Questions about Dell Vostro 3550 and External Battery

[Enargins]

I'm a complete noob when it comes to electrical stuff, but I have a question about something, so please bear with me.

I have a Dell laptop (Vostro 3550), and recently bought an external battery for it ("Dell Power Companion"). The external battery is rated at 12,000 mAh. Since my laptop voltage is 11.1 V, I assumed I'd be getting 12 * 11.1 = 132 WHr. Turns out, though, I'm only getting the equivalent of about a 43 WHr battery with it.

To further confuse things, my laptop voltage is 11.1 VDC. But the DC power converter's output voltage is 19.5 V.

Anyway, below are the specs for the laptop and for the external battery. If someone can explain to me why I'm only getting 43 Whr from a battery that's rated at 12,000 mAh with 11.1 volts, I'd appreciate it.

Thanks!

Laptop specs:
90W power supply: 19.5V 4.62A
Original 43 Wh laptop battery: 2.2 Ah
Laptop Voltage: 11.1VDC

External battery specs (copied from back of battery):
12000 mAh
14.8V 43 Wh
19.5V 2.3A

 

[jedishrfu]

What region of the world are you in? What are your AC specs? 120v, 240v, 50 or 60 cycles / sec or something else...

 

[Grinkle]

Power is Voltage times Current. The battery capacity is 19.5V * 2.3A = ~45Wh. It looks like the advertised capacity was de-rated to 43Wh from the specs you are showing.

To know your actual capacity, you would need to know the average current draw of the computer over the life of a single charge. You could approximate the battery voltage at a constant 14.8V, it would be lower towards the end of charge in reality but you can ignore that for first order calculations. I don't think its easy to measure the current draw of your laptop, though.

edit - 2.2Ah means 2.2A for one hour, so the more accurate calculation is -

19.5V * 2.2A * 1h = 42.9

It wasn't de-rated, I just made a mistake and missed the 2.2Ah part.

 

[Drakkith]

The external battery is rated at 12,000 mAh. Since my laptop voltage is 11.1 V, I assumed I'd be getting 12 * 11.1 = 132 WHr. Turns out, though, I'm only getting the equivalent of about a 43 WHr battery with it.

Battery capacities are given at a certain rate of discharge and going above or below can get you more or less total energy than the stated amount. If I remember correctly, drawing more amperage from the battery than what was used to rate it will give you less watt-hours/amp-hours. In other words, the higher the current you draw from the battery, the less total energy you can get from it. I'm not quite sure why.

Also, looking at the external battery, it appears that it has its own built in power/voltage converter (not sure of the technical name for this), otherwise it can't output 19.5 volts with only 4 lithium-ion cells (each cell has a maximum of 4.2 volts, and 4x4.2 = 16.8v). I'd bet you're losing energy in this conversion process.

To further confuse things, my laptop voltage is 11.1 VDC. But the DC power converter's output voltage is 19.5 V.

I've read that 19 volts is high enough to charge 4 lithium-ion cells in series and still leaves a little "headroom" for other things like the converter and controlling electronics. The laptop itself steps down the 19 volts to whatever it needs to run off of or just uses the voltage from the battery.There's a long discussion here on stack exchange about it.

 

[russ_watters]

So is this just misleading advertising? They rate the pack as if the batteries were in parallel when they are really in series?

 

[Drakkith]

So is this just misleading advertising? They rate the pack as if the batteries were in parallel when they are really in series?

No idea. I don't know if they rate each cell and then add the ratings together, rate them in parallel, or rate them in series.
Now that you mention it, I'm not even sure the cells are in series. I assumed they were, but I could be wrong. None of the technical specs I looked at gave that information. They all just said "4-cell lithium-ion".

 

[russ_watters]

No idea. I don't know if they rate each cell and then add the ratings together, rate them in parallel, or rate them in series.
Now that you mention it, I'm not even sure the cells are in series. I assumed they were, but I could be wrong. None of the technical specs I looked at gave that information. They all just said "4-cell lithium-ion".

