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DIY powertank complete


parallaxerr
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Well I've had the bits for ages but finally got around to assembling my home made powertank. After researching the options such as Tracers etc. I figured I could make my own, cheaper...

Being an ex RC geek I had quite a few of the parts already and more importantly, the knowledge. The tank uses a 14.8V lithium polymer battery with a 10Ah capacity driving a 12V 4.5A switching voltage regulator. At 12V I should have about 12Ah available assuming 98% regulator efficiency.

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I assembled the workings into a project box and added a switch, battery capacity meter and output voltage meter and audible low battery alarm. All DC power plugs are 2.1X5.5mm Celestron standard size and due to a lack of coiled DC power cable available locally, I used mono audio cable which is fine for a few amps. A lanyard with slip knot allows the tank to hang below the mount.

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The tank powers my 8SE mount and 2 dew bands via a Hitec Astro controller, max draw being about 2.9A with heat on full and slewing on both axes. Charging is done through the small white "balance" plug that sticks out of the bottom. I saved money here as I have a specialist lipo charger already, meaning I didn't need a charging circuit.

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All in all I'm pleased with the reuslt. I may add a 5v regualtor that I have lying around and USB port for charging mobile phone/tablet etc.

The battery was the main cost at £40 with the whole lot costing about £70 (expensive Maplin components), not bad when compared to some of the branded equivalents!

Edited by parallaxerr
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Ohh, now that is a brilliant idea, I have a Quadcopter habit and I've been wondering about doing something similar when I finally pick up a motorised mount. Looks like you made a really neat job, well done. Just one thing where did you get the battery percentage monitor from?

Thanks,

Matt H

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Hello Parallaxerr,

Nice and clever setup. I would like to make a few additions.

 

You are using 4 Lipo cells in this pack, as I presume, normally providing a nominal voltage of 4x 3.7V = 14.8 V. Fully charged they will provide a voltage of even 16.1V, but this drops fast on Lipo cells when being discharged. Never discharge a Lipo cell more than 80% depth-of-discharge. Your pack will then still provide 14.4V and your voltage regulator still will make out 12V of this. So you normally won’t notice this. You adapted a voltage meter over your pack, which is an absolute must-have.

When you discharge a power pack more than 80% DOD, the change is realistic that the voltage of one cell will drop under 3V and this cell will be damaged permanently, resulting in less capacity and even increasing this problem the next cycle. If the cell voltage drops under 2.5V, the cell will be damaged immediately. If the other cells then still provide a proper current, there is a change of this cell running to hot and even catching fire.

Unless one has a security battery management system, measuring the voltage of each cell independently and cutting off the discharge when a single cell runs under 3V, one should really provide for a low-discharge security over the pack, cutting off the discharge when the voltage of the whole pack drops under 14.4V at 80% DOD. I’m glad you provided a low-battery alarm, which is the second must-have on these Lipo packs. You set the sound alarm probably at this 14.4V voltage, but, if this is the only security, have to make sure to cut the discharge shortly after by hand and never leave the power pack discharging unattended.

 

You have a 10Ah 14.8V Lipo battery pack you can discharge to a maximum of 80% DOD. The useful capacity of the battery pack will then be 80% x 10Ah = 8Ah. Your voltage regulator is probably a buck converter with a 92% efficiency, so at 12V the useful capacity of your power pack will be 92% x 8Ah x 14.8V / 12V = 9Ah, but only when new and under 20 °C conditions.

But you have to take into account that these pack will lose some capacity during their lifetime (about 300 full load/discharge cycles for Lipo cells), the mean loss will be around 15%. The useful capacity of your power pack will then be 85% x 9Ah = 7.7Ah as a mean value during its lifetime at 20 °C conditions.

The last thing to take into account is the use at low temperatures. Even a few degrees above freezing a capacity loss of 20% is very realistic. Under low temperature conditions the useful capacity of your power pack will probably be around (100%-15%-20%) x 9Ah = 5.9Ah. So you should be a bit careful expecting a 12Ah useful capacity of this power pack.

