Hello it's my first time here and I will try to address the issue as best as I can.

I left my keys in the ignition a few days back and hence my battery drained itself.

It is a 12V maintainence-free battery and I want to charge it enough for just one crank. I found a 16v 1.5A adapter and 12V 4A adapter. They are probably old laptop chargers. Could I use anyone of them to charge it?

I have been reading up about it and have seen cases where the wires have melted due to overheating and there are chances of explosion too. Normally a 14V adapter would be ideal but 12V or 16V should work too right? But I am worried about the Amperage since a heavy current will fast charge and damage the battery.

Personally I feel the 12V 4A adapter might not be able to charge the battery that efficiently,since a multimeter reading shows the battery to be at 11V and a 4A current might be too high. So the 16V seems like a better option.

Plus my multimeter's Amperage measurement is damaged so I have no way to take current reading when connecting the battery to the 16V adapter. But could I connect them without a resistance since the current is only 1.4A and constantly check for overheating? I just need enough for one crank and the battery is new, not even a year old with no corrosions on the terminals or leakage.

Sorry if this was too long and thanks for any help/advice.

PS: I really want to charge it on my own and understand there are risks involved. I'm planning to make a proper battery charger on my own in future, but for now I lack the technicalities involved.

  • 1
    I'm no electric engineer, but won't a regular 12V 4A power supply be way inadequate, besides the fact that charging with a regular power supply is extremely dangerous by itself? Without a special pulsing battery charger the current should shoot up instantly (as the lead battery internal resistance is very low). Since the adapter is rated for no more than 4A, the consequences are hard to predict. I would most definitely not try this. Please correct me if I am wrong, I might be. Dec 23 '15 at 14:51
  • 1
    @IhavenoideawhatI'mdoing There is only a 1v difference between his battery and the adapter 12v output, so the current drawn by the battery shouldn't be excessive. His adapter has more than likely more over-current protection than the majority of general purpose car chargers, so I can't see it being a problem as a way to get him going.
    – HandyHowie
    Dec 23 '15 at 15:21
  • 1
    @HandieHowie A lead acid battery has an internal resistance of 3-20 mOhm. That would mean that even at a voltage difference of 1 volt the 20 mOhm battery will pull 50 A. As far as I know a discharged battery at 11V should have a little higher resistance (twice as much maybe), but you're exceeding the rating even if it is ten times as high. I wouldn't trust the overcurrent protection, even if there is one it is probably intended for emergency situations, since it's a laptop charger which normally doesn't go near such currents. Once again, you can't predict the outcome. Dec 29 '15 at 10:48
  • Just buy one of the cheap chargers from the auto parts shop. They are "analog" so the voltage will clamp down to the battery voltage and it will only provide so much current, so it works. As the battery charges, it takes less and less of the waveform (rectified AC) so it acts like a pulse charger. I have charged batteries with a 4 amp solar panel, it will "boil" the battery when it is fully charged, so don't do that for long. Never had a problem with gasses failing to vent fast enough (as mentioned in a comment). Measure current using a shunt: 1 foot of #10 AWG is 1 milliohm. 1 amp = 0.001 V.
    – user15009
    Apr 13 '16 at 20:41

Your 12v adapter would only be able to raise the battery voltage to 12v, but to charge a car battery fully you would need to raise the voltage to 13.8v. The 16v adapter would raise the voltage too high and could cause damage. A 4A maximum current would be fine and would not cause any damage, but you need a 13.8v source. Batteries like NiCd need an accurate constant current to charge them, lead acid batteries (e.g. car batteries) need an accurate constant voltage to charge them.

  • can you suggest some homemade ways to reduce the voltage then? should I try to charge it with the 12V adapter and see if I can get one crank? Dec 23 '15 at 8:43
  • 2
    You would do no harm with the 12v charger. Disconnect the battery from the car first, to ensure you cause no harm to the car electronics. Make sure you get the polarity correct. You would need to build a 13.8v regulator to use the 16v adapter.
    – HandyHowie
    Dec 23 '15 at 8:53

I'll preface this by saying, I don't think using a laptop adapter to charge a car battery is the way to go. If it's your only means to get it done, do it at your own risk ... that said:

If you can get your battery back to 12v using the 12v adapter, it should be enough to get your motor started. Then the alternator can take over and recharge the rest of the way. I had a battery in my truck going bad for some time. When I finally took it out to change it, it read a little over 10vdc, yet it was still able to start my truck (albeit the starter was complaining about it). Point is, 12vdc should be adequate to get the vehicle started. 13.1vdc at the battery is optimal (fully charged battery at rest), but 12vdc should get the job done. Ensure you don't have any extraneous power draws going on when you do this (ie: radio, lights, door open, etc). This will take away from the batteries ability to get the vehicle started.

