Using a charger to power a load and charge the battery

[wacky_aki]

I'm designing a portable device that will use a 4S2P Li-ion battery pack as its main power source. I also would like to be able to hot swap the battery without having a backup battery, basically to keep the device on while I insert a fully charged battery.
To minimize power jacks, I want to use the charger to charge the battery and be able to power the load as well. In simple terms, if the charger and the battery are plugged in - the battery will be charging, and either charger or the battery can provide power to the load (depending on what's doable); if the charger is plugged in and the battery is removed from the unit, I want the charger to serve as the power source and keep the device turned on. I do understand that I would need either a hot swap IC or an ideal diode to limit the current when the battery is plugged in/out.

Is this doable or am I silly for even thinking this? Would it be a better idea to use a wall power adapter to power the unit and charge the battery (via a charger IC)?
Thoughts?

Any help is appreciated. Please do ask questions if I made anything unclear.

[NiHaoMike]

Just limit the cell voltage to 4.1V (16.4V for your 4s pack) and limit the maximum charging current to less than 1C.

[Buriedcode]

Dpeends on what exactly you're trying to do, and the sort of charger you're using.  As you know most 'chargers' in terms of commercial products are just voltage-regulated power supplies - the charge management circuitry that controls current and voltage is usually inside the product.  For hobby RC its a different story, the charger is both the supply and charge management. The battery packs just have the mandatory protection cricuit (back to back FET's, monitors current in, out, and max/min voltage).

For multicell setups there's things like cell balancing to contend with that I'm sure many chargers do just fine, but it also means that there's several connections/voltages that vary to maintain current.

If you are designing this yourself, you will obviously need charging circuitry in the device - unless you plan on having users remove the battery to charge it.  So, if you're already designing the charger you'll have a power input from a regulated supply that will probably be converted, using a buck/boost by the charger.  In which case a very simple load sharing circuit will do providing your load can cope with the range of input voltages you're planning on using for the charger.

That is, 4S2P = 12.8 - 16.8V (for 4.2V cells) or  12.4- 16.4V (for 4.1V cells).  If you have a linear charger then you'll need a supply higher than peak voltage to account for losses, current sense etc.. so say 18V.  If you're using a switcher, many can be sepic, or buck/boost, in which case the input voltage range can be above, below, or in the middle of the range, say 12-18V.  If the load has its own regulator, then it can cope with this range so the following 'add-on' circuit will power the load whilst also charging the batteries:

http://ww1.microchip.com/downloads/en/AppNotes/01149c.pdf page2.

[KL27x]

It sounds like you want to build this device except for the power supply and charging circuitry... and you want to kill two birds with one off-the-shelf charger?

If power draw of the device is significant, you will probably want to tap the DC voltage off the charger and add your own regulator. Basically you would just be using the charger's psu for double duty, not the charging circuitry. And switch battery out of circuit when the charger is plugged in, sort of thing? (Or just rectifier diodes, if you can stand the voltage drop). And make sure the charger PSU has enough output to handle the load and the charging at the same time. And you will also have to make sure the charger behaves when it is unplugged... which it probably won't. reverse current draw and what not. 

Just limit the cell voltage to 4.1V (16.4V for your 4s pack) and limit the maximum charging current to less than 1C.

This is fine, but if the power draw is significant, this means the battery isn't charging fast as it could be while the device is on.

[Buriedcode]

Agreed, whilst some might say its overkill, I've never liked charging a lithium battery with a load attached.  It can play havoc with the charge termination, perhaps not damaging the battery at all, but rather you don't know when its finished charging because the load draws more current that the minimum set to indicate it done.

As the OP is charging a multicell pack anyway, it'll be complicated enough without having a varying load throwing a spanner in the works.  I believe one can buy multicell chargers/balancers, bare boards, off amazon and ebay - as to whether these sources are trusted...

[David Hess]

A current shunt in series with the battery would allow measurement of the battery current for limitation of the charge current and end of charge determination.

[wacky_aki]

It sounds like you want to build this device except for the power supply and charging circuitry... and you want to kill two birds with one off-the-shelf charger?

If power draw of the device is significant, you will probably want to tap the DC voltage off the charger and add your own regulator. Basically you would just be using the charger's psu for double duty, not the charging circuitry. And switch battery out of circuit when the charger is plugged in, sort of thing? (Or just rectifier diodes, if you can stand the voltage drop). And make sure the charger PSU has enough output to handle the load and the charging at the same time. And you will also have to make sure the charger behaves when it is unplugged... which it probably won't. reverse current draw and what not. 

Just limit the cell voltage to 4.1V (16.4V for your 4s pack) and limit the maximum charging current to less than 1C.

This is fine, but if the power draw is significant, this means the battery isn't charging fast as it could be while the device is on.

That is exactly what I want to do - "kill two birds" with one off-the-shelf charger (which is really a cheap Li+ 16.8V bought directly from China).
My peak load is roughly 5A while in idle the system load current is 1A. I've purchased a few chargers to test: 6A, 8A, and 10A.
I would place ideal diodes in the circuit to protect the circuit when the charger or the battery are being plugged/unplugged.

[wacky_aki]

Dpeends on what exactly you're trying to do, and the sort of charger you're using.  As you know most 'chargers' in terms of commercial products are just voltage-regulated power supplies - the charge management circuitry that controls current and voltage is usually inside the product.  For hobby RC its a different story, the charger is both the supply and charge management. The battery packs just have the mandatory protection cricuit (back to back FET's, monitors current in, out, and max/min voltage).

For multicell setups there's things like cell balancing to contend with that I'm sure many chargers do just fine, but it also means that there's several connections/voltages that vary to maintain current.

If you are designing this yourself, you will obviously need charging circuitry in the device - unless you plan on having users remove the battery to charge it.  So, if you're already designing the charger you'll have a power input from a regulated supply that will probably be converted, using a buck/boost by the charger.  In which case a very simple load sharing circuit will do providing your load can cope with the range of input voltages you're planning on using for the charger.

Thanks for the application note link! It contains some very useful information.

Thanks for the application note link! It has some very good information.

That is, 4S2P = 12.8 - 16.8V (for 4.2V cells) or  12.4- 16.4V (for 4.1V cells).  If you have a linear charger then you'll need a supply higher than peak voltage to account for losses, current sense etc.. so say 18V.  If you're using a switcher, many can be sepic, or buck/boost, in which case the input voltage range can be above, below, or in the middle of the range, say 12-18V.  If the load has its own regulator, then it can cope with this range so the following 'add-on' circuit will power the load whilst also charging the batteries:

http://ww1.microchip.com/downloads/en/AppNotes/01149c.pdf page2.

The battery pack that I'm using only has the mandatory protection circuitry that you've mentioned, which is somewhat of an issue as the battery will take as much current as the charger is able to provide. As a result I would certainly need to have a charger management IC/circuit regardless if I use a charger or a wall adapter. What I'm not sure about is if the charger IC will "play nicely" with the charger. Also, how would the charger respond to frequent changes in the  load? As I mentioned in the reply to KL27x, I've purchased 16.8V Li ion (charging voltage can range from 10V to 16.8V) chargers from China; these are switching power supply chargers.
The load will definitely have its own regulator so I'm not worried about that. I will also spec all the components to be able to handle >=20V.

[montreuilg@sympatico.ca]

I'm doing the same that for a project using 4.2v lipo batteries, thermistors and 6 volts/2watts solar panels.
The distributor mentions that concern in its design notes and offers a solution, although not in the voltages range you require. Maybe it will inspire you or others?
https://learn.adafruit.com/usb-dc-and-solar-lipoly-charger?view=all#design-notes

_
g