charging and using a 12v NiCad battery

[neilalexrose]

So the challenge has been set, construct an oil lamp that will incorporate a wireless dimmer, so the flicker effect can be controlled from the LX desk.

We have the wireless unit, (mentioned in the recent 12v dimming post), from theatrewirless.com on hire from Sparks.RC2 wireless DMX

The spec for the practical says that it needs to be lit for around 30 minutes (the length of an act in a 3 act play).

The bulb the designer wants is a 20w 12v capsule lamp, as it is quite often the key light for the scene.

And of course all the electronics need to be mounted in a very small enclosure, the base of the oil lamp.

The lamp itself has to be picked up and moved around (hence the need for 12v battery power)

The battery I've found from RS is a 10 cell AA rechargeable NiCad battery pack, as it's pretty much the only unit that will fit in the available space in the Lamp. (RS code 544-0201)

 

Because of the length of time the unit needs to be illuminated it has been decided to run it from mains power when stationary and sat on a table, and when it needs to be moved around for short periods of time, to run on battery power. At present the plan is to have the actor switching between mains and battery power manually, and plugging and unplugging the lamp every time it needs to be moved.

 

So my question is this: In order to eliminate the 'Actor Factor' as much as possible, and negate the need for a manual switch, is it possible to power the circuit from a 12v power supply, that will also charge the battery at the same time? Thus when the mains is disconnected, the battery will take over and continue powering the unit. (like how a laptop works when you disconnect from the mains). One thought was to put the unit on something like a kettle base, which would also get rid of the need for a mains cable to be plugged in.

My knowledge of electronics is lacking in this area, and whilst I know you do need to charge NiCad batteries in a certain way using a charger, I have read that it's possible to recharge batteries from an appropriately regulated mains supply.

 

My thought/hope was to be able to run a 12v transformer from the mains to power and charge the unit, without any extra circuitry, but I am sure this is wrong, hence why I'm asking the question to get others opinions/expertise.

 

Any help and pointers gratefully received

 

Thanks

 

Neil

[boatman]

You need two high current diodes inside the lamp to automatically switch between an external DC supply and the internal battery. Connect one anode to the battery positive, one anode to the external supply positive and join the cathodes together to feed the positive power input to the control system. Having the external DC supply slightly higher than the battery voltage will ensure that the lamp always works off the external supply when it is available. You need to devise a way of connecting the external DC supply without the actor having to do anything except put the lamp down in the right position.

 

You will need remove the battery to charge it, but this is the simplest switch possible with a limited knowledge of electronics.

[david.elsbury]

You need to devise a way of connecting the external DC supply without the actor having to do anything except put the lamp down in the right position.

 

A pair of rings on the table/surface where the lamp sits, and a couple of pins for the lamp to electrically connect to these. Provided the actor sits the lamp down in the right place, power is applied.

[neilalexrose]

You need two high current diodes inside the lamp to automatically switch between an external DC supply and the internal battery. Connect one anode to the battery positive, one anode to the external supply positive and join the cathodes together to feed the positive power input to the control system.

 

 

boatman

 

Thanks for the response. Just browsing RS, would something like this be suitable? RS code 485-4312

Or am I barking up the wrong tree with rectifiers? It's rated at 20a and 150v, or should I be looking at lower values (which are considered high in this situation?) Also the example I've found has 3 poles instead of 2, would that matter in this situation? Again my electronics knowledge is limited when it comes to components, so please forgive my ignorance.

 

Thank you!

[mutley]

Simply couple a resistor of about 100 ohms (DMX terminating type resistor would be ideal) across the diode from the battery pack, and then when the light runs off the mains-fed power, it will charge the Ni-Cd battery as well.

 

As for getting mains into the unit, what about dismantling (ie, hacksawing!) an old cordless kettle? The type that uses a central circular connector would be ideal.

[dbuckley]

For a pair of switching diodes, I'd use a 1N5820G, which is a Schottky diode, 3A 20V. The advantage of Schottky diodes is they have a much lower voltage drop across them than "ordinary" diodes so more power available to the lamp, or conversely, a little less stress on the batteries.

 

For power pickup I'd use a series of copper strips across where the lamp will be placed, which are alternatively connected to supply +ve and -ve, and have pins or contacts or something on the base of the lamp. The lamp has to have rubber feet, of course, to prevent the metalwork of the lamp shorting the strips. There have to be enough pins so that no matter how you orient the lamp on the surface, at least two pins touch the strips, and again, of course, the contacts have to have dimensions such that they cant short the rails. Each pin has a pair of diodes (ordinary diodes will do for this (anything with PIV 50V, IF 3A)) as you dont need to conserve power when on mains) connected to it going to a common positive and negative rail.

 

This way the actor can just place the lamp naturally on a reasonable size flat surface and it'll all work. Otherwise the actor has todosomething right, and thet'll probably be artistically obvious.

 

12V 20W for 30 mins is 833ma/hr, so your 650ma/hr batter pack will not go the distance. You do need torun from mains preferably much of the time.

 

To quote the Sci.Electronics NiCd Battery FAQ V1.00:

1.3) Credo #1: Charge control is the key to battery management

More batteries are destroyed or damaged by bad charging techniques than all other causes combined. Once a battery reaches full charge, the charging current has to go somewhere -- most often, generating heat and gases. Both are bad for batteries.

