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  1. #1
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    Basic electronics question.

    Okay...I'm wondering about this PCB protection board with "fuel guage" function. Do you just solder two cells in series and connect the remaining positive and negatives to the "B+ & B-" terminals? What's the "common" terminal used for?



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    Is it like this?


  3. #3
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    correct
    Attached Thumbnails Attached Thumbnails Basic electronics question.-correct.png  


  4. #4
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    Quote Originally Posted by chelboed
    Okay...I'm wondering about this PCB protection board with "fuel guage" function. Do you just solder two cells in series and connect the remaining positive and negatives to the "B+ & B-" terminals? What's the "common" terminal used for?


    Not that I know for sure but I think it has to do with keeping a balanced charge on both of the batteries. Since these are usually used in building batteries that are sealed it is probably necessary to make sure both batteries get an equal charge. If I'm wrong someone please correct me.

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    Quote Originally Posted by Cat-man-do
    Not that I know for sure but I think it has to do with keeping a balanced charge on both of the batteries. Since these are usually used in building batteries that are sealed it is probably necessary to make sure both batteries get an equal charge. If I'm wrong someone please correct me.
    No need to correct you. In order to provide protection against Deep discharge, you need to detect the voltage for each separate cell. Therefore you need the Common connection. It will prevent any cell discharging below ~2.3V (depending on the exact circuit you use).

  6. #6
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    Looks like that particular board may also have an optional temperature function. There is a T called out in the upper right. Probably dot need to worry about temp unless your running a lot of charge or load current.

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    Quote Originally Posted by bikerjay
    Looks like that particular board may also have an optional temperature function. There is a T called out in the upper right. Probably dot need to worry about temp unless your running a lot of charge or load current.
    Since you can see the traces on the board from that terminal, you can tell how that works. It goes to what looks to be a unpopulated 603 surface mount component pad. The other end of that component pad is wired to ground. So, you're right it is for pack temperature measurement. But it's just the wiring for a simple thermistor to ground. So if you want to solder a surface mount 603 thermistor onto that pad, then a charger could read the resistance value of that thermistor to determine pack temperature during charge. This assumes you have a charger that measures temperature in that manor. For li-ion packs this is only used as a safety measure. It's not used for end of charge detections as it is in nimh packs.

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    The 18650 battery pictures you used in your diagram look like they could be protected cells. They may sell unprotected cells with a similar wrapper, but it does bring up a couple of important points. First, you don't want to use protected cells with a protection PCB.

    But, more importantly, it brings up the discussion of how to connect the cells to build the pack. You really don't want to solder directly to a li-ion battery. That can easily damage the pressure based safety cutoff in the cell. The problem is there is no way to tell if the heat from soldering has damaged that safety cutoff until something really bad happens. But, if you're buying individual cells meant for flashlights and building them into a pack, kinda like the pictures suggest. You actually may want to buy protected cells and take them apart. The reason for that is that protected cells have spot welded straps attached to the actual bare cell in order to connect the internal protection PCB. I've taken them apart before and used the bare cells with the attached straps to build a pack. That way you solder the straps together without having to solder to the cells. It's effectively a cheap way to get bare cells with solder tabs already attached.

    Just be aware that the bare cells may or may not have shrink wrap covers on them after you take apart all the protection stuff. It's about 50/50 based on who made the cell. So you need to buy the right size heat shrink to cover the individual cells. If the individual cells don't have covers, the pack will short out and cause a fire. The protection PCB can't prevent that because this is a short behind the protection circuit.

    If you're buying bare cells to build the pack. Make sure you get cells with solder tabs attached.

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    Yeah...I have a couple of Trustfire 2500mAh cells that I was planning on taking the boards out of for the project. Good call on checking the indiv. shrink wrap. Thanks.

  10. #10
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    Quote Originally Posted by MtbMacgyver
    ...( edited )....The 18650 battery pictures you used in your diagram look like they could be protected cells. They may sell unprotected cells with a similar wrapper, but it does bring up a couple of important points. First, you don't want to use protected cells with a protection PCB.

    But, more importantly, it brings up the discussion of how to connect the cells to build the pack. You really don't want to solder directly to a li-ion battery. That can easily damage the pressure based safety cutoff in the cell. The problem is there is no way to tell if the heat from soldering has damaged that safety cutoff until something really bad happens.


    If you're buying bare cells to build the pack. Make sure you get cells with solder tabs attached.
    Glad you mentioned this stuff. After checking BatterySpace I don't see Li-ion cells with tabs anymore.

    Is it really a big issue using protected cells with an external pcb? I was thinking about buying one of those new BatterySpace 18650 holders with pcb. I'm not soldering anything but the leads but I might have to buy unprotected cells. What are the dangers if I use protected cells? link to holders > http://www.batteryspace.com/Li-Ion-1...installed.aspx

  11. #11
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    Quote Originally Posted by Cat-man-do
    After checking BatterySpace I don't see Li-ion cells with tabs anymore.
    You can buy any cell and add tabs for $0.25. Use the "tab options" to select tabs.

  12. #12
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    Battery space? They charge a $25 HazMat fee.

  13. #13
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    Quote Originally Posted by desolder
    You can buy any cell and add tabs for $0.25. Use the "tab options" to select tabs.
    Thanks... ...I don't think I would have seen that unless someone pointed it out.

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