Expert advice for 3.7v LI-ON battery project - Page 1
Forum section: electronics / beginners

author: Captain Morgan
author_rank: Contributor
timestamp: October 14, 2017, 07:20:50 pm
content: Hello folks,I've been reading up on swapping the battery on my Logitech G930 and I have a battery in mind. The battery is a 3.7v 3500mah flat battery for the Sprint HTC phone.I measured the voltage and after all these years and there is still voltage across all 4 pins.The pins appear to be negative (labelled) and negative and negative and positive (labelled). If I keep the + from the volt mater on the + of the battery and move the negative to any of the other 3 pins, I get reading above 3v.This makes me think this is a multi-cell battery for the additional capacity. So here's the deal...I want to put this in to the Logitech headphone - it has 3 wires that go to a 600mah battery with a built in controller ON the battery.The project calls to removethatcontroller and wire it to the new battery.Question #1 - IF the 3500mah battery does not have it's own controller, then would I just reuse the controller to all 3 cells via SHARED negatives? Just splice 3 wires and run in parallel.Question #2 - I can't find any mfr data on this battery, so unless I tear it apart and LOOK is there a way to know if the 3500Mah battery has a charge controller? (I assume it doesn't or there wouldn't be 4 pins)Question #3 - since the charge voltage going to the battery is 4.1v, do we r e a l l y need that charge controller? No explody?Thanks in advance!!

author: KL27x
author_rank: Super Contributor
timestamp: October 14, 2017, 09:34:10 pm
content: Most likely, it is not multiple cells; it's probably just multiple ground terminals that are all going to the same thing. If you remove the case, you can see this for sure.Whatever you're calling a controller is probably just max output limiter in case the positive and negative rails get shorted on the pcb somewhere, and to prevent the battery from overdischarging. I imagine the most common circuit includes a voltage detector and a FET and some kind of flipflop. If you swap between batteries, it may not be calibrated properly, anymore. It might allow overdischarge to occur. Or it may limit your huge battery to miniscule output current, unnecessarily, which would be the most likely result in the scenario you just stated.Leave the little circuit on the battery it came with. If you want additional limits (to protect your load/circuit rather than the battery), such as fuse/polyfuse, or custom voltage cutout, then add it. But this is generally not necessary. The circuit will draw what it needs. Unless you want to poke around and modify the circuit, you don't need additional limiting, and if you ARE poking around, you would generally want to use a current limited PSU.

author: Captain Morgan
author_rank: Contributor
timestamp: October 14, 2017, 09:46:02 pm
content: Thx for the quick replies!This is the mod that I want to follow, the difference is the number of pins available on the battery I have >http://www.instructables.com/id/Logitech-G930-Battery-Upgrade/Logitech G930 supply has three wires, 2 for charging (red and black) and one for telling when the battery is fully charged or not (yellow) and changing an LED color from red to green.So anyways, sure let's scrap that protection circuit. Done.I still question why there are three varying voltages when I touch one of the three ground pins. No problem in wiring to all three then?

author: Nusa
author_rank: Super Contributor
timestamp: October 14, 2017, 11:02:51 pm
content: I'd say the person that wrote that instructable doesn't know a whole lot more than you do on the subject.If you get different voltages between A and B vs A and C, then B and C are NOT the same pin. Batteries are a bad place to make assumptions about what can connect to what!Google "cell phone battery pinout" or similar such search strings. Even the general results should quickly convince you that all such batteries are not the same, even when they have the same number of pins.I'd use the labeled + and - pins only, unless you actually know and understand how the other pins work. If you leave the yellow line unconnected, your status indicators probably won't work, but otherwise it will likely function.On the 4-pin battery, one of the unknown pins is most likely to a thermistor (a resistor whose value varies with temperature), so the phone can determine how hot the battery is.

author: KL27x
author_rank: Super Contributor
timestamp: October 14, 2017, 11:09:45 pm
content: ^ agreeQuotePay attention. This is illogical statement. The same wires used to charge are the only way the charging circuit "can tell" if the battery is fully charged. (And in a li ion charging circuit, the circuit doesn't actually need to know this.) An extra wire is there for something else, like to read a thermistor.Because the battery you are putting in is WAY bigger than the battery you are replacing, it is 99.999% safe to say you do not care about temperature. When you increase the battery size for a given charging circuit and load, you are increasing your margin of safety when it comes to battery temperature. (Barring chance that the original battery is made specifically for high C/drain..... which is highly improbable for a set of headphones which last longer than 5 minutes on a charge.)

author: Captain Morgan
author_rank: Contributor
timestamp: October 15, 2017, 12:22:14 am
content: Condescension aside, thanks for the info. Do you not think I googled the battery pinout etc?If you had watched the video it describes at the 19 minute mark that the yellow wireis requiredin order to charge the battery. It requires 10k resistance. Blame Logitech.No yellow wire, no resistance, no worky.I read the resistance on the new battery by setting the VM to 200k- I got 33.3on the next pin and 99.9on the next. My meter wouldn't read anything if set to a lower range than the 200k.

