Wireless 12v charger

[Earl]

Would anyone know if there is a way to charge a small leisure battery wirelssly? like some mobile phones work?

[Lonestar70]

Hi Earl,

Sorry but it cannot be done... the battery would need to have an inductive charging receiver circuit built in to it... same as phones have.

There is very little advantage anyway, you still need to plug the inductive charging mat (as used by phones) into the mains

and the rate of charge would be considerably less than a conventional mains charger, hence taking much longer to charge even a small leisure type battery.

Stick with a conventional charger that has good battery condition checking.

Keep Happy.

Sandy.

[Earl]

Your negative points are in fact the posatives to be honest, Time is not an issue, and the need for power is also not an issue, the wirleess induction between the two devices is perfect for its use

I want to be able to change the battery on my dome shutter when it is in the parked posistion, without any physical contact, as the dome does not get used fore days at a time, it not imporant for it to be quick. the current Solar option is not doing it.

[Gina]

Very interesting   I don't have an answer but I admire your question   As someone once said, I believe

[just_ed]

A DIY wind driven charger?? Just just an idea.

[Earl]

Like wind power in genral it would cost more than it produces

[Earl]

Can you use an electromagnet to induce a voltage in an oposite electromagnet?

[just_ed]

I was thinking a DC generator. That's what was used to measure wind speed at one time. It would have to put out more than 12 volts of course.

[Oddsocks]

Can you use an electromagnet to induce a voltage in an oposite electromagnet?

Yes, you are basically describing a type of transformer commonly referred to as "Air Cored"

Two electromagnets do induce voltages in either one providing the magnets are either continually moving relative to each other for DC magnets or are stationary relative to each other when fed with AC power.

Air core transformers are used in specialist applications such as high vacuum vessels where you can not have a hole drilled in the vessel to feed in outside power for internal devices, they just have one coil glued to the inside of the vessels' (non magnetic) wall and the other glued outside, the gap between the coils can be quite large if the magnetic fields are strong enough.

Air core transformers are very inefficient when working at normal AC power frequencies (50hz) and might have loses up to 75% or so in the transfer process but become much more efficient if you step up the frequency to around 1khz or more (which also has the added bonus of reducing the size of coil required.

Problems to overcome when using "Air Core" transformers include ensuring the transformer can not be active when the coils are not close together, as in a moving observatory dome, as the inductance of the supply coil will drop as the other coil moves away, the current will ramp up in the supply coil and begin to heat significantly so you need to fit some kind of physical interlock to prevent this happening, or control the primary coil current through electronic regulation.

An air cored transformer would be sufficient to drive a trickle charger for your battery but it is not likely you could buy one, you would have to wind the coils yourself.

William.

[Lonestar70]

Yes, you are basically describing a type of transformer commonly referred to as "Air Cored"

Two electromagnets do induce voltages in either one providing the magnets are either continually moving relative to each other for DC magnets or are stationary relative to each other when fed with AC power.

Air core transformers are used in specialist applications such as high vacuum vessels where you can not have a hole drilled in the vessel to feed in outside power for internal devices, they just have one coil glued to the inside of the vessels' (non magnetic) wall and the other glued outside, the gap between the coils can be quite large if the magnetic fields are strong enough.

Air core transformers are very inefficient when working at normal AC power frequencies (50hz) and might have loses up to 75% or so in the transfer process but become much more efficient if you step up the frequency to around 1khz or more (which also has the added bonus of reducing the size of coil required.

Problems to overcome when using "Air Core" transformers include ensuring the transformer can not be active when the coils are not close together, as in a moving observatory dome, as the inductance of the supply coil will drop as the other coil moves away, the current will ramp up in the supply coil and begin to heat significantly so you need to fit some kind of physical interlock to prevent this happening, or control the primary coil current through electronic regulation.

An air cored transformer would be sufficient to drive a trickle charger for your battery but it is not likely you could buy one, you would have to wind the coils yourself.

William.

