The spec sheet I have says it has a 28 amp hour battery. So, with the 24 watts (2amps x 12w) that the device I intend to use is going to draw it should be good for 14 hours...or in my pessimistic (all battery life estimates are BS) mind it should last for about 7-8 hours. They claim that if it is charged from the DC port that the maximum time for recharge will be 4 hours (from completely drained). Shouldn't that indicate that it will charge faster than it will drain?
I figure I'll start out with the device not being charged and let it run for a few hours (4-5) and then plug in the DC cable and see what happens. Rather than test with the harddrive based unit I'll use something of less value out of my closet. I'll have my wife sit next to it so she can tell me if/when it catches on fire :)
Your calcs seem right for the 28AH (ie, 28AH@C20 = 1.4A for 20 hours etc), and your BS reference reeks of wisdom**.
I'd charge the unit FIRST and then test.
Reason - thru storage they are usually run down; most appliances state "charge before use" to blow away the cobwebs (sulfate etc) and prevent further deterioration. (FYI- battery capacity usually increases after a few cycles.)
Besides, then you start from a known condition - ie, fully charged.
Re other specs, I am going from the spec sheet I downloaded, and some of the info is ambiguous - ie, a 2.5A dc-plug charging limit.
But that's what their techs should be able to clarify - including charge and use simultaneously (why not? - after all it's not an ac-dc charger)?
Not that I necessarily trust company or professional info (a recent example involving incorrect Rostra supplied and website info being typical), but you seem safe with your Amazon return situation.
** Re batt capacity estimates & bullsh - I find them ok and on the conservative side (they need to cover 97% or 99% etc of their batteries), but I realise how they can vary.
My last estimate was a few years ago after losing my alternator. A 12 year old (10 year design) 38AH UPS battery and a one-hour trip home at night.
I estimated a 2 hour reserve for an ok battery with whatever load (~15A with lights etc). That gave me my initial confidence.
And then monitoring my dash voltmeter... With a ~0.3V drop over ~30 minutes I was confident I'd make it home - not that a battery cannot collapse at any point in time...
The best was the next morning after (battery) resting. A voltage check confirmed a discharge of ~50%, hence supporting my original 2 hour reserve-time estimate, and suggesting the battery was as good as new (at least to 50% capacity...).
Sure - somewhat variable and approximate, but I think my estimated errors balanced out (15A for headlights & lights & engine and occasional braking; was the 38AH rating @C15minutes) or C1, C10 or C20; how charged was the battery (it had been used for 1 hour cranking my car out of a bog); how aged was it; how conservative were the manufacturers specs?).
But with people even on this site thinking that simple component or equipment specs are bullsh (they fail to understand things like the test frequency or conditions & 95% etc rule), I can see the difficulty people have with batteries (SOC, temperature, age) - especially at loads different to the C rating. (LOL - especially when they think 38AH @ C20 means 38 Amps for 20 hours!)
[ ... end Incidental ]
I'm curious - will your wife be wearing your pants?
Yes, I'm going to fully charge before testing.
The battery estimates I have issues with revolve around the ones uses in mobile electronics. The problem being of course that the amount of power consumed by these devices can vary wildly depending on use. It really would be nice if there was a set benchmark for it like the audio industry finally settled on years ago with receiver/amp specs.
And yet so many amp specs don't use RMS!
But you pointed out the problem with mobile electronics - too many load changing variables. Even mobile comms vary their transmitter power depending on distance...
As for configs and attachments, one disk drive, SSD or magnetic, processor cores at what speed, USB devices, etc etc.
But there is one similarity - amp specs like load PSUs provide a maximum rating. Not that that means you use a constant 1kW RMS speaker power or the full 550W capability of the ATX PSU etc.
Even your 2A plugpack may only reflect the "average max" current - ie, at startup or with the disk spinning up or writing etc. The average may be under 1A.
[ I used average max to mean an average "max" current, not the surge currents which are usually handled as a matter of course - eg, peak power/current = twice RMS values for most devices, though even then the instantaneous peak can be higher. Max ratings are usually related to heating, though sometimes a max punch-through value can be the limit - eg current may rupture, and voltage may arc through insulation (and hence create a low resistance path leading to further breakdown). ]
That's were ammeters are useful, though generally only temporarily to get an idea of typical or max currents. Even then, most will add up the load specs for the attachments or accessories that they have installed and add some margin.
The voltmeter is still usually the best monitor - it shows if your system is up to it. IE - Who cares what the current is or should be? If the voltage drops, something is wrong - even if it's a mere 10A from a 80A alternator etc, or 12.1V after a few minutes of 10A from a fully charged 100AH battery.
And there's the final suck it and see approach. That is the ultimate measure anyhow... (The problem is if you can't refund the mistakes.)
Did a bit of testing this weekend with the Xantrex 600HD and all seems to be working well.
Charged the Xantrex to full and put it in the car, ran an extension cord to the WD Live Hub and powered the inverter on and then the WD. All worked well. I then let it run on battery for six hours and all was fine.
I then plugged the DC to DC cable into the Xantrex and the car and turned the car on and off. There were no ignition issues...the device kept running.
I've now got to see if the car will charge the battery to full within the four hours they state while running. That test will come later.
Finally, I've got to find a nice box that will hold the WD Live and cushion it from some of the road vibration while allowing it to get air to stay cool (fan is on the bottom).
Best of all...using my phone's 4G tethered connection could even run Netflix and other services quite well. Obviously, not going to use that most of the time as it would burn through my allotted data very quickly, but it's nice to have the option.
I've also decided I am going to setup a full network in the car with the option of it connecting to the internet...for no other particular purpose than I can :).