Safely charging multiple Li-ion battery pack

[camerart]

Hi All,

I would like to build a safe li-ion cell battery pack charger. I have read lots about it, but so far haven't seen a good circuit for charging packs.

I have tried reverse engineering a bought one, but I found feedbacks that I didn't understand, and it charged cells over the 4.2v recommended.

It should be able to charge protected and unprotected cells. (Over/under voltage)

I think the best way might be to charge each cell separately, that is each cell still in it's pack would be charged on it's own 1 then 2 then 3 and so on. There are good chips for accurate charging, so all I need to do is make a switch circuit, that would allow any number of cells (perhaps) I intend to use 4 to 6, 18650 cells.

I will be charging them using a solar pack (I've yet to make) which will have a buck controller for varying solar output. The solar cells will probably produce 5 ish volts boosted to 'say'12V.

Any ideas welcome. Cheers, Camerart.

 

[audioguru]

Li-o battery packs are used to power electric radio controlled model airplanes, helicopters, cars and boats.
They also have high quality but inexpensive chargers that charge series cells with a "balancing circuit" so that each cell is separately charged.
The under voltage circuit in a model airplane pulses the motor when the voltage becomes low then shuts off the motor when the voltage is too low but leaves controls operational.

 

[canadaelk]

National, Maxim, Linear and others manufacture single IC solutions. I am using the Linear LTC4054-4.2 with 18650 batteries and I am happy with that. For a multiple cell charger have a look at www.rcgroups.net in their forum section go to batteries and chargers. E

 

[camerart]

Li-o battery packs are used to power electric radio controlled model airplanes, helicopters, cars and boats.
They also have high quality but inexpensive chargers that charge series cells with a "balancing circuit" so that each cell is separately charged.
The under voltage circuit in a model airplane pulses the motor when the voltage becomes low then shuts off the motor when the voltage is too low but leaves controls operational.

Thanks Audioguru,

I have a circuit for balancing, but I would like to know if it actually charges each cell, differently? I might build it and do some tests.

Cheers, Camerart.

 

[camerart]

National, Maxim, Linear and others manufacture single IC solutions. I am using the Linear LTC4054-4.2 with 18650 batteries and I am happy with that. For a multiple cell charger have a look at www.rcgroups.net in their forum section go to batteries and chargers. E

Hi Canadaelk,

I looked at the one you use, it is very neat.

Some good chips to choose from. I didn't find any circuits on the RCGroups site, if you see any can you post the link please? (Low voltage input)


Cheers, Camerart.

 

[canadaelk]

Here it is: https://www.rcgroups.com/forums/showthread.php?t=215550
I have not tested the unit, schematic in post 46, eagle files attached to #41. I do not use eagle, so I can not veryfy anything.
Another thread is: https://www.coolcircuit.com/project/lipo_charger/lipo_charger.html
E

 

[audioguru]

"http://www.coolcircuit.com/project/lipo_charge/charger_circuit.jpg'
The webpage cannot be found.

 

[Sangoma]

Most RC chargers do not charge each cell individually, they charge the whole battery and discharge the higher cells to balance, then charge again and repeat until all are charged at the same level.

To charge multiple cells individually, you will need an isolated supply for each cell, ie an inverter circuit for each cell, and a control circuit for each cell. Expensive RC chargers do this, which partly explains the huge price difference between cheap and expensive ones.

From an RC modellers point of view, the only safe way to charge them is in a fire-proof container, either metal or a "lipo bag".

Seems that boeing haven't found a safe way yet either

 

[pbrgdmkj]

Some good chips to choose from. I didn't find any circuits on the RCGroups site**broken link removed**

 

[camerart]

"http://www.coolcircuit.com/project/lipo_charge/charger_circuit.jpg'
The webpage cannot be found.

Hi Audioguru, I think this is it? I'm not too keen on a circuit with heat sinks, for me it's not a good sign

Cheers, Camerart.

 

[camerart]

Here it is: https://www.rcgroups.com/forums/showthread.php?t=215550
I have not tested the unit, schematic in post 46, eagle files attached to #41. I do not use eagle, so I can not veryfy anything.
Another thread is: https://www.coolcircuit.com/project/lipo_charger/lipo_charger.html
E

Thanks Canadaelk, I'll keep these in mind. I found myself following many posts, all interesting. It's difficult for me to choose, hopefully, someone will have tried and tested them and chosen the best one that uses the most up to date components.

Cheers, Camerart

 

[camerart]

Most RC chargers do not charge each cell individually, they charge the whole battery and discharge the higher cells to balance, then charge again and repeat until all are charged at the same level.

To charge multiple cells individually, you will need an isolated supply for each cell, ie an inverter circuit for each cell, and a control circuit for each cell. Expensive RC chargers do this, which partly explains the huge price difference between cheap and expensive ones.

From an RC modellers point of view, the only safe way to charge them is in a fire-proof container, either metal or a "lipo bag".

Seems that boeing haven't found a safe way yet either

Hi Sangoma,

This is my worry, but I'm sure as they are on sale to us, there must be a good way of making a good charger. From what I have read, if they are charged incorrectly, they get very warm and the heat can build up till they must 'blow off', if this is contained as in a sealed torch for example, then it will explode.

I haven't gone into the Boeing story, but perhaps they weren't using this site to design their chargers

Cheers, Camerart.

 

[audioguru]

Thanks Audioguru,

I have a circuit for balancing, but I would like to know if it actually charges each cell, differently? I might build it and do some tests.