  • I guess he will get the pleasure of having an car battery exploding in the engine bay and making extensive damage. I've seen it enough times to be ultra careful with them.
    – race fever
    Dec 23 '15 at 15:38
  • 1
    @race I know the risks involved, but can you tell me what exactly goes wrong which causes the explosion? Dec 23 '15 at 19:51
  • Sure! Basically stuff inside gets heated up due to too much current at once. The battery cannot vent the resulting fumes fast enough and it ends up being a sort of pressure cooker. It reaches a point where the plastic casing won't hold up and explodes. It can happen to sealed and open batteries. I would really just take it somewhere to get charged or replaced. You can end up paying much more. Last one I saw was an SUV who needed about $2000 worth of work and parts (including paint) after the explosion.
    – race fever
    Dec 23 '15 at 20:05
  • 3
    @racefever. 4 amps is hardly a lot of current when it comes to charging a car battery, an alternator is capable of providing a lot more current than that.
    – HandyHowie
    Dec 23 '15 at 22:57
  • @HandyHowie - Yah, you see battery explosions all the time because of 2/10/50A battery rechargers ... WAY too much amperage for the battery to handle <cough><cough> Dec 24 '15 at 14:03


I did not hook my battery up with the 16V charger as I felt the risk would be too much (its a pretty new battery will update if I try it though).

With the 12V charger things worked fine. I did not find any heating in the adapter or the battery. The adapter just got mildly warm when the voltage reached close to 12V. The first time I charged my battery for nearly 3 hours, did get a crank which was better than the state I found my battery in, but not enough to start the car engine. Next day I charged it continuously for 6 hours and it worked.

I wouldnt recommend charging this way but, its a nice handy trick to have in case you need it someday.

  • Good to know you didn't die, it's a good trick to remember. Jan 4 '16 at 13:11
  • I did it with a motorcycle battery multiple times and it was working fine. Apr 17 '17 at 13:33

I have used such power supplies on small motorcycle and jet ski batteries for many years. Output Voltage has ranged from 12-16v depending on the power supply, and the current is typically in the 500-850 milliamperes range. Never a problem. The key is not to leave them connected as if they were a "smart charger." They have no auto-shut-off feature, so you simply have to be careful to disconnect them after a few hours and check the battery. Using this method, a full size car battery would need at least 6 hours of charging... hardly a risk of instantaneous explosion. As a side note, I also have a very old "dumb" battery charger: No-load output is 14.7v and loaded output is around 15.5v. Current rating is 1.0 amp. These old style battery chargers work just fine, and in fact, are nothing more than very simple, non-pulsing power supplies. They served us very well for many decades before smart chargers came into being, and such devices can continue to do so today.


There are three stages of charging a lead-acid battery:

  • Constant current stage, where you limit the current and voltage slowly rises to about 14.5V
  • Constant voltage stage, where the voltage is limited to 14.5V and the current slowly decreases close to zero
  • Float stage, which is just like constant voltage stage, but now you limit voltage to 13.5V-13.8V

All voltages refer to room temperature voltages. Charging a battery during a freezing winter outside may require higher voltages.

Now, what does your laptop charger do when you exceed its current limitation? It may not be designed for use cases where you even attempt to draw too much current. Well-designed chargers will limit the current and let the voltage be lower, but then again do you trust some cheap Chinese charger to do that? Especially if the charger is designed only to work with a laptop that never draws too much current.

If the charger is a genuine 16V/1.5A charger that limits current to 1.5A and lets the voltage be lower whenever you are trying to draw too much current, you can do the initial charging stage (constant current) if you disconnect it early enough. E.g. a 45Ah battery can be charged at most 30 hours if it's completely empty. Leaving the battery connected for too long may cause the electrolyte to be broken to hydrogen and oxygen, meaning your battery is ruined if there's no way to top it up with distilled water.

A 12V/4A charger does not have high enough voltage. You can do the initial charging stages with it, but your battery will never become truly full.

I would recommend purchasing either a genuine lead-acid battery charger or if you have more money to spend, a current/voltage limited laboratory power source that can be used in various power supplying tasks.


I've done this, and it works to about 80% full. Enough to get you started, BUT THEN, go to Auto Zone where they can charge it fully for FREE! I'm sorry I'm late to the party, but if anyone else is searching this, it's ok but only in a pinch!

It sounds like you had a few days so...AutoZone.com will NEXT DAY AIR a Real charger for $34.99: https://www.autozone.com/test-scan-and-specialty-tools/battery-charger/noco-75-amp-6-12-volt-genius-battery-charger/388458_0_0

Or a SOLAR charger in case your battery dies in the middle of nowhere for $29.99: https://www.autozone.com/test-scan-and-specialty-tools/battery-charger/noco-2-5-watt-solar-battery-charger/227340_0_0

  • Hi Taylor, welcome to the site. It's unlikely that this would be of any help to the OP, as AutoZone only ship to US addresses, and they were in Kolkata. Being a global site, we generally prefer to recommend solutions rather than particular products/suppliers.
    – Nick C
    Apr 4 '19 at 14:26

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