Power consumption

[AlistairHowie]

I'm trying to size power tank requirements for a basic imaging set-up, comprising something like a QHY5 guider, QHY8L CCD, laptop and NEQ6/iEQ45 mount.

I'm just trying to do a bottom up sizing of Ah requirements,. The big ticket items are presumably the laptop and cooled CCD. I'm trying to see whether a 24Ah LifePO4 battery (which can be safely discharged all the way down, so the full capacity is available, unlike with a SLA) would give me, say, an evening's worth of imaging ... so 5 or 6 hours max.

All thoughts greatly received. Many thanks.

[Stephen]

Unlikely. The QHY5 will run from USB, but the QHY8L requires a 5A power supply I think?

5x5=25

That's before you have factored in the laptop (7Ah or 12Ah?) and mount (between 1 and 2Ah)

[AlistairHowie]

Unlikely. The QHY5 will run from USB, but the QHY8L requires a 5A power supply I think?

5x5=25

Thanks, Kheldar. I don't quite get your calculation - one of the 5s is Amps, but what's the other??

I've found this in the manual for the QHY8L ...

"QHY8L’s power is from 3.6W to 30W, which is depended [sic] on the TEC’s PWM power. So the power supply should be larger than 4A. QHY8L’s power is from 3.6W to 30W, which is depended on the TEC’s PWM power. So the power supply should be larger than 4A."

I'm being a bit thick here, but how does that translate into Ah? The battery is rated at 30A continuous output.

[Stephen]

A 24Ah battery will provide one amp for 24 hours, or 24 amps for one hour.

My calculation was based on my "assumption" that the QHY8L requires a 5A power supply and your time requirement of 5 hours.

5A for 5h = 25Ah

[AlistairHowie]

Ahhhh, got it - thanks.

So if I read the manual correctly, is that saying that when the CCD is being cooled to, say 20 or 30 degress below ambient, the camera could by drawing up to 30A per hour???

[Stephen]

Ahhhh, got it - thanks.

So if I read the manual correctly, is that saying that when the CCD is being cooled to, say 20 or 30 degress below ambient, the camera could by drawing up to 30A per hour???

No, no. Watts and Amps are different things all together.

Modern Astronomy sell a dedicated power supply for the 8L which is rated at 5A. Worst case, you camera will draw 5A per hour, that's under 5 hours on your proposed power supply for the camera alone.

If you can't run from mains power, you are going to need a bigger battery (85Ah or 105Ah leisure battery) to power your camera, mount and laptop.

[AlistairHowie]

OK, I got it ... I think. At least I've got the conclusion - bigger is better! And certainly bigger than 24Ah.

[AlistairHowie]

You know, I've been looking again at my power requirements, and think it's not nearly as bad as I initially thought. Can someone check my reasoning here, please?

The extract from the QHY8L manual says ...

The DC201 requires a regulated 12V input. The normal input range is 11V-13V ... QHY8L’s power is from 3.6W to 30W, which is depended on the TEC’s PWM power. So the power supply should be larger than 4A.

As Kheldar pointed out before, I was confusing Watts and Amps. If the maximum power is 30W and the input voltage is 11-13V, then worst case the camera must draw 30/11A = 2.7A ... which over an hour would be 2.7Ah. Is that right? If it is, I'm not sure why the manual says you should use at least a 4A power supply. I guess just to give some head room so a 3A power supply is not being over worked and running near it's capacity?

Plus my intended iEQ45 mount uses 1A when slewing, or 0.2A when tracking - so call it 0.5A on average ... or 0.5Ah over an hour.

So my running total is about 3.2Ah.

I've found a 12V in-car laptop charger with the following specs:

Input voltage: 12V DC

Output current: 4.62A

Output voltage: 19.5V DC

Power: 90W

So I guess that means add in another 4.6A. So grand total worst case would be 8A per hour required from my battery, and if I go for Lithium Ion or LiFEPO4 where I can use about 95% of the capacity with no issues, then a 40-50Ah battery would suffice for an evening's imaging.

Can anybody spot a flaw in my logic?

[Tinker1947]

A flaw, what about when you start down the dew heater route, electric focusor, i'm sure there will be more things, aim for 50% more then you think might be safer.

[AlistairHowie]

Thanks, Tinker. I have a 10Ah Li-Io battery already which would comfortably cover off those items, but your point is well taken.

Ignoring additional items for a moment, are my calculations and assumptions above correct?

[idbvideo]

Watts = Amps x Volts. Amps therefore = Watts/Volts

The supply will provide 13.8V normally. Thus the current consumption will be approx 2.2A

Your mount will tend to dram more current with more weight attached to the OTA as more weight is attached (even ballanced it still has to move the mass) The EQ5 Synscan draws 2A when slewing, so your going to be running at 4.2A. So your 12v in car charger is going to be working hard when slewing. Remember the camera remains powered up during this time too.

When choosing a power source its probably better to assume that ALL devices attached will be powered up at the same time and draw their maximum current for the duration that they are powered up. Then as Tinker suggested add 50% more capacity for expansion.

Hope that helps

[AlistairHowie]

Thanks, Ian. You've validated my calculations, which is great. I went with 11V for the battery as worst case as I'm thinking of lithium ion or LiFEPO4 rather than SLA. Actual voltage will depend on the battery chemistry and might not be as high as the 13.8V you get with SLA batteries.

I'll be mounting a 3.5kg 98mm refractor on a iEQ45, so a light(ish) telescope on a light(ish) mount, but I take your point about everything being powered while slewing. The batteries I'm looking at can cope with such an Amp draw, it's just capacity I'm concerned with.

At least with Li-Ion or LiFePO4 I can use about 95% of battery capacity, which all helps :-)