So...I own a 10ah USB charger battery. Powering USB ports is all it does, so I'm pretty sure that's 10ah at 5V. Seems like it should be just that clear-cut here.

 

[Drakkith]

So...I own a 10ah USB charger battery. Powering USB ports is all it does, so I'm pretty sure that's 10ah at 5V. Seems like it should be just that clear-cut here.

I doubt it's even that clear cut with your own battery. Everything I've read says that the capacity of a battery is reduced as you increase the current drawn from it (perhaps a lot of power is eaten up by internal resistance?). Also, since lithium-ion batteries don't provide 5 volts, there's a DC-DC converter and some other circuitry that eats up power as well. How much I don't know.

 

[Enargins]

What region of the world are you in? What are your AC specs? 120v, 240v, 50 or 60 cycles / sec or something else...

I'm in the U.S. 120V. Don't know about the Hz.

 

[Enargins]

The battery capacity is 19.5V * 2.3A = ~45Wh.

But that's the thing. WHY is the battery capacity * 2.3A? If the battery is rated at 12,000 mAh, wouldn't that be 12 Ah, not 2.3A? (I realize I'm mixing A and Ah; but it seems that the A should be the same whether it's A or Ah.)

Anyway, that's my question -- where does the 12,000 mAh come into play? Seems that that's not there at all, and it's going by 2300 mA instead of 12000 mA.

Again, I'm a total noob, so forgive me if the answer is obvious. But I'm just not seeing why 2.3 instead of 12 is being used in the calculation.

 

[Drakkith]

External battery specs (copied from back of battery):
12000 mAh
14.8V 43 Wh
19.5V 2.3A

Okay, some further reading has revealed that lithium ion battery cells have an average voltage of 3.7 volts, and a max of near 4.2V. The 14.8 volts here is likely from four 3.7 V cells in series, and your laptop's battery voltage of 11.1V is just 3.7V x 3. Assuming the battery can actually provide 43 watts for an entire hour (for 43 watt-hours), dividing 43 W by 14.8V gives 2.9 A, while dividing by 19.5V gives about 2.2A, which is close to the advertised 2.3A. Since watt-hours is actually a measure of energy, while amp-hours aren't, the 43 Wh is the number you should be looking at. As you've already discovered, this is likely correct. Also, if we divide 43 Wh by 12 Ah we get about 3.6 volts, close to the 3.7V for each cell. Perhaps the advertised mAh rating is simply the rating of a single cell that was then multiplied by four? I can't see any other explanation at this time as to why the mAh rating and the Wh rating are so different when you do the math. Very confusing...

 

[jedishrfu]

This article describes the amps vs amp-hours for batteries and why they aren't the same:

https://www.allaboutcircuits.com/textbook/direct-current/chpt-11/battery-ratings/

For example, an average automotive battery might have a capacity of about 70 amp-hours, specified at a current of 3.5 amps. This means that the amount of time this battery could continuously supply a current of 3.5 amps to a load would be 20 hours (70 amp-hours / 3.5 amps). But let’s suppose that a lower-resistance load were connected to that battery, drawing 70 amps continuously. Our amp-hour equation tells us that the battery should hold out for exactly 1 hour (70 amp-hours / 70 amps), but this might not be true in real life. With higher currents, the battery will dissipate more heat across its internal resistance, which has the effect of altering the chemical reactions taking place within. Chances are, the battery would fully discharge some time before the calculated time of 1 hour under this greater load.

 

[russ_watters]

Okay, some further reading has revealed that lithium ion battery cells have an average voltage of 3.7 volts, and a max of near 4.2V. The 14.8 volts here is likely from four 3.7 V cells in series, and your laptop's battery voltage of 11.1V is just 3.7V x 3. Assuming the battery can actually provide 43 watts for an entire hour (for 43 watt-hours), dividing 43 W by 14.8V gives 2.9 A, while dividing by 19.5V gives about 2.2A, which is close to the advertised 2.3A. Since watt-hours is actually a measure of energy, while amp-hours aren't, the 43 Wh is the number you should be looking at. As you've already discovered, this is likely correct. Also, if we divide 43 Wh by 12 Ah we get about 3.6 volts, close to the 3.7V for each cell. Perhaps the advertised mAh rating is simply the rating of a single cell that was then multiplied by four? I can't see any other explanation at this time as to why the mAh rating and the Wh rating are so different when you do the math. Very confusing...

