Best balanced power supply for astro photography

[AstroAndy]

Hello all

 

I've looked through threads, and also online for a good power supply for AP, and haven't come to a conclusion, so I'm going to ask in here: What I'm after is a power supply with minimal installation (or DYI skills required, as I have 2 left hands) that runs the following equipment for about 12hrs.

Now, I'm in the field with 4 power supplies (power packs), but they are not deep cycle, and are getting on in years (except the 17a/h one, but which is also more of a car battery charger).

Laptop (5a/h)

Dew heater (5a/h)

Atik cam (0.8 a/h)

EQ5 mount (0.5 a/h average)

Filterwheel (0.3 a/h)

Hitec Focus (? a/h)

QHY5 guide cam. (0.1 a/h)

Roundabout, my equipment will draw 12 amps per hour. I would also need a 6 way female cigarette lighter socket with one male socket (don't know whether they even make them, I only found a 3 way, to keep from overcharging?) to the battery (as well as whatever I'd need to make the battery connectable, not blow myself or my equipment up, and make it safe from dew).. Again, I may have to rig it a little bit, but I'm no expert. All in all, I'd need to connect 6 things to the battery (not the guide cam., as it runs through the PC).

Since many APers face the same problems, I thought to ask in here.

Regards

Andy.

 

 

 

 

[symmetal]

Hi Andy, no-one's come to your aid so I'll chip in.

Your quoted usage figures should be just in amps. Amps/hour is a measure of rate of change of current which is normally used as to how fast a power supply can cope with a sudden change in load, usually Amps / second or millisecond.

Amp Hours, (amps * hours), symbol Ah is the measure of the capacity of a stand alone power source (battery) to deliver a current for a certain time. A 24Ah battery can supply, in theory, 24 amps for 1 hour, 12 amps for 2 hours, or 4 amps for 6 hours etc. In practice it wouldn't supply 24A for 1 hour as when delivering a high current its capacity is reduced.

Your rig will be taking much less than 12A in practice. My dual rig setup with two cooled cameras, 2 filter wheels, 2 focusers, 3 dew strips and AZ-EQ6 mount draws about 4.5 amps. When slewing it's about 6.5A. I don't know what scope you have but I expect the dew strips takes less than 1 amp. Filter wheels and focusers will be insignificant as they only draw any significant current when actually moving which isn't often and even then will be small. Your laptop will be the biggest current draw but that will be less than the 5A in practice which may be for running full speed and charging a flat battery. I'd have thought less than 2A would be a more realistic figure.

I'd estimate that 4A is a more realistic figure for average current draw for a single scope with laptop. Slewing will take more current but I assume you won't be doing that for long periods. What's the total capacity of the four power packs you currently use for 12 hours and do they last with no problems.

48Ah will give 4A for 12 hours. 12V,48Ah LiFePO4 will be very expensive at around £600. Personally I'd use a deep cycle leisure battery (caravan type) as you can get an 80Ah one for about £80 which would easily cover your needs. The drawback is the battery is heavy if that's a concern for you. Sealed lead-acid batteries are about twice the cost. Some people sneer at lead acid batteries but they do the job well as long as they are charged regularly.

If you want to use cigarette lighter style sockets then as you say, 3 outputs are the common ones available though there are a few 4 output ones. The plugs/sockets themselves are rated at 10A each though the 3 way splitters are usually made internally using a printed circuit board, so 2 to 3A a socket would be a reasonable limit. I would get 2 good quality plug to 3 socket adaptors and connect them to your battery using a couple of these adaptors.  Larger capacity batteries like leisure batteries usually have screw terminals for connection. You won't need to do any extra wiring or soldering to trouble your two left hands. 

Hope this helps with your decision.

Alan

[hughgilhespie]

Hi Andy,

Alan has covered this very well but I thought it might be worth adding that if you do go for a lead-acid battery, you really shouldn't plan on discharging below about 50% of the battery capacity. So, if you do need 4 amps for say 10 hours = 40 amp-hours you are best with a battery of 2 times this capacity, i.e. an 80 amp-hour rated battery. The life of the battery will be badly affected if it is regularly discharged below about 50%. The problem is of course that an 80 amp-hour lead acid battery is going to be very heavy. A lithium phosphate battery would be a lot lighter, about one third the weight, and they can also be discharged to a higher percentage of their capacity, I believe 90% discharged is considered acceptable. So your decision will be between an 80 amp-hour lead acid battery - and please only consider sealed units - spilling sulphuric acid while moving the battery is not good - and something like a 45 amp hour LiPO4. 

Here are a couple of links to batteries that should be suitable for what you want. The only other thing I would recommend is that whatever type of battery you get, buy a smart charger for it (The LiPO4 battery in the link includes a charger - and so it should at £600!) Leave the battery permanently on charge when you are not using it. It will keep it in good condition and will always be ready for use when you need it.

https://www.ultramax.co.uk/sealed-lead-acid-batteries/ultramax-np80-12-12v-80ah-20ahr-as-75ah-90ah-high-capacity-deep-cycle-sealed-lead-acid-battery.html

https://www.ultramax.co.uk/lithium-vrla-batteries/li50-12-12v-50ah-lithium-iron-phosphate-lifepo4-high-capacity-deep-cycle-battery-charger-included.html

Hope this helps.

Regards, Hugh

[symmetal]

You're right Hugh about the 50% max discharge recommended for lead-acid batteries. I should have mentioned that.  You can buy 4 80Ah sealed lead acid batteries for the price of one 45Ah LiFePO4 and the total number of cycles and life  would be similar so either pay a big amount up front or spread it out over the years. The price of LiFePO4 batteries or similar may get cheaper as the technology progresses, or may increase as the demand for Lithium etc. exceeds economical supply. I'd get a sealed lead acid for now and see what happens in the future. 

Alan