Need help for building a car jump starter 5 questions

[protocol217]

I have a big arc welding AC transformer around 100 amps. and i want to build an 12v car starter from it, so that i can start a car or a truck without a battery.

First i will unwind the secondary coil until i get strong 12-13V of AC, and then add a rectifier and i don't know if i need a capacitor to smooth out the voltage.
So my questions are:

1. how much should i unwind the secondary coil - what ac output voltage should i have?
2. what rectifier should i buy - with how much amp rated ?
3. do i need a capacitor to smooth out the voltage if i need one what capacity and voltage rating should i buy ?
4. if i add a capacitor it will steep up voltage from rectified 12v dc to 15-18 v why is that and can the car starter 12v dc motor take 15-18v with capacitor or should i unwind the secondary even more like to 9 v of AC and then when rectified and the capacitor comes it gives 12 v of dc precise ???
5. will this work for car stater, and can this damage any of car electronics or the actual starter 12v dc motor ?

 

[crutschow]

I have a big arc welding AC transformer around 100 amps. and i want to build an 12v car starter from it, so that i can start a car or a truck without a battery.
.....................................

That's a no-no. You can jump-start an engine with a low battery but you can't with no battery if the engine has an alternator. That's called a load-dump condition which will likely cause high voltage transients that can zap any car electronics including the engine controller.

 

[MrAl]

Hi,

I agree fully with Carl. Better to use a regular battery, then "boost" charge it with a heavy duty charger, that you make yourself with the right transformer and rectifiers.

This kind of booster can start the car in about 15 seconds even with a dead battery. They typically sell for around $60 but you can get them cheaper like $50 on sale. They already have everything you need built in, and nothing to assemble. You plug them into a 120vac line but they probably make them for 220vac too. They are might lighter in weight than a welder also.

 

[protocol217]

so you say i can do this if i have an old dead battery in my car witout a capacitor to smooth out the voltage - the battery will act as a capacitor, am i right?

 

[MrAl]

Hi,

That is sort of correct, the battery will act like a capacitor but a very large one, and also stabilize the voltage to near 12 to 14 volts or so. This will help prevent burning any other component out in the car.

 

[protocol217]

ok, now i have two questions left:
1. how much should i unwind the secondary coil - what ac output voltage should the secondary of the transformer give?
2. what rectifier should i buy - with how much amp rated ?

 

[MrAl]

Hi,

If you want to charge at 50 amps (which is pretty high really) then you need diodes that are rated for continuous 50 amps or better (maybe 100 amps), and the secondary voltage should be adjusted as you go, by taking turns off until you get the right current, starting at say 18vrms output.
It would be better if you could put turns back on, but that's probably harder to do. Because we are taking turns off that means we might have a current that is too high to begin with, so some caution has to be exercised. Maybe use some resistance in series to limit current for the first try, then eventually reduce that resistance to zero or nearly zero as you take turns off.

What comes into question is the initial voltage of the battery and the drop of the diodes as well as the series resistance of the primary and secondary, and the inductance also. We can predict to a certain degree, but it's better to test for the right value rather than try to predict this exactly because we are dealing with higher than usual current (50 amps) and some of the variables are not easy to measure.
You can start by measuring the primary and secondary resistance, that might help get started.

The current is limited by several things:
The primary voltage, leading to the secondary voltage,
the primary resistance,
the secondary resistance,
the equivalent series inductance,
the wire resistance,
the battery initial voltage,
the diodes initial and final voltage drops,
connection resistances.

If the resistances are all low and the output voltage is too high, we might pump too much current into the battery and cause it to heat up and even blow up. These things are very nasty when they blow, so you really have to be careful to limit the current to a safe level. That's the whole goal, and since we are taking turns off, that means we will be starting with a current that is too high to begin with uless we limit it by some other means like extra resistance. Once we get closer to the right value, we can decrease the extra resistance and that will eventually leave us with the right output voltage.
There also could be variation with input voltage if the line varies.

You also have to realize that diodes this big will need heat sinks. Also the battery can not be left on charge for too long as it will overcharge because there is no active charge control.
There is also the question of whether or not the welder is rated for continuous duty or intermittent duty. It will overheat and shut down if it is for intermittent duty and has built in protection if it is left operating too long at high current.

What is the current rating of the welder?
Is it rated for continuous or intermittent duty?
Can you measure the primary and secondary resistance?
What is the input line voltage?

 

[protocol217]

I don't know the exact current rating of the welder, i have only this big transformer that i picked up in the scrap yard ( got it cheaply ) with no housing, i tried it with 220 v input and it works as a welder but there is no screws for adjusting the current on it.
I thought i can unwind it to 12 v and rectify the current and start a car, i thought the more current the better, but i was wrong.

i will use it for a welder, because i am not that good with electrics and i don't want to destroy my car electronics and ignition, i will charge up a battery and then start car.

Thanks for all the useful info.

 

[MrAl]

Hi,

You're welcome.

12v isnt high enough. For one thing, a car battery will charge with a voltage of around 14v, Second, the diodes (bridge rectifier configuration) will drop about 1v each, so that's another 2v about, for a total input requirement of 16v. That's probably the minimum needed.

Check out those boost starter chargers for sale. They are not super expensive if you really want one.

 

[tcmtech]

Making a large low voltage high current transformer into a high current battery booster is not really all that hard. AS has been mentioned before getting your AC side output voltage in a range that a automotive electrical systems can handle is the main issue.

