2015
6,451
I keep a car battery in the garage connected to a car radio. When I'm out there I play the radio. The battery remains on a 13.8 volt float charge at all times. The battery has been sitting there for a few years now. This is Utah and summers have been getting over 100 degrees for more than 10 days a year. Some times close to 20 days. It can get hot here and stay hot. If not in the 100's it's always a high in the 90's. Come fall, around late September it starts cooling off to below 90. Maybe keeping your battery on a constant charge regulated at 13.8 volts will keep it alive for several years. I don't KNOW that to be the case - I just wonder if that's why mine has lasted so long. And on top of that - it's a used battery. Neighbor burned up his van. Gave me the alternator and the battery along with a hand full of relays.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
521
I keep a car battery in the garage connected to a car radio. When I'm out there I play the radio. The battery remains on a 13.8 volt float charge at all times. The battery has been sitting there for a few years now. This is Utah and summers have been getting over 100 degrees for more than 10 days a year. Some times close to 20 days. It can get hot here and stay hot. If not in the 100's it's always a high in the 90's. Come fall, around late September it starts cooling off to below 90. Maybe keeping your battery on a constant charge regulated at 13.8 volts will keep it alive for several years. I don't KNOW that to be the case - I just wonder if that's why mine has lasted so long. And on top of that - it's a used battery. Neighbor burned up his van. Gave me the alternator and the battery along with a hand full of relays.
I have decided to make changes to my current setup until I find or setup a suitable high current bench power supply. I've decided to obtain one or two of those conventional type flooded lead acid car batteries that need to be topped with distilled water. Next I'll get a smaller trickle charger like you (not the heavy duty car battery chargers, as they are harsh on the battery if plugged in continuously). I'll move this setup from my garage to the smaller store inside my house which has cooler temps (like close to 70F) due to air conditioning in summer months. This should help the battery survive the summers and get longer battery life. Also any lost electrolyte can be topped since it is a flooded type battery.

Well you got lucky I guess. You got so many auto-electric stuffs to play around with..:)
I too once got a bunch of auto-electric stuff salvaged from a blown engine E36 318i. Apart from that I do help out close folks with rebuilding motors and electrical/electronic repairs when I'm free.
 

RAMBO999

Joined Feb 26, 2018
256
Hi everyone,

Does anyone have any info on how much current does a car starter motor draw at no load? The motor specs:

Power max: 1.4kW
Voltage: 12VDC
Armature resistance: 0.7 ohm
Brand: Bosch
Model No.: 1740374

The starter was removed from the engine when tested.

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 the starter motor outside the engine after rebuilding or replacing brushes (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 & a Imax peak of about 400A for a very short burst as per spec sheet . 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 I guess since the wires are 6AWG, 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.

Thanks in advance,
R
A good starter motor will normally draw 60 to 150 amps with no load on it, and up to 250 amps under load @ 12 VDC. So whatever you come up with it is going to have to be able take a 3 kW load bare minimum. I would double that. Maybe triple.
 

MisterBill2

Joined Jan 23, 2018
9,778
At one time I was responsible for running cold starting tests on cars with 383 engines and experimental fuel systems. They were run using a battery simulator supply so that the current could be recorded for starting analysis.
The instant current would spike to over 200 amps and then drop to about 120 for the 2 to 3 seconds before the engine caught. But one time during a no-start test the current spiked to about 1200 amps.
The supply was a BIG lab supply and had output cables about #2/0 size. It was powered by a 480 volt 3 phase connection, much bigger than any normal big bench supply. That 1200 amp spike did not trip it's overload system.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
521
A good starter motor will normally draw 60 to 150 amps with no load on it, and up to 250 amps under load @ 12 VDC. So whatever you come up with it is going to have to be able take a 3 kW load bare minimum. I would double that. Maybe triple.
Looks like I might have to add multiple transformers and 150A rectifiers to each and then connect them in parallel. I do have three transformers. Hopefully this will have the juice to power them up.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
521
At one time I was responsible for running cold starting tests on cars with 383 engines and experimental fuel systems. They were run using a battery simulator supply so that the current could be recorded for starting analysis.
The instant current would spike to over 200 amps and then drop to about 120 for the 2 to 3 seconds before the engine caught. But one time during a no-start test the current spiked to about 1200 amps.
The supply was a BIG lab supply and had output cables about #2/0 size. It was powered by a 480 volt 3 phase connection, much bigger than any normal big bench supply. That 1200 amp spike did not trip it's overload system.
Yes, which engines connected these starters pull a lot of current. That sounds like an awesome setup. Were they gasoline or Diesel engines? I know that Diesel engine starters are larger and they pull a lot more current than gasoline starters. Even a regular diesel cars battery is much larger than a similar side gasoline cars lead acid battery.
 

MisterBill2

Joined Jan 23, 2018
9,778
Yes, which engines connected these starters pull a lot of current. That sounds like an awesome setup. Were they gasoline or Diesel engines? I know that Diesel engine starters are larger and they pull a lot more current than gasoline starters. Even a regular diesel cars battery is much larger than a similar side gasoline cars lead acid battery.
These were standard gasoline engines but with experimental fuel systems. Other wise, stock passenger cars. But at -20 degrees they do take more to turn over.

