Electronic – ny sense in this technique for charging Li-Ion batteries

batteriesbattery-charginglithium ion

This is a replication of this question on Android SE. I feel that it is in fact an electrical engineering question, since it covers batteries behavior in details.

Basically there's a claim that the following procedure will significantly (two times improvement claimed) improve the battery runtime (time a device can run on a single battery charge):

  • Turn on your phone;
  • Connect the charger and wait until the battery is full-charged (green indicator is on);
  • Disconnect the charger;
  • Wait until the green indicator is off and connect the charger back;
  • When green indicator is on, turn off the phone.

Now, continue with switched off phone.

  1. Disconnect the charger;
  2. Wait until the green indicator is off;
  3. Connect the charger, wait for green indicator and disconnect the charger again;
  4. Repeat the "3" step 10 times. Each iteration might take from 30 seconds to 30 minutes, usually that's about 1 minute.

The procedure description is taken from this article in Russian which I've read and the English translation is correct.

Here's how I understand what is going on. While the green indicator is on, the battery is being actively pumped energy into. At some point the charging circuit decides that the voltage is high enough to declare the charging complete (this requires some heuristic for customer convenience).

Then once the charger is disconnected the voltage on the battery gets lower, so when the charger is connected again the charging circuits sense voltage below the "stop charging" threshold and turn the green indicator on to show that it decided to charge the battery a bit more.

Since all this is happening while the battery is near the "stop charging" threshold the charging current is minimal and also when the green indicator is off it doesn't mean the battery is not being charged – it is just being charged much slower. So simply leaving the device connected to the charger for another hour would be just as efficient.

What is likely happening during the described procedure? Will it improve the battery runtime? Are my assumptions correct?

Best Answer

I think what they're trying to do here is 'trick' the phone's battery charging intelligences. Li-On batteries are very touchy and have somewhat complex charging strategies. It all boils down to determining something called State-Of-Charge (SOC). SOC is just a percentage in the end, but arriving at the SOC number relies on a large number of factors that are not always easy to read and sometimes must be indirectly inferred. For instance, let's assume that you have a cell phone with a Li-On battery that is 3.7V and 1000mAH. We'll start with it being fully charged, so we know SOC is 100%. As you use your device you're drawing current out of the battery and the battery's voltage will drop - eventually. By measuring the current and monitoring the voltage you can guess what the SOC is. One problem is that the voltage isn't terribly useful in determining SOC because it doesn't change very much until the battery is nearly empty - that is NOT something you want to do to a Li-On battery. So you're mainly relying on the current.

So your SOC is being estimated throughout the usage. It gets low - 50% maybe - so you plug it in to charge. While it's charging, it monitors the charge current and battery voltage to determine when SOC is 100% again. Only, due to errors in measurement it says that the charge is complete when SOC is actually only 95%. Now your phone thinks 95% is fully charged - and it remembers this for future reference because it doesn't want to over-charge the batteries (this is also very bad). So essentially it's trying to read when the battery is full by measuring what goes in and guessing where that puts the SOC based on past results.

The errors aren't large so during normal charge/discharge usage you won't notice a problem. But sometimes the errors can stack up and your phone thinks its fully charged when it has little or no charge - it goes straight from full to empty and due to the incorrect SOC calculation the phone won't try to charge the battery more because it doesn't want to damage it.

In these cases you have to reset the SOC state. I have a Droid Incredible 2 and I've done it by removing the battery and holding the power button for 30 seconds, then putting the battery in and charging the phone while its off. This always fixes the issue where the battery thinks its full but drops down to something like 10% very quickly and the issue where it thinks its at 10% but has much more charge left.

The strategy outlined in your post is obviously trying to recalibrate the SOC or trick the algorithm somehow. Having never developed a charger that relied on SOC I can't say whether it will work but it seems like a lot of effort for a questionable amount of benefit. If your battery is acting really funny try what I suggested first.