This is ,in effect, exactly how wireless phone chargers work.

The problem would be building the transformer coils and getting them to couple well enough to transfer sufficient voltage and current to charge the battery... you would also need to ensure proper polarity and regulation.

Not overly difficult for a phone battery... not so easy for a much larger SLA type battery.

Great idea if you can get it to work... who knows, you could make a killing on the market with a good design.

Enjoy.

Sandy.

[Oddsocks]

Great idea if you can get it to work... who knows, you could make a killing on the market with a good design.

Enjoy.

Sandy.

Nice idea, I could do with the cash, unfortunately some chap called Nikola Tesla already patented the design back in c1891, didn't do him much good however he still died impoverished and in debt.

The biggest user of the Air Cored transformer, also called a Tesla Coil when configured as a tuned HF coupled resonant transformer turned out to be Hollywood in all those old 1930 Frankenstein movies.

If you ever visit Munich make a trip to the high voltage exhibition at the Deutches Museum, they put on a live demonstration a couple of times daily of Tesla Coils in action....quite literally, hair raising, and very very loud!

[Earl]

I have been ponderig this problem over the day.

As crazy as it might sound a model railway track on the inndedr ledge of the dome, connected to the shutter control and a set of springed bogies sending power to the rails could be a solution.

[Lonestar70]

Hi Earl,

That might work but you would need to make the tracks and bogies from something that did not tarnish  (Gold plated tracks and bogies anyone) or you would get poor connection problems quite quickly with any moisture in the air... quite probable in an un-attended and un-heated obssy.

Probably easier to achieve/build than a wireless set-up.

Keep Happy.

Sandy.

[Oddsocks]

Earl,

If you don't mind a suggestion the way we did this in power engineering was via carbon brushes and brass slip rings.

In your case the dome roof is always in one fixed location when it is closed?

The battery charger is on the fixed non-moving part of the dome wall and the + /- 12V charger cables, both protected at the charger end via in-line fuses, run up to a pair of brackets facing the rim/flange of the dome roof and carrying a single carbon brush on each bracket.

On the dome roof a pair of short brass slip rings, approx 10cm long and with a gap between them equal to twice the length of the slip rings, in this case i.e. 20cm can be epoxied to the rim/flange and a pair of cables + / - continuing up the wall and connected via in-line fuses to the battery terminals.

The spacing between the bracket centres carrying the brushes should be the gap between the slip rings plus the length of one slip ring, in this case 10cm + 20cm = 30cm. in this way the circuit can not be bridged or reversed and the complete circuit is only made when the dome roof is in the home position.

The brushes used should be of the radiused type and the edges of the slip rings need to be chamfered to avoid chipping the brush as the dome rotates away from the home position.

Most, but not all, battery chargers supply a maximum of 4A and detect a no-load condition, you would need to confirm yours is this type, in which case you could leave the charger on supply all the time, when the dome is rotated away from the home position the charger would sit idle, when the dome is in the home position the carbon brushes sit in contact with the slip ring and the charging circuit is complete, the charger will trickle charge the battery.

A pair of carbon brushes 5mm x 8mm will take a current of 30A and the brass slip rings 10mm x 1.5mm would take 100A.

As the carbon brushes run over the brass slip rings each time the dome rotates the contacts are self cleaning.

The brushes are rated for years of continual use and in an observatory with very occasional use should last several lifetimes...

The brackets can be mounted to the observatory wall using thick rubber washers between the wall and between the head of the screw and the bracket, this will provide the spring effect to keep the brushes firmly pressed against the slip rings when in the home position but free of the dome flange when the dome is rotated away to a working position.

If your battery charger is not of the auto sensing type then you would need to fit an IP44 environment protected 240V proximity switch activated when the dome is in the home position to turn the charger supply on / off as the dome roof is in or away from the home position, these are available from most electrical suppliers and are not costly, the proximity switch can be the microswitch and cam type, doesn't need to be anything that sophisticated but it would be cheaper to buy an auto-sensing charger that does not need to ever be turned off.