Cheers, Camerart.

The circuit is too simple and seems to be very old. A lithium charger IC is MUCH better:
1) The circuit is supposed to measure the voltage of a discharged battery and either refuse to charge it if its voltage that is too low or it might attempt charging at a low current until the voltage is normal. If the voltage does not become normal then it will refuse any more charging because the battery might be defective (then it might catch on fire).
2) REGULATED charging current is not needed. Simple current limiting is fine for a charger.
3) A lithium battery is only 70% charged when its voltage reaches 4.2V per cell. A circuit must detect that the charging current has dropped to a few mA then disconnects the charger from the fully charged battery.

 

[camerart]

The circuit is too simple and seems to be very old. A lithium charger IC is MUCH better:
1) The circuit is supposed to measure the voltage of a discharged battery and either refuse to charge it if its voltage that is too low or it might attempt charging at a low current until the voltage is normal. If the voltage does not become normal then it will refuse any more charging because the battery might be defective (then it might catch on fire).
2) REGULATED charging current is not needed. Simple current limiting is fine for a charger.
3) A lithium battery is only 70% charged when its voltage reaches 4.2V per cell. A circuit must detect that the charging current has dropped to a few mA then disconnects the charger from the fully charged battery.

I was looking at this PDF, as it has circuits and explanation, of the sort of thing, I had in mind.

Sangama mentioned the Boeing problem, ( Note page 7 and the patent at the bottom of the page.!!) I just had a look for what caused the fire. It looks like it could have been a too high charge voltage, but it's not clear. So I think any design should have temperature monitoring.

https://www.electro-tech-online.com/custompdfs/2013/03/Li-IonChargeBalancingpdF.pdf

Cheers, Camerart.

 

[Sangoma]

I was looking at this PDF, as it has circuits and explanation, of the sort of thing, I had in mind.

Sangama mentioned the Boeing problem, ( Note page 7 and the patent at the bottom of the page.!!) I just had a look for what caused the fire. It looks like it could have been a too high charge voltage, but it's not clear. So I think any design should have temperature monitoring.

https://www.electro-tech-online.com/custompdfs/2013/03/Li-IonChargeBalancingpdF.pdf

Cheers, Camerart.

My reason for mentioning Boeing was solely to point our that even with huge resources, LiPos can still be a problem (I'm a Boeing fan, great believer in "If it's not Boeing, I'm not going". I won't fly Airbus!!!)

I must say that out of all the options in that article, the Boeing one I like most, and my second choice is individual chargers per cell.

My RC chargers are all the B6 variety, they all work well for the purpose, I have checked the voltages and all are OK. The cheapest one was from Hobbyking, and was about $22 when I bought it. I have never had a problem, but I charge them in a steel container, and they are stored in a steel toolbox with drawers. There are other issues as well, like if you charge them at 20C, then the battery falls to 0C, it is effectively overcharged and will probably puff up.

If you don't need the high current capacity, it may be better to use LiIon. I use LiIon packs on the workbench as portable/extra 12V power supplies, and in many projects I use cell phone batteries, one of which gives the ideal voltage for the new PIC chips, and three give the ideal 12V supply, and two are nearly the same as a 9V PP3 battery. LiIon are also safe to charge in the device, whereas LiPos are risky.

When I charge them, I often just set the bench supply to 4.22V and set the current limit to .5C. On projects that use more than one, I add a connector for the balance plug on a B6 charger. For projects that just use one, I use a simple IC circuit (will look for the number later). Most cell phone LiIons have built in over/under voltage protection, and at less than $2 each, offer a very good power source for small projects.

 

[Sangoma]

The chip I use is a MCP73826-4.2V, about 75c each (had to go and look in the parts drawers)

 

[camerart]

The chip I use is a MCP73826-4.2V, about 75c each (had to go and look in the parts drawers)

Hi Sangama,

I understood why you referred to Boeing, I had the same thought, but they did something wrong, and that's why we're here

I prefer the second one too.

I looked at your chip, and it looks good. However I think there must better chips available, that can sense temperature as well. Also there might be a chip that can check each cell for problems before charging. I'll start with MCP73826-4.2V, and depending on other suggestions eliminate them till one is chosen.

If the chosen chip is expensive, I originally thought that a switching circuit, between all of the cells instead of the chips could be used, then any number of cells could be charged. Perhaps if necessary, a pic chip could be used to control it all.

Cheers, Camerart.

 

[Sangoma]

That chip will a cell that is too low and reduce the current until it comes up. It is quite low capacity though, I think 500mA, but it suits my need OK at a good price.

Your switching idea is good, as long as you are not in a hurry.

I think the B6 idea is probably the best compromise, charge them all and then discharge to high ones until they are all full. Simple to implement, though of course some time is wasted, but not a lot if the battery is in good overall condition.

I suppose you could also use a relay bypass system, which would minimise the charge time and use easy to get components, take their idea and adapt it

 

[camerart]

I'm looking into this chip, it seems to cover everything. MCP73841/2/3/4

I am now thinking that for the extra cost versus the complication, it will be better to just get an ic for each cell.

Cheers, Camerart.

 

[camerart]

Hi all,

Can anyone tell me (Remember safety is important)

If a battery pack has 1X Charging i,c for each cell. These charge ic,s monitor cell condition and charge the cell independently. Is it ok to have the ic,s on and working, while the pack is being used?

Cheers, Camerart.