Estimate run time

[250]

Hello all,
I'm trying to put together some instructions for a piece of equipment we are using.
It consists of two 20 watt halogen lights powered by a NiCad 1.8 MaH 12 volt battery.
I'm looking to know the run time and candlepower/lumens. The company quotes 80,000 CP for 30 minutes but I'd rather get a real estimate from the experts here.

Sorry I can't add any more details but I can not post from work and left my notes there. I also have questions about the charging but don't know the output charge from the wall wart. It did say after a full charge it goes into trickle mode. Can this be relied on?

Thanks for any replies. If more details are needed I'll post back when I know them.

 

[TigerhawkT3]

Those numbers sound about right, but the light will be dimmer at the end of that half hour. As for the charger, trickle charging is just fine for a NiCad pack.

What lights are these, exactly?

 

[Mr Happy]

Hello all,
I'm trying to put together some instructions for a piece of equipment we are using.
It consists of two 20 watt halogen lights powered by a NiCad 1.8 MaH 12 volt battery.
I'm looking to know the run time and candlepower/lumens. The company quotes 80,000 CP for 30 minutes but I'd rather get a real estimate from the experts here.

You can calculate it.

Two 20 W bulbs is 40 W. On a 12 V supply that would be 40/12 = 3.3 A. Drawing from a 1.8 Ah battery that would be 1.8/3.3 = 0.55 h = 32 m.

So the absolute longest run time on a full battery would be about 32 minutes. However, figure that the battery voltage tails off before running out, the battery may not in actual use deliver 1.8 Ah, and all the numbers are only approximate. So maybe 25 minutes is more likely in actual use.

To really know you have to do a test. Theoretical calculations only get you so far and true life circumstances may differ from assumptions made when calculating.

 

[250]

Thanks thats the info I was looking for. I did the calculation but thought it was too easy, that I was missing something.

The lights are an intergral part of ballistic shields we are replacing. There are two lights (Approx 4" X 3" reflector) on the front activated by a tape switch on the handle. 25 minutes is more than enough time for what they are used for. (If they were issued per capita, I might be inclined to up the wattage a bit on mine)

The charger puts out 12.07 V at 180 MA. It doesn't list the rate for the trickle charge. I'm suggesting they charge until they notice the trickle charge light on then remove from the charger. This may be hooked up to the charger for months at a time without use. I'm assuming this, even at a trickle, can't be good.

 

[TorchBoy]

Those numbers sound about right, ...

Except presumably it should be 1.8 Ah or 1800 mAh.

The charger puts out 12.07 V at 180 MA.

And that's hopefully mA. The actual output will very likely change depending on the state of charge of the battery, providing more current when the cells are flat. Generally speaking, NiCd cells can supposedly cope with trickle charging, whereas NiMH cells don't like it. In practice I haven't seen such good results from continuously trickle charged NiCd batteries. It would be interesting to know just what the trickle charge rate is.

 

[Mr Happy]

And that's hopefully mA.

True. Here's an idea of what 180 MA might do: http://www.youtube.com/watch?v=-iClXrd50Z8

 

[250]

Sorry,
Yeah it's little mA on the charger. Its a little bigger than normal for a wall wart transformer. If it did put out MA I might have to "lose" one. I'd love to run some tests on the current and other stuff but I don't think the department would be happy with me screwing around with it and cutting up the wires (everything is soldered together). They are kind of pricey, not because of the lights but the other technology.

 

[Mr Happy]

I'd love to run some tests on the current and other stuff but I don't think the department would be happy with me screwing around with it and cutting up the wires (everything is soldered together).

No no no

When I said "do a test" I meant do a run time test to find out how long it would run on a full charge. You can try to calculate these things but there is no substitute for actual measurement. Using a stopwatch, switch it on and off in a pattern similar to normal use, and time the on periods until the light output is no longer satisfactory. Perhaps repeat with two or three samples to get a good average, depending on how keen you are.