When the car battery is dead, can you use AA batteries to crank the engine?
So there is a way, that is if you can put the AAs together and wait for 10 minutes. Could be useful if you are in the middle of a desert with a dead battery and a bunch of AAs! I would probably take a taxi if my job was on the line.
AA batteries can provide a lot of energy. The issue is that they can’t output that charge in a burst of high current. Every battery has some internal resistance, and that increases as the battery depletes.
While the internal resistance of a car battery can be less than 10 mOhm, the resistance of a AA battery is around 250 mOhm. This means if you draw 2 A from the battery, its voltage drops by 0.5V out of its total 1.5V. So ideally if you short the AA battery you can get a maximum of 6A.
A car’s crank motor typically needs over 50A of current and to start it may take an in-rush current of over 150A. 150A from a charged car battery would mean around only 1.5V drop out of its 12.5V no-load voltage. Like I mentioned, you would need maybe over 1000 AA batteries, 10 in series to get 15V no load, and 100 of those in parallel so that each string can provide 1.5A of the in-rush current. That should work, and probably you won’t need 1000 batteries either for smaller cars. That another thing to try I guess.
Now, every battery has a capacity, typically shown as Ah (Amp-Hour). The Ah number means you can draw that much current from the battery for an hour before the battery completely depletes. Of course if you draw more current, the battery dies faster. If you draw IL (Load Current) amount of current from the battery, ideally the following formula shows the amount of time you can run with your battery:
Time to Discharge = Ah / IL
In actual usage, if you draw less than Ah you can get more charge out of your battery, and if you draw more than Ah you will get less charge. Also the environmental conditions like temperature also effect the capacity. Ah value drops by battery age and all these are also dependent on the battery chemistry.
An alkaline AA battery has roughly around 1Ah of capacity. I tried this by connecting a 1 Ohm resistor across a new dollar-store AA battery and got the following voltage versus time curve. As I’m using 1 Ohm load, the curve can also be current versus time (V = R x I). The curve starts at no load voltage of around 1.58V and drops right away below 1.3V for around 1.3A. At the end were the battery voltage start dropping at a faster rate is where the battery is done.
AA battery loaded with one ohm resistor
Now assuming a AA battery can do 1A for 60 minutes, using the formula above, ideally it can do 360A for 10 seconds if it didn’t have the internal resistance. It means that the needed charge is there. So if I can pour that charge into a car battery than can deliver huge current with little voltage drop, I’m good to go.
To charge the car battery, I need to create a voltage above its level. I used 12 AA brand new batteries that gave me 19.4V at no load, or 1.62V per battery. I also put a one ohm resistor in series to measure the current as well as limit the current flowing through batteries.
As soon as I was drawing 2A into the car battery, with car battery around 12V and 2V drop on the 1 ohm resistor, I had 14V on my AAs, or 1.167V per battery. This indicates an internal resistance of around 225 mOhm for the new battery.
Now as the AAs discharged, their current dropped as shown in the curve below. I charged the car battery for 10 minutes, and I roughly had an average of around 1.5A going into the battery. This means that I could draw an average of 90A from the car battery for 10 seconds which is more than enough for most engines for a single crank.
To understand how I roughly calculate the amount of current I can draw, below is the formula I use to estimate the times or currents:
Current_1 x Time_1 = Current_2 x Time_2
Current versus Time curve for charging car battery with AA batteries
As soon as the engine starts going, it quickly charges the car battery in a matter of a few seconds so you can crank with it again. So make sure that one crank counts.
Something you should note is that because you are charging the car battery with relatively small current, you should make sure all car devices and lamps are off. Otherwise they will draw the AA current away from the car battery and waste it.
You can also use a 12V or 14V drill battery. They can deliver much higher current and will charge the car battery much faster, like in a minute.
And again, don’t connect the batteries backwards. A car battery, even when it can’t crank the engine, can still deliver some significant amount of current and can smoke your AAs. There is no shock hazard though as the voltage is pretty low.
Oh Great Mehdi,
My car is dead and all I have are a bag of lemons I bought at the market, a box of galvanized nails, my vintage penny collection, and a spool of wire. What should I do?
Call the mobile mechanic and make him some lemonade and use the vintage penny collection to pay him.
Not sure about the nails.
It’s quite difficult. Thank you for this video. It’s helpful b/c it’s a visual representation.
Do we need a resistor?
Thanks
Can I use 2 x 9V Batteries in series (Since it has snap connectors, no soldering required) and charge the car battery? Thanks
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Hi, please reply fast, would much appreciate it. I’m flying back home tomorrow, and I’m pretty sure my battery will be flat. I don’t want to wait the hour for the tow truck so planned on buying 12 AA batteries, a roll of tape and a bit of wire instead. Do I need the resistor or will it be safe to just hook it up like this?
Love all your vids
You don’t need the resistor
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This is a really great article. Interesting and informative and makes several excellent points!
