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SLA battery

I have a few questions regarding an old SLA charger (Powakaddy 1999) and I hope you can help me to sort out some problems. First, there is no voltage across the alligator clips (with no load and just the voltmeter) and the regulator measured 24V. Is it true some chargers register 0V until connected to a battery?

The black negative output lead consists of three wires and is sort of terminated as one single wire before it’s soldered to the alligator clip. I think the two wires incorporate a thermistor measuring the temperature near the battery. Would it be an NTC type? What is a typical value? 500Ω?

Unfortunately, I replaced the leads and have had no luck surfing for a schematic of the charger. (E. W., via email).

• It is possible that your charger does not deliver an output unless a battery is connected. Try connecting a large electrolytic capacitor (say 1000μF) across the output leads together with a shunt resistor of 100Ω or so. This will simulate a battery but depending on the complexity of the charger, even this may not work when you make a measurement.

The thermistor would be a NTC (negative temperature coefficient) type but you need to know the specified value as there is little point in making blind guesses.

indicator query

Could you tell me if any circuit modifications or misprints are associated with the “Frequency Indicator For Generating Equipment” featured in the Circuit Notebook pages of your May 2008 issue? I have built the unit, checked the circuit, connections and components several times. I can only get the LEDs running continually through a decade count and no variation on adjustment of VR1.

I am testing it with transformer T1, plugged directly into the mains. I know these are contributors’ circuits and not yours. (R. C., via email).

• If the LEDs just run as a chaser, it suggests that Q1 is on permanently. You need to carefully check the connections around ICIc, IC1d & Q1. 

Time To Get A Bigger Battery

I am working on a project that will turn on several different incandescent 12V lights using relays powered by a small 1.3Ah SLA battery. What I have found is that the battery voltage drops quite a lot when the lights are turned on, due to the large load, but what I am concerned about is that if this voltage drops too much it could reset the microcontroller that is being used to switch the lights on.

The light ratings are: 

Relay 1: 3 x 6W

Relay 2: 1 x 18-21W

Relay 3: 1 x 18-21W

Relay 4: 2 x 18-21W

No more than two relays will be on at the same time. Is there any way of reducing this drop in the battery? If I added largish capacitors on the output of the 5V regulator, would they be able to provide a short supply of power until the lights are switched off, as they are only turned on briefly, for less than two seconds? Or should large capacitors be added across the input to the regulator?

Can you provide any ideas? (B. W., via email).

• By the sound of it, the battery is not big enough. You have to consider that an incandescent lamp will pull surge currents of 10-15 times its normal rating at the instant after switch-on. With a 21W unit, its surge rating can be anywhere between 17.5A (10 times rating) to 26.5A. The battery certainly cannot supply this current without drooping.

The solution is to use a bigger battery or use high-brightness LEDs instead of incandescent lamps.

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