DIY LiPo Protectors

Spiderman’s Uncle Ben was known to say, “With great power comes great responsibility.” The same holds true for battery power. [Andreas] wanted to use protected 18650 cells, but didn’t want to buy them off the shelf. He found a forty cent solution. Not only can you see it in the video, below, but he also explains and demonstrates what the circuit is doing and why.

Protection is important with LiPo technology. Sure, LiPo cells have changed the way we use portable electronics, but they can be dangerous. If you overcharge them or allow them to go completely dead and then charge them, they can catch fire. Because they have a low source resistance — something that is usually desirable — short-circuiting them can also create a fire hazard. We’ve covered the chemistry in depth, but to prevent all the badness you’ll want a charger circuit.

The little circuit fits on top of a standard 18650 cell and uses two chips (one of which is just a dual MOSFET) and three discrete components. It does add about 3 mm to the cell. [Andreas] found that battery holders with a coiled spring would accommodate the extra length, but those with metal leaf springs would not.

We have looked at LiPo charging a few times in the past. We’ve even looked at a site with obsessive battery testing.


24 thoughts on “DIY LiPo Protectors

    1. And it’s surprising just how robust those 18650’s are. I tried and failed to get one to ignite, and I was pretty brutal with it – shorting it out, puncturing it with a nail, etc etc. Actions that make LiPo pouch cells go boom were just shrugged off. Charging when damaged is the biggest risk, and cell protection handles that pretty well.

      There’s a good reason electric cars companies and laptop manufacturers are generally pretty confident in their use.

      1. The older cells from 1998 and older equipment really do go up like fireworks! at least in my experience by chucking them into a fire for LOL-sake (They barely held a charge).

        Modern ones for that matter… Smoke and the occasional hiss, that’s it! How disappointing.
        Unless, of course, you bag yourself a china-mart backstreet aftermarket laptop battery pack… and even then those are starting to improve in safety.

    1. I like the video, but I also totaly agree with the “video” tag… it would allow me to stack on tabs that I wanna watch later instead of just reading during coffee break. +1 for the tag proposal. (cant watch video at work)

  1. Useful tip, thanks!
    I too have “tinkered” with 18650s and found that if you want to prevent them going into thermal runaway adding protection especially against overdischarge is wise.
    Intriguingly the biggest problem with cheap devices using them is lack of cell balancing, a dead giveaway is that battery capacity deteriorates at about 2% per cycle but if caught in the early stages it can be remedied with a simple manual balance and adding the failsafes that manufacturers did not. Typical culprits are cheap portable DVD players and other items using more than one in series.
    Actually to prevent overcharging is a lot simpler than people think and a simple 4V1 zener will work but capacity generally suffers as leakage on these is atrocious. A regulator such as a MN1381 with external drive+shunt resistor and indicator LED is a better option.

  2. Andreas’s video was interesting in its own right and quite relevant to me, as I am still very much inexperienced in DIY-electronics and I am still learning things, and recently I decided that I want to design and build my own, low-power, LiPo-powered device from scratch. The video reminded me that I have bought quite some time ago some random battery-protection boards on eBay and I dug them out, and sure enough, there’s a DW01 protection-IC and one FS8205A dual N-channel MOSFET on-board that I could desolder for my own uses. I also remembered that I have some TP4056-based charger-boards without any protection on them, so I might as well salvage the charger-ICs from them.

    Alas, I don’t quite like the DW01: its discharge-voltage protection kicks in at 2.4V, which is really low — I’ve read in many, many places that one generally shouldn’t let the cells discharge below 2.8V or 2.9V, and so I was thinking of opting for a FS312F-G with the cut-off point at 2.9V instead. The boards that Andreas is using and seemingly almost all such boards on eBay/Aliexpress/etc. use the DW01.

    1. I think 2.4v is fine in practice, maybe a chance of some accelerated performance loss over normal aging but I don’t think it’s a real danger. Maybe in the dim dark days it was a problem or using those very high discharge pouch cells that RC hobbies do, those always seem to be on the raggedy edge of what is “safe”. Some datasheets I have seen for 18650 cells actually have the nominal discharge voltage at 2.5v.

      Big Clive did a video about totally discharged cells a while back (as in 0v), I have myself recently recharged cells salvaged from laptops which were 100-200mV, some still managed 2Ah capacity (at 500mA discharge).

  3. I don’t see so much sense in DIY replicating what you can buy for some cents from AliExpress – where you mostly have to get the DW01 anyway. Buying the parts separately (PCB, DW01 and 8205) is normally more expensive than buying ready made 18650 protection “discs”.

    1. He’s not making his own protection boards, he’s just installing them to the battery. For 40 cents each it’s not worth making the board, sourcing the chips and soldering them. Plus, in that price you also get top and bottom cap for battery, heat shrink sleeve and flat leads.

        1. The chips are worth a few cents, but the PCB itself (considering it’s round which means you can’t do a simple v-score panel) and your time to design and assemble are worth considerably more.

  4. Before these supercheap DW01 based protectors were widely available I used old cell phone batteries, they usually have easy-to-remove protection (usually same DW01+2FET configuration). They fit perfectly on two parallel 18650s salvaged from laptop battery and can provide 2-3A, enough for powering small microcontroller based circuits or LED lamps.

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