Sonntag, 10. Mai 2015

DIY LiIon-Charger

There are plenty of readily built chargers for LiIon batteries available on the market, and many are even quite cheap. But they all are usually limited to a form factor like 18650 cells, special for certain LiPo batteries like for quadcopters, mobile phones or digital cameras. Also, you never really know what you get technically, and sometimes it's not even worth being called a charger.

DIY battery holder with TP4057 DIY charger.
For example, I ordered a chinese wall-plug-charger with sliding connectors so I could charge all kind of flat batteries. One problem though, this charger showed "ready" when the battery had 4.4V - the limit is 4.2V +/- 0.05V, so correct result should have been 4.15-4.25V. Other chargers behave like LiIons are NiMH/NiCd batteries and don't stop charging, but continue to trickle-charge with a few mA; unfortunately the one I bought as universal charger with changeable cradles is one of these. LiIons have very little self discharge, so this will overcharge and damage the battery. Other chargers like the Trustfire TR-001 and TR-002 have electrical problems and also don't work properly with many produced lots; it's like playing the lottery to get a good charger.

"Damage the battery" doesn't sound as bad as it possibly is. Cheap NiCd/NiMH can blow up and leave an ugly mess with the acidic electrolyte. LiIons not only can produce fire and a hydrogen explosion, they also contain Lithium which is one of the most reactive alcaline metals which burns by itself. Destabilizing such a battery with a cheap or simply broken-by-design charger is something you certainly want to avoid. Just remember the Boeing Dreamliner LiIon issues.

TP4056 IC on the eBay/Aliexpress PCBs - original.
This was the reason I was looking around for alternatives. The first thing I stumbled upon a few years ago were TP4056 boards which received quite good feedback. They are small, extremely cheap (less than a buck per board on Aliexpress) and reliable.

These boards use the original TP4056 charger IC (datasheet). There seem to be a few knock-offs or clones around, which nearly all seem not to work well like the original ICs (WARNING!). But this shows that you always need to check at least one charge cycle to verify that your charger is working correctly.

I ordered a few handful of these chips for about 2 Euros including shipping and today finally built my own PCB with it. It is a bit smaller than those boards from eBay/Aliexpress.
Size comparison: DIY board vs. eBay PCB.

On the picture you can see that it is possible to save some space. But it was a challenge to get that on a perfboard without shorts and solder bridges. The LEDs are in the usual colours - red and green, instead of blue (charging) and red (ready/stand by) on the properly manufactured boards.

Finally it worked correctly. Using my improvised battery holder I had a decent CC CV charging cycle with very little thermal throttling. During the constant current phase, close to 1A current were flowing. The battery came out with 4.175V, the charger completely stopped charging the battery. So the IC works like wanted and expected. This was only a random result though, see the newer article about fake and clone TP4056 ICs.

Close-up view of the DIY TP4056 PCB.
Knock-off TP4056 - the logo is missing.

I also ordered a few different ICs which I wanted to test for building my own (universal) chargers .
The LTC4054 DIY PCB.

There is a small SOT-23-5 chip available called LTC4054 (datasheet). It is dirt cheap on Aliexpress, I got 50 pieces for less than 3 Euros. That again includes shipping fees. It can charge with up to 800mA.

At least in theory. At that rate the IC gets quite hot and thus runs into thermal throttling. This not only prolongs the charging cycle, it also leads to a premature end of the CV phase. My batteries had only around 4.15V after charging with this IC.

The LTC4054 only supports one status LED. With a capacitor for the battery connection, without a battery attached it shows a dimmed light with bright pulses at a frequency depending on the capacitance. During charge, it shines bright; when finished, it is in the dimmed state again - without flashing.

It is reliable and doing a good job, but it is hard to cool down due to its small footprint. I like it anyways. Not charging a LiIon battery to the full 4.20V is putting less stress on the cell, leading to a longer life span.

Another charging IC I recently found is the TP4057 (datasheet). The datasheet is only available in chinese, but Google Translate delivers a sufficient translation.
A TP4057 DIY charger board.

The SOT-23-6 IC is also hard to put on a perfboard. But it works fine after all.

The TP4057 delivers up to 500mA charge current. As the other ICs I tested, it properly stops the charging cycle at the end. The final cell voltage was exactly within the datasheet specs of 4.20V +/- 1% with the batteries having 4.158V after the charge cycle.

The TP4057 also has a very good reputation and as noticeable special feature a reverse-battery protection. It got used on some Miller ML-102 versions, where two of the ICs were used in parallel for increasing the charge current up to 1A. The LEDs are connected to only one of the ICs then, which is why those ML-102 versions continue charging a bit longer even when their green LED lights up already.

With these ICs it is quite easy and fun to build your own LiIon charger. The result is a reliable charger which does a proper job without overcharging or trickling the battery. For very little money as well!

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