And my suggestion for checking starter motors is both a power supply and a good battery, and a low power float charger to keep the battery close to full charge. That is a lot less expensive than an adequately regulated adequate capacity power supply.
 

be80be

Joined Jul 5, 2008
2,057
Just buy this 39 is the bid right now on ebay


https://www.ebay.com/p/6031170561?i...-ZBqykgiJITxVhrRARGaTjG1tPj_6ZnAaArT5EALw_wcB
Almost made the same as the one I used 37 years ago
buttons and meter are newer looking I couldn't find the one we used used !

We used a booster charger 200 amp starters not tested under load just bump test to see if they engaged and was they binding
if they spin up good and engage was about all you need to no
You could tell if they was binding really easy by how they sounded.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
9,778
Certainly right about the binding of starter rotors. I had two Mopar starters fail that way, the first on a new van with only about 10K miles. The problem on both was a worn out sleeve bearing for the armature. In the first instance the replacement was $4.95 for a new bearing at the dealer parts department, the second time it was $2.49 at an auto parts store. In neither case, 2 different cars, was there ever another starter problem. So it was really a Mopar quality issue.
 
If you think an automobile starter draws lots of power, you'll be surprised what a construction machine like a bulldozer or a motorgrader draws. Even with a 24 volt system.

Because of the outdoor nature of their work, many times their batteries may not have enough cranking power to start the engine during a long weekend. More so during the winter.
Even when using starting fluid and the glow plugs, those large diesel (petrol?) engines require several seconds of cranking before they start.

The problem is compounded because many construction sites are remote.
The only thing that you can usually find, to assist in the starting? And that is commonly used in a construction site? A DC electric welder.
Like others have mentioned before, find yourself a cheap used welder.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
521
If you think an automobile starter draws lots of power, you'll be surprised what a construction machine like a bulldozer or a motorgrader draws. Even with a 24 volt system.

Because of the outdoor nature of their work, many times their batteries may not have enough cranking power to start the engine during a long weekend. More so during the winter.
Even when using starting fluid and the glow plugs, those large diesel (petrol?) engines require several seconds of cranking before they start.

The problem is compounded because many construction sites are remote.
The only thing that you can usually find, to assist in the starting? And that is commonly used in a construction site? A DC electric welder.
Like others have mentioned before, find yourself a cheap used welder.
That kinda reminded me of my Grandad's old Diesel car. He had always kept his old diesel car (kinda looked like similar to those VW beetle shapes) until a few years ago he sold it after he could no longer drive. I remember that car was very difficult to start in cold mornings and its glow plug system was manual. So, you need to toggle the ignition key to ON position and then pull the glow plug pull-switch (it was an annoying switch, it had a high tension seeing that made it uncomfortable to hold for long). After about 10-15 seconds you need to try to start the car. It's a hit or miss the first attempt. Usually the glow plug needs to be fired again and then only the engine would fire up. I remember once when the battery was drained by leaving the lights ON accidentally, they had to take the battery out to put on charger. The battery was almost twice the size of a 800CCA modern car battery. It took two strong people to put it in the car. The battery cable on the car is as thick as a toe! I can only imagine how those 24V systems would be like. :D
I have currently decided to get a few 200A bridge rectifiers and put my other UPS transforms also together and make a high current supply in parallel. Meanwhile I've also started looking for cheap used DC welders.
 
...
Power max: 1.4kW
Voltage: 12VDC
Armature resistance: 0.7 ohm
Brand: Bosch
Model No.: 1740374
...
I wanted to calculate the stall current, because as mentioned, when the motor is spinning up, the current will be the stall current and drop, as back EMF increases. 0.7 Ohm cannot be right, or the stall current would be around 17 Amps, which we all know cannot be right. And 0.7 mOhms doesn't seem right either.
 

be80be

Joined Jul 5, 2008
2,057
Could be right starter are not made as they was years ago. Newer starters spin around 28,000 to 32,000 rpm and have gears to slow them down to get torque
needed to start the engine.
Older ones where made to run slower and used lots of amp to get the torque needed.
 

MisterBill2

Joined Jan 23, 2018
9,778
I wanted to calculate the stall current, because as mentioned, when the motor is spinning up, the current will be the stall current and drop, as back EMF increases. 0.7 Ohm cannot be right, or the stall current would be around 17 Amps, which we all know cannot be right. And 0.7 mOhms doesn't seem right either.
If you read 0.7 ohms resistance for a starter motor that probably includes the connection resistance for the ohms meter, which is usually about 0.5 ohms for a typical meter. To verify that, put both meter connections on the same motor terminal and read the resistance. Probably it will be the same, or very close.
AND the stall current is actually only at the first instant of power applied, it drops as soon as the armature starts turning. So really you need a fairly fast analog scopeand a current shunt to see it.
 
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