 

A last remark I would like to make a general advice on charging Lipo battery packs. The use of a balancing Lipo charger, measuring the voltage of each Lipo cell and preventing it from rising over 4.2V, is also an absolute must-have. Your pack has, next to the main power connector, the necessary second connector to provide the balancing charger with these measuring voltages and gives the charger the possibility to prevent overloading of single cells in your battery pack. Overloading can be dangerous to with Lipo cells and also cause overheating and even fire.

A lot of people don’t know that the life-time of a Lipo battery (normally around 300 full load/discharge cycles) is based on a mean of 40% charging of the lipo battery. At a 100% mean charging (so always fully charged) a Lipo battery has a life time of 3 till 4 times less (so about 100 full load/discharge cycles). When not using the Lipo battery pack for a longer time (even more than 2 weeks) it’s is better to store it only 40% charged. Some Lipo battery pack chargers therefore have the possibility of a special storage (40%) charge. In this Lipo batteries differ a lot from lead acid or Nickel batteries.

 

Have fun with your power pack. I really like it.

 

Robert.

Edited by rweust
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36 minutes ago, Gina said:

LIPOs sound very complicated - I think I'll stick to sealed lead acid batteries.

Agreed Gina.  To take nothing away from the OP I think I'll stick to the simple, easy devil I know for the time being (aka lead acid)!

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18 hours ago, chaotician said:

Ohh, now that is a brilliant idea, I have a Quadcopter habit and I've been wondering about doing something similar when I finally pick up a motorised mount. Looks like you made a really neat job, well done. Just one thing where did you get the battery percentage monitor from?

Thanks,

Matt H

Hi Matt,

The battery monitor was from eBay...

http://www.ebay.co.uk/itm/12V-Lead-acid-Battery-Indicator-Intuitive-Voltage-Display-LED-Display-Meter-/151561812441?hash=item2349c9b9d9:g:Wr0AAOSwAYtWQZ7z

The instructions were poor but it wasn't hard to figure out the setup.

 

Robert...thanks for the post. Having used LiPos for many years I am well aware of all the things you mention in your post. With regards to capacity, 80%DOD is not a hard and fast rule and in such a low discharge application I have no issues using the batteries full capacity down to the alarm @ 3.3V/cell, something I have done on many occasions previously with no ill effect. Also, the battery I have used has a low C rating, specifically for low discharge applications. I'll let you know how much I get out of it after 1st charge. So far I've used it for 5 or 6 observing sessions, with dew heaters and it's hardly touched the battery. And don't worry, I do have a specialist charger, several in fact and that's why I have routed the balance leads externally.

 

 

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1 hour ago, Gina said:

LIPOs sound very complicated - I think I'll stick to sealed lead acid batteries.

 

27 minutes ago, kirkster501 said:

Agreed Gina.  To take nothing away from the OP I think I'll stick to the simple, easy devil I know for the time being (aka lead acid)!

Ah they're not that bad really. There's lots of scaremongering about them but I've never had an issue in all the years I've used them. Lots of commercially available power supplies such as Tracers use them nowadays. The only difference is they have charging circuits and low voltage protection built in. I didn't go in to such detail as Robert in fear that I'd put people off them!

I found my mount slowed considerably towards the end of my lead acids charge so I built this, the beauty of it is that even when the battery is near flat, I still get 12 volts.

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We had RC cars and followed the heavy NiCad to NiMh to light LiPo battery changes a few years ago, suddenly our toys that struggled to run for 5 minutes on cherry picked expensive batteries (with complicated discharge/charge) ran for an easy 15 minutes and the extra speed and grunt was/is outstanding.  Given an understanding of LiPo charging, care and cost we would never go back.  I have seen LiPo's swell and stories of fire from mistreated LiPo batteries are rife, exploding NiMh batteries was not unknown.

