charging dual 18650 batteries in series

[kingsob]

I am looking at something like http://ww1.microchip.com/downloads/en/DeviceDoc/20002190C.pdf (this is the same controller Dave used in his uSupply design at http://www.eevblog.com/files/uSupplyBenchRevC.pdf)

I was talking to a friend who is in to RC cars, and all of his packs have a third wire between the two cells for balanced charging. I assume the cells are being charged individually, using something like two MCP73113 (the single cell variant of the above MCP73213).

In the data sheet above, the "Typical Application" on page 2 have the 2 batteries in series, without any connection to the middle of the two cells.

Is it safe to charge the cells like this? I assume Microchip knows what they are doing, and would not recommend something that is not safe... but I have read countless threads stating that it is not safe without the balanced charging. I can't find a single dual cell charger (I checked Microchip and TI) that supports this center wire.

If anyone could shed some light on this for me, it would be greatly appreciated!!

[franksanderdo]

Hi Kingsob

technically you can load LiIon batteries in a 2S configuration.

Some preconditions are given then:
1. The batteries should be pretty much matching
2. They have to be loaded before once to the same level
3. Every now and then you have to compare them.

IMHO it is not the best solution to load LiIon batteries in series configuration without balancing.
Load balancing is not very difficult to achieve though. in fact you are not using to loaders, one for each batterie.
It is just that you compare the voltage over each battery and the one with the higher voltage gets bypassed partially or (if it is fully loaded before the second one) it gets fully bypassed.

Unfortunately in a quick shot I can't find the app note MC has done for that. In there was a pretty nice and simple circuit based on a MCP73213.

Hoping this helps you a little
Frank

[kingsob]

Hey franksanderdo, thanks for the info!

I think I found the application note you were referring to, I was at work when I found it, and can't seem to locate it again. I will post the like tomorrow. Unfortunately the part it referenced was at its end of life, and I couldn't find a similar part from Microchip.

I was however able to find a demo board from TI which does exactly what I was looking for. http://www.ti.com/tool/bq40z50evm-561

Thanks again
Derek

[sleemanj]

technically you can load LiIon batteries in a 2S configuration.

English as a second language?  The word for putting power into a cell is "charge" not load, which can have quite a different meaning.

[TerraHertz]

I hope you see the problem with series connection? When charging, 18650 voltage should not be raised over 4.2V (with slight adjustment for temperature.) When discharging, they should not be allowed to go below 2.75V. Exceeding either limit can damage the battery.

But with two batteries in series, you cannot know how they are sharing the voltage in charge or discharge. If one is a bit bad, you may exceed the limits for the other battery, and kill it too.

[Paul Price]

When identical new cells are connected to make batteries with Li-Ion cells connected in series, they charge equally without a problem..that is if both cells are not abused by over-current charge/discharge or over-voltage charging. When cells don't match, the prodigal cells need to be replaced

A MCU can monitor the voltages of each cell using A2D voltage measurement, and an optical coupler controlling a transistor that switches in a reasonably small valued resistor across each battery. This idea can be used for balancing,  In this way a shunt across a cell that is charging faster than it's buddies in series can be made to match its charge with its peers. But all this is really not necessary, because if one or more cells fails to match its peers in charging, it needs to be replaced. A voltmeter can easily check the cells for balance after charging if the terminals of the cells are accessible.

[franksanderdo]

English as a second language?  The word for putting power into a cell is "charge" not load, which can have quite a different meaning.

Hi sleemanj

yep, sorry I am not native english speaker / writer and sure i should have used "charge"
Thanks for the hint, will try my best to improve.

All the Best
Frank

[Siwastaja]

2s li-ion packs are widely used without a balance wire or mid-point sensing by reputable companies, but you have to do it exactly like this:

Edit: 0) Make sure the cells are exactly identical, from a high-quality manufacturer, and in identical shape.

1) Charge the cells separately to 4.20V. Verify that they indeed have the same open-circuit voltage (OCV) within 0.01V. 4.19 and 4.18V for example, is ok.

2) Now connect the cells in series, permanently. Don't connect anything to the center tap. Make extra sure there is nothing that can draw current from the midpoint.

3) Now treat it as a fixed product; never charge or discharge the cells separately and never take them apart. You may have the center-tap available for occasional checking with a multimeter, but don't connect anything permanently.

4) Add a little bit leeway to the voltage limits, especially at bottom. For example, for a typical LCO cell, low-voltage cutoff (LVC) is 2.80V and high-voltage cutoff (HVC) is 4.20V. For a two-cell pack, increase the LVC to maybe 3.10V per cell and add a bit of extra margin to HVC side too. So, the final limits might be 2*3.10V=6.20V and 2*4.15V=8.30V. You will lose about 5% of the energy storage doing this but you can expect better lifetime and safety doing so.

For example, Bosch seems to use 4s packs (in their li-ion hand drills) this way without balanced charging or even cell-level LVC, so it clearly works and is clearly safe when done right. When I changed the cells to such a pack after 5 years of use, the cells were still in perfect balance.


If you want to charge/discharge random cells in series (holders for cells you can change, for example), or if you have more than just a few cells in series, you need to monitor the taps between each and every cell to make sure they all stay within the limits while both charging and discharging. Just stopping the charge when one of them reaches 4.20V is fine and easy, but then the others won't get full and will limit the discharge cycle; so actually balancing the cells increases the amount of energy storage.

[MrAl]

Hi,

There are some serious issues that come up when using Li-ion cells in series or in parallel.

When used in series, it is best to have a center connection so both cells can at the very least be monitored individually.

There is an exception to this rule though, but that is only when the two cells in series have BUILT IN protection circuitry.  Some cells have very small circuits mounted right inside the case with the battery cell itself and this circuit is there to prevent over charging and over discharging.  In this case, with two in series, if one voltage gets to 4.2v before the other some of the charge current will be diverted around the cell.  This means the other cell will still charge but that one cell will not charge any more.  During discharge, if the cell voltage drops too low the circuit will shut off or drastically decrease current flow.

I should also note that i can not comfortably recommend doing it this way though, even though it 'should' work.  The reason being is that the safety of the pack then relies solely on the built in circuitry to do all the safety checks and hopefully work as expected if there really is a problem.  If something EVER goes wrong with that circuitry, it could spell disaster.
Also, the data sheet or manufacturer would have to be consulted to be sure that the actual cells being used really do contain this special protection circuitry, because NOT ALL cells contain this circuitry, and it would be very presumptuous to believe that they are all exactly the same and work exactly the same.

Perhaps the cells to be used could be tested somehow beforehand.  If they pass the test then they have the circuitry, but if not then they dont.  Even so, it is still wise to use a center wire for monitoring to be sure.
The time when this really gets tricky is when there are more than two cells.  Then the extra wires really become a problem.  For only two cells adding one extra wire cant be that hard to do for the peace of mind.