Charging Multiple Lithium Batteries

[Overclocked]

I need a way to charge multiple Lithium Ion batteries, Yes I know they do make chargers for two lithium batteries in series but as far as I know, they dont balance them. This is one solution ive thought of, but I want to get other's thoughts before I make it. Since Im messing with lithium batteries, I may not see the short comings of the design and (hopefully) prevent disaster.

So basically I have a single Lithium charger sitting on each battery. The Inputs to the chargers are in parallel with each other, and the chargers are hooked up to each battery separately. I will be using a MCP73831 charger (They are small batteries. This IC doesnt have temp monitoring but its a small stepping stone for one that will). After the charger, the batteries are hooked up in series, so you could say the chargers are hooked up in series with each other as well.

Any suggestions on if this would work or how I could improve the idea?

 

[ChrisP58]

I need a way to charge multiple Lithium Ion batteries, Yes I know they do make chargers for two lithium batteries in series but as far as I know, they dont balance them. This is one solution ive thought of, but I want to get other's thoughts before I make it. Since Im messing with lithium batteries, I may not see the short comings of the design and (hopefully) prevent disaster.

So basically I have a single Lithium charger sitting on each battery. The Inputs to the chargers are in parallel with each other, and the chargers are hooked up to each battery separately. I will be using a MCP73831 charger (They are small batteries. This IC doesnt have temp monitoring but its a small stepping stone for one that will). After the charger, the batteries are hooked up in series, so you could say the chargers are hooked up in series with each other as well.

Any suggestions on if this would work or how I could improve the idea?

If the inputs are in parallel, and the outputs are in series, the chargers need to have their inputs and outputs isolated so you don't create a short circuit condition.

 

[Overclocked]

If the inputs are in parallel, and the outputs are in series, the chargers need to have their inputs and outputs isolated so you don't create a short circuit condition.

But theres a internal Pfet in the charger IC itself that controls the current/voltage, wouldn’t this be sufficient enough? Or would a fuse be sufficient enough as well?

 

[ChrisP58]

But theres a internal Pfet in the charger IC itself that controls the current/voltage, wouldn’t this be sufficient enough? Or would a fuse be sufficient enough as well?

No, you need galvanic isolation. The negative net of your charger circuit is common to both input and output. As soon as you connect two charge circuits to two cells, you will create a dead short across the lower cell. Adding a fuse will just blow the fuse every time you try to charge a battery.

 

[Overclocked]

No, you need galvanic isolation. The negative net of your charger circuit is common to both input and output. As soon as you connect two charge circuits to two cells, you will create a dead short across the lower cell. Adding a fuse will just blow the fuse every time you try to charge a battery.

How would I do that then, even though their physically separate cells?

 

[ChrisP58]

You either need two independently isolated power sources, or a charge circuit that provides isolation.
Far easier is just to select a charger designed for two cells.

 

[Overclocked]

I meant to ask, is balancing really all that important if Their just gonna be in a flashlight? They wont get damaged or catch fire if their not balanced, will they? Fire is what Im most concerned about. Or will they just get shorter and shorter life as time goes on?

 

[ChrisP58]

The purpose of cell balancing is to keep the maximum charge voltage below the danger point on all cells in the string.

For instance, if you have two cells and you are charging to 4.2 volts per cell, or 8.4 volts. If you have 1/2 volt of imbalance, then one of the cells will reach 4.7 volts, which will start to plate metallic lithium out. This is a very bad thing as lithium metal will burst into flame if exposed to air.

 

[ronv]

Assuming you are using a dc adapter for the PSU you can get around the problem by using 2 separate adapters.

 

[Overclocked]

Thats True, But rather inconvenient. The other way Ive thought of doing this is basically sever the connection between the two batteries so they aren’t in series, And then Once charging is complete, have it become reconnected again. Ive thought of using a relay, as power isnt always going to be connected. Ive also just thought of using a simple Switch that has to be turned switched on to charge it (basically switching that connection between a open and short). This would ensure they would both be equally charged and it will still work to get the desired output. However this doesnt really seem practical either. I dont think there is any solid-state switch that remains on when power isnt supplied (if there is, Id like to know!) FETs and Transistors Need power to allow current to flow so those wont work.

