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External batteries mAh scaling

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truongster

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Hey fellas, found a forum that might be of great use in the future.

Firstly to starts things off. How does mAh scale in portable electronic device batteries.
E.g. The batteries in phones.

For example the SG3 has a 2100mAh battery at 4.35 V - It starts here when fully charged and it's rated for 3.8V. with 7.98 Wh.

Just from my year 9-10 Electronics class I have a very basis understanding of what it all means.

My question is, does mAh scale linearly, e.g. 2000mAh is lasts twice as long as 1000mAh in normal operation.
I know it's milliAmps per HOUR so I just want to affirm this.

Secondly if a battery that is in use has a 2100mAh battery, and on the markets there are advertised 3000mAh external batteries for recharging your phone on the go.
What I am confused is the external batteries are outputing 5Vs, does this make a difference in terms of mAh. So if a 4200mAh external battery can recharge a depleted phone battery twice?


Lastly, I understand some physics and distribution of energy. If both the external battery and the phone battery are rated at 2100mAh and the external battery has 5v Output into the phone's input, wouldn't connecting the two charge the depleted battery to half so that there is equal energy in both batteries?

Thanks in Advance~
 
My question is, does mAh scale linearly, e.g. 2000mAh is lasts twice as long as 1000mAh in normal operation.
Generally speaking, yes.
The devil is in the detail, it could be that for a given load the 2000mAh battery lasts more than twice as long as the 1000mAh battery.


I know it's milliAmps per HOUR so I just want to affirm this.
WRONG!
Lets forget the milli part for now.

It is AmpereHours. The product of current (in Amps) and time (in Hours).
So a battery which can provide 2 amps for 5 hours is rated at 10AmpereHours.

Amperes per Hour would be current divided by time.
In relation to a battery as far as I know, this has no meaning.

So, it is milli-AmpereHours, NOT milli-Amperes per Hour.

JimB
 
[quote[For example the SG3 has a 2100mAh battery at 4.35 V - It starts here when fully charged and it's rated for 3.8V. with 7.98 Wh.

It takes 4.35 volts to fully charge the battery. Once charged it's output will be 3.8 volts.


My question is, does mAh scale linearly, e.g. 2000mAh is lasts twice as long as 1000mAh in normal operation.
I know it's milliAmps per HOUR so I just want to affirm this.

Yes, it may actually last a little longer because most batteries have their maximum capacity at lower discharge rates.

Secondly if a battery that is in use has a 2100mAh battery, and on the markets there are advertised 3000mAh external batteries for recharging your phone on the go.
What I am confused is the external batteries are outputing 5Vs, does this make a difference in terms of mAh. So if a 4200mAh external battery can recharge a depleted phone battery twice?

I think most on the go chargers output 5 volts as most phones recharge from USB voltage. There will be some losses from boosting the charging battery voltage to 5 volts and some losses in the cell phone charger.


Lastly, I understand some physics and distribution of energy. If both the external battery and the phone battery are rated at 2100mAh and the external battery has 5v Output into the phone's input, wouldn't connecting the two charge the depleted battery to half so that there is equal energy in both batteries?

If you just hooked the 2 batteries together that would be true - although the current would be very high when you connected the discharged battery to the fully charged one. This is why the charger has a circuit to raise the battery voltage to 5 volts even when it may be discharged to 3 volts. This allows the charger to pull all the capacity from the charge battery. The charger also has a circuit to limit the charge current to a safe level.

Thanks in Advance~[/QUOTE]
 
It takes 4.35 volts to fully charge the battery. Once charged it's output will be 3.8 volts.

No it won't. If you charge such a cell at that voltage, you will damage it.

This is why the charger has a circuit to raise the battery voltage to 5 volts even when it may be discharged to 3 volts. This allows the charger to pull all the capacity from the charge battery. The charger also has a circuit to limit the charge current to a safe level.

Are you trying to cause a fire?

Li-Ion batteries or cells as they are single devices charge at constant current to 4.2 volts. Then charge at constant voltage until the current drops to usually Capacity/10.

If you put such cells across a 5V supply you are going to cause serious damage which could result in a fire.

They hold 4.2 volts for a small time then stay at 3.7v during their discharge. Eventually running flat when this voltage starts to drop below about 3.6V.

Mobile phones contain complex charging regimes that allow a 5V input (USB or Wall plugs) and manage the charging of dead cells to fully charged cells to 4.2V.

Your advice on this subject is nothing short of dangerous.
 
