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alvinlwh

Where did the missing Vs go?

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mvJ1rYK.jpg

Quick question.

Charging my battery tonight and noticed this. 

8.43V is what comes off the main battery connector. It shows even when the balance plug is not plugged in. 

4.185V is off the balance plug. 

4.185V x 2 = 8.37V. Yet it is reading 8.43V off the mains? Where did the missing 0.06V go? When it is done, it does read 4.2V x 2 off the balance and 8.4V off the main. Should I be concerned about it going above 8.4V or is this just something that will happen during charging? 

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From my experience, typically a battery will show less voltage than 8.4v , especially the more cycles it has had.

It could be a poor cell, it could be the charger reading incorrectly,

Do you have another charger or a low voltage alarm that could give you a second reading?

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7 hours ago, Snappy1 said:

From my experience, typically a battery will show less voltage than 8.4v , especially the more cycles it has had.

It could be a poor cell, it could be the charger reading incorrectly,

Do you have another charger or a low voltage alarm that could give you a second reading?

Well, I should have included a picture of the readings at the end of the charge, which was 2 x 4.2V = 8.4V. So it seems that the charger is reading correctly at the end. This difference in cell vs total seem to only happen when charging.

Yes I do have an alarm that reads V, but I also notice that the readings on the charger changes the moment I disconnected the input (charger stays on with battery attached as it can do reverse charge in the other direction). So I will not get the same reading off the alarm as shown on the picture as it will not be charging at that moment.

OR do you mean check at rest readings? If so, I can probably try that. 

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I suspect it's a question of Ohm's Law and measurement methods the charger uses.

The charger is probably showing 8.43 V and 3.00 A right at the charger output jacks.  There is a small amount of resistance associated with all connector contacts, lengths of wire used in the charging leads and battery leads, and the cells themselves.  The current (3.00 A) multiplied by the series resistance of the connectors, leads, and cells (about 20 milliohms) is what results in the 60 mV discrepancy you noticed.  The short answer is that the 60 mV difference and 3.00 A of current results in about 180 mW of dissipated power in the wires, connectors, and cells; a very small amount of heat you probably can't feel since it is distributed across the full length of wire, multiple connector junctions, and surface area of the battery.

The balance lead itself serves double duty as a Kelvin connection to the battery as well as to steer a small amount of current in/out the center connection between the two cells in the battery.  Because the balance lead is connected directly to the cells and is not carrying substantial current, it doesn't exhibit the same voltage gradient between the battery cells and the charger.  The individual cell readings from the balance plug more accurately represent the state of the cells in the battery, not the overall voltage shown at the charger output jacks.

My LiPo chargers do the same thing, and I've even noticed over the years certain charge leads have some wear and tear on them leading to larger differences between the overall voltage and the individual cell voltages added together.  This seems to be attributed to connector cycling, as new charge leads with tighter connectors have reduced the difference a good amount, but not entirely.

If you charge your battery at a lower current, like 1.00 A instead of 3.00 A, you'll probably see the voltage difference reduce proportionately.  Also, if you happen to watch your battery charger as it gets close to the end of the cycle, it will reduce current from the maximum 3.00 A to lower levels as the cells get close to their target voltages.  In that case you'll also see the difference between the main voltage the summed individual cell voltages will be less, too.

In summary:

  • The difference between the charger voltage and the summed cell voltages, multiplied by the current, is the power being dissipated in the connectors, wires, and cells.  You should not be able to feel any heat.
  • The individual cell voltages are more important than the charger's output voltage. 
  • Connectors do wear with cycling, and internal resistance of cells will rise with cycling.  If you start noticing large differences between the charger output voltage and summed cell voltages, it's worth investigating if connectors or the battery itself needs replacing.  Heat and LiPos, large voltage differences, cell imbalances = no bueno.
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