I very much like parallaxerr's power pack, possibly a bit more than 12 volts output may be liked by some devices, rweust's comments are all valid and all batteries need to be treated with respect.  But for some that have grown used to LiPo and the use of a dedicated balance charger keeping each cell the same every recharge I cannot see many problems, a good charger will give a read voltage for each cell and refuse to charge over discharged batteries.  Most RC cars being 2 cell LiPo, flight Lipo's more 3 or 4 cell I cannot begin to estimate how many are charged/recharged without problems each week.

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A lead acid battery can't be discharged under 80% DOD either, Gina. That is if you own a deep-cycle lead acid battery. A normal starting (car) battery can't even be discharged under 40% DOD without being damaged permanently, resulting in loss of  useful capacity and life cycles. I really recommend a voltage meter and low-discharge protection on a lead acid battery too. True, there is no risk on running hot if you do discharge them to low, but they do contain sulfuric acid.

 

A lead acid battery is best stored fully (100%) charged for the best life time, instead of 40% charge with Lipo batteries. This is a bit less complicated, but lead acid has a higher self-discharge so you have to recharge your battery every few months (if you own a AGM or gel battery every 3 till 4 months, a wet lead acid battery even every 6-8 weeks) when in storage. A Lipo battery can stay in storage about half a year unattended, even at 40% charge.

 

A lead acid battery also loses capacity during its lifetime and drops in capacity when becoming cold. So there isn’t much difference between the two battery types according to this.

 

Charging the Lipo battery is more complicated, but a good quality balancing Lipo charger will fully take care of this. Charging can be done very quickly with Lipo batteries, sometimes even in an hour. With lead acid batteries you always have to take the initial current into account. With deep-cycle AGM batteries this normally has a maximum of 0.2 x C, or a maximum charging current of 7.2A for a 36Ah battery. Fully charging will then take 5 till 6 hours. Charging with higher current will also damage the lead acid battery.

 

Lipo batteries are about 5 times lighter in weight and 3-4 times smaller in size then lead acid batteries. Lead acid batteries are easier to charge and cheaper. An AGM deep-cycle lead acid and a Lipo battery both have about 250 till 300 full load/discharge cycles life time. Modern LiFePO4 batteries have even a much higher cycle life time, are a bit more friendly to discharge, have an in build battery management system, but also a bit heavier and larger to build than Lipo batteries. They still are even more expensive than Lipo batteries.

 

So it is difficult for me to prefer lead-acid over Lithium. The fact is that Lithium batteries are becoming better and better and cheaper and cheaper, but I myself also still own an 12V 36Ah AGM deep-cycle lead acid battery in my power pack. It is heavy, though.

 

Robert.

 

Edited by rweust
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I like LiPos much from my RC Helis. I have the knowledge and the chargers. If that is given (like for the OP) they are save and very comfortable. NiMH and Lead batteries are prone to be empty if you need them, normal leads need to be taken care of not cycle them, so the capacity you should use is like 50% or so. Special cycle resistant Leads are good but also expensive (I have a multipower 50Ah).

And one more: The current even a big scope and heater will draw from a RC Lipo is a piece of cake for these power packs. I doubt you will ever see a single cell beeing out of balance here (assumed you take care of balancing while charging). So you only need a voltage check for the whole pack, in my Arduino dew heater this is a "circuit" of two resistors. Easy.

So far for what I like. Your mileage may and will vary :-)

Carsten

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27 minutes ago, parallaxerr said:

Funny old world,  just had to have one of these, found one a bit cheaper for after the Chinese new year............  many thanks
(yea, brushed motors - who would have them :icon_biggrin:)

I think/know we will see many more power packs running off LiPo, LiFe etc but do understand the need to keep this tech safe.

Edited by Mick J
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1 hour ago, parallaxerr said:

 

Ah they're not that bad really. There's lots of scaremongering about them but I've never had an issue in all the years I've used them. Lots of commercially available power supplies such as Tracers use them nowadays. The only difference is they have charging circuits and low voltage protection built in. I didn't go in to such detail as Robert in fear that I'd put people off them!