This really annoys me. I have trouble interpreting it as anything but false advertising. It's like when fan manufacturers give the airflow and pressure of a fan at different operating points. I guess it's ok to screw with consumers (?), but if I fell for it, I'd get sued for installing the wrong fan.

...And we have an electrical department at my office. Can you imagine sizing an electrical system for an amperage capacity without specifying the voltage? Oops, bought the wrong size transformer!

This article describes the amps vs amp-hours for batteries and why they aren't the same:

https://www.allaboutcircuits.com/textbook/direct-current/chpt-11/battery-ratings/

That's fine(difference between theory and reality), but it isn't the OP's problem: the problem (as i see it) is the theoretical numbers don't even compute because the manufacturer is intentionally misleading about what voltage goes with what amperage to mislead about the battery capacity.

I don't think they play that game with the internal batteries, because it is less plausible when the pack only outputs one voltage.

...though I'm not even sure about that, since one I just Googled says 11.1V *and* 7200 mAh, instead of 7200 mAh *at* 11.1V.

 

[Grinkle]

But that's the thing. WHY is the battery capacity * 2.3A? If the battery is rated at 12,000 mAh, wouldn't that be 12 Ah, not 2.3A? (I realize I'm mixing A and Ah; but it seems that the A should be the same whether it's A or Ah.)

Anyway, that's my question -- where does the 12,000 mAh come into play? Seems that that's not there at all, and it's going by 2300 mA instead of 12000 mA.

Again, I'm a total noob, so forgive me if the answer is obvious. But I'm just not seeing why 2.3 instead of 12 is being used in the calculation.

The answer is not obvious, you are asking valid questions. Can you post a picture of the 2 labels you reference in the original post? I wonder if there is any other context that might be on the label to explain the 12Ah number - its very odd, I agree. I can't think of any better explanation than others have already posted - hard to think of a pair of use-contexts where 12 and 2.3 can each be correct.

 

[jedishrfu]

BAsed on the previous article I posted then your battery having a capacity of 12 amp hours and maximum output of 2.3 amps would mean the battery would last for about 5.2 hours before its exhausted.

 

[Grinkle]

BAsed on the previous article I posted then your battery having a capacity of 12 amp hours and maximum output of 2.3 amps would mean the battery would last for about 5.2 hours before its exhausted.

I read the article this way as well, and imo it is a reasonable interpretation of 12Ah / 2.3A as provided specs. I can't make that seem consistent with the other specs printed on the battery; 2.3A @ 19.5V * 5.2H = 234Wh, and the label also says the battery capacity is 43Wh at 14.8V.

 

[russ_watters]

BAsed on the previous article I posted then your battery having a capacity of 12 amp hours and maximum output of 2.3 amps would mean the battery would last for about 5.2 hours before its exhausted.

...assuming the capacity and output ratings are provided at the same voltage. From the numbers provided by the OP, it looks to me like 12 Ah / 2.2 A = 1 hr

From the OP, it looks to me like Dell is selling two different versions of the same battery, one accurately rated at 2.2ah and one inaccurately rated at 12ah.

 

[Enargins]

The answer is not obvious, you are asking valid questions. Can you post a picture of the 2 labels you reference in the original post? I wonder if there is any other context that might be on the label to explain the 12Ah number - its very odd, I agree. I can't think of any better explanation than others have already posted - hard to think of a pair of use-contexts where 12 and 2.3 can each be correct.

I don't think I missed anything, but I uploaded a picture from the back of the external battery with the specs.

