hello everyone! newbie here. average dude who knows very little about electronics. good thing i stumbled upon this site. been surfing it for a while now.

here's my problem, i just bought a cheap iPad clone (the China-made aPad that runs on android) and my first frustration was the battery life! it only lasts for a few hours and then you have to recharge it again. the charger indicates:

input: 100-240VAC 0.5A
output: 9v 1500mA

since it was 9v, i immediately thought about the piles of 9v batteries lying around. i also have some 9v rechargeables. i then soldered 2 battery clips (in parallel) to a switch and then to the socket that fits my tablet. when i tested it, it has an output of 10v+

my observations were:

1. the tablet says 100% charged when the batteries are on and back to normal battery level when off.

2. when the batteries are low, the apad dies. (makes a weird sound) this happened with the NiCd. (i hope i didn't damage it, still works though)

now what i want to do is

1. be able to really "charge" (transfer the power from the batteries my device)
add a safety feature?

2. resistor or capacitor to regulate power coming from the 9v batteries

3. add a power meter of some sort (a single led that indicates low power)

4. add a usb charger port (i already found guides for this)


i would also appreciate any tips, hints, and precautions with this diy project. thank you very much. have a nice day.

 

9V rechargeables have a capacity of about 100 - 150 mAhr. You are trying to power equipment rated at:

input: 100-240VAC 0.5A
output: 9v 1500mA

They won't last long

 

I think the battery description refers to the output current, not energy, as it writes mA, not mAh.

The mAh rating could be measured approximately by seeing how many hours can the pad be on full operation (WiFi, GPS and other modules on).

 

A little 9V rechargeable Ni-cad or Ni-MH battery has a voltage of only 7.2V (6 cells) or 8.4V (7 cells). The voltage is not high enough to charge the battery in the ipad clone.

 

I think the battery description refers to the output current, not energy, as it writes mA, not mAh.

The mAh rating could be measured approximately by seeing how many hours can the pad be on full operation (WiFi, GPS and other modules on).

NO, the rating I gave is mA-hr (It is printed on the battery) and I actually have measured some. 150 mA-hr is all they can do.

And as audioguru points out, they are not really "9V".

 

You guys overlooked the 8cell batteries, and good 9v Ni-MH batteries come with over 200mAh now.

Can you get any specs off the internal battery. At this point all we know is the charger outputs 9v at 1500mA. One can only hope the design is meant to charge and run the device at the same time.

Where in the device exactly is the battery switch ? If it's on the other battery terminals I foresee bad smokey things. If it's on the charge port, and the source supplies sufficient current, it's already transferring to the devices battery.

Schematics and or pictures would be helpful.

As to your questions.
1: Is already happening as safe as it was before, assuming you connected at a charge port.
2: is a little more complicated than that, which voltage-current dance you do depends on the battery chemistry.
3: Waste of power but lots of ways to do it, a pair of resistor and an led being about the simplest.
4: USB only specs 500mA for a high powered device. In theory you could gang them together, but USB hubs aren't always setup to supply more than one high power device. And the USB power bus in the device might not handle the current flow, if it doesn't expect more than 500mA.

The Eken M003, uses a 9v 1.5amp charger. I'm guessing this is your device.
http://www.circuitdb.com/articles/17/5
The last photo here shows a 2cell Li-, maybe a 1600mAh rated for 3C based on the "1603". But i can't find where to check, they're standard at least on a factory to factory basis and there aren't many factory..

 

definitely doing it wrong. even if your batteries were a full 9v and you magically find some extreme capacity cells, you cant reliably plug them into your charger port.

when the laptop is taking power from your charger, it expects that power to be at 9v and up to 1500ma of current. it does not have many safeguards to protect your apad in case the voltage changes at this point. it simply expects 9v, 1500ma no matter what.

when you use batteries on that port, even if they are 9v to start with, it wont be long before that drains to 8v, 7v and lower. this is why you hear a whine/weird noise coming from the apad... because the dc-dc converter inside is not made to handle undervolting. yes, you probably can damage your tablet doing this.

if anything, install a low-voltage cutoff circuit inbetween your battery pack and the apad charger port. the lowest voltage that your tablet can safely run on is anyones guess. you would need to know what type of power converter is built inside the tablet. as a pure guess though, i doubt you want to feed much less then 8v to that port....... .....

now if you wanted to be able to charge your external battery pack, you would probably need to scrap the a/c adapter you have now, get a small automatic charger for your batteries that can output a charging current of at least 2a (1500ma for the laptop, 500ma to charge your batts), and put a voltage regulator in your circuit right at where you plug the cord into your tablet. this way the batteries can charge at a higher voltage like they need to, and the regulator keeps a steady 9v going into the laptop. do this with nicad or nimh only!, as youre basically using a dummy charger as a power supply too. if you want to use lithium you would need a bit more circuitry to automatically switch the batteries off so they can charge without being drained at the same time (for safety reasons)

 

You guys overlooked the 8cell batteries, and good 9v Ni-MH batteries come with over 200mAh now.

Here is what the spec for an Ipad says about power (don't know if the Chinese knock off is identical, but it has to be close):

• Built-in 25-watt-hour rechargeable lithium-polymer battery
• Up to 10 hours of surfing

That means it takes about 2.5 Watts per hour of operation. Assuming the magical 200 mA-hr @ 9V battery, that's 1.8W. It will run less than 40 minutes on that. In reality, it will probably run maybe 15 minutes since the mA-hr ratings are at discharge rates of about 1/10 C. At a rate of about 1.5C, the mA-hr capacity will be much less than half.

 

Here is a graph from the Energizer made in Germany Ni-MH 9V battery:

 

thank you all for replying.

i looked for the manual and it said Li-ion Battery 1450mAh as for opening the tablet, it's still under replacement warranty for the month. it's true that my device drains those 9v batteries like crazy. a friend of mine suggested adding more batteries to somehow compensate? i can still pack 2 more inside my enclosure. he also said i should put the batteries in series and try to raise the voltage to 18v and step it down to 10v.

 

Even if the rechargeable "9V" has the proper voltage, it will not be able to provide enough current to operate the device. And if it could, maybe for 10 minutes...

 

charging the ipad clone off of a single 9V battery is not practical, as this would not even give you a full charge. you could try using 6 AA batteries instead of the 9V as this technique would last (a little bit) longer

 

charging the ipad clone off of a single 9V battery is not practical, as this would not even give you a full charge. you could try using 6 AA batteries instead of the 9V as this technique would last (a little bit) longer

You can get NI-MH "AA" cells with 2000 mA-HR capacity (and maybe a bit higher) in four packs at a decent price. I think you would need at least seven cells in series.