# Laptop power supplies, does current matter?

I have two laptops (same manufacture), with the same type of power connector.

However, the power supplies/transformers are slightly different.

The output on the first laptop's power supply is 15.6 V at 8.0 A. The output on the second laptop's power supply is 15.6 V at 5 A.

Clearly the voltages are the same, but the currents are different. I assume the second laptop's power supply can not be used on the first, because it can't supply enough power to the laptop.

However, can the first laptop's power supply be safely used on the second laptop?

• My question pre-dates the "duplicate" by a year and the other question is asking about adapters with different amperage AND voltage requirements. – BIBD Feb 18 '11 at 16:10
• Lol, that's pretty epic. @Sathya: How about reopening this just so it's not closed because someone posted a duplicate? :\ – Mehrdad May 5 '11 at 20:32
• Wish someone had left a link to that other question... – jpmc26 Apr 25 '16 at 3:49
• @jpmc26 From the edit history on this question I got this: superuser.com/questions/172257/… – BIBD Apr 25 '16 at 15:00

Using a lower current rated brick (the 5 A on the 8 A laptop) would result in one of the following:

• Melted power supply or cord, as the laptop starts drawing too much current
• Working laptop, with little to no charging of the battery (or charging the battery, but no working laptop) as 5 A is enough for one, but not the other
• Perfectly working laptop, as even though the brick is rated at 8 A, your laptop only draws 5 A (or the 5 A is perfectly capable of pumping 8 A)

Using a higher current rated brick (the 8 A on the 5 A laptop) should be fine - the laptop will only draw a theoretical maximum of 5 A, so that's the maximum that will be put out by the power supply.

This, of course, assumes that the polarity is correct - otherwise you'll likely just go poof (or unlikely catch fire). Sometimes there's a diagram, sometimes you have to check, sometimes you just cross your fingers and pray (the latter is not recommended for expensive toys like laptops).

• Assuming same manufacturer like in the question, the third option will happen. – Magus May 4 '09 at 20:49
• Certainly for dell laptops and power supplies the third option will obtain. I have no idea how this is accomplished. – Christopher Edwards May 5 '09 at 1:00
• Dell power supplies have some sort of communications protocol they use to inform the laptop of their capabilities. If you poke around in the BIOS you may even be able to find readouts of its capabilities. – bdonlan May 5 '09 at 6:38
• If the power brick is made for the series of laptop it's plugged into, higher AMPs in the powerbrick is just fine. Lower APMs in the power brick will result in something not working correctly (running laptop, charging battery, or both). It it's not the correct power brick, it'll do nothing (or maybe a warning will pop up). If it's completely different (different voltage for instance) you could do damage to both. – Chris S Oct 25 '10 at 14:56
• If the supply can't provide enough power, it may overheat, or the voltage may sag. If the voltage sags, the machine may just charge slowly, or it may crash, or it may stress the internal voltage regulators. I'm not sure whether that last case is likely or what would happen. – poolie May 26 '13 at 23:53

Basic rule of thumb for power supplies: The voltage has to be right; the amperage just has to be high enough. Your laptop will draw differing amounts of power depending on what it is doing.

Remember that the power supply has to meet the maximum possible power consumption of the laptop. e.g. burning a DVD whilst simultaneously running CPU at max, connecting to a wireless network, and thrashing the hard disc. In "normal" usage, you'll be drawing much less than 8A.

So I would suggest:

1. Don't rip any DVDs while using the lower-rated power supply.
2. If you're worried, get a Kill-a-Watt (or equivalent for your country), plug your laptop in using the right adapter, and measure the power usage during various tasks.
• The kill-a-watt suggestion isn't ideal, as this will measure the power usage (in Watts) on the mains supply side of the laptop PSU. This will always read higher than the actual power usage of the laptop due to the inevitable inefficiencies in the power supply (typically 10-40%) – tomfanning Jun 30 '09 at 12:40
• Also you'll be needing to use P=V*I (or more specifically I=P/V) to calculate the number of amps current the laptop PSU is drawing from the mains. – tomfanning Jun 30 '09 at 12:42

Basically the current rating is a maximum value. This means that a 15.6 V/8 A can replace a 15.6 V/5 A powersupply. What's important in a power supply for a laptop is that the voltage output is the same and the current are the same or above the original power supply.

You'll want to make sure the connector is really the same (same polarity, same size, it does not wiggle or anything). Ideally they would be from the same manufacturer.

• How do you check polarity? There is a symbol on charger something like `(-)--0--(+)` but not on netbook (i am have 65W laptop charger that I intend to use on lower power netbook). – Salman A Sep 13 '12 at 5:30
• If no indication is on the laptop, you can try it. Inverted polarity will result in no power sent to the laptop but should not damage the laptop. You have, of course, to make sure that the voltage, amps and connector size are compatible. Do a test without power to make sur the connector is good. then apply power and see if you see the charging light come on. – Pierre-Luc Simard Sep 25 '12 at 14:22

I purchased my HP 6730B business laptop used. It was in a very nice condition, had Windows 7 Professional and ran great until the charging unit started beeping. I could shake it at first and tap on it and it would stop beeping and work again. This got more frequent within days.

