• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Do i get more Amp hours at lower resistor voltage?

Electro Noob

New Member
So i built powerbank from laptop batteries to charge my phone by adding three 3.7 volt 2200mAh Li ion batteries in series and then another three in parallel giving 4400mAh and 12.6 volts when fully charged.Then i hooked the batteries to charger i used for charging my phone on lighter socket in car giving me 5 volts.It works fine but i dont get does it now have 4400mAh or 4400×2.4=10560mAh(since voltage is 2.4 times lower).
Any help will be much appreciated.
 

crutschow

Well-Known Member
Most Helpful Member
You need to look at the battery energy, which is always conserved during any conversion.
The energy available from the battery is Ah*V so, for a given battery Ah, lowering the voltage increases the Ah available (assuming the voltage reduction/conversion is done with high efficiency by a switching regulator and not with a linear regulator or resistor).
 

Electro Noob

New Member
Thanks for the clarification so i guess i get around 10000 mAh assuming charger is power efficient. I am wondering is there a way to charge it from car battery through male to male lighter connector? I guess it would need some sort of voltage regulator such as lm 7812 in series since car alternator can give more than 12.6 which is maximum for charging.
 

Electro Noob

New Member
Yes im aware it can explode if i exceed rated charging voltage which is 4.2 volts for each cell but to be sure i will go even lower and charge it only up to 12 volts for three cells in series.
 

addysoftware

New Member
But apparently not aware that each of the battery MUST NOT go higher than 4,2V and no lower than 3.0V and the batteries are not equal. There is need of a specialised electronic circuit called BMS which take care of each of the cells (in your case, two parallel cells means one cell, the BMS is a called 3S Li-Ion BMS.
 

Electro Noob

New Member
I tried to make it as simple as possible since im just starting to learn electronics i found now that there are buck converters with usb and all sorts of protections.For the purpose of monitoring voltage i considered adding led voltmeter.
 

sagor1

Active Member
In theory, you should not parallel two Li Ion batteries together. All it takes is for one bank to get a bad cell, and the other bank will provide huge surge currents to try and charge from one battery bank to the other.... Potential for overheating and other "damage".
There are many commercial "USB power packs" out there that can provide over 4400mAh of charging at 5V
 

Electro Noob

New Member
Would it draw too many amps and blow fuse if i put 12v voltage regulator and one amp fuse and hook it up to cigarette lighter socket in car?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Would it draw too many amps and blow fuse if i put 12v voltage regulator and one amp fuse and hook it up to cigarette lighter socket in car?
DON'T DO IT!!!!

It's VITALLY important that Li-Ion cells are charged correctly, you need an accurate 12.6V supply, that is also constant current - when you place the cells on charge the current is constant, and the voltage will be low - as the cells charge up the current will remain constant, and the voltage will rise. Once the voltage gets to 12.6V the voltage will remain constant, and the current will decrease - once it's dropped below a certain level the batteries are fully charged.
 

Electro Noob

New Member
Then i guess i would have to stick to using universal laptop charger, its voltage fluctuates between 12.55-12.6 is that ok for charging them? I charged them several times that way and nothing heats up at all.Thanks for warning me i wont try it but im curious what is wrong with charging them with only 12 volts?I thought it would just charge slower because it has less potential difference.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Charging Li-Ion isn't something to mess with, as Samsung found out with their phones setting on fire!.

If you try and charge from only 12V, then it will only charge partially.
 

Mickster

Well-Known Member
Electro Noob
Please get hold of a safe means of charging your Li-Ion pack/cells.
You have experts here trying to guide you in this thread, for your safety, so please take their advice.
The last thing you need is for this pack to go up in flames in your car, or home.
 

sagor1

Active Member
Not at all, it's perfectly normal, and pretty well every USB power pack out there does exactly that, as do the majority of actual Li-Ion applications.
True, it is normal in commercially produced power packs. My point was that a "user" should not experiment with such an idea, especially if one does not know if the bateries are even identical or from different vendors, etc. Doing this "home brew" can be dangerous. Commercial products have the internal regulators/controllers in the battery packs to prevent overcharging, sensing heat, etc...
 

Pommie

Well-Known Member
Most Helpful Member
As already explained, all that matters is the wattage available so it would be much safer to place all batteries in parallel. However, they must all be fully charged to the same voltage before doing this. You can then use a circuit designed to charge a single cell and a boost converter to step up to whichever voltage you need. Personally, I'd just buy a power bank and use that. Power bank -> USB cable -> boost converter -> device. Here is one that takes 7 18650 batteries and contains the needed protection circuitry - what size are your batteries?

Mike.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
True, it is normal in commercially produced power packs. My point was that a "user" should not experiment with such an idea, especially if one does not know if the bateries are even identical or from different vendors, etc. Doing this "home brew" can be dangerous. Commercial products have the internal regulators/controllers in the battery packs to prevent overcharging, sensing heat, etc...
As do home built ones, or they SHOULD - protection boards are freely and cheaply available.
 

Electro Noob

New Member
Cells are all from same laptop battery they are all 18650 so im sure they are all the same but now i have another problem with this pack. I monitored voltage and it lasts for six charges but voltage drops dramaticaly after.Here are results i got after each discharge:
12.56 - fully charged
1. - 12.28
2. - 11.97
3. - 11.69
4. - 11.49
5. - 11.06
6. - 7.13 (only 2.3 each)
My phone battery is 1500mAh.I will get powerbank case with protection circuits but it seems my cells are not quite good, or that sort of voltage drop is normal idk.Thank you for your help everyone i will not leave cells unmonitored especialy while charging.
I tried to charge them after this voltage drop and they heat up i unplugged the charger and took cells outside.I guess some of the cells has gone bad.
 
Last edited:

Nigel Goodwin

Super Moderator
Most Helpful Member
Cells are all from same laptop battery they are all 18650 so im sure they are all the same but now i have another problem with this pack. I monitored voltage and it lasts for six charges but voltage drops dramaticaly after.Here are results i got after each discharge:
12.56 - fully charged
1. - 12.28
2. - 11.97
3. - 11.69
4. - 11.49
5. - 11.06
6. - 7.13 (only 2.3 each)
My phone battery is 1500mAh.I will get powerbank case with protection circuits but it seems my cells are not quite good, or that sort of voltage drop is normal idk.Thank you for your help everyone i will not leave cells unmonitored especialy while charging.
I tried to charge them after this voltage drop and they heat up i unplugged the charger and took cells outside.I guess some of the cells has gone bad.
Well you're wasting a lot by using 12V worth, power packs usually use the cells simply in parallel, with an up-converter to give 5V for charging the phone.

As you've discharged the cells to 2.3V, you've quite probably damaged them now anyway, one of the functions of protection circuits is to prevent over-discharge of the cells.
 

EE World Online Articles

Loading

 
Top