If it is a single LiIon battery charger with a 4200 mAH battery it uses a boost switching regulator to boost the 3.5v to 4.2 vdc LiIon battery voltage from the external battery up to 5 volts for the charger. The switcher circuit in the range of 85% efficiency. In simple terms the 4.2 AH @ 3.85v median from the external battery yields about 16 WH's. 85% efficiency through the switcher reduces this to 13.7 WH. Delivered at 5.0 vdc, this translates to 13.7 WH's / 5 vdc = 2.75 AH's. This means your 4200 mA external battery is only about 2750 mAH's effective for phone charging. Biggest loss is using 5 vdc for charger input.

Now things depend on cellphone manufacturer and their highly custom and different method of charging more the 0.5 amp limitation of USB. This is usually a particular voltage level set by resistors on the USB data lines. For example Apple sets different voltages via divider resistors from the 5 vdc Vbus of the USB to tell the iPhone or iPad how much current the particular charger can supply. An iPad with its charger will draw up to about 2.1 amps during charging where an iPhone will only draw about 900 mA based on the capacity of their batteries when hooked to their respective model's charger. Max charge current to charge the battery is about 80% of mAH's of battery in mA's. A 1000 mA battery is charged at a maximum of about 800 mA.

Achieving maximum charge current means the external battery powered charging station must have the right setup on the USB data pins. If it does not then the charge may be limited to 0.5 amps or perhaps even 100 mA which the maximum USB load without negotiating approval from the external USB host to move from 100 mA to 500 mA. A dumb charger will not be able to complete the negociation. Many phones just ignore the letter of the law of the USB spec and go ahead and and try to suck 500 mA. Many laptops will just shut down the USB port power if a phone just blindly tries to suck 500 mA from them. The laptop's reason for shutting down the USB port power is for survival of its own limited battery life.

Most desktops will supply 500 mA without proper USB negotiation.

If your head is not spinning yet, there is more. Many phone's unique matching chargers have current limiters within their switching power supply. The phone has two regulating modes for charging. First, if it recognizes its particular correct mating unique charger, it will go into a switching mode. In this mode the phone's series pass regulator transistor fully turns ON to a low resistance state. This causes the external charger to go into current limit which is matched to the charging current safe for the phone's battery mAH size. The 5v output from the charger will drop in voltage to near phone battery voltage. The advantage of doing this is it minimizes the heat generated in the phone's internal series pass transistor.

If the phone does not detect its unique charger, it can go to the 500 mA limit or maybe even the 100 mA limit of the USB spec. To do this the phone's charge regulator must operate in linear mode. The series pass transistor in the phone heats up a little but the current is also reduced to keep heating under control. Most of the time it can regulate to 500 mA, unless the phone's battery voltage is very low which causes more voltage drop across the phone's regulator transistor that causes more heating. If you totally sucked the phone's battery to point the phone shut itself off, the battery charging will start off at 100 mA until battery voltage reaches about 3.3v to 3.4 v then it switches to full current recharging. This is a LiIon safety function.

Now if your external battery/switcher charger is uniquely matched to phone with proper current limiting in it boost switcher to 5vdc, it too will regulate the current and the 5v will drop to near battery voltage. The switcher efficiency in this mode is a bit lower then 85% but it is more then made up by now the power conversion is to near phone's battery voltage instead of 5vdc. So you would effectively get more then the previously calculated effective 2750 mAH. A generic external battery pack charge will not likely operate in this current limit mode since there is such a wide variety of current limits necessary for a given phone's battery size. A pack specifically designed for the phone model is more likely to have the ability to operate in the more efficient charge current limited mode. Best bet is a pack specifically designed for an iPhone since it is well defined what the current limiting point should be.
 
You are correct on the 4.2 volts, but perhaps you should read the rest again.

It takes 4.35 volts to fully charge the battery. Once charged it's output will be 3.8 volts.

Bad science. I stand by my original post. 4.35V will overheat the battery and at the best will kill its lifespan. Moreso if it is not fitted with a protection circuit.
 
Hi Guys

Looking briefly at what has been posted here applies to batteries in general. For example the little PP3 9V NiMH Rechargeable which I know well.

I am throwing the cat amongst the pigeons here. So be it.

The little PP3 rechargeables I own are rated at 250 mAH. They need to be charged up to a maximum of 10.15V....even though they only output 8.4V on load....So, the charger's job is to pull them up to the ideal 10V and hold it there forever. The battery likes that and tapers off and trickle charges forever and a day without any worries of overcharging.....

Nothing complicated at all.

Have I missed something :confused:

Regards,
tvtech
 
a 2000mAh Battery outputting at 5v compared to 2000mAh battery outputting at 2.5V

In terms of charge do they have the same "energy storage"

The external batteries output at 5V and have a rating at 4200mAh in comparison to the 3.8V 2100mAh of the phones.
 
