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How to get a stable 12v supply for CEM25P from a 12v VRLA battery

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#1 curry7714

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Posted 19 July 2019 - 12:10 AM

Hello everyone.

I recently bought an iOptron CEM25P mount and I have a 12v 7.5ah VRLA battery that I need to use to power the mount. The mount needs 12v 1.5A input. When I tested from my multimeter the output from the battery at full charge was 13.1v. Will I damage the electronics if I connect the battery directly to the mount? How can I get a clean 12v output from the battery? 

I've read about buck and boost regulators and such and they need to have a difference of at least 3v between input and output. Also I cannot buy a jump starter or a dedicated power bank at the moment.

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Edited by curry7714, 19 July 2019 - 12:15 AM.


#2 BGRE

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Posted 19 July 2019 - 12:31 AM

Its trivial to do with the correct topology switching regulator such as the Cuk and other variants. 

However its not necessary given the mount specifications:

https://www.astronz....o-eq-mount/172/


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#3 RaulTheRat

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Posted 19 July 2019 - 12:33 AM

Yeah LTC3780 don't need a difference between input and output voltage, but your mount will be fine up to 14.something volts anyway as it needs to be in order to safely be able to draw power from a battery at different states of charge or when charging from a vehicle alternator.
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#4 BGRE

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Posted 19 July 2019 - 12:36 AM

spec is 9-15V input.



#5 sg6

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Posted 19 July 2019 - 01:24 AM

2 simple ideas are to add some form of anti-surge inline. If (and a distant if) I recall is a simple choke that causes a back EMF.

 

The other if the voltage concerns you is add an LED inline. Does the dual thing of dropping voltage by around 0.7v and shows it is in use also (3) is that it can act as reverse polarity protection.

 

Not sure of iOptron but Skywatchers are rated at 12v but it is often recommended that 13.2 v is a better voltage. And that recommendation comes via the retailers. Skywatcher say 11-15v 2A.

 

Personally I would first opt for the anti-surge, then concern myself with the actual voltage. Remember that what a battery indicated initially is going to be higher then it will drop to after only a short time.


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#6 BGRE

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Posted 19 July 2019 - 02:41 AM

Randomly adding inductors (chokes) to circuits without understanding why almost inevitably leads to disaster. Clamping of transients with shunt devices can be far more effective and safer.


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#7 curry7714

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Posted 19 July 2019 - 03:45 AM

Its trivial to do with the correct topology switching regulator such as the Cuk and other variants. 

However its not necessary given the mount specifications:

https://www.astronz....o-eq-mount/172/

Yeah I saw its rated 9-15v. I was just being a bit extra careful.



#8 curry7714

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Posted 19 July 2019 - 03:46 AM

Yeah LTC3780 don't need a difference between input and output voltage, but your mount will be fine up to 14.something volts anyway as it needs to be in order to safely be able to draw power from a battery at different states of charge or when charging from a vehicle alternator.

I see. I'll try LTC3780 and see if it works. Thanks.



#9 jdupton

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Posted 19 July 2019 - 06:50 AM

curry7714,

 

   Welcome to Cloudy Nights! It looks like this was your first post here.

 

   I would (and do) use a synchronous buck-boost converter between the battery and astronomy gear when you want to regulate the voltage. Synchronous converters such as those based on the LTC3780 control chip suggested by user RaulTheRat are seamless in their regulation. You can regulate a 12 volt source to a 12 volt output if you want. They do not have a voltage dead zone like most simpler SEPIC style converters.

 

   That said, the mount will operate fine within a voltage range rather than exactly 12.0 volts. In fact, many mounts prefer a voltage of around 12.5 to 12.8 volts as that is near the center of the range of a lead acid battery. A regular lead acid battery will start off between 12.9 to 13.1 volts after charging and slowly drop in voltage to about 12.4 volts when it is time to recharge them.

 

   Lead acid batteries are considered 100% depleted when they reach 10.5 volts. However if you use them down that low more than a few times, they will die completely and refuse to ever take a full charge again. They should generally be drained only to about 50% or before recharging to ensure a long life. At 50% of full charge, the voltage will be about 12.3 volts.

 

   Using a synchronous buck-boost converter allows you to regulate a changing battery voltage to a single stable voltage. As the battery slowly drains and it's voltage drops from 13+ volts to 12.3 volts or lower, the output of the converter will provide a steady voltage without dropping out of regulation due to the 1 v to 3 v dropout range. 

 

   Find a reasonably good quality synchronous converter (like those based on the LTC3780) and use that rather than random voltage dropping elements. A good synchronous buck-boost regulator can be purchased for as little as about $10 (USD). The regulator is much more effective than any simple voltage dropping circuit you might add. Adding an LED inline with yous mount will not work and will only serve to act as a fuse. LEDs generally have a maximum current rating of a few milliamps unless you use one made for high intensity lighting which is sort of contrary to astronomy use. As soon as the mount draws current for slewing (up to 2 amps), the LED will fail and open the circuit. Chokes and other simple circuits are not likely to give you what you want. Part of the problem with a simple voltage dropping circuit is that it not only works when the battery is fully charged, it also works as the battery is drained and voltage declines.

 

   Good luck with your new mount. Use it with just the battery and have fun until you can add the regulator you want. Just be careful not to charge the battery while using it to power the mount at the same time. A charger may reach a voltage that could be close to the limits of the mount.

 

 

Best Regards,

John


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#10 BGRE

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Posted 19 July 2019 - 07:25 AM

Using a standard switchmode converter or even a standard linear regulator to supply power to any gear that has a motor can be a spectacularly bad idea should the motor act as a generator and deliver reverse current to the supply. Its possible to design the regulator or converter to safely deal with reverse current but most off the shelf designs do not do this. The classical solution as espoused in the HP power supply handbook and elsewhere is to ensure that a resistive load that draws more current than the motor is always present. Although this works well it is somewhat inefficient and other solutions are possible. 