Then the tapping and the shaking stopped working, so U placed it in the freezer for 20 minutes. This also worked for another week or so. Then the beeping became louder and neither the freezer trick nor the tapping worked. So I blasted it with a hammer a few quick times and it squeeled like a stuck pig and the beeping got louder and is now steady (whereas before it was intermittent).

In any case, I got one from a friend that is lower in volts by .5. It's 18.5, not 19. Also it has less amps (3.5, not 4.7). The wattage is different: 65 watts, not 90 watts.

There hasn't been a noticeable difference, except that when the old charger had completely charged the battery the icon would read "battery fully charged", "plugged in, not charging" — now it only does this sometimes and I noticed my cooling fan is on more often, although it has gotten considerably warmer where I live, so I'm thinking this is a temporary fix.

I'm going with the original manufacturer specified unit ASAP, and although lower watts/amps/voltage works, original gear is always better.

Not quite so simple as presented above: there's a critical difference between regulated and unregulated power supplies. Unregulated ones only hit their target output voltage at their rated load, and will generate a higher than desired voltage at lower loads. A regulated power supply will always generate the expected output voltage.

The most direct way to work out whether your laptop PSUs are regulated or not is if you can get a volt meter and check the voltage directly from the plug: if it's the rated 15.6V, then they're regulated power supplies and the above comments apply: so long as the power rating is high enough, you can substitute. If you find that the output voltage is a few volts higher than you expected, then the supply is unregulated, and will drive lower-power equipment at a higher-than-specified voltage, which may cause it damage. For example, I have an unregulated 9V "wall wart" power supply for a 'phone, which from its plug (the one that goes into the 'phone) appears to generate 12V.

That said, much equipment supplied with an unregulated PSU will have internal regulation circuitry, so you may be ok anyway.

(Unregulated power supplies have fewer components, and are cheaper as a result.)

I would say that it would probably be ok to use the first power supply for the second. It really depends on what the hardware is like if the second laptop will like this or not, but as the manufacturer put the same plug on them it would seem to be OK. This is similar to how Dell, for example, has a 45 watt and a 90 watt power supply for different machines. I have used the 45 watt with the laptop that came with the 90 watt and vice versa and the only difference I have seen is battery charging time. Your mileage may vary however.

I really wouldn't try to swap them around, clearly one machine is capable of drawing more than the other and if you get it wrong then the PSU will go pop and potentially be dangerous too.

• Not necessarily, the higher amp power supply might just be the same one that is used for another computer that does require the higher amps, and they don't want to have to worry about stocking two nearly identical power supplies. – Brad Gilbert May 5 '09 at 3:25
• We have some laptops that came with a 120w power brick, but only take a 90w to run and charge a single battery. The 120w brick is only necessary in certain configurations (which we don't have). It happens occasionally; though for the most part you're right. – Chris S Oct 25 '10 at 14:58

The power supply can provide electricity at the specs given. Assuming the connectors are the same polarity and the supplies have the same voltage, they are interchangeable in a pinch. The difference in current has to do with how fast it can charge the battery. Higher capacity batteries take longer to charge, and a beefier power supply can supply more current to the battery to charge it faster.

The thing about charging a battery is that the charge/discharge cycle wears the battery out. The faster you charge it, the faster the wear. Over time this makes the battery lose its capacity. Slower charging is better (less bad, really) for the battery. Also, Lithium-Ion batteries don't like to be completely discharged or topped off.

That's probably more than you needed to know. The difference in the power supplies boils down to one being designed to provide more power to charge the battery faster. If the voltage and connector are the same, you can swap them without significantly damaging anything.

Yes, it matters, but not all the time. You can successfully use a compatible laptop power adapter of lower amperage than that of the stock adapter for your laptop. Max power draw for a laptop is at max. cpu + gpu utilization + constant optical drive use + battery charging + max. display brightness + using an external display. Unless you do all of these things, you are unlikely to run into problems when using a lower amperage power adapter. The most likely problem you'll run into is that your battery will stop charging.

Note: I make no guarantees with this advice, if you fry your laptop and/or burn your house down, don't blame me.

To share my experience, I've used a 90 W adapter to replace a 65 W adapter and the battery is dead. The battery indicator is now permanently blinking amber, on the very same day on my Lenovo laptop. The voltage rating is the same on both the adapters. So beware, not all laptops work with higher power- or current-rated adapter even though voltage is the same!

If you use a higher current rated with your laptop, your battery will lose its charge capacity more quickly over time than it would with the correct charger. The manufacturers, however, would have you believe otherwise. I have witnessed this time and time again with generic replacement adapters rated at 6 amperes used with units rated 3.42 amperes. The battery charge capacity always fades to useless in a very short period of time.

• Your side effects have nothing to do with amperage and everything to do with power line quality (the "steadiness" of the DC voltage). Cheap power bricks are cheap because they cut corners that they shouldn't. – Chris S Oct 25 '10 at 15:01