Amp Power, how much is enough and how much is a certain value?

[04Stefan07]

Hey all.

I have no idea about all this power stuff.

Been looking at power supplies and I know what I want to get but I want to ask about the Amps, specifically how many hours I can get out of a scope. This is only to power a scope, nothing else.

- 900 Amp, 12V, 17Ah (is what I am looking at)
- 500 Amp, 12V, 9Ah (is what I am considering since I found one for $45).

So how many hours can I get out of these two different powers in one night?

Also can someone clarify what the different between the Amp value and Ah value and what they contribute to so I know for future reference.

Thanks guys!

[mithrandir]

Stefan,

Amps tells you what current it can deliver but not for how long it can deliver that amount.

Ampere hours lets you work out how long it can deliver a given current.

900 Amp, 12V, 17Ah means it can deliver an instantaneous 900A for 17/900=0.18 hours (which drain would probably damage the cells), a continuous 17/12=1.4 amps for 1 hour, and so on.

Lots of slewing is what really determines how long it will last. The manual says the optional power supply is 12 VDC-750 mA so the absolute minimum you could expect would be 17/0.75=22.6 hours, and probably several times that.

[04Stefan07]

Ah yep I think I understand it a bit more now.

So would 500 be a bit weak you think?

[Allan_L]

Hi Stefan,
For JUST powering the scope, the 9 ah will keep you going all night.

But if you start running dew heaters (as most need to eventually) then the 17ah may even struggle after about 4 hours.

The ah rating needs to be taken with a grain of salt, as you should not completely drain a deep cycle battery. But in theory if drawing 1 amp (and most scopes are less than that) it should run 9 and 17 hours respectively.

A lot of people use the 17ah pack at the pony club.
I use a 33ah deep cycly battery but I run upto 4 dew heaters.

[MrB]

Allan is correct regarding the deep discharge of Lead-acid batteries, it kills them very quickly.
Regular 50% discharge though will give you a reasonable life from a battery.
At shallow discharges, like 20% or less, they can last 20 years or more if maintained.
This ofcourse reduces the effective Ah rating. With a 50% max discharge, your 17Ah is effectively an 8.5Ah battery.

Another thing that most salesman will never tell you(many of them probably have no idea) the Ah rating of a Lead-acid battery is calculated with a 20hour discharge, or... 17Ah/20h = 0.85 amp draw for 20 hours. This is unique to Lead acid's as they have very high internal resistance and a 1hour discharge figure doesn't look as good for sales purposes ;) Other battery chemistries (NiCd, MiMH, Lithium etc) have very low internal resistance so are calculated with a 1hour discharge.
Higher current draws will derate this figure, at 2 amp draw the battery could probably be considered a 15 or 14Ah, then at 50% max Discharge, ~7Ah.
At 5A draw, maybe 10Ah, at 50% ... 5Ah
For these reasons, I used to tell my customers(portable solar setups) to come up with a $$ figure they would like as a limit, and then spend all of that on the largest battery they could afford.

Edit: The depth of discharge is determined by measuring the batteries open circuit voltage(no load) after resting for a while.
A 100% full battery should be approx 12.7 or 12.8v, at 50% discharge approx 12v. Going deeper is getting into low-life territory(regular replacement) but you won't be doing it daily so it may be okay. Anything below 10.5v(loaded) regularly will kill it. All of this does depend on cell chemistry and design, but generally is pretty close for all lead-acid types.

Edit2: haha
Attached a couple of images to show what I'm talking about, these are lifted from the datasheet for a Panasonic 17Ah AGM battery.
Which reminds me, try to steer clear of the GellCells, they are fussy little so and so's, go for the AGM type.

[bmitchell82]

and for the nutty astrophotographers we run 2 or 3 100+ah batteries! :D hehehe

[Sarge]

Like you Stefan, power supply stuff is not my portfolio. So this is really getting difficult.:confused2:

Simon,

I was looking at this one from jaycar
http://www.jaycar.com.au/productView.asp?ID=MB3594&keywords=jump+starter&form=KEYWORD
It has a 12V 18 Ah SLA battery (which I now know means Sealed Lead Acid)
Max current 1000A
Cranking amps 500A
400W (800W surge) Inverter
2 x 12VDC sockets for running 12V appliances
and lots of other stuff (it can even pump up your tyres, with a work light :nerd:).

So what do all these figures really mean? Could I power a mount, laptop and one or two dew heaters for a reasonable time say 5-8 hours, or would something more powerful (or two units) be required? :question:

Clear skies

Rod
:D:D