(An alternative arrangement can be both brushes on a single bracket, arranged vertically and the slip rings one above the other on the dome roof flange but this is more difficult to implement since the brush bracket has to be rigid and the brushes carried on sprung bogies so that the pressure between brush and slip ring is consistent for both contact sets, it is also easier to accidentally short between the contact sets and blow the fuses)

Below are a few pictures of contact sets and slip rings to give you an idea of what is available, you could even scavenge most of the components at a good scrap yard.

 

 

 

William.

[hughgilhespie]

Adding a thought to William's excellent idea, I suggest using silver for the slip rings, rather than brass. Sheet silver is not horrendously expensive, two pieces 10 x 50 mm of 3 mm sheet would cost less than £30 and silver is so much more tarnish resistant than brass.  Brass in an external environment, particularly in an urban location, will tarnish badly in a fairly short time.

I suggest 3 mm sheet as it's is thick enough to use countersunk M3 or M4 bolts for fixing so no problems with the brushes hitting the bolt heads.

Regards, Hugh

[mhard26339]

On 29/01/2015 at 11:11, Oddsocks said:

Earl,

If you don't mind a suggestion the way we did this in power engineering was via carbon brushes and brass slip rings.

In your case the dome roof is always in one fixed location when it is closed?

The battery charger is on the fixed non-moving part of the dome wall and the + /- 12V charger cables, both protected at the charger end via in-line fuses, run up to a pair of brackets facing the rim/flange of the dome roof and carrying a single carbon brush on each bracket.

On the dome roof a pair of short brass slip rings, approx 10cm long and with a gap between them equal to twice the length of the slip rings, in this case i.e. 20cm can be epoxied to the rim/flange and a pair of cables + / - continuing up the wall and connected via in-line fuses to the battery terminals.

The spacing between the bracket centres carrying the brushes should be the gap between the slip rings plus the length of one slip ring, in this case 10cm + 20cm = 30cm. in this way the circuit can not be bridged or reversed and the complete circuit is only made when the dome roof is in the home position.

The brushes used should be of the radiused type and the edges of the slip rings need to be chamfered to avoid chipping the brush as the dome rotates away from the home position.

Most, but not all, battery chargers supply a maximum of 4A and detect a no-load condition, you would need to confirm yours is this type, in which case you could leave the charger on supply all the time, when the dome is rotated away from the home position the charger would sit idle, when the dome is in the home position the carbon brushes sit in contact with the slip ring and the charging circuit is complete, the charger will trickle charge the battery.

A pair of carbon brushes 5mm x 8mm will take a current of 30A and the brass slip rings 10mm x 1.5mm would take 100A.

As the carbon brushes run over the brass slip rings each time the dome rotates the contacts are self cleaning.

The brushes are rated for years of continual use and in an observatory with very occasional use should last several lifetimes...

The brackets can be mounted to the observatory wall using thick rubber washers between the wall and between the head of the screw and the bracket, this will provide the spring effect to keep the brushes firmly pressed against the slip rings when in the home position but free of the dome flange when the dome is rotated away to a working position.

If your battery charger is not of the auto sensing type then you would need to fit an IP44 environment protected 240V proximity switch activated when the dome is in the home position to turn the charger supply on / off as the dome roof is in or away from the home position, these are available from most electrical suppliers and are not costly, the proximity switch can be the microswitch and cam type, doesn't need to be anything that sophisticated but it would be cheaper to buy an auto-sensing charger that does not need to ever be turned off.

(An alternative arrangement can be both brushes on a single bracket, arranged vertically and the slip rings one above the other on the dome roof flange but this is more difficult to implement since the brush bracket has to be rigid and the brushes carried on sprung bogies so that the pressure between brush and slip ring is consistent for both contact sets, it is also easier to accidentally short between the contact sets and blow the fuses)

Below are a few pictures of contact sets and slip rings to give you an idea of what is available, you could even scavenge most of the components at a good scrap yard.