As far as that goes to be honest if you can get anywhere between 13 and 17 volts as the open circuit voltage your good to go. The rest is just designing for pure amp capacity beyond that.

I have a pair of battery chargers I built out of old 2 - 2.5 KVA mainframe type computer power supply transformers that had a as built AC output winding set that produced about 14 - 15 VDC open circuit that could handle 100+ amps load all day long.

With that I simply used some salvaged stud diodes from old welding power supplies and called it good. No fuses or breakers except for what ever is on the power line that they are plugged into which to be honest given our 120 VAC 20 amp typical household power circuits that gives me about 200 continuous cranking amps and about 300 short term until the breaker or fuse pops. ~400 amps if I am in my shop where I have 30 amp breakers on my circuits.

FWIW at that power level it takes about 1 minute to recharge a stone dead car battery to the level it will start the engine.

 

[chemelec]

Hopefully the Bad Battery you use isn't Totally DEAD or Sulfated.

If it is REALLY BAD, It won't take any charge.
That is Just the same as No Battery and That is a NO, NO.

 

[MrAl]

Hi,

Some VERY good points that came up with the last two posts. I agree with almost everything except for the range of voltage at the output of the transformer.

Usually they want to measure the open circuit secondary voltage and then ask a question like, "is this the right voltage to charge the battery with?"
But it's not just a matter of what the open circuit voltage is because the current they want is usually quite high, and high current demands a little more attention to detail than just what the secondary open circuit voltage is.

At 50 amps, even 1 ohm drops 50 volts, and even 0.1 ohm drops 5 volts, and to charge a battery at a given desired current level (preset by whoever wants this charger) requires a given DC voltage that appears at the battery terminals *under load*.
To get this to work, we have to take into account *every* voltage drop in the system, including the diodes and lead wires that connect the circuit to the battery and secondary to the diodes, and the winding resistances, as well as other factors mentioned before.
There are so many variables that it's better to come up with a way to adjust the current while you find the right voltage, and once you find the right voltage then you no longer need to adjust the current unless you want to include that little adjustment too.

Another way to do it is to use a variac on the input of the transformer. Thia works nice because you can adjust the output voltage by adjusting the input voltage as you take off turns, and eventually you'll work your way up to full line voltage at the input so you can then remove the variac. Of course if you have the luxury of using a variac ALL THE TIME, then you never need to adjust the secondary turns. Just adjust the variac for the correct output current each time or find the setting that works for a given current.
The variac has to be able to handle the full power, but a 10 amp variac would work well for 50 amps output i think.

 

[tcmtech]

Personally I disagree with the concern over the short term peak charging amps limits.

I have done a number of rests suing my Miller Shopmaster 250 CC/CV welder as a battery charger being at the minimal output its very close to 14.5 volts plus the output is regulated and can easily do the high side of 700 amps short term in CV mode.

Given that I have deliberately hooked up large dead LA batteries to it just to see how many amps they would draw once they started taking a charge.

To be honest the most I have ever seen is about 400 amps draw of which lasted all of maybe 4 - 5 seconds before the batteries voltage started coming up causing the charge rate to quickly drop below 100 amps tapering off to under 50 in maybe a minute or two. From the batteries tend to hold that level of draw for some time before they drop off.

To me right there that pretty much told me that the short duration peak amps are not really an issue. A few hundred amps input for a few seconds followed by the rapid leveling off to a far more reasonable rate is not something I have ever had any reason to worry about.

To be honest the several tens of milli ohms of resistance your unit is likely going to have throughout its charging circuit plus clamps is likely going to limit your realistic peak charging current enough as is that you will never see any real problems arise anyway.

Add in the fact that you are using a welder transformer which by design is built to handle highly abusive current and power demand as and the odds are you would never hurt let alone kill the thing anyway. Simply put the odds are you will never have a battery big enough and dead enough or vehicle that can crank long enough before popping your primary line side fuses or breakers to ever hurt your transformer anyway.

 

[MrAl]

Hi,

Yeah but what if it puts out 15.5v, or 16.5, or 20.5 volts, what is the max current then?
When batteries like this blow they blow big...fragments and acid all over the place.

 

[tcmtech]

Thats the whole point of making it have an open circuit voltage that is within the working range that would be found in a typical vehicle.

If it has that it's highly unlikely it will do any serious damage to a battery in the normal time frames that a person would leave it connected to do what it was intended to do.

 

[MrAl]

Hello,

It's fine to do that, but after you do that, the current may not be high as you want it to be because of the circuit resistances which are many.
An open circuit voltage of 16 volts with only a mere 1 ohm series resistance can only pump 4 amps into a 12v battery. If the battery voltage rises to 14v, it can only put out 2 amps. That's not nearly even close to the target current for this project.
Once you take the turns off, it's too late unless you can put some back on. The turns you put back on will be there to make up for the series resistance, which you didnt know to begin with.

 

[tcmtech]

If you're getting an ohm of resistance between your power source and battery you have some severe connection and or cable sizing issues.

 

[MrAl]

Hello,

The problem was exaggerated to bring out the main problem. It's not the one ohm, it's the combination of any series resistance and us wanting to reduce the number of turns while not taking off too many, and the same time get the right current. It's still not perfect though because line voltage variations will change the current anyway, but we still can try to get close.

 

[tcmtech]

Perhaps but to be honest in a realistic emergency situation if the OP can get anywhere between 11 and 15 volts DC at any reasonable current level the thing will work.

Emergency boosting is no where close to precision battery charging.

 

[MrAl]

Hi,

Yeah, but long before the emergency we have plenty of time to optimize