 

You can use simplistic charge mechanisms (as simple as the resistors alluded to above) but over time, that will roger your batteries. How long is "over time"? - I have no idea, but if your show only has to run for a short poeriod of time, or you are not adverse to short lives of batteries, then a simple resistor charger may give you a useful gain in battery power per show at an acceptable cost. As the FAQ states, C/10 is an sort-of acceptable charge rate, your chosen pack is 650ma/hr so a charge current of 65ma. At this rate, your batteries take about 10 hours to charge, which means its pointless even attempting to do mid-show recharge. You need to work out how much charge you need based on how the lamp can be used.

 

Between shows you should use a proper nicd charger to charge the battery "properly".

[neilalexrose]

Thanks for the replies guys, very helpful as always.

 

Mutley - I'd had the thought about butchering a kettle already, and it's probably what will happen (surely actors can operate a kettle on their own?)

 

dBukley - The show is a 10 week tour, so the unit will be doing 7 shows a week, so I see what your saying about the battery not being charged properly mid show, and being damaged. Until I get all the components together, and wire the unit up, I can't tell how long the whole thing will last for on battery power. I believe that the lamp is not mobile for more than 5 - 10 minutes at a time, so hopefully the battery will last that long.

And yes, the idea was to take the battery away and charge for each show.

 

Keep the ideas coming guys. I'll let you know how things turn out.

 

Many thanks

Neil

[DJ Jules]

Pretty much all of the NiCad battery chargers are basically just a half or full wave rectifier plus a resistor, and even leaving batteries on charge for days at time with these you should still get a lifespan of 6+ months out of the cells as NiCads are pretty tolerant to this kind of behaviour. What is far worse for the batteries is leaving the light on when the batteries are flat which puts them into a strte known as "Deep Discharge" (basically permanent damage to the electrodes in the battery) which reduces the batteries overall capacity permanently.

 

If you want higher capacity, go out onto the high street and buy a set of NiMh or Li-Ion cells (e.g. standard rechargable batteries) as these have capacities up to 2500mah in AA form (e.g. 4 x that of NiCads), but can be charged using the same simple diode/resistor circuit as NiCad batteries - however, overcharging is more important with these kinds of cells as they will become physically damaged if overcharged for extended periods of time (I've seen some nice deformed NiMh batteries which have been put into a cordless phone designed for NiCads!) You can simply get round this effect though by carefully choosing the transformer voltages and diodes you use so that as the batteries charge to their full voltage the total voltage of the cells become almost the same as the transformer voltage, reducing the current flow into the cells (but bear in mind that most transformers output 10-15% higher voltages than quoted (so 12V usually comes out as something more like 13.5v AC - which is usually fine as if you full wave rectify this voltage to make a DC voltage, then you lose 1.4v across the rectifying diodes anyway!)

 

Email me on jburr@lockin.org if you want more info/diagrams on how to do this - it's difficult to explain wiring diagrams without the help of pictures

 

Julian

[Tomo]

I would suggest avoiding charging the batteries at all during the show.

You can't do it properly, and you're not going to get much charge into them either so it's not worth bothering.

 

Buy a proper fast (4 hour) charger for the batteries you choose and an easily changed spare pack. (Or spare lantern!)

That way you can charge in four hours, and you can swap in the spare pack mid-show if it becomes necessary.

 

Use diodes the way Boatman described to ensure you can't put current into the battery.

 

Things to watch out for:

NiCad can safely supply a *lot* of current. Choose the right fuse.

 

You *must* avoid Lithium Ion (and Lithium Polymer), for two reasons:

1) Charging them incorrectly results in fire. Every time.

2) Lithium Ion has special transport laws in most countries due to Issue 1. (It can't go in airfreight for a start)

 

For capacity:

Calculate how long to lamp will be lit for, then double it because you don't want to go past half discharge. (1 hour)

Your system probably draws 25W total, which is just over 2A.

 

So you need 2Ah capacity at a minimum, but I'd double it if possible.

[DJ Jules]

Sorry - wasn't aware of the restrictions with LiIon, as Tomo says though, if you use NiCads, definitely fuse them as they can output alot of current very quickly if you have a wiring fault (due to their low internal resistance).

 

Have you considered using a mix of LED light sources to get the same effect - I'm assuming that this is probably out of the question because of getting the right colour temperature, effect, etc?

[adam2]

Since the OP states that the lamp only needs to be lit for 30 minutes, I would avoid the complication of an external power source, and simply work the lamp from a battery for the whole 30 minutes.

 

AA Nickel metal hydride cells are readily available in 2.4 A/H , ten such cells would be sufficient to power the lamp for over an hour.

These batteries can supply dangerous currents if short circuited and therefore a 5 amp fuse should be fitted in one battery lead, as close to the battery as possible.

Simply ensure that the cells are fully charged before each performance. Such batteries are a little more fussy regarding correct charging, but proper chargers are readily available.

[neilalexrose]

Thanks all for the posts so far.

Quick update: have gone for NiMH batteries as suggested (we had some lying around the office!).

I've ordered the diodes for inserting into the project at a later date. The main concern was to get the thing working first, in the simplist way possible (mainly because of time, as we fit up tomorrow). The same goes for the kettle base idea, to be tried at a later date, once we see the table its on etc. So for now it'll be a manual switch from mains to battery (as its easy to construct), with a plug in lead from unit to table.

I'll update again if I expand the project further.

Many thanks again.

Neil

[Vanjast]

I made a few similar (hurricane) lamps, but used sealed Lead Acid 12V batts, for a scene shoot.

All the 'stuff' (including battery) was contained in the bottom of the lamp (where the parafin goes).

 

It lasted for 15 minutes at 3 amps, was dimmable, and was good enough for their purpose. I'd put a jack plug in the bottom for AC/DC charging power, and they boost charge it over an hour. Batteries eventually died after a few cycles , but you can replace these.