(title continuation: and testing a lead-acid battery charger may not be as simple as you think)
We have vehicles that have 12, 24 or 36 V lead-acid batteries in them. 12 V for starting gasoline or diesel vehicles ... 24 or 36 V for electric vehicles, and these are actually banks of 6-volt batteries in series. There are chargers for all three voltages. Occasionally there has been a question as to whether a charger is operating correctly. So, a test load that would draw a fixed number of amps was needed. I wanted something in the neighborhood of 10 amps.
For 36 V, 10 A would mean 360 W, which was more than my BK electronic load can manage. Besides, I wanted something more robust electrically and physically to use off the bench. I decided to use wire-wound high wattage resistors. The concept was to use three resistors in series with end taps and two center taps. One resistor for checking a 12 V charger, two for a 24 V charger, and three for a 36 V charger.
For each 12 V increment at 10 A, a 1.2 ohm, 120 W resistor would be needed. I don't like to run power resistors at more than 1/2 their rated wattage, and even then they get hot ... as we'll see. A 1.2 ohm, 240 W resistor wasn't feasible. So, to drop the power load per resistor in half and still have 1.2 ohms, two 2.4 ohm, 120 W resistors in parallel would be needed. Good luck in finding 2.4 ohm, 120 W resistors! Sometimes initial specs have to be changed a bit to accommodate parts availability and budget.
The sweet spot in price per watt for power resistors was 100 watts, which are available in many resistances ... the closest to, but greater than, 2.4 ohms was 3 ohms. So final design was three sets in series of parallel pairs of 3 ohm, 100 W resistors. This would provide 3 taps of 1.5, 3, and 4.5 ohms to provide 8 A of current draw for 12, 24, and 36 V, respectively. Any resistor receiving power would carry 4 A at 12 V, so 48 W per resistor, which is about 1/2 the power rating.
As mentioned above, at 1/2 rated power, the resistors would get hot, and only passive cooling was desired. Initially, the resistors were mounted on a 300 x 100 x 6 mm Al plate that was mounted horizontally on plastic feet. At full load (36 V producing 288 W), the feet started to melt. So large heat sinks were added to the opposite side of the Al plate, and it was positioned vertically. See the pictures.
Thermal paste was used on the resistors and heat sinks. The Al plate was isolated thermally from the support brackets with PTFE bushings (25 x 6 mm with 6 mm hole). Thin PTFE washers were used under the metal washers used as part of the hardware for mounting the plate on the brackets. The brackets were isolated thermally from the wood base with PTFE sheeting. This last step proved unnecessary when the device was tested. The connecting wires are 14 AWG (1.63 mm) solid copper.
The device was tested at full load (36 V = 288 W) at 39 C ambient with no wind or forced ventilation. Steady state temperatures were:
upper row of resistors average: 119 C
lower row of resistors average: 61 C
support brackets average: 41 C
I consider these temperatures to be acceptable, and the resistors have had no problems with prolonged use.
Testing some types of battery chargers is not as simple as just connecting them to a resistive or electronic load because some chargers won't try to charge a dead load. Yup, some chargers check whether there is voltage applied to their output terminals before they initiate or continue a charge cycle. So, they will do nothing with a completely discharged battery or a tester. If you look inside these beasts, the positive and negative output terminals each have a large and small wire connected to them. The large wires are used for the charging current; the small wires go to the voltage sense circuitry. This circuitry not only determines whether to start a charge cycle, it also determines when to end it.
To test the charger, the sense wires need to be separated from the charge wires at the output terminals. The charger tester can then be attached to the output terminals, and a voltage source can be applied to the sense circuitry. This can be done in a temporary fashion, but I install a DPDT switch that can direct the sense wires to the charge terminal or a small pair of test terminals. I also install a 1N4002 diode on one of the small test terminal wires to guard against reverse polarity application. If your voltage source is a mains-powered power supply, be certain it has isolated outputs ... the charger sense wires may not be isolated.
A picture below shows an old Lester 24 V charger modified to allow convenient testing. In addition to allowing testing, independent access to the sense wires also allows some exploration of the workings of the charger. For example, the specs for the Lester charger stated that 5 V needed to be present for a charger cycle to begin. Actually, 11 V was needed to start a cycle and 6 V to maintain it. These values would have been a volt or so lower without the protection diode. Also discovered that that during charging, the charger continuously monitors voltage until it reaches 26 V, then it continues for 45 minutes and shuts off.
After carefully checking that there would not be a mains short, I checked the Lester charger output with a scope during a session with the test load. It put out a distorted rectified sine wave, 122 Hz, Vpeak = 33.6, Vmin = 7.0,Vrms = 28.6, Vavg = 27.7. Presumably, when charging a functional battery, the control electronics monitor Vmin to determine when to end the cycle.
I've used the battery charger tester a number of times and have been very happy with it. Hopefully a few folks will find this post useful.
Mike in California
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