I found my mount slowed considerably towards the end of my lead acids charge so I built this, the beauty of it is that even when the battery is near flat, I still get 12 volts.

 

This in-build buck-converter bringing the voltage to 12V in combination with the higher voltage of the Lipo battery pack is the beauty of your set-up.

 

The problem with the ready-to-use 22Ah Tracer Lipo power pack is the voltage dropping to even below 11V at 2/3 of their discharge capacity. My NEQ6 mount doesn’t like voltages under 11.5V which a Tracer Lipo power pack will already reach at 30% DOD, so it won’t supply my mount for long. My 12V 36Ah cyclic AGM lead acid battery won’t drop till 11.8V until 80% DOD, so it works well with my NEQ6 mount, even under some load.

 

Capaciteit_zpsq1viocvl.jpg

 

Tracer does have an 12.8V 24Ah LiFePO4 power pack in their product line also without, as one might expect, this dramatic voltage drop during discharge. This power pack will fit better for the use with telescope mounts, I think.

 

Robert.

 

Edited by rweust
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Hi parallaxerr, I will also be making one of these after being inspired by you! the only bit I am not clear on is the   12V 4.5A switching voltage regulator , could you please give me more details on it or post a larger picture so that I can identify it for purchase.

kind regards

Mike

 

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  • 4 months later...

20160703_121912 (1).jpg20160703_123909.jpgSorry about the delay in posting, thought you might be interested in my completed power supply ! Thanks  parallaxerr for showing the way!, the only changes I made were to site the outputs, switches, and meters to be more ideal for my pier , and to go up to a 16000Ah lipo . Please people, do not be afraid of Lipo's they are not hard to re-charge as long as you pay attention to a few simple rules. I always feel that people (as have done on this thread) seem to impart the dangers about these things in such a way as to almost suggest that 'common people' should avoid them! The best simple advice about using Lipo's is if you feel you would like to use then then  go to your nearest radio control model shop and they will put you right without scaring the pants off you! 

 

 

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Hey Mike,

For some reason I stopped receiving notifications for this thread!?

Good job though, looks cracking. Good that you've got your XT60 external for faster charging, I can only pump 1A in through the balance leads on mine which means an overnight charge. However, as you no doubt will find out, the battery lasts for AGES anyway!

I've been using a 1500mAh 3S LiPo fitted with a 2.1x5.5mm DC plug directly into the mount for quick sessions through the winter. Checked my power supply last night though, which I left at a storgae charge level and it's still perfectly balanced ready to go :)

Did you manage to set up the % meter OK? I seem to remember it took a bit of adjustment to get it right.

Edited by parallaxerr
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On 2/10/2016 at 12:37, parallaxerr said:

Hi Matt,

The battery monitor was from eBay...

http://www.ebay.co.uk/itm/12V-Lead-acid-Battery-Indicator-Intuitive-Voltage-Display-LED-Display-Meter-/151561812441?hash=item2349c9b9d9:g:Wr0AAOSwAYtWQZ7z

The instructions were poor but it wasn't hard to figure out the setup.

 

Robert...thanks for the post. Having used LiPos for many years I am well aware of all the things you mention in your post. With regards to capacity, 80%DOD is not a hard and fast rule and in such a low discharge application I have no issues using the batteries full capacity down to the alarm @ 3.3V/cell, something I have done on many occasions previously with no ill effect. Also, the battery I have used has a low C rating, specifically for low discharge applications. I'll let you know how much I get out of it after 1st charge. So far I've used it for 5 or 6 observing sessions, with dew heaters and it's hardly touched the battery. And don't worry, I do have a specialist charger, several in fact and that's why I have routed the balance leads externally.

 

 

ooo, that's battery meter is shiney.  I'm going to add one of those to my super mega battery pack.

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Hi parallaxerr, thanks for the kinds words!, setting the battery health meter still has to be done!,....are you able to remember how you did it as I have no instructions!

 

I like the idea of three cells! will look at that too!

 

Kind regards

Mike

 

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  • 3 months later...

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