Also, here is the spec document for the external battery (discussing only the power DC jack here, not the USB port): http://www.sets.com.lb/wp-content/uploads/2015/05/Dell-Power-Companion.pdf

And here is the Owners Manual for the laptop, which has the specs for 4 different laptop models. The electrical specs for the 3550 are only pp. 108-110. To save you the trouble, here's what it says:

Battery:
Vostro 3450/3550/3555 Type • 6-cell "smart" lithium ion (2.2 AHr)
Voltage 11.1 VDC
AC Adapter:
Input voltage 100 VAC to 240 VAC Input current (maximum) 1.5 A / 1.6 A / 1.7 A / 2.3 A / 2.5 A
Input frequency 50 Hz to 60 Hz
Output power 65 W, 90 W, or 130 W [My model uses the 90W power supply.]
Output current: 65 W • 4.34 A (maximum at 4- second pulse) • 3.34 A (continuous) 90 W • 5.62 A (maximum at 4- second pulse) • 4.62 A (continuous) 130 W 6.70 A (continuous)
Rated output voltage 19.5 +/– 1.0 VDC

 

[Grinkle]

And on the battery itself, it says -

12000 mAh
14.8V 43 Wh
19.5V 2.3A

?? The datasheet does not say anything about 43Wh @ 14.8V. The datasheet very clearly calls this a 12Ah battery.

 

[Drakkith]

?? The datasheet does not say anything about 43Wh @ 14.8V. The datasheet very clearly calls this a 12Ah battery.

Indeed. Hence the immense confusion in this thread. The battery is labeled as 43 Wh with no Ah listed, but the datasheet does the opposite.

Edit: Wait. On the back of the battery, middle right, it clearly says EU 2200 mAh...

 

[Grinkle]

The product overview sheet (its not really a datasheet, I don't think) is wrong. I don't know what else to conclude.

 

[Enargins]

The datasheet does not say anything about 43Wh @ 14.8V. The datasheet very clearly calls this a 12Ah battery.

That was taken from the information on the back of the external battery. See the picture I attached.

 

[Enargins]

On the back of the battery, middle right, it clearly says EU 2200 mAh...

You're right! If EU stands for European Union, then here's a thought. The battery is a 43 WHr battery. In the EU, perhaps they use the maximum voltage (19.5 V) to specify mAh. In the US, though, perhaps they use average voltage (3.7 V) to specify mAh. (That's just speculation on my part.)

Thus, the EU rating is 43/19.5 = 2.2 Ah, whereas the US rating is 43/3.7 = 11.6 Ah, rounded up to 12 Ah (either that, or they used 3.6V and got 12 Ah).

Thus, it could be a difference between which voltage is used -- the maximum voltage, resulting the more-realistic 2.2 Ah, or the average voltage, resulting in the higher 12 Ah.

(Which raises the new question: why not just list the Whr, instead of the mAh, in the first place? They do that with batteries they sell for and include with the laptops.)

 

[Drakkith]

Thus, the EU rating is 43/19.5 = 2.2 Ah, whereas the US rating is 43/3.7 = 11.6 Ah, rounded up to 12 Ah (either that, or they used 3.6V and got 12 Ah).

Thus, it could be a difference between which voltage is used -- the maximum voltage, resulting the more-realistic 2.2 Ah, or the average voltage, resulting in the higher 12 Ah.

That's a fair guess in my opinion.

 

[Grinkle]

In the US, though, perhaps they use average voltage (3.7 V) to specify mAh.

They rate the pack as if the batteries were in parallel when they are really in series?

I think these two lines of thought get one to the same end point more or less, but I suspect its closer to what Russ said.

Do capacities add? Sure. Add up the capacities of the individual cells for the pack product sheet, then.

 

[russ_watters]

I just bought this:
https://www.amazon.com/dp/B01HEVEDTG/?tag=pfamazon01-20

Since I'm wondering about its performance due to this thread, I'm currently testing it with my laptop and a Kill-a-Watt.