Hi Guys

Looking briefly at what has been posted here applies to batteries in general. For example the little PP3 9V NiMH Rechargeable which I know well.

I am throwing the cat amongst the pigeons here. So be it.

The little PP3 rechargeables I own are rated at 250 mAH. They need to be charged up to a maximum of 10.15V....even though they only output 8.4V on load....So, the charger's job is to pull them up to the ideal 10V and hold it there forever. The battery likes that and tapers off and trickle charges forever and a day without any worries of overcharging.....

Nothing complicated at all.

Have I missed something :confused:

Regards,
tvtech

NiMH charge system is different then Li-Ion. NiMH can take a continuous overcharge of between 5% and 10%C rating. LiIon cannot take overcharge. When the LiPO battery rises to 4.2 vdc and current tapers to about 5 % C rating the charging is terminated. It will damage a LiPO cell and potential exposive result if left on continuous charge at 4.2 vdc.

Other safety details on LiPO, if cell is less then 1.0v trash it. If between 1.0 v and 3.0 vdc start out with a 0.1 C charge rate, when (and if) cell voltage gets above 3.0 vdc (no more then 30 mins to get to 3.0v or trash battery) then charge current can be increased to 0.8 C. There is no such thing as a 1 hour quick charge of LiPO that is totally depleted. There are companies that apply 1.0 C on battery, which stresses the battery and results in about 80% capacity when it hits 4.2v max charge voltage cap. It takes about 2 1/2 hours to fully recharge a fully depleted LiPO battery.

Many of the new battery powered tools are using LiFePO4 batteries which can take rapid charging but their nominal cell voltage is 3.2 vdc compared to 3.85v for LiPO. These LiFePO4 must also have a terminated charger system and cannot take a continuous overcharge.

Swithing back to NiMH batteries. They have a nominal cell voltage of 1.25vdc. A nine volt battery will have six in series, just like their old NiCd cousins. They will go to up to 1.78v per cell on a high charge rate but should not be allowed to go higher then this. Shortly after charging they will be about 1.45 vdc, dropping to about 1.35v over several hours off the charger.
They can be controlled rapid charge up to about 1.0 C but must be terminated by cell voltage or heat detection. NiMH have a lot of similar characteristics of NiCd battery but NiMH do not have the 'memory' problem of NiCd batteries. Toyota Prius Hybrids uses NiMH batteries.
 
Hi RCinFLA

Interesting to read about Li-Ion. My dealings have been mainly with NiMH PP3 over the last few years. I have practiced the attached guide to the letter with PP3's from various Manufacturers and all I can say is that it is spot on advise. With a tweak of course...

My PP3 250mAH are charged with a Constant Voltage charger with Current Limiting. Voltage set at 10V. Current limiting set at 120mA.

This brings a totally flat PP3 up to full charge in under Three Hours without any drama or heat issues or complicated cut-off circuitry :)
And you can leave the battery in the charger for as long as you want without any overcharging issues/worries.

After literally hundreds of charging cycles even my very first PP3 is still going strong after three Years with no sign of falling capacity. PP3's are beauts once you understand them ;)

Regards,
tvtech
 

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Hi RCinFLA

Interesting to read about Li-Ion. My dealings have been mainly with NiMH PP3 over the last few years. I have practiced the attached guide to the letter with PP3's from various Manufacturers and all I can say is that it is spot on advise. With a tweak of course...

My PP3 250mAH are charged with a Constant Voltage charger with Current Limiting. Voltage set at 10V. Current limiting set at 120mA.

This brings a totally flat PP3 up to full charge in under Three Hours without any drama or heat issues or complicated cut-off circuitry :)
And you can leave the battery in the charger for as long as you want without any overcharging issues/worries.

After literally hundreds of charging cycles even my very first PP3 is still going strong after three Years with no sign of falling capacity. PP3's are beauts once you understand them ;)

Regards,
tvtech

That one looks like it has seven cells. There are two versions of 9V NiMH around, one is seven cells with 8.4v nominal (seven times 1.20v) and ones with 7.2v nominal (six times 1.20v)
 
Your 9V PP3 Ni-MH batteries last a long time because they are never completely discharged and are never fully charged.
 
Your 9V PP3 Ni-MH batteries last a long time because they are never completely discharged and are never fully charged.

Spot on AG :)

It is called a happy balance. Thank you for seeing this and replying and understanding too. Much appreciated.

Regards,
tvtech
 
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