Wattage calc question

[Xanthe_2871]

Hi! I have a battery (185Wh/50000mAh) which can output 12V/2.5A DC. I want to use a 12v car adaptor thingy that converts to a US AC 120V outlet. How many watts can I pull at 120V AC off this battery?

 

If someone just gives me the answer that's cool, but if you have a link to a website with a calc that'd be even better! Because it can also do 20V/3A DC, and some 12V DC to AC 120V adaptors will also accept 24V DC, so maybe they'd work with 20V too? I don't know. I don't think anything would break testing that. If they pull 24v that might be within the battery tolerance, and if not it will just turn off. In theory at least. And maybe the adaptor will be happy with 20V... 

[Dujith]

Wattage stays the same. So at the other end you'll get 30W / 120 =  0.25A That will be your maximum load.

(not counting losses in the conversion)

Formula wise:

Power = Voltage x Current, so Current = Power / Voltage

[Enderman]

4 minutes ago, Dujith said:

Wattage stays the same. So at the other end you'll get 30W / 120 =  0.25A That will be your maximum load.

(not counting losses in the conversion)

Formula wise:

Power = Voltage x Current, so Current = Power / Voltage

Inverters are typically not very efficient.

[Xanthe_2871]

4 minutes ago, Dujith said:

Wattage stays the same. So at the other end you'll get 30W / 120 =  0.25A That will be your maximum load.

(not counting losses in the conversion)

Formula wise:

Power = Voltage x Current, so Current = Power / Voltage

I don't understand this math would the 20V/3A DC scenario give significantly more watts than the 30 available on the 12v?

[Dujith]

Just now, Enderman said:

Inverters are typically not very efficient.

You're assuming 100% efficiency which is impossible.

 

5 minutes ago, Dujith said:

(not counting losses in the conversion)

This was just a calculation example  

[Dujith]

1 minute ago, Xanthe_2871 said:

I don't understand this math would the 20V/3A DC scenario give significantly more watts than the 30 available on the 12v?

Well, following the forumula:

Power = 20V x 3A = 60W vs the 30W you get at 12V. So yes. You doubled the power it can deliver.

The converter will have the specs somewhere including how much loss there is. Most of the times its a table that will tell you what you get when you input a certain voltage.

[Xanthe_2871]

2 minutes ago, Dujith said:

Well, following the forumula:

Power = 20V x 3A = 60W vs the 30W you get at 12V. So yes. You doubled the power it can deliver.

The converter will have the specs somewhere including how much loss there is. Most of the times its a table that will tell you what you get when you input a certain voltage.

Ah, ok thanks!

 

Any idea if a 12V/24V adapter would be happy with 20V? 

[Dujith]

2 minutes ago, Xanthe_2871 said:

Ah, ok thanks!

 

Any idea if a 12V/24V adapter would be happy with 20V? 

I doubt it. But without knowing which adapter you mean its just guess work.

[mariushm]

7 hours ago, Xanthe_2871 said:

Hi! I have a battery (185Wh/50000mAh) which can output 12V/2.5A DC. I want to use a 12v car adaptor thingy that converts to a US AC 120V outlet. How many watts can I pull at 120V AC off this battery?

 

If someone just gives me the answer that's cool, but if you have a link to a website with a calc that'd be even better! Because it can also do 20V/3A DC, and some 12V DC to AC 120V adaptors will also accept 24V DC, so maybe they'd work with 20V too? I don't know. I don't think anything would break testing that. If they pull 24v that might be within the battery tolerance, and if not it will just turn off. In theory at least. And maybe the adaptor will be happy with 20V... 

Such a battery is most likely fake.

 

To answer your question an inverter won't be 100% efficient, assume that at best you're going to get 85% efficiency.  So if you want 120v AC at 1A  (120 watts), then the inverter will take around 120 watts x 100 / 85 = 140 watts.   So the battery must be able to provide 140 watts / 12v = 11.6 A continuously.

Assuming the battery bank produces 12v from the internal battery at 100% efficiency - which isn't the case, but let's go with it - then to produce 120v 1A  your 185Wh battery would keep producing that for 50000mAh / 11600mAh = ~ 4.3 hours ?  I'm probably wrong. In reality, probably less than 4 hours.

 

Power banks typically have lithium batteries which have a voltage between 3.7v and 4.2v and they put multiple such batteries in series to get higher voltage and then multiple such packs in parallel to increase capacity. So a battery bank like that one you mention, most likely would have 4 batteries in series to create a battery with a voltage between 14.8v and 16.8v and a capacity of around 2500-5000 mAh depending on the battery type (2500mAh for standard 18650 batteries, more for those flat rectangle style batteries)

So each of these packs would output around 15v and up to around 4-5A of current. 

The battery bank would have to put multiple such packs of batteries in parallel to get that big capacity of 50000mAh ... with typical batteries available, you would have to stack up to 10 such battery packs to get your 50 000mAh, and that's why I think the values are fake.

