I am making a power bank and read that in order to charge a 3.7 V battery I should supply it with no more than 4.3V battery source. I want to use a 3.7V battery in my power bank and would like to step it up to around 4V in order to provide enough current to charge the battery in the device I want to charge. I was wondering if there was a way to boost the battery up to 4V without boosting it to 5V and regulating it back down to 4. I ask this because 5v step up boosters are pretty common when searching online. I am very new to this area and have a very basic knowledge. Any help would be appreciated.

 

I would use a 5V booster and put a diode in series with the output to drop it to 4.3V,

 

I am making a power bank and read that in order to charge a 3.7 V battery I should supply it with no more than 4.3V battery source. I want to use a 3.7V battery in my power bank and would like to step it up to around 4V in order to provide enough current to charge the battery in the device I want to charge. I was wondering if there was a way to boost the battery up to 4V without boosting it to 5V and regulating it back down to 4. I ask this because 5v step up boosters are pretty common when searching online. I am very new to this area and have a very basic knowledge. Any help would be appreciated.

If you are willing to carefully specify your requirements you can construct a DC-DC converter with a boost configuration that will boost a discharging battery in the range say 3.0-3.7 volts up to 4.2 volts. There is no need to regulate it further. The reason you want to limit the lower bound of the battery is that as it discharges more current will be drawn from it causing the discharge rate to increase. IIRC, Li-Ion batteries don't like to be discharged all the way. I don't recommend you build such a DC-DC converter on a breadboard for anything except experimentation. You need to layout a PCB if you want to use this thing day in and day out. There are any number of chips that you can use that come with design aids and component selection information.

You begin the design by doing a "back of the envelope" power budget. Say you need 4.2VDC at 500 mA. Assume your conversion is 80% efficient.
4.2VDC x 500 mA = 2.1 Watts of output power
To get the required input power, you divide 2.1 by 0.80
2.1 Watts / 0.80 = 2.625 Watts of input power.

Now here comes the important point:
This 2.625 Watts of input power is REQUIRED regardless of the input voltage from a discharging battery. Therefore
2.625 Watts / 3.7 VDC ≈ 710 mA --- AND
2.625 Watts / 3.0 VDC ≈ 875 mA
You can see where this is going, and let me tell you that designing a PCB for an SMPS with those kinds of currents is not for the faint of heart.

You have to learn this stuff at some point so dig in and have at it -- just be prepared to throw the first one or two away. You'll end up doing that regardless of any other consideration.

 

Is this a lithium battery that you're wanting to charge? And are you planning to feed the 4V directly into the battery, or into a charging system?

I ask these questions because lithium batteries have very specific, somewhat difficult charging requirements. It's not like charging a simple lead acid battery. If you don't do everything right, they're really dangerous.

 

Is this a lithium battery that you're wanting to charge? And are you planning to feed the 4V directly into the battery, or into a charging system?

I ask these questions because lithium batteries have very specific, somewhat difficult charging requirements. It's not like charging a simple lead acid battery. If you don't do everything right, they're really dangerous.

Lead acid batteries are not so simple to charge either, at least not simple when done properly. But lithium is at a whole other level. With the safety factor leading the top of the pack. Anyone that needs to ask how to charge a lithium batttery should not be designing a circuit to charge a lithium battery. Buy a proper charger.

 

Lead acid batteries are not so simple to charge either, at least not simple when done properly.

Agreed. I shouldn't have implied that.

It's all relative though. I don't have the knowledge or nerve to mess with either one, but many people here that I'd consider experts make lead acid charging systems and make it look easy, but even they (or at least most of them) aren't trying to roll their own lithium charging systems.

 

Is this a lithium battery that you're wanting to charge? And are you planning to feed the 4V directly into the battery, or into a charging system?

I ask these questions because lithium batteries have very specific, somewhat difficult charging requirements. It's not like charging a simple lead acid battery. If you don't do everything right, they're really dangerous.[/

If you are willing to carefully specify your requirements you can construct a DC-DC converter with a boost configuration that will boost a discharging battery in the range say 3.0-3.7 volts up to 4.2 volts. There is no need to regulate it further. The reason you want to limit the lower bound of the battery is that as it discharges more current will be drawn from it causing the discharge rate to increase. IIRC, Li-Ion batteries don't like to be discharged all the way. I don't recommend you build such a DC-DC converter on a breadboard for anything except experimentation. You need to layout a PCB if you want to use this thing day in and day out. There are any number of chips that you can use that come with design aids and component selection information.

You begin the design by doing a "back of the envelope" power budget. Say you need 4.2VDC at 500 mA. Assume your conversion is 80% efficient.
4.2VDC x 500 mA = 2.1 Watts of output power
To get the required input power, you divide 2.1 by 0.80
2.1 Watts / 0.80 = 2.625 Watts of input power.

Now here comes the important point:
This 2.625 Watts of input power is REQUIRED regardless of the input voltage from a discharging battery. Therefore
2.625 Watts / 3.7 VDC ≈ 710 mA --- AND
2.625 Watts / 3.0 VDC ≈ 875 mA
You can see where this is going, and let me tell you that designing a PCB for an SMPS with those kinds of currents is not for the faint of heart.

You have to learn this stuff at some point so dig in and have at it -- just be prepared to throw the first one or two away. You'll end up doing that regardless of any other consideration.

