Thinkpads Forum

I have a new ThinkPad T400 and it came with a 20 volts 3.25 amps (20v 3.25a) power adapter. Can I use my old power adapter from my ThinkPad T60 on this T400. The Old adapter has the same "power tip" on it but it has a 20 volts 4.5 amps (20v 4.5a) output.

I've searched around on the Lenovo site and this site for the answer but have not been able to find anything. I've had people tell me that it would be fine to use it but I would prefer to see something from Lenovo in writing that states it's OK to use it. I don't want to damage the battery or make it less efficient in the future.

Thanks for the help.

Lots of T400 units ship with the 90W adapter. It's about integrated versus dedicated/switchtable graphics, but really you can always use a higher-wattage adapter, as long as the voltage is the same.

dr_st wrote:Lots of T400 units ship with the 90W adapter. It's about integrated versus dedicated/switchtable graphics, but really you can always use a higher-wattage adapter, as long as the voltage is the same.

Is the 20v 4.5a power supply 90W? I see that this 20v 3.25a one that came with the T400 is a 65W power supply.

scheumanrj wrote:Is the 20v 4.5a power supply 90W?

From the physics class we know that:
P [W] = U [V] * I [A]

so:
P [W] = 20 [V] * 4.5 [A] = 90 [W]

As you see, the 90W comes straight from multiplication of the voltage with the amperage. To answer your question, yes, 20V 4.5A power supply is a 90W one.

Same applies to 3.25A:
P [W] = 20 [V] * 3.25 [A] = 65 [W]

dr_st: Thanks for the quick reply
Yak: Thanks for the education. I never new that.

Can anyone point me to information from Lenovo that states that it's okay to use the 90W power supply or at least confirm dr_st's statement. No offense dr_st, I just don't know you and I want to be sure that I'm not going to damage the battery. I confirmation from another source or forum user would be great.

.

Short answer is yes, you can use a 90W adapter with a ThinkPad that ran with a 65W one.

But lets elaborate a bit. First of all, the voltage must be the same (20V in this case). Second, while running, the laptop constantly draws x watts of power. The number changes depending on many factors: current LCD brightness, CPU load, etc. It's important that the wattage of the power supply is always higher than the number of watts drawn by the laptop at any given time. This means that if 65W was enough for your T400, 90W will be more than enough. In other words if it was running smoothly on a 65W adapter, it definitely will on a 90W one too.

From what could be observed, Lenovo seem to ship their laptops with either 65W or 90W adapters depending on the graphics option. If a laptop has a power hungry dedicated graphics chip, a 90W adapter will be delivered with it. 65W ones are added to laptops with graphics integrated into the chipset which draw less power but are also less capable. Usually used for office work only.

The reason for two different power supplies is that 65W ones are smaller and lighter so they are preferred if they can cope with the appetite of the laptop for power.

This also means that 65W adapters should not be used with power hungry laptops, i.e. laptops with dedicated graphics chip that originally came with 90W adapters.

I hope that explains a bit.

PS: I would just like to add, that I've read a lot of dr_st's posts and as far as I could tell, they were always correct.

yak wrote:PS: I would just like to add, that I've read a lot of dr_st's posts and as far as I could tell, they were always correct.

Wow, you have more confidence in me than I have in myself. I'm flattered.

But I do try to keep my knowledge up to date, and expand it. This forum and its users have been a great source of knowledge to me for Thinkpads and computer technology in general.

P.S. Your elaborate explanation was excellent, I think all questions should be answered now.

One analogy I'd like to make - think of desktop computers. When buying a power supply for a desktop, one is always concerned with getting enough wattage, right? We are not concerned of getting too much wattage. Because, as yak said, you only pull what you need anyways, so if the PSU is capable of delivering more, it won't damage you in any way, but the opposite situation is risky. Same with laptops.

Wow! Quick replies with comprehensive explanations. This forum is awesome.

Thanks everyone.

This addresses a question I've had. Having taken physics many years ago I was trying to figure out whether I could use my T43 power supply with the T400 that I have on order (looking at purchasing an advanced mini dock on Craigs that is missing power cord). T43 V is 16, Amperage is 4.5. So even though multiplying those two exceeds the 65w thresh hold I cannot use it because I would need 20V instead of 16. Correct?

revcp wrote:This addresses a question I've had. Having taken physics many years ago I was trying to figure out whether I could use my T43 power supply with the T400 that I have on order (looking at purchasing an advanced mini dock on Craigs that is missing power cord). T43 V is 16, Amperage is 4.5. So even though multiplying those two exceeds the 65w thresh hold I cannot use it because I would need 20V instead of 16. Correct?

Correct - you cannot use it. The voltage is wrong.

Not to mention that the plug doesn't fit in the hole.