author: Nusa
author_rank: Super Contributor
timestamp: October 15, 2017, 02:59:11 am
content: No, I hadn't watched the video, and I still haven't except for the small bit you pointed at.What that says to me is that charging electronics is entirely in the other earpiece, and the yellow wire is either a thermistor line or a battery ID code. Either way, it's expecting a particular resistance to enable the charging circuit, in this case about 10K. (If you want to know for sure, chill the original battery in the refrigerator and see if that resistance changed or not.)In the video case, the replacement battery used the same system, just with a different value resistor that he chose to utilize in series with his external resistor.In your case, I'd suggest simply putting an external 10K resistor between ground and the yellow wire, and ignoring the two unlabeled pins, which you've already proved aren't going to give you a 10K result.

author: KL27x
author_rank: Super Contributor
timestamp: October 15, 2017, 07:51:30 am
content: QuoteOops. I am sorry for leaving this out.If the yellow wire is for thermistor, yes, it has to be show "safe" for the battery to charge. This depends on the thermistor that was used and how it was connected. What I meant is that this doesn't have anything to do with "knowing when the battery is fully charged" or not.QuotePutting the yellow wire on a pullup/down resistor is just spoofing the sensor to make it think that the voltage it is reading corresponds to a temp that is under the cutoff. The way the charge circuit knows when the battery is charged (and when to change the LED) is by measuring charge voltage and current (which is going thru the same red/black wires which it is using to charge the battery). *In the original circuit, Logitech did not design the yellow wire to be connected to a 10K pullup resistor. (Or w/e this guy connects it to.... I'm not going to watch the video). It is connected to a thermistor on the battery, itself, which acts as a temp sensor. And there's a comparator or a logic circuit/micro which shuts down the charging if the thermistor indicates that temp has exceeded a safe limit.What I meant was you can safely ignore it in the sense that you do not need it to be connected to an actual thermistor reading an actual temp in this scenario, due to the reasons I explained in my last post. You can connect it to a dumb voltage and not worry as long as you are using a battery with a much larger discharge/charge rate (and higher mass/heatsinking). You are BYPASSING a safety feature which Logitech deemed necessary (and indeed may be). When you bypass a safety feature, you should know what you are doing and why. I don't aim to be condescending; if I am using fancy language to repeat something you already know, let's pretend it is for clarity because it is unclear to me that you know what I do. I've been using li ion batteries since the only response you'd ever get from a post like yours (in forums LIKE this, before this one existed) was essentially "don't do it." The only way I learned to use li ion batteries was from ignoring other peoples' advice, ignoring "battery university" website, and actually reading and understanding battery charge IC datasheets; and these datasheets have all the information you ever need to know if you can decify the technical language.Going larger in the battery takes care of 99% of potential issues with heat while charging. Lower internal resistance means it will handle the max charge rate that the headset can give it without flirting with danger. But you should be aware that if you let the battery go completely flat and don't charge it for a long time, letting the voltage go much below 2.7V, bypassing the thermistor could result in a poof/fire. I suppose this could be a Michael Jackson Pepsi commercial type of problem. A smart charger would charge the battery with a small current until it goes back somewhere north of about 3V. But not all devices will have a smart charger, and in this case the thermistor safety circuit could have been handy. The internal resistance of a li ion battery also goes up over time, so using a battery that is barely beefy enough might result in a problem down the line.I am not sure what the multiple "-" tabs are for on your battery. If the voltage readings are not the same, they are not redundant ground terminals. Looking at the circuitry on the other side (whatever they connect to) might give a clue.*Now, in the old days of NiMh batteries, a sharp rise in temp WAS, indeed, how the charge circuit knew when the battery was full. Li ion are much, much more predictable. Float voltage (or charge voltage plus current < arbitrary cutoff point) is all you need to know.

author: Captain Morgan
author_rank: Contributor
timestamp: October 15, 2017, 04:05:02 pm
content: Thx again. Although I understand the function and purpose of a thermistor circuit, I wanted to think about it all and wrap my head around before going forward.In summary, only use theNEGandPOSpoles, tie the yellow wire to theNEGusing a large value resistor, plug the headset in and see of it will start to charge the battery, and watch long term to see if the LED will change from the charging state to the fully charged state. The supplied 4.1v is acceptable and the larger battery capacity should be fine without including the old protection circuit, for which there is no room for in the cavity given the size of the new battery.As a side note, I can monitor the voltage while being charged and also the temperature of the battery pack.

author: Captain Morgan
author_rank: Contributor
timestamp: October 23, 2017, 09:38:11 pm
content: This weekend I soldered the 3 leads to the 2 posts on the battery, and added a 10k resistor to ground as prescribed.After a bit of testing and watching I reassembled the headset and left on charge until the logitech software indicated fully charged with 10 hours of charge remaining.It appears that 10 hours is the cap it will show no matter the size of the battery.I ran the headset on full volume for 4+ hours before the charge dropped to 9 hours remaining. I left it on another 6 hours and watched the charge status drop hour by hour.At this point I'm confident that I can get a full days music or so out of it without it dying on me.Thx again for all the assistence.

author: Audioguru
author_rank: Super Contributor
timestamp: October 23, 2017, 10:10:43 pm
content: Your guessing game about the pins on the battery is asking for van explody.

author: Captain Morgan
author_rank: Contributor
timestamp: October 24, 2017, 12:03:44 pm
content: Thanks for showing up after the fact with the least helpful and least technical post!