Lipos revisited

[BaDaBooM]

I don't think you can make a comparison like this without taking consideration of the different voltages that the batteries are using to supply the board. The 8.4 volt supply with only have to carry half as much currant to the chip as the 4.2 volt supply for an given output, I think. Its comparing apples with oranges. I stand by what I said before though, to get the best battery life from a regulated device use a higher resistance coil, higher volts and lower amps and the battery will be more efficent.

You are right about the coil, best option would to build a 3ohm coil on it and I think it would in prove battery life.

 

[RicModMaker]

Am I way off here by comparing batteries to tanks of water? If 2 tanks are joined together, they hold twice the amount of water as 1 tank? Surely drawing the same amount from each setup will mean the 2 tanks will last twice as long as the single tank?

 

[Chegs]

I've just finished recoil of my Mephisto(1 ohm is now 1.5 ohms) and I'll try to time the discharge, but the in line voltmeter I have installed is way out(cells read as high as 8.56V straight off the charger which is wrong as the chargers meter says 8.4V)

I was then going to test the newly constructed pack,(but just remembered that's also configured for 8.4V) at 4.2 V but brain fade disease sucks.

If I decide to do the first cell pack in similar fashion to todays(made a little square of fibreglass to add support to the contacts on the lipo cells)I'll do a 2 cell pack at 4.2 V whilst the fibreglass cures.

 

[fasteddie600]

There seems to be some confusion re battery capacity.
I will have a go at explaining this. This is my 3rd attempt at writing this post so hopefully it will make sense.
Note-I have used 3.7V & 7.4V to simplify matters, the voltage obviously depends on charge. Also capacity changes under different loads but we will use the rated figures.

MAH's & Wh's

First mAh's. We all use this to indicate battery capacity, which in simple terms it does. However, it is not a particularly useful measure to compare batteries across different voltages without properly understanding it. The mAh rating indicates that the battery can deliver ---mA for 1 hour at it's rated voltage.

For example- An 18650 3.7V 2000mAh battery can deliver 2000mA(2A) at 3.7V for 1 hour. It can also do 1A for 2 hours. or 4A for 1/2 hour etc

This is the- current capacity -of the battery.



To compare a 3.7V battery to a 7.4V battery we want to know what the power(watts) capacity is. This is Wh(Watt hours), you may see this on some batteries(some of my AW's have this on)
This allows us to compare the actual amount of energy stored in cells across voltages as this figure takes account of voltage.

To calculate the Wh of a battery you multiply the voltage by the Ah rating(if you have mAh, divide it by 1000 to get Ah). V x Ah = Wh


Using the same 18650 for example-3.7V 2000mAh(2Ah) battery. Multiply voltage by Ah. 3.7V X 2Ah =7.4Wh
So this 18650 has a power capacity of 7.4Wh which means it can run- 7.4W for 1 hour, or, 3.7W for 2 hours, or, 14.8W for 30mins,


Multiple batteries-

If we add multiple batteries to make a parallel pack (3.7V) we add the mAh rating of the batteries together and the voltage stays the same.
If we make a Series battery pack we add the voltage of the batteries together and the mAh rating stays the same as one battery.


Say we take 2 of those 18650 2000mAh batteries and run them in parallel. We add the mAh ratings- 2000mAh + 2000mAh. Voltage stays the same.
We get a Parallel(1S) 3.7V 4000mAh(4Ah) battery pack.

Now if we take 2 and run them in series then we add the voltage- 3.7+3.7. The mAh rating stays the same as one battery.
We get a Series(2S) 7.4V 2000mAh(2Ah) battery pack.


It now looks like we have less power in the 7.4V pack because 2000mAh is less than 4000mAh, but comparing the power capacity shows they have the same.


Using VxAh=Wh here is how it works out-

Parallel- 3.7V X 4Ah = 14.8Wh

Series- 7.4V X 2Ah = 14.8Wh

Both packs can run 14.8 Watts for 1 hour, or, 29.6W for 30mins, or, 59.2W for 15mins.


Assuming a chip (like DNA30 or SX350) to be running at 100% efficiency then it would make no difference(if the board allowed) to use either series of parallel packs from the example. They would both run the same wattage for about the same time.




Board efficiency, wiring losses, battery discharge profile, load, temperature, etc all play a role in battery life and are affected by voltage so other factors have to be considered to determine battery life and configuration. Typically 7.4V will give lower losses/longer life given the lower loads to achieve the same power as 3.7V.


Hopefully that explains something, let me know if anything is incorrect.




A little bit I found which may also help explain battery capacity-

The energy stored in a battery (or its capacity) is expressed in watt-hours (Wh). For historical reasons, the rated voltage in volts (V) and the charge in ampere-hours (Ah) are sometimes also stated. In this case, the energy stored in the battery is calculated by multiplying the voltage by the charge.
Unfortunately, there are very few areas of technology where manufacturer's data are as confusing as with battery capacity. For example, you might think that a battery specified as 12 V and 100 Ah, which therefore stores 1200 Wh of energy in nominal terms, should be able to deliver 100 A of current, or 1200 W of power, for one hour. However, you would be wrong.
As a result of the internal resistance of the battery, the available capacity falls off with rising current.

 

[RicModMaker]

There seems to be some confusion re battery capacity.
I will have a go at explaining this. This is my 3rd attempt at writing this post so hopefully it will make sense.
Note-I have used 3.7V & 7.4V to simplify matters, the voltage obviously depends on charge. Also capacity changes under different loads but we will use the rated figures.