 

Most likely a battery bank claiming such high capacity would be at around 15000 - 22000 mAh  real capacity, stacking 4-6 lithium batteries to get 3.7..4.2v at 4-6 x 2500-4000mAh

 

[Xanthe_2871]

7 hours ago, mariushm said:

Such a battery is most likely fake.

 

To answer your question an inverter won't be 100% efficient, assume that at best you're going to get 85% efficiency.  So if you want 120v AC at 1A  (120 watts), then the inverter will take around 120 watts x 100 / 85 = 140 watts.   So the battery must be able to provide 140 watts / 12v = 11.6 A continuously.

Assuming the battery bank produces 12v from the internal battery at 100% efficiency - which isn't the case, but let's go with it - then to produce 120v 1A  your 185Wh battery would keep producing that for 50000mAh / 11600mAh = ~ 4.3 hours ?  I'm probably wrong. In reality, probably less than 4 hours.

 

Power banks typically have lithium batteries which have a voltage between 3.7v and 4.2v and they put multiple such batteries in series to get higher voltage and then multiple such packs in parallel to increase capacity. So a battery bank like that one you mention, most likely would have 4 batteries in series to create a battery with a voltage between 14.8v and 16.8v and a capacity of around 2500-5000 mAh depending on the battery type (2500mAh for standard 18650 batteries, more for those flat rectangle style batteries)

So each of these packs would output around 15v and up to around 4-5A of current. 

The battery bank would have to put multiple such packs of batteries in parallel to get that big capacity of 50000mAh ... with typical batteries available, you would have to stack up to 10 such battery packs to get your 50 000mAh, and that's why I think the values are fake.

 

Most likely a battery bank claiming such high capacity would be at around 15000 - 22000 mAh  real capacity, stacking 4-6 lithium batteries to get 3.7..4.2v at 4-6 x 2500-4000mAh

 

https://www.amazon.com/gp/aw/d/B00YP823NA#Ask

 

Lots of reviews both on Amazon and on other websites. Given it's size and weight vs known 20000 mAh packs I think it seems reasonable.

[mariushm]

Yeah, lots of reviews but nobody opens it up to check the batteries or test the capacity of individual batteries.

Noticed in one video a drawing of the insides, which makes me think there's 8 batteries ... 2 groups of 4 batteries in series .. see video below at around 0:25

If that's the case, that would make each battery around 6250 mAh in capacity, which is doable, but I suspect it's more like wishful thinking... 5000-5500mAh per battery in reality.

Anyway, yeah... that would make the DC voltage from batteries at around 15v and you get up  around 4A from a group of 4 batteries (15v x 4 = 60w), so with two groups of batteries I can see that maximum value of 130 watts they advertise doable.

 

Anyway yeah ... if you connect a 12..24v inverter directly to the battery to go around the voltage regulators that convert battery voltage down to 12v, you can probably get the inverter to output around 100w of AC voltage for a reasonable amount of time.  How much ... well, you can sort of estimate 186 wh ... one hour if consumption is steady at 186w ... two hours at around 90w and so on...

 

The most you can get out of the bank is 130 watts of DC, they say that's the maximum, though i suspect the batteries will heat a bit at continuous 130w out. Accounting for the efficiency of the inverter that means you'll have around 100 watts or around 1A at 120v.

 

 

 

anwyay, you can build your own power wall

[Xanthe_2871]

10 hours ago, mariushm said:

Yeah, lots of reviews but nobody opens it up to check the batteries or test the capacity of individual batteries.

Noticed in one video a drawing of the insides, which makes me think there's 8 batteries ... 2 groups of 4 batteries in series .. see video below at around 0:25

If that's the case, that would make each battery around 6250 mAh in capacity, which is doable, but I suspect it's more like wishful thinking... 5000-5500mAh per battery in reality.

Anyway, yeah... that would make the DC voltage from batteries at around 15v and you get up  around 4A from a group of 4 batteries (15v x 4 = 60w), so with two groups of batteries I can see that maximum value of 130 watts they advertise doable.

 

Anyway yeah ... if you connect a 12..24v inverter directly to the battery to go around the voltage regulators that convert battery voltage down to 12v, you can probably get the inverter to output around 100w of AC voltage for a reasonable amount of time.  How much ... well, you can sort of estimate 186 wh ... one hour if consumption is steady at 186w ... two hours at around 90w and so on...

 

The most you can get out of the bank is 130 watts of DC, they say that's the maximum, though i suspect the batteries will heat a bit at continuous 130w out. Accounting for the efficiency of the inverter that means you'll have around 100 watts or around 1A at 120v.

 

 

 

anwyay, you can build your own power wall

That's a lot of useful information. Thank you! 

 

I've had issues with my inverter but it might just be a bad inverter. I can't get anything to run off of it right now. I'll try a different one later.