William.

Hi William

 

Do you know the supplier for the slip rings?

 

Regards

 

Mike

[Oddsocks]

Hi Mike.

When I used to source contact and slip ring sets for proper (paid!) work projects we used a UK supplier, Ekaton Ltd.

http://www.ekaton.ltd.uk/products.asp

For stock items they used to supply in small quantities, for a specific bespoke design it was expensive for one-offs and wasn't economic unless ordering in the hundreds so in most cases we took standard products and adapted the machine we had to build or repair to take the standard catalogue items. Many of the standard products have remained unchanged for forty years plus and were widely interchangeable over many generations of machines.

Last time I dealt with them was ten years or so back and they were always helpful, if you tell them what you are trying to do they used to be able to suggest suitable components for you rather than having to trawl through all the different product catalogues and specs.

H.T.H.

William.

[Adam J]

I am interested as to why you would want to do this?

[Oddsocks]

I believe Mike has a problem with his observatory dome roof power supply (if it was his post I saw earlier). 

Some of the commercial observatories use a battery powered shutter on the dome that uses a solar panel to charge the battery and at certain times of the year the solar panels may not see any direct sunlight due to nearby buildings, trees etc, and after a while the battery runs flat. This can be disastrous if you are running your observatory remotely from hundreds of miles away and the battery goes flat with the dome shutter open and a rain front moving in.

Because the dome rotates right through 360 deg without any limitation, you could rotate a full turn clockwise over and over again, a power cable between the fixed wall of the observatory and the rotating dome is not possible.

The only options then are some kind of sliding contact set to take charging power up to the battery, similar to the way an electric tram or train works, or use an inductive air-coupled transformer where two coils are placed in close proximity and low voltage AC power is transferred over the "gap" between dome wall and dome roof to then be AC-DC converted and used to trickle charge the battery while the dome is closed and parked during the day.

[mhard26339]

8 hours ago, Oddsocks said:

I believe Mike has a problem with his observatory dome roof power supply (if it was his post I saw earlier). 

Some of the commercial observatories use a battery powered shutter on the dome that uses a solar panel to charge the battery and at certain times of the year the solar panels may not see any direct sunlight due to nearby buildings, trees etc, and after a while the battery runs flat. This can be disastrous if you are running your observatory remotely from hundreds of miles away and the battery goes flat with the dome shutter open and a rain front moving in.

Because the dome rotates right through 360 deg without any limitation, you could rotate a full turn clockwise over and over again, a power cable between the fixed wall of the observatory and the rotating dome is not possible.

The only options then are some kind of sliding contact set to take charging power up to the battery, similar to the way an electric tram or train works, or use an inductive air-coupled transformer where two coils are placed in close proximity and low voltage AC power is transferred over the "gap" between dome wall and dome roof to then be AC-DC converted and used to trickle charge the battery while the dome is closed and parked during the day.

Got it in one Oddsocks, you have summed up my plight in November perfectly, I have increased my solar capacity significantly but however many panels you still have to have sun!!. Steve (Steppenwolf) has detailed as he states a "clunky" method on my thread New Observatory in Norfolk UK and I will probably install a modified version using spring loaded carbon brushes, However doing further research I found this http://www.robotshop.com/uk/12v-600ma-wireless-charging-module.html which could work, for such small money it could be worth a punt. I am back in UK mid December so if it is back in stock by then I may give it a shot, I will have about 6 weeks before return to Spain to play but by then the sun will be back over the buildings and the panels should work again, frustrating but the problem has to be solved for November 2017.

Whatever, these forums are great when you get a problem, you realise that you are not alone!

 

Regards

 

Mike

[steppenwolf]

Mike, at that price, it has to be worth a punt. The efficiency of this type of 'coupling' is quite low but trickle charging is all that is required for your purposes. You need to be sure that the output voltage is higher than the battery voltage so I'm guessing that the specs shown are giving a nominal voltage output. Again, at that price, it has to be worth a punt!