MAH's & Wh's

First mAh's. We all use this to indicate battery capacity, which in simple terms it does. However, it is not a particularly useful measure to compare batteries across different voltages without properly understanding it. The mAh rating indicates that the battery can deliver ---mA for 1 hour at it's rated voltage.

For example- An 18650 3.7V 2000mAh battery can deliver 2000mA(2A) at 3.7V for 1 hour. It can also do 1A for 2 hours. or 4A for 1/2 hour etc

This is the- current capacity -of the battery.



To compare a 3.7V battery to a 7.4V battery we want to know what the power(watts) capacity is. This is Wh(Watt hours), you may see this on some batteries(some of my AW's have this on)
This allows us to compare the actual amount of energy stored in cells across voltages as this figure takes account of voltage.

To calculate the Wh of a battery you multiply the voltage by the Ah rating(if you have mAh, divide it by 1000 to get Ah). V x Ah = Wh


Using the same 18650 for example-3.7V 2000mAh(2Ah) battery. Multiply voltage by Ah. 3.7V X 2Ah =7.4Wh
So this 18650 has a power capacity of 7.4Wh which means it can run- 7.4W for 1 hour, or, 3.7W for 2 hours, or, 14.8W for 30mins,


Multiple batteries-

If we add multiple batteries to make a parallel pack (3.7V) we add the mAh rating of the batteries together and the voltage stays the same.
If we make a Series battery pack we add the voltage of the batteries together and the mAh rating stays the same as one battery.


Say we take 2 of those 18650 2000mAh batteries and run them in parallel. We add the mAh ratings- 2000mAh + 2000mAh. Voltage stays the same.
We get a Parallel(1S) 3.7V 4000mAh(4Ah) battery pack.

Now if we take 2 and run them in series then we add the voltage- 3.7+3.7. The mAh rating stays the same as one battery.
We get a Series(2S) 7.4V 2000mAh(2Ah) battery pack.


It now looks like we have less power in the 7.4V pack because 2000mAh is less than 4000mAh, but comparing the power capacity shows they have the same.


Using VxAh=Wh here is how it works out-

Parallel- 3.7V X 4Ah = 14.8Wh

Series- 7.4V X 2Ah = 14.8Wh

Both packs can run 14.8 Watts for 1 hour, or, 29.6W for 30mins, or, 59.2W for 15mins.


Assuming a chip (like DNA30 or SX350) to be running at 100% efficiency then it would make no difference(if the board allowed) to use either series of parallel packs from the example. They would both run the same wattage for about the same time.




Board efficiency, wiring losses, battery discharge profile, load, temperature, etc all play a role in battery life and are affected by voltage so other factors have to be considered to determine battery life and configuration. Typically 7.4V will give lower losses/longer life given the lower loads to achieve the same power as 3.7V.


Hopefully that explains something, let me know if anything is incorrect.




A little bit I found which may also help explain battery capacity-

The energy stored in a battery (or its capacity) is expressed in watt-hours (Wh). For historical reasons, the rated voltage in volts (V) and the charge in ampere-hours (Ah) are sometimes also stated. In this case, the energy stored in the battery is calculated by multiplying the voltage by the charge.
Unfortunately, there are very few areas of technology where manufacturer's data are as confusing as with battery capacity. For example, you might think that a battery specified as 12 V and 100 Ah, which therefore stores 1200 Wh of energy in nominal terms, should be able to deliver 100 A of current, or 1200 W of power, for one hour. However, you would be wrong.
As a result of the internal resistance of the battery, the available capacity falls off with rising current.

Thanks fella. That's what I thought. Thanks for confirming it for me. I thought I was missing something for a while there. :-)

 

[RicModMaker]

Got my Hobbyking delivery so now have an Eco Six balance charger and some LiPos.
Can I get the bugger to work?
I've tried a laptop power supply: Input Vol Err
I tried another power supply with variable voltages: Input Vol Err.
I'm guessing the laptop power supply is too high as it's at 19v and the other doesn't have enough amps.
Has anyone else got one of these and if so, what power supply did you use?
Thanks
Ric

 

[fasteddie600]

I have one. 19v won't work(I did try). I am using a bench power supply set at 12v and it works without issue. Take a look at Maplins they do several 12v supplies but they are expensive(£30ish)

I can't remember what it needs for input but I think it is 10-18v and 3A but don't quite me on that.


Sent from somewhere inside a huge cloud!

 

[BaDaBooM]

Got my Hobbyking delivery so now have an Eco Six balance charger and some LiPos.
Can I get the bugger to work?
I've tried a laptop power supply: Input Vol Err
I tried another power supply with variable voltages: Input Vol Err.
I'm guessing the laptop power supply is too high as it's at 19v and the other doesn't have enough amps.
Has anyone else got one of these and if so, what power supply did you use?
Thanks
Ric

[h=3]Input power is 11-18v, perfect for car batteries or pc/laptop powersupply.[/h]
Tried it on your car battery ??

 

[fasteddie600]

If you don't mind how it looks, this should do the job nicely
CIT Micro ATX 300W Fully Wired Efficient Power Supply - Ebuyer

Will require a little modding, but I am sure that would be no issue to you.


Note- you will get voltage errors from those little 12v plug type adapters as they do not give a true 12vDC due to the way theAC is converted, the actual voltage output looks a bit like an AC wave with the bottom chopped off.


Sent from somewhere inside a huge cloud!

 

[RicModMaker]

Input power is 11-18v, perfect for car batteries or pc/laptop powersupply.


Tried it on your car battery ??

Not yet. Hold on, I'll run my car through my office wall so I can charge up my lipos