Portable Car Jump Starter School project

Thread Starter

Electrical Dude

Joined Jan 25, 2021
1
Hi, I am currently going to college at a technical school in Missouri, learning how to be an electrician, and for one of my classes I am required to make a project of some sort that incorporates electricity and that would be a practical solution to the every day world. I started brainstorming and thought of an idea to make a portable jump starter for your car, if your cars battery was dead or low, you could hook another battery to it and jump start your car. I know they sell something like this at auto stores, but I want to make one for myself. I have seen YouTube videos of people jump starting their car with a lipo hobby battery, and thought I could do the same. I have a lipo remote control car battery that I would like to use for this which is rated at 11.1 volts and 7600mah at 75C. Would this battery be enough to jump a car battery? I was also thinking of a way to safely do this, and the ability to use it more than once. I understand that if you were to get the car started with the battery I have, the cars alternator could charge it and could cause some safety concerns, I also thought of a way to prevent this would be to add a diode to the battery I would be using to jump the car. Another one of my questions would be what ratings on the diode would I need to successfully do this without blowing myself up. I understand that a cars alternator can charge a car battery up to 14volts or more. Is this too crazy of an idea, or would it even work? Is there anything I am missing here?
Any help is appreciated!
Thank you!
 

shortbus

Joined Sep 30, 2009
8,591
Is there anything I am missing here?
What jumps out to me is the size of the battery you want to use. Compare the " 11.1 volts and 7600mah " you show to the same thing for the battery of your car. Just the volts alone of your proposed battery are disqualifying for the project, since a good car battery is only considered good when it is 12.6 volts. The 12.6 comes from - https://www.autobatteries.com/en-us/battery-testing-and-maintenance/car-battery-voltage-and-testing
 

LowQCab

Joined Nov 6, 2012
560
A car "Jump-Starter" Battery is a bad idea no matter how you look at it.

If you want to use a LiPo Battery it must be a "4-S" (peak Voltage of 16.8V) and
it must have a SERIOUS current Rating, like at LEAST 12,000mAh, that's a very expensive Battery.

The good heavy duty ones use a SLA ( Sealed Lead Acid ), or an AGM ( Advanced Glass Mat ), Battery.
LiPos are simply too expensive.
They can also be very dangerous when abused.

Any electronic component like a Diode, that can withstand ~200 Amps is going to be very expensive,
and require substantial Heat Sinking to survive, that's why the manufacturers don't use them.

The problem is, nobody wants to WAIT for the Car Battery to be CHARGED UP by the "Jump-Starter" Battery.
They want to hook it up and twist the switch, they will do it every time.

If the Car Battery will not accept a charge, what is the purpose of "Jump-Starting" it anyway ??????
It's just going to die again, and at the worst possible time and place.

You need at least 4ga. Wire to successfully "Jump-Start" a car, otherwise you will be dependent on
waiting to see if the Car Battery is going to accept enough of a partial charge,
within a reasonable length of time, to successfully operate the Starter.

If the Battery is completely dead, a portable Battery "Jump-Starter" is not going to start the engine alone.
If the Battery is merely too low to spin the starter fast enough, the portable "Jump-Starter" may
provide enough Current assistance to get the engine running,
but you've got about 3 good tries, and then you're done.

"Re-Charging" the Car Battery is the objective, this takes at least ~15 minutes in most cases.

Sorry about the rant, I get frustrated with people sometimes.
.
.
 

MrAl

Joined Jun 17, 2014
8,251
Hi, I am currently going to college at a technical school in Missouri, learning how to be an electrician, and for one of my classes I am required to make a project of some sort that incorporates electricity and that would be a practical solution to the every day world. I started brainstorming and thought of an idea to make a portable jump starter for your car, if your cars battery was dead or low, you could hook another battery to it and jump start your car. I know they sell something like this at auto stores, but I want to make one for myself. I have seen YouTube videos of people jump starting their car with a lipo hobby battery, and thought I could do the same. I have a lipo remote control car battery that I would like to use for this which is rated at 11.1 volts and 7600mah at 75C. Would this battery be enough to jump a car battery? I was also thinking of a way to safely do this, and the ability to use it more than once. I understand that if you were to get the car started with the battery I have, the cars alternator could charge it and could cause some safety concerns, I also thought of a way to prevent this would be to add a diode to the battery I would be using to jump the car. Another one of my questions would be what ratings on the diode would I need to successfully do this without blowing myself up. I understand that a cars alternator can charge a car battery up to 14volts or more. Is this too crazy of an idea, or would it even work? Is there anything I am missing here?
Any help is appreciated!
Thank you!
Hello,

You really should be experienced in power supply design to attempt this kind of project. A project like this is electrical that's true, but it may also be deemed 'electronic' meaning that it requires certain design tips that are common to electronic devices rather than just pure wiring.

For example, you cant really use a diode in series if you expect the battery to charge enough to start the car, or a voltage lower than the 12v nominal will start the car. That is because any diode will drop some voltage and although that may still supply some power to the battery it reduces the ability to transfer energy to the battery, and even though the diode voltage drop may be only 0.5 volts, that is still much worse than 0.0 volts, and the wire used to connect drops some voltage already so a diode is not a good idea. If you are just charging the battery it may work, but it will take longer to charge the battery and it will never be as good as a direct connection.

