# Using LiPo cells without a protection circuit

I designed a PCB to provide the power circuit for my robot's servos. This has a LiPo battery protection IC (undervoltage/overvoltage) and a voltage regulator IC. I sent this PCB in to be manufactured, and went to order the parts...

Only to find out noone sells the IC (S-8253) I planned to use for protection. Seiko Instruments does not list this as out of production/outdated, and my quick initial search did show the IC on Digikey's website, but I did not apparently notice that it is not stocked. And I cannot buy 3000 chips ;)

Anyway, the PCBs will be here soon, and I would like to do some work on my robot during this Xmas break. How bad will it be if I bypass the protection circuit for the time being? I plan to charge my LiPo using a proper charger, which I would hope balances the cells anyways... But I have no way to control even discharge.

Also, if anyone has any suggestions for undervoltage/overcurrent 2-4 cell protection ICs that can be hand soldered, I'd appreciate it :)

Thanks!

• Try Master Electronics masterelectronics.com . They also distribute Seiko parts. – brhans Dec 19 '14 at 18:36
• Unfortunately they require a minimum of $100 order for Canadian customers :( I actually did spend some time trying to order everything else from them (capacitors, either ICs, etc), but they don't seem to carry as much stuff as Digikey, making it difficult to get to the$100 limit even if I order lots of extras. – Mewa Dec 19 '14 at 18:39

## 1 Answer

The protection circuit protects the LiPo from many dangerous conditions. Overvoltage can be ignored here, as you are not going to charge the cells from your circuit. Undervoltage (i.e., overdischarging the battery) can be fatal to the cell, although not immediately dangerous (the big issues might arrive later, when charging). The other protections that are common are overcurrent/short-circuit, which is a very important one, as a shorted LiPo can potentially explode, and overtemperature, which is mostly useful while charging, or while discharging at very high rates.

For your particular application, if you take care all the time of the battery voltage (making sure it does not overdischarge), and only use a proper charger to charge it, I don't see a major problem in running your circuit for test purposes without the protection. However, make sure you add a fuse between the battery and the power input of your circuit, to prevent dangerous conditions in case of overcurrent or short-circuit. This is very important. LiPo batteries can be very dangerous. If you have a LiPo protection bag, use it, as long as you're testing without the protection: better be a bit too careful than not enough.

Concerning a hand-solderable protection IC: I recently used a DS2777 (for a 2-cell system), it works pretty well, and even if it is a SMD part, it can still be soldered by hand, although it's not simple. But is possibly a bit "overkill" for your application, as it requires two external MOSFETs, and is not only a protection circuit but also a battery monitor system that can estimate remaining charge, read battery parameters (voltage, current, accumulated power, etc.) over I2C, etc.

• Thanks Ale! I have a fuse in circuit (on the PCB), thinking of placing a 10A one in there, though originally I was going to do 15A. I don't have a protection bag, but I'll stop by the local hobby store and get one this weekend (I should...). Thank you for your IC recommendation as well! I am going to look at the datasheet. SMD parts aren't too scary, but if they have the tucked-away legs, it gets a bit tricky. I suppose I could just reflow it though :) – Mewa Dec 19 '14 at 18:42
• Most protection devices need the 2 external MOSFETs though, so best to prepare yourself mentally ahead of time. – Ignacio Vazquez-Abrams Dec 19 '14 at 18:48
• @IgnacioVazquez-Abrams: actually the S-8253 mentioned in the quesiton also requires two MOSFETs, I didn't look long enough at the datasheet when answering... ;-) – Ale Dec 19 '14 at 18:54
• @IgnacioVazquez-Abrams, yup, I already had the 2 MOSFETs on my PCB, so it's not that much of a shock anymore. The main downside of DS2777 is that its 2-cell, and while I don't need it right now, I will need something capable of supporting a 4-cell pack as well. I could of course make two different boards, buuuut I would prefer the same one. – Mewa Dec 19 '14 at 19:01