Without using a external Balancer, is there IC that does balancing? Ive been searching around the net (including major IC manufacturers), and Cant seem to find much of anything that will balance two batteries. I need something that’s solderable, Ive been able to solder up to a MSOP-10 package.

 

[ronv]

Here are some 2 cell chargers. It doesn't ballance but looks like it floats the batteries at low current.

https://www.linear.com/product/LTC4002

 

[audioguru]

I have an RC airplane that came with a 2-cells lithium battery charger that does balancing. Each 2-cells battery has 3 wires.
The charger is an E-Flite Celectra and charges each cell at about 250mA. It is powered by a car battery or a 12VDC wall-wart.

 

[ChrisP58]

The bq29209 from TI has cell balancing. https://www.ti.com/product/bq29209

 

[strantor]

I'm not really qualified to advise (someone correct me if I'm wrong), but my thinking is that if you use 2 separate power supplies like you proposed in post #1 (and you made sure they were floating supplies) you should be able charge the batteries individually while the batteries are still in series. No need to put the supplies in series and then worry about balancing. No need to worry about switching the batteries around, etc.
Like this:View attachment 65263

As long as your load is disconnected (no current flowing through the series string of batteries) and as long as your supplies are isolated and floating, potential only exists across their own 2 leads, so they shouldn't care that there's another battery chargin next door. This would allow you to charge each cell (even much longer series strings) at it's own pace.

 

[Overclocked]

The bq29209 from TI has cell balancing. https://www.ti.com/product/bq29209

I wish there was a way to solder that. I dont think I could solder a 8SON, even though I do have a hotplate.

Ive been wondering something about the power brick idea. If I used say a 12V PSU, and then have two 5V step down converters, for each charger. That way each charger would have its own supply. The Only downfall is more overall cost, but easiest to implement. The Load will be switched on and off though, but not on during charging.

 

[ChrisP58]

Sorry, I didn't mean to imply that that was TIs only part with balancing. They have many LI charger/protection chips, with different footprints. Maxim has some too I believe. I'm sure there are other manufacturers.

Remember, if there is a common need for some electronic function, someone will make a chip to do it.

 

[ronv]

here is a descrete circuit one.

https://www.coolcircuit.com/project/lipo_charger/charger_circuit.jpg

 

[Overclocked]

Just gonna throw out an idea Here. Im thinking a Comparator circuit that checks the battery's voltage. Cells only need to be balanced if they are Over 4.2V, Correct? Under charging really isnt too much of a issue. So, If i have the comparator trigger a small load (say 100 mA) if the batteries voltage is over 4.3V and then turn off once the voltage reaches 4.2V. Im thinking this would achieve balance, and not risk damaging the cells. The only thing is, would I use a Comparator with Hysteresis?

 

[ChrisP58]

Just gonna throw out an idea Here. Im thinking a Comparator circuit that checks the battery's voltage. Cells only need to be balanced if they are Over 4.2V, Correct? Under charging really isnt too much of a issue. So, If i have the comparator trigger a small load (say 100 mA) if the batteries voltage is over 4.3V and then turn off once the voltage reaches 4.2V. Im thinking this would achieve balance, and not risk damaging the cells. The only thing is, would I use a Comparator with Hysteresis?

You can do it that way, but the voltage level needs to be no more than 0.5% above 4.2V, and the bypass current needs to equal the charge current. It needs to work like a precision zener. Be aware, if you are charging at 1 amp, you will be dissipating 4.2 watts.

 

[Overclocked]

You can do it that way, but the voltage level needs to be no more than 0.5% above 4.2V, and the bypass current needs to equal the charge current. It needs to work like a precision zener. Be aware, if you are charging at 1 amp, you will be dissipating 4.2 watts.

I was under the assumption that balancing takes place after charging. I mean, while your charging at 1A, You would also be consuming 1A at the same time, so the battery wouldnt get charged, would it?