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"Exploding" Battery |
R. W. Stuart One of our better looking fliers recently gave me a battery to examine which had "exploded" when he was trying to get voltage readings. Apparently he had pushed pins into the battery connector to bring out the voltage and then they shorted, which I am sure attracted his attention. He grabbed a pair of pliers and pulled the wires out of the battery. I presume the battery leads and connector were destroyed as he provided a spare 3 wire servo extension connector with pigtails. On opening the battery I found that the wire leads were gone as were 3 out of the 4 nickle cell connector straps inside. The cells looked pretty good, but several burned spots showed on the plastic cell tubes and what was left of the nickle straps were badly "wrinkled" by heat. The tube spots were dabbed with good old Duco. Each cell was voltage checked and all showed one or more volts indicating they might be useable. The battery was reassembled using fine gauge wire for the jumpers from cell to cell. The stub end of the spot welded straps may be removed by rolling the strap around a needle nose plier as the plier is rotated. Smooth the weldment burrs and tin each end of each cell while keeping the cell heating to a minimum. Assemble into a square or flat as you wish and solder on the jumpers. The arrangement for this battery came out with +, -, and midpoint all at the same end. The plus is red, negative black and white midpoint will not be used. Since the plug on my charger is a mini phone plug, I put a plus pigtail and a neg pigtail on the battery so I could charge it. The battery was charged ( 1/10 rate ) for 30 hours after which the following capacity test was done.
JH BATT TEST, 4.8 v. RCVR
TIME E Ea OHMS OHMa ma ma/12 mahr
---- ---- ---- ---- ---- ---- ----- ----
0 5.44 5.2 0 0 0 0
5 4.81 5.13 5.6 5.40 950 79.2 79.2
10 4.76 4.78 5.7 5.65 846 70.5 149.7
15 4.71 4.74 5.7 5.70 831 69.3 219.0
20 4.62 4.67 5.7 5.70 819 68.2 287.2
25 4.50 4.56 5.7 5.70 800 66.6 353.8
30 4.40 4.45 5.7 5.70 780 65.0 418.8
35 4.23 4.31 5.6 5.65 762 63.5 482.3
40 3.74 3.98 5.5 5.55 717 59.7 542.0
45 2.16 2.95 5.4 5.45 541 45.1 587.1
50 0.98 1.57 5.3 5.35 293 24.4 611.5
55 0.47 0.73 5.2 5.25 139 11.6 623.1
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Where TIME is elapsed time in minutes.
Where E the voltage across the load.
Where Ea is the average voltage thru the interval- the
voltage at the interval start plus the voltage at the
interval end, the sum of which is divided by two. This
average interval voltage provides a conservative Ea.
Where OHMS is the resistance of the load measured in ohms.
The load resistance increases as the resistor heats.
Where OHMa is average ohms across the interval.
Where ma is the instantaneous current ( Ea/OHMa x 1000 )
average for the interval.
Where ma/12 is 5 minutes worth of the ma, ( 60/12 = 5 ).
Where mahr is the summation of the intervals of the test,
which is the milliampere hour capacity of the battery.
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The test shows 623 mahr capacity which is not bad for a 600 mahr battery and is very good for a 500 mahr battery. After a recharge, the proper plug pigtails will be soldered in, and the white wire will be folded in the battery case and will provide strain relief for the battery leads. Test setup connects the battery right across the load resistor and the voltmeter across the load where it reads both the load drop and the battery voltage. About 5 ohms is a suitable load and the higher the wattage of the resistor the smaller will be the excursion of ohmage due to heat- 10 watts or better. |
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