7.4 V 3000 mA LiPo battery charging

Discussion in 'General Electronics Chat' started by Tonyr1084, Feb 12, 2019.

  1. Tonyr1084

    Thread Starter Distinguished Member

    Sep 24, 2015
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    I have a 7.4 volt 3 amp Lithium Polymer battery. Has three wires, Red ( + ), Black ( - ) and White ( ? ). What's the white for?

    What's the proper way of charging it?

    Does the white wire have anything to do with the charging circuit?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Probably a temperature sensor to determine when the battery is overheating or the temperature has started to rise, indicating the battery is charged.
    With a dedicated Li ion charger that has a temperature sensor input.
     
  3. Tonyr1084

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    Looking at those now. There are cheap ones for less than $10.00 and expensive ones closing in on $200.00. The thing I most want to be sure of is to choose a charger that has the correct plug for my battery. OR if need be I can rewire a plug of my own.

    I know LiPo's can be potentially dangerous. That's why I ask. The one I have hasn't been charged for 3 years but is still sitting at 7.5 volts. I've been cleaning shop and have come across this battery before. I just scrapped a 6 volt SLA battery for not being charged and now won't charge. I don't want that to happen to any more batteries.

    I'll look more closely at LiPo Chargers in the days to come. Anyone have any recommendations? I only have the one battery and don't want it to go the way of the other SLA battery.
     
  4. -live wire-

    Active Member

    Dec 22, 2017
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    Measure the voltages between them. I bet it's a balance connector. You need to charge them independently to ensure the voltages don't drift, so that requires a ground connection and one to each battery.
     
  5. Tonyr1084

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    Three wires: Red, White (middle pin), Black.

    Red to White = +7.57 V
    Black to White = 0 V
    Red to Black = +7.57 V
     
  6. crutschow

    Expert

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    Measure the resistance black to white.
     
  7. -live wire-

    Active Member

    Dec 22, 2017
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    Can you share a link or datasheet for it? Also, maybe try measuring in millivolts if you don't have auto ranging on your meter.
     
  8. Tonyr1084

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    10.28KΩ

    Nope. Don't have one. No part numbers.

    Did. 0.001 mV
     
  9. MisterBill2

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    Jan 23, 2018
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    Probably the white to black connection is a temperature sensor, either thermister or a temp sensing diode. For safe charging you can use an adjustable DC supply and set it for 20 milliamps charge current. It will take a lot longer but it is safe and won't damage the battery.
     
  10. Tonyr1084

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    Thanks @MisterBill2. I'm in no hurry. Just don't want to lose another battery the way I lost the 6 volt SLA.

    What voltage should I charge to?
     
  11. MisterBill2

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    Since 7.4 is the published voltage and you measure 7.57, the state of charge is not obvious. You can put an ammeter in series and raise the voltage until the current is 30 mA, which should be less than 8 volts, I believe. I recall charging some kind of lithium cells to a bit over 4 volts. For a more expert opinion, I suggest going to the website of a known high quality maker, such as Tadrian, and seeing what voltage and current they recommend. Those folks are very much into very reliable products and so their advice should be very reliable. My advice will get the battery charged up to some point eventually, but no explosions or fires either.
     
  12. Tonyr1084

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    Does anyone know what a good load test would be? I know that with coin cells, particularly the 2032 you put a resistive load of 1500 ohms for 2 seconds. The voltage should not drop below 2.9 volts and should recover quickly when the load is removed. A weak cell will drop below 2.9 volts. So I know how to test a coin cell. However, this is not a coin cell and has substantially more current capabilities than a coin cell, so before I attempt anything I would like to have reasonably good advice on how to test a 7.4V LiPo battery capable of 3 amps.

    The battery's intended use was backup power for a home security system. The wires coming off the battery are quite small. Probably can handle 3 amps but likely not any more than that. As for the wire gauge - I don't have that information. If push comes to shove I could measure and report the wire plus insulation diameter. But to get a wire diameter I would have to cut the connector off - an idea I'm resisting.
     
  13. MisterBill2

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    Typically, in better quality systems, a heavier gauge is used to provide adequate mechanical strength. Quite a few manufacturers have that requirement on production machinery. For an alarm system one easy check would be to monitor the battery voltage while triggering the alarm sounders, whatever they are, sirens of bells or flashers. 5 seconds should be adequate to see the voltage drop. For a longer term test, a load of 10% of the amp-hour rating should be revealing.So for 3000 Milliamp hours that would be 300mA.
    Doesn't the alarm system charge the backup battery??
     
  14. MrSoftware

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    Oct 29, 2013
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    If it's rated 7.4v then it's most likely a 2 cell battery, in which case I would be surprised if the 3rd wire wasn't a tap between cells for the purpose of balance charging. Not that they don't exist, but I have not myself seen any multi-cell LiPo batteries that didn't have wires for balance charging. But if that were the case then you would surely see a voltage between the middle wire and the other two..
     
  15. MrSoftware

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  16. Tonyr1084

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    @MrSoftware: There IS a control board on the top of the battery(ies). The construction appears to be two flat batteries taped together to form a larger cell. Exactly how they're built I don't know but I can see the edge of the board where the wires go under the cardboard insulation. As for the alarm system charging the battery - that part of the system is long gone, so I don't have a way of charging it from the original system. Nor do I have the sirens that may have once been associated with it. But the 300mA advice seems reasonable. I can select a resistor that can give me 300mA at 7.5 volts, meaning a resistor wattage of 2.25 watts. 25 ohm light bulb should do it. Especially a light bulb of 120 VAC. That way the resistance won't change much when the filaments warm up. Even if I use an automotive bulb and it begins to light up - the draw of current should quickly tell me if the battery capacity is good or not.
     
  17. MisterBill2

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    Yes, I had expected that the balancing circuit would be in the battery assembly, because it is usually done that way.
    I have used banks of 1000 watt bulbs as a test load and they are certainly very prone to resistance change as they heat up. Interesting, though, is that OSRAM has somehow come up with a 1000 watt photoflood that has a lower inrush surge than most lights. That might have been because of all of the hundreds of those floodlights that our company bought in that era.
     
  18. Tonyr1084

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    Using an auto tail light bulb, cold resistance on the high intensity filament I read 4 ohms. When I connect the LiPo 7.4 volt battery (reading before light = 7.54v) the light illuminates. Not bright but it definitely lights. The battery voltage drops down to 7.47 volts. I'd say the battery seems fairly healthy. That's good to know. Of course, the light bulb resistance goes up once lit, but I can't tell just what that resistance is when lit.
     
  19. MisterBill2

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    If you have a variable supply with an ammeter you could set the supply to 7.47 volts and read the current.. But my thinking is that the battery does need recharging, because the discharge curve on those batteries is fairly flat for quite a ways.
     
  20. Tonyr1084

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    Actually I do have a variable supply. Came from a school. They were getting rid of their old equipment. I said "Old". This is a unit that has a ferroresonant isolation transformer that feeds an auto transformer that feeds a bridge rectifier with filter capacitors. They ( I have a few ) need to be repaired to be safe. Kids were tough on them and the power cables need attention. I'll probably get around to restoring one of those units and powering it up. Turn it to 7.47 (with the load attached) and see what kind of current I'm drawing. But right now the wife wants me to start dinner (slow cooker).

    ")
     
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