Power supply

[Galaxyfaraway]

Hello,

What is the recommendation for a reliable power supply for a Skywatcher mount? (such as AZ EQ6 or HEQ5).

I read people recommend Maplin's power supplies for mounts but it seems more expensive than the powertank:

http://www.maplin.co.uk/p/17ah-ppack-21a-usb-compressor-200w-inverter-charge-station-a25tb

vs  http://www.firstlightoptics.com/batteries-powerpacks/skywatcher-powertank-17ah.html

Also will 17ah be enough for a night and would I need a separate source for a CCD etc?

Most imaging will be done in my backgarden (with power source nearby) but 5% away. So perhaps a combination of mains and some sort of battery?

Thanks, GFA

[daz]

Many people will use the bench supply in a permanent set up - it gives a regulated 13.8v instead of a fluctuating 12v.

The powertank is not regulated so you will get fluctuations in voltage - this should not normally be a problem, just be aware of it.

As for the question will 17Ah be enough....

You'll need to add up all your devices current draw, and then do a simple division.

So, the EQ6 draws an amp I think when normally tracking, so 17/1 = 17 hours of use.

However, when you start adding in CCDs and dew straps, it soon eats into that time.

[ronin]

I suspect that a 17Ah tank will last you 3  maybe 4 hours. After that the voltage and current will have dropped off and things will start going wrong, there are many tales of the skywatcher mounts doing odd things when the voltage and current gets low. Low seems to be 12v.

Additionally when it starts going wrong the battery inside is drained and likely some damage has occurred so will not hold as much charge the next time.

Unfortunately the standard lead acid batery is a good item but not suited 100% to the way that astronomers use them so eventually something goes wrong.

Mentioned elsewhere: If you have a new power tank and it is all working fine then at 3 hours into your session are you going to stop, pack up and head home when there is no sign of any problem? Because that is likely the approach that has to be taken. On a good clear night with everything tracking and smoothly running when the 3 hour alarm goes Ding, you may need to stop simply to protect the power tank.

[Dr_Ju_ju]

depending on how much of a bodger you are, a deep-cycle caravan battery e.g. http://www.ebay.co.uk/itm/111513953238?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT built into a suitable case with appropriate charger etc. will give you enough for a full night or 2

There are some threads under DIY Astronomer, for you to peruse...

[brantuk]

Yup - marine/caravan leisure battery for longer sessions - you can get 80ah or 110ah commonly. For a short 3 or 4hr session where portability and lightness is priority, and you're only drawing amps for tracking/slewing - many folks use the 17ah models.

[Carbon Brush]

Good advice from Ronin.

I will add a few more bits about the behaviour of the gel batteries that are promoted for astro and other use.

1/ The battery capacity is usually quoted at the 20hour rate. Discharging at higher current, the capacity is lower.

So a 20Ah battery will give 1A for 20 hours. But with a 5A load it may only give 3 hours use.

2/ The capacity stated above will reduce daily, regardless of how well the battery is treated. This is not recoverable in any way. The loss sounds small, typically 0.015% at 20C, increasing with temperature. This may not sound much. But if you store the kit in the airing cupboard, or near the central heating boiler to keep it dry, the situation is much worse.

3/ The capacity stated is often on the basis of discharging to 1.8V per cell. That is a 12V battery dropping to 10.8V. A low enough voltage for many mounts to misbehave.

So you don't really get to use the full capacity anyway.

4/ Battery performance declines at low temperature. Both capacity (Ah) and internal resistance suffer. On cold nights some people try to overcome this by using warm (from the house) leisure batteries in insulated boxes.

5/ Many gel batteries are not suited to deep discharge. When recharged there is often a loss of performance that cannot be recovered. Leisure batteries are generally designed with deep discharge in mind, so provide better immunity to this problem.

6/ Commercially available 'powertank' and 'start/charge' type of products are generally cost conscious. This can mean a low quality battery is used. This may well degrade quicker than expected. Or simply fail unexpectedly.