Second, you must incorporate some form of reverse polarity protection.
Again you can not use a series diode because that does not provide protection against a reverse connection (do a simulation of you dont think this is true).
The way to do this is to provide a high current switch and either LED or light bulb. The light bulb must have a diode in series with it, and when you first connect the jumper to the battery the switch is in the 'off' position, but the LED or bulb and diode connects to the battery. Now the diode (and LED) is oriented such that if the two connections are right, the LED or bulb lights up, but if backwards, it does not light up. This means that if the LED or bulb lights up it is OK to close the switch, but if not then you have to reverse the connections and try again. Without an elaborate electronic detection circuit this is the simplest way to get reverse polarity detection.

The typical battery in most jump starters is a sealed lead acid type rated fairly high like 10 Amper Hours or better. The one i had had a 16 Ampere Hour battery. The battery however should also have a high surge current rating. Some of them go up pretty high but you have to check. You can tell to some degree by the kind of terminals they have. If the battery has those little snap on connectors then it is not going to cut it, a decent rated battery will have bolt on terminals of some kind or those big studs that a regular car battery has. The other thing is that once the car starts you have to be ready to disconnect the jumper right away and that prevents it from being charged by the alternator for any serious length of time. Of course lead acid will take the abuse better than anything like a lithium based cell.

So there are a few things you have to beware of:
1. Reverse protection using a high current switch and LED or bulb with diode in series to detect the correct polarity (NOT to detect reverse polarity but to detect the correct polarity so it lights when connected properly.
2. A battery with a high enough capacity (Ampere Hours) and a high enough surge rating. You can get away with a lower surge rating than the starter might actually draw because it will be for a short time but the battery terminals must be of the bolt on type or heavy duty studs.
3. Disconnect the jumper as soon as the car starts up.
4. Decent size large alligator clips to connect the cables to the car battery, and the cables should be as short as possible and high rated like 8 AWG or better but 6 AWG or 4 AWG is better if you intend to start the car with the jumper rather than just charge the car battery.
5. A decent charger for the jumper battery. This must comply with the charge regimen required for the battery chemistry. No shortcuts here either.

The procedure would go something like this:
1. Make sure the switch is turned off. You can use a second LED to detect this too.
2. Connect the jumper. If the first LED lights then you can close the switch. If this LED does not light, then reverse the connections and try again. If the LED lights close the switch, if it does not light do not close the switch there is something wrong with the battery it could be shorted and closing the switch may blow up the jumper. If you suspect it is just dead, then use a high current light bulb or low value resistor in series with the jumper to see if the car battery starts to increase. If it does, let it increase so that when you try again the LED lights up with correct polarity.
3. Have someone else start the car, and as soon as it starts disconnect the jumper.
4. Turn off the switch.

As a last tip, you can get DC fuses that are rated at very high current but they are expensive too. You would also need a very heavy duty fuse holder unless you can find a bolt on one. For this reason i dont think any jump starters use one.
Littlefuse apparently sells low cost 300 amp fuses but i am not sure what the voltage drop is for these things at full current. You may want to check into this for ultimate protection against a shorted car battery or accidental shorting of the jumper itself.
.
 
Last edited:

Audioguru again

Joined Oct 21, 2019
3,201
He does not want an antique lead-acid battery powered monster. Instead he wants to make a modern lightweight and small one using a lithium battery. Here is one that is rated at 500A. On a video test it smoked but did produce 500A.
 

Attachments

MrAl

Joined Jun 17, 2014
8,251
Stick your tongue on to see if it's hot !!!
Hi,

Funny you should mention that. In the old days before Alkalines came about, we used to stick our tongue across the two terminals of a regular 9v battery to see if it had any life left in it. Not sure if i'd want to try that with an alkaline though.
 

Audioguru again

Joined Oct 21, 2019
3,201
In the old days before Alkalines came about, we used to stick our tongue across the two terminals of a regular 9v battery to see if it had any life left in it. Not sure if i'd want to try that with an alkaline though.
Why not? 9V from an antique carbon-zinc low current battery is exactly the same 9V from a modern high current alkaline battery.
Your wet tongue senses the voltage. Today my tongue measures about 2.5M ohms.

I haven't "tasted" a 9V battery since about 65 years ago so I tried it now with the 8.79V from an alkaline 9V battery. It tastes the same as I remember.
 

MrAl

Joined Jun 17, 2014
8,251
Why not? 9V from an antique carbon-zinc low current battery is exactly the same 9V from a modern high current alkaline battery.
Your wet tongue senses the voltage. Today my tongue measures about 2.5M ohms.

I haven't "tasted" a 9V battery since about 65 years ago so I tried it now with the 8.79V from an alkaline 9V battery. It tastes the same as I remember.
Hi,

Yeah it's funny now that you mention it, it tastes to me kind of like something salty.
I guess the alkaline type may not be too bad as like you say i think the tongue has high enough resistance to limit the current.
These days i use either a high precision 5 1/2 digit bench meter or a decent regular digital meter that reads up to a 'count' of 9999 which helps when reading 9v batteries as the reading comes out something like 9.123 volts rather than 9.12 volts on a regular 3 1/2 digit unit. The 3 3/4 digit (3999) are quite common now, and the 4 full digit (9999) i think are becoming more common too. Beats the tongue by a long shot, but maybe we cant get dogs to lick them and stamp on the floor the voltage they detect as they have less sensitive taste than we do. Maybe they can smell the voltage with there average 50000 times better sense of smell. :)
Now a hammerhead shark, that's a different story. They can probably tell us the voltage at 1 mile away :) In fact, i can hear them talking now ..."Those dang humans with their dang 9v batteries". :)
 
Top