Hello,
I’m planning on putting together a starter battery:
With 32 elements of 3.7V @ 2.3Ah
max. discharge current for 10s 120A
4 series of 8 would make 15.2V 18.4Ah 960A
920A are required for a coldstart to crank up the engine
How does it sounds ? Are 15.2V okay ? Or do i need to keep it at 13-14V for safety of the alternator and/or other components ?
Thank you
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hi. i like your vidio. i kike to use rechargeable 2.3 aa batries. how many do i need ?
What if you use 6 AAs = 9V ? Will it work? If it does it will take more time obviously, but is it going to deliver more energy since it discharges slower than the 12 AAs? Thank you!
You need to have a voltage above the battery voltage you are charging. So over 12V would work. Otherwise the car battery sources current in the AAs and discharges. The higher the voltage, the faster charge as the current increases.
hi i am a middle school student from Beijing China and love your vedio very much
How about a motorcycle battery with rating 5aH(10 hours). .how many minutes can jump start a car battery.
Can you use C batteries in lieu o AA and what would the difference be in charge time?
Can you use an 18 volt cordless drill battery to jump the car and if so how can you tell on the battery which is + or – to connect to the car battery?
C batteries are stronger and can charge faster, so yes. You would need less of them too. I don’t know the required charge time without testing but since their internal resistance is smaller, for the same 12 batteries it would provide much more current and so charges the battery faster, maybe over 3 times faster.
You can use 18V cordless drill batteries, but then those guys can provide a lot of current and connect it to car battery can damage the 18V battery. I wouldn’t go over 14.4V for larger batteries.
what about using say 36 aa’s at 18v (1.5v x 12batterys, 3 sets parallel) ?
what are the requirements for the resistor that you added, and how would it best be wired into that configuration?
thanks
The resistor is only for current measurement, you don’t need it. more batteries means your can battery will charge in 3 minutes enough for a jump start.
thanks for the fast reply. another question. what is the voltage range that can be used as the charging source?
i get the impression that 14-18v seems ideal but why? how much is too much? why not 25v or 50v? (theoretically, that is. i do realize it is less practical)
It is not really about the voltage but more about current. Every power source has some current limitation and series resistance. I had to set voltage of the AA batteries higher (like 19V) so that they could provide over 1A when connected to the car battery. See, the car battery voltage wouldn’t change much because it is strong, and has a very small series resistance but the AAs have larger resistance. So as soon as we connect them, the AA bank voltage drops to 12V feeding close to 2A and dropping to the car battery. So the only reason you would increase the voltage would be to increase the charging current. If your supply can deliver 100s of amps with low resistance, then you want its voltage not to be too far away from the battery under charge. Usually we use a current source to charge a battery though, which provides more reliable and fixed current and its voltage changes to provide that fix current.
Hi Mehdi.
I’m building a wind turbine that i plan on connecting to a 12v (lead acid) battery. Can I use a dc-dc buck converter (step-up, step down combo) in order to get a stable voltage needed for the battery?
Is there anything else i will need to think of while charging a battery? Will the charging slow down (and eventually stop) once the voltage of the battery is high enough?
Also, if possible, what voltage would you set the buck converter to output to the battery?
Thanks in advance!
Hi Erik,
You shouldn’t connect a voltage supply to charge a battery unless it is current limited. When the battery and supply voltage are greatly different, like 1V off, the current draw could be huge (>10A). So Make sure your power supply is current limited to, like… 5A if your turbine and circuit can take it.
Then the battery can be charged to 14V. So set your supply to 14V. It will charge up with a limited battery and when reaches 14V the current drops and the battery voltage remains at 14V.
Good luck!
Thanks for the quick reply.
Is a current limitation needed even though my turbine won’t be able to output anywhere near those amps? (or is that the problem?)
I suppose if have a regulator that has an enable input with hysteresis, Then you can set it such that it the input rises above some level it starts pumping and if the input drops below a level it stops driving. You can have a large input capacitor so it charges to a point where the driver runs and pushes that charge into the battery.
Great vid. And what happens If I use 2 9V batteries? Thanks
9V batteries are notoriously weak and can’t provide much current. So it probably won’t charge much.
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I think using AA battery charger is quite difficult. Thank for your guide
It’ll definitely work if the car battery is not too degraded from being in a state of discharge too long. On a hot summer day, the plates in a discharged car battery can corrode beyond help in a matter of a few hours depending on the age of the battery, in which case this would be an exercise in futility as even the a heavy duty charger will not recharge the battery enough to crank the engine.
Either way, this exercise demonstrates a good reason why we all need to cram cordless soldering irons, solder wire, and a half meter of wire in our glove compartments. 🙂
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me gusta los video me parese gracioso
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How many batteries at lease once of a time?
Thank You.
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You could also use a car battery charger if you have one. Some chargers even provide enough current to crank the engine.
The challange would be to use an AA-battery charger 🙂
*challenge
Everyone has a few Ah notebook battery.
I wonder if it would work with a single notebook battery and if you would actually want to try that. I’m not sure this doesn’t mess up your batteries.