The Amazon page says very little about its capacity other than the 24000 mah, but it does have a little hint: 16.8V @ 2.6A input takes 3 hours to charge. If we get 80% charge efficiency, that would be 104 watt-hours. Since the main output is 5V, that equates to 20800 mah, so life appears good. This one's a much simpler problem due to the lack of other voltage options makes it harder to justify any other rating voltage. I have another power tank which might have the issue though, since it will power a laptop on DC...

The poorly written user manual pamphlet does give better info. Among other things, it gives efficiency (!), at 85%, so updating the above numbers stream-of-consciousness style gives 22100 mah. 8% off; not bad. It also gives a sample run time of 1.5hr+ at 65W, which equates to 95 watt-hours+.

My laptop burns about 15W at idle, 40W under a healthy graphics load. So I can expect 4-5 hours of life from this thing. I'll take that. Using this for my laptop and my other power tank for my telescope will power me through the eclipse if needed...

 

[russ_watters]

Well, that was quite a bust! I think in AC mode and/or at high current it must be much less efficient at getting the power out. I only got 1:24 to 20%. And the kill-a-watt only reads in hundredths of a kWh, so not enough precision for a good estimate (but on the order of 15-30 Watt-hours).

 

[Enargins]

I think these two lines of thought get one to the same end point more or less, but I suspect its closer to what Russ said.

Do capacities add? Sure. Add up the capacities of the individual cells for the pack product sheet, then.

An interesting followup: I was reading an article recently about a megabattery pack, with about 420 Wh of power. The interesting thing is that the author of the article cited the mAh and then in parenthesis stated what that was in Wh. When you do the math, turns out he was using 3.7V to do the conversion as well. So that seems to be the standard, as the average voltage, I guess. Anyway, found that interesting.

 

[russ_watters]

Well, that was quite a bust! I think in AC mode and/or at high current it must be much less efficient at getting the power out. I only got 1:24 to 20%. And the kill-a-watt only reads in hundredths of a kWh, so not enough precision for a good estimate (but on the order of 15-30 Watt-hours).

I tried my other one, a Spirit model A8, rated at 18,000 mAh. It has a voltage converter to directly output DC to a laptop. It gave about 2 hours before kicking off (2:15 before I noticed, and my laptop was at 89% battery). Better, but still not great considering its rating.

 

[Tom.G]

External battery specs (copied from back of battery):
12000 mAh
14.8V 43 Wh
19.5V 2.3A

• The de-facto standard these days for power input to laptop computers is 19.5VDC. The laptop internally converts this to whatever voltage the internal batteries need to charge. The current the laptop draws is typically between 2A to 5A.
  
• According to the "Tech Spec" sheet you supplied, the external battery pack internally has 4 Lithium-Ion cells with a total CURRENT rating of 12000mAh or 12Ah capacity. There is no indication of what the internal VOLTAGE is.
  
• If the cells are in series, the nominal internal voltage would be 14.8V, yielding an energy content of 14.8V x 12Ah = 177Wh.
  
• If the cells are in parallel the nominal internal voltage would be 3.7V, yielding an energy content of 3.7V x 12Ah = 44Wh. This matches the rating of 43Wh stated in post #1, copied from the back of the battery.
  
• Therefore there are four 3Ah cells wired in parallel for 12000mAh at 3.7v, yielding 4x3x3.7 = 44Wh

All the above relates to the amount of energy the external pack can store.Now the pack has some internal voltage converters to supply the 5V USB outputs and to supply the 19.5V to recharge the laptop.

The 19.5V output has a maximum current capability of 2.3A. This is 44.8W, or at full load about 1 hour to full discharge, if there is no load on the USB outputs.

Conclusion: Even though it is scattered, all the information is supplied. Probably doesn't reach 'False Advertising' level, but is certainly obfuscated and misleading.

p.s. I have no idea where the 14.8V comes from on the battery sticker ( as shown in post #1)

 

[Drakkith]

p.s. I have no idea where the 14.8V comes from on the battery sticker ( as shown in post #1)

See post #11 and see if helps.