 

I've seen the destructive result of reverse current from a motor flowing into the output of regulated power supply innumerable times when the supply isn't designed to handle it.


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#11 BGRE

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Posted 19 July 2019 - 07:26 AM

Using a standard switchmode converter or even a standard linear regulator to supply power to any gear that has a motor can be a spectacularly bad idea should the motor act as a generator and deliver reverse current to the supply. Its possible to design the regulator or converter to safely deal with reverse current but most off the shelf designs do not do this. The classical solution as espoused in the HP power supply handbook and elsewhere is to ensure that a resistive load that draws more current than the motor is always present. Although this works well it is somewhat inefficient and other solutions are possible. 

 

I've seen the destructive result of reverse current from a motor flowing into the output of regulated power supply innumerable times when the supply isn't designed to handle it.



#12 curry7714

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Posted 19 July 2019 - 08:32 AM

curry7714,

 

   Welcome to Cloudy Nights! It looks like this was your first post here.

 

   I would (and do) use a synchronous buck-boost converter between the battery and astronomy gear when you want to regulate the voltage. Synchronous converters such as those based on the LTC3780 control chip suggested by user RaulTheRat are seamless in their regulation. You can regulate a 12 volt source to a 12 volt output if you want. They do not have a voltage dead zone like most simpler SEPIC style converters.

 

   That said, the mount will operate fine within a voltage range rather than exactly 12.0 volts. In fact, many mounts prefer a voltage of around 12.5 to 12.8 volts as that is near the center of the range of a lead acid battery. A regular lead acid battery will start off between 12.9 to 13.1 volts after charging and slowly drop in voltage to about 12.4 volts when it is time to recharge them.

 

   Lead acid batteries are considered 100% depleted when they reach 10.5 volts. However if you use them down that low more than a few times, they will die completely and refuse to ever take a full charge again. They should generally be drained only to about 50% or before recharging to ensure a long life. At 50% of full charge, the voltage will be about 12.3 volts.

 

   Using a synchronous buck-boost converter allows you to regulate a changing battery voltage to a single stable voltage. As the battery slowly drains and it's voltage drops from 13+ volts to 12.3 volts or lower, the output of the converter will provide a steady voltage without dropping out of regulation due to the 1 v to 3 v dropout range. 

 

   Find a reasonably good quality synchronous converter (like those based on the LTC3780) and use that rather than random voltage dropping elements. A good synchronous buck-boost regulator can be purchased for as little as about $10 (USD). The regulator is much more effective than any simple voltage dropping circuit you might add. Adding an LED inline with yous mount will not work and will only serve to act as a fuse. LEDs generally have a maximum current rating of a few milliamps unless you use one made for high intensity lighting which is sort of contrary to astronomy use. As soon as the mount draws current for slewing (up to 2 amps), the LED will fail and open the circuit. Chokes and other simple circuits are not likely to give you what you want. Part of the problem with a simple voltage dropping circuit is that it not only works when the battery is fully charged, it also works as the battery is drained and voltage declines.

 

   Good luck with your new mount. Use it with just the battery and have fun until you can add the regulator you want. Just be careful not to charge the battery while using it to power the mount at the same time. A charger may reach a voltage that could be close to the limits of the mount.

 

 

Best Regards,

John

Thanks for such a detailed explanation, jdupton.

I have ordered a LTC3780 Buck-Boost converter which should arrive by Monday. I will try to use the mount with just the battery tonight taking care of all the details you gave me and see how well it goes. 
Clear skies :)



#13 jdupton

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Posted 19 July 2019 - 09:03 AM

curry7714,

 

   Let me add one word of caution. 

 

   Check, double check, and triple check the polarity of connections before turning on the mount. If you accidentally wire the battery to your mount with a reversed +/- connection, it will usually spell instant death to the mount's internal electronics.

 

   It is best to make permanent connections at the battery with a 12v automotive style cigarette lighter socket. Wire that up and then check the socket with a voltmeter to ensure the polarity is right. Leave the socket always connected to the battery and simply plug the mount's DC power cord into that.

 

  Something like these can save you from making this mistake.

https://www.amazon.c...&ref=nb_sb_noss
https://www.amazon.c...&ref=nb_sb_noss

You may be able to find these sorts of connectors at a local automotive parts store.

 

   Taking extra care to make sure you will never accidentally connect the battery to the mount backwards will save you much grief in the long run. 

 

 

John


Edited by jdupton, 19 July 2019 - 09:04 AM.


#14 curry7714

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Posted 19 July 2019 - 12:18 PM

curry7714,

 

   Let me add one word of caution. 

 

   Check, double check, and triple check the polarity of connections before turning on the mount. If you accidentally wire the battery to your mount with a reversed +/- connection, it will usually spell instant death to the mount's internal electronics.

 

   It is best to make permanent connections at the battery with a 12v automotive style cigarette lighter socket. Wire that up and then check the socket with a voltmeter to ensure the polarity is right. Leave the socket always connected to the battery and simply plug the mount's DC power cord into that.

 

  Something like these can save you from making this mistake.

https://www.amazon.c...&ref=nb_sb_noss
https://www.amazon.c...&ref=nb_sb_noss

You may be able to find these sorts of connectors at a local automotive parts store.

 

   Taking extra care to make sure you will never accidentally connect the battery to the mount backwards will save you much grief in the long run. 

 

 

John

Ah yes I did take care of that already, wired the battery to a dc male jack in correct center positive connection (as my mount needed). Thanks for linking the cigarette lighter ports, the fused connection is definitely a plus-point!




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