Essentially I am looking to make a 3.7 V 700mAh Lipo battery a power bank. It would only be used to charge a 3.7V 200mAh Lipo battery. I want to know if this is possible. The goal is to at least be able to charge the battery fully once.

 

Is this a lithium battery that you're wanting to charge? And are you planning to feed the 4V directly into the battery, or into a charging system?

I ask these questions because lithium batteries have very specific, somewhat difficult charging requirements. It's not like charging a simple lead acid battery. If you don't do everything right, they're really dangerous.

I am trying to turn the 3.7 V 700mAh battery into a power bank that will only ever charge another 3.7V mAh battery. I just want it to charge safely and at least be able to charge it fully once. I am hoping that this is even possible with the 3.7 V 700mAh battery.

 

I think what ebeowulf17 and others are getting at is how you are connecting these two batteries together. Let's say that the voltage question is resolved. Then what. Are you going to connect them directly together to charge up the smaller, discharged battery? Or are you going to use the recommended commercial charger for the battery? Or something else?

If you are going to power a real, honest-to-goodness Li-ion charger then what matters is what is the input voltage requirement for the charger. If you are hoping to just connect them together to share the charge then you are asking for serious trouble and, no, it is not safe.

 

I am trying to turn the 3.7 V 700mAh battery into a power bank that will only ever charge another 3.7V mAh battery. I just want it to charge safely and at least be able to charge it fully once. I am hoping that this is even possible with the 3.7 V 700mAh battery.

I'm sure what you want to accomplish can be done, but I don't have the expertise to safely guide you. If I were in your shoes, I'd look at dedicated LiPo charge management ICs and build a circuit around one of them. You might also look at the schematics for proven, open source projects with similar requirements.

Here's an example of a boost circuit implementation (albeit one that delivers higher voltage than you requested)
https://learn.adafruit.com/minty-boost/download

Here's a tiny LiPo charging solution.
https://learn.adafruit.com/adafruit-pro-trinket-lipoly-slash-liion-backpack/downloads

Looking at the schematics for these projects might help you figure out the best approach for yours. Perhaps more importantly, look into the datasheets for the relevant parts (charger IC in particular) and look at recommended circuit layouts, etc.

A simpler approach like you've described *might* be ok in very specific circumstances, but everything I've heard makes me suspect otherwise. I'd be very careful with lithium batteries unless you're sure you know what you're doing, or that you're getting solid guidance from someone who does (not me!)

Regardless of which approach you take, I'll be following this thread with interest to see how it goes. Good luck!

 

A "Power Bank" uses one 3.2V (dead) to 4.2V (fully charged) lithium rechargeable battery cell to drive a 5V regulated voltage boost circuit to feed the charger circuit in a phone. The battery in the power bank has a charger circuit fed from 5V USB.
Your idea is missing important battery charger circuits.

 

I think what ebeowulf17 and others are getting at is how you are connecting these two batteries together. Let's say that the voltage question is resolved. Then what. Are you going to connect them directly together to charge up the smaller, discharged battery? Or are you going to use the recommended commercial charger for the battery? Or something else?

If you are going to power a real, honest-to-goodness Li-ion charger then what matters is what is the input voltage requirement for the charger. If you are hoping to just connect them together to share the charge then you are asking for serious trouble and, no, it is not safe.

I was hoping to hook up the 700mAh battery to this power module and then use the commercial charger in the usb to charge the 200mAh battery. The battey is inside a device that I desire to charge.

 

A "Power Bank" uses one 3.2V (dead) to 4.2V (fully charged) lithium rechargeable battery cell to drive a 5V regulated voltage boost circuit to feed the charger circuit in a phone. The battery in the power bank has a charger circuit fed from 5V USB.
Your idea is missing important battery charger circuits.

So I intended on using this power module with the 3.7V 700mAh LiPo battery. I would then use an original charging cable that would plug into the usb portion of the module to charge the other 3.7V 280mAh LiPo battery inside a device I desire to charge.

 

That picture tells us nothing except that it's a small board with a couple of different style USB jacks on it.

How about a link to the data sheet or at least a manufacturer and part number?

 

That picture tells us nothing except that it's a small board with a couple of different style USB jacks on it.

How about a link to the data sheet or at least a manufacturer and part number?

So all that it says in the description is that it is a 3.7V to 5V 800mA USB mobile charger power module

 

A "power module" that boosts voltage is not a proper charger circuit for a lithium battery. The battery being charged might explode or catch on fire.
Usually a lithium battery inside a device (a cell phone?) has a proper charging circuit in it, but you said you want to charge (blow up?) its battery yourself.

 

So all that it says in the description is that it is a 3.7V to 5V 800mA USB mobile charger power module

My guess is that it is a power module to power a mobile charger and not a mobile charger itself.

So, if I'm understanding correctly, you intend to use this power module to take power from your battery bank cell and use the output from the module to then power a proper Li-ion battery charger and NOT to connect it direct to the battery. That should work as long as the battery bank cell can meet the current demands that are placed on it, which is probably can.

 

OK so the power module will boost the 3.7, 700ma volt battery to 5 volts to run a standard USB charger to charge the smaller 280ma Lipo.
That will work but how are you charging the 3.7,700ma battery?

SG

 

I think it's a 700 mAh battery, not 700 mA. It can probably provide a few amps of current.

 

Does anybody make a Li-PO charger circuit with a 5V input and a low current output for such a small battery? The battery in a cell phone is much larger and uses a much higher charging current.