Probably a temperature sensor to determine when the battery is overheating or the temperature has started to rise, indicating the battery is charged.I have a 7.4 volt 3 amp Lithium Polymer battery. Has three wires, Red ( + ), Black ( - ) and White ( ? ). What's the white for?
With a dedicated Li ion charger that has a temperature sensor input.What's the proper way of charging it?
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.a dedicated Li ion charger
10.28KΩMeasure the resistance black to white.
Nope. Don't have one. No part numbers.Can you share a link or datasheet for it?
Did. 0.001 mVtry measuring in millivolts if you don't have auto ranging on your meter.
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.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?
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.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.
Yes, I had expected that the balancing circuit would be in the battery assembly, because it is usually done that way.@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.
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.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.