Laptop power supplies, does current matter?

Question

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?

Answer 1

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).

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.)

Answer 6

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.

Answer 7

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.

Answer 8

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.

Answer 9

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.

Answer 10

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.

Answer 11

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!