[Starlight 1]

One of the most interesting DIY posts, my simple solution thinking back to early hatchbacks are The two switches in contacts in the base of the boot lid they control  wiper and heater is all made ready to take 12v supply.

[Oddsocks]

3 hours ago, mhard26339 said:

 I found this http://www.robotshop.com/uk/12v-600ma-wireless-charging-module.html

 

3 hours ago, steppenwolf said:

 The efficiency of this type of 'coupling' is quite low but trickle charging is all that is required for your purposes.

This device is what I had been trying (rather poorly) to explain further up this post last year. In essence it is a pair of tuned coils separated by an air gap, AKA, an air coupled transformer.

If the output voltage is too low then a cheap B.U.C. (Boost Up Converter) module can be added to raise the voltage to whatever is needed.

i.e. https://www.amazon.co.uk/Converter-Voltage-Adjustable-Module-LM2577/dp/B00HI7G7HS

The waveform used by the coils will likely be a square wave so check that the cameras do not pick up any odd interference while it is running, wavy or straight lines, dots or dashes etc in the image. If this is the case a time switch to only allow it to run during the day will be a better option than a cam switch on the dome that is operated in the home position (as Murphys Law will come into force when you find yourself imaging with the dome in the "home" position and the transmitter operating)

When you mount the coils suggest gluing them to wooden or plastic brackets so no metal within 5cm of the coils otherwise efficiency is reduced, and mount coils horizontally so that the gap between them can be minimised ( if vertical mount you have to allow a bigger gap to account for the radius through which the dome turns ) 

 

[steppenwolf]

Quote

If this is the case a time switch to only allow it to run during the day will be a better option than a cam switch on the dome that is operated in the home position (as Murphys Law will come into force when you find yourself imaging with the dome in the "home" position and the transmitter operating)

Good point - I did also wonder about RF interference but I doubt that this would affect Bluetooth or WiFi connections to any extent. However, Murphy's Law is a powerful tool when problem solving so your advice about the cam switch is more than valid!

[mhard26339]

22 hours ago, Oddsocks said:

 

This device is what I had been trying (rather poorly) to explain further up this post last year. In essence it is a pair of tuned coils separated by an air gap, AKA, an air coupled transformer.

If the output voltage is too low then a cheap B.U.C. (Boost Up Converter) module can be added to raise the voltage to whatever is needed.

i.e. https://www.amazon.co.uk/Converter-Voltage-Adjustable-Module-LM2577/dp/B00HI7G7HS

The waveform used by the coils will likely be a square wave so check that the cameras do not pick up any odd interference while it is running, wavy or straight lines, dots or dashes etc in the image. If this is the case a time switch to only allow it to run during the day will be a better option than a cam switch on the dome that is operated in the home position (as Murphys Law will come into force when you find yourself imaging with the dome in the "home" position and the transmitter operating)

When you mount the coils suggest gluing them to wooden or plastic brackets so no metal within 5cm of the coils otherwise efficiency is reduced, and mount coils horizontally so that the gap between them can be minimised ( if vertical mount you have to allow a bigger gap to account for the radius through which the dome turns ) 

 

Decision made, with the item out of stock in the UK I have ordered one from the US to make sure I can play when I get back to UK. Re interference, in my case I only intend to use it with the dome in the parked position for battery charging so no imaging problems as it will also be switched off when the job is done. The battery charger will be connected via the APC 7920 and will be switched on/off remotely when not in use ie from December to October, I only get this problem in November, at other times I will continue to use the solar!!

 

Belt and braces, the brackets will also be designed and made to carry the slips and brushes just in case, research tells me that the air gap between the coils, for a 13V output should be 1mm, we will have to see how achievable that is on the dome. I also have fears about how an auto sensing battery charger will react, but as I said earlier, at that price worth a punt. I will let you know how I get on, probably in January.

 

Regards

 

Mike