7/ The correct charging regime is essential for battery survival. If you exceed 2.3V/cell (13.8V on a 12V battery) and the battery is anywhere near full charge, then it will gas. The water (in the acid) is split into hydrogen and oxygen. The battery has a vent to prevent explosion and acid spray. But you end up with impaired performance. Gel batteries cannot be topped up. However, sparks in the vicinty of a venting battery are inviting explosion. I have seen many chargers that if left on for too long, will wreck the battery. If the battery is warm, then the 2.3V/cell must be reduced. Not many chargers do this.

So why on earth do we use lead acid batteries?

Answer. They are cheap and easy to manufacture. Having been around since Queen Victoria was on the throne, they are well understood.

Any portable power source that uses any rechargeable batteries should be regarded as a complete package comprising:

A/ The battery.

B/ Discharge protection circuit for overload, and prevening over discharge.

C/ The correct charging regime for the battery.

Nickel Cadmium and Nickel metal hydride batteries offer various advantages over lead acid. But are less well understood by the casual user.

Lithium based batteries offer the best solution for portable power. Their power per kilogram is streets ahead of the competition. They generally come with the the protection and charge circuits in the box. Life expectancy is better than lead acid. The only stumbling block here is cost. But when you take weight and lifetime into account, they often come out on top.

Sorry to go on. Just my two pennorth having worked with numerous battery types over many years. Hope this helps.

[Galaxyfaraway]

Wow. That's some info, many thanks. Now will find links to those DIY threads...I somehow doubt I will be able to build one without my life insurance increasing the premiums drastically.

[Galaxyfaraway]

Are there by any chance any step by step instructions on how to adapt this beast: http://www.ebay.co.uk/itm/12V-85-Ah-Leisure-Battery-For-Caravans-Motorhomes-And-Marine-/251222123532?pt=LH_DefaultDomain_3&hash=item3a7e01640c

to power the mount without casualties?

Also that doesn't quite solve the issue of having to power the CCD separately.

[Galaxyfaraway]

Found a link. Is this roughly how to do it? 

[Dr_Ju_ju]

well this is wonderfully done, if more than your comfortable with, http://stargazerslounge.com/topic/230078-oh-no-another-power-tank-project/?hl=%2Bpower+%2Btank#entry2500458  but should give you some ideas

[Galaxyfaraway]

Amazing! But I might as well have a go at building the AZ-EQ6 with the same success...

[Galaxyfaraway]

Incidentally if I power from the mains with this: http://www.firstlightoptics.com/power-accessories/maplins-xm21x-7a-138v-regulated-mains-power-supply.html

do I need to get anything else or does everything just plug in the unit? This seems to give the regulated 13.8V supply mentioned above. In the meantime I will learn how to build a portable battery...

thanks, GFA

[Dr_Ju_ju]

it looks complicated, but it comes down to a power source = deep cycle battery(s), a distribution system (sockets) for 12 volt & with some dc-dc regulators 5\8 volt sockets, and then an off the shelf case to put it into...

[Galaxyfaraway]

Does anyone know approximately how long it will take until a fully charged 17ah battery gets below the 40-50% charge if left unused?

This all seems rather cumbersome. Are there no batteries in powertanks that can go completely flat without breaking/getting damaged?

How come no one manufactures cases with gel or agm type batteries in cases with all the right plugs? If someone could make this, I would gladly buy them!

[Carbon Brush]

Gel lead acid batteries tend to significantly self discharge over several weeks, to months.

NiCd (ie chinese built cordless drill) tends to self discharge over days.

Lithium self discharge tends to be measured as a few % annually.

You can leave a lead acid battery on charge permanently without harm. This applies to conventional, gel and AGM types.

BUT (a big but) the charger output must be less than 2.3V/cell. That is 13.8V for a 12V battery. Otherwise the battery will gas.

If you stick to 2.2V/cell, or 13.2V, then you won't notice any significance reduction in performance when in use.

You do though have a margin for error on voltmeter accuracy, charger temperature drift and battery temperature rise.

An earlier post asked about anyone making a good quality battery to put into a 'powertank' or whatever.

Gel lead acid batteriy case sizes tend to be standard, give or take a millimetre, between manufacturers.

All manufacturers offer options on terminal type and layout.

This means that replacing a dud battery is relatively easy.

But if you replace one 'Wish Washy Electric Co' battery with another, don't expect much.

If you stick to known names, you reduce the risk of replacing with more junk.

Hope this helps.