Use temp. comp. voltage to calculate watts?

[madlabs]

Hi All,

I think the answer is no, but I just want to be sure. Should I use the temperature compensated voltage for calculating watts in/out? I'm guessing I should use the TC value for charging purposes only, but I was wrong once, long ago, so I thought I'd check.

Jonathan

[electronbaby]

it depends on what you want to calculate. If you want to calculate total PV array output, then yes, to be accurate, you should use the TC voltage (after all, thats whats actually going into your batteries).

[Ungrounded Lightning Rod]

For batteries the thing you're probably after is state of charge, which is more closely related to amphours than watts.  An ampere-second moves a columb of charged ions from one plate to the other, regardless of the temperature or voltage.  Put one in and you can take one out (less leakage losses) and return to the same state-of-charge.  Losses are mainly from having to put it in at a higher voltage than you take it out.  (I think that in principle you could run the battery as a heat engine by slowly charging it at a low temperature, heating it up, slowly discharging it at a higher temperature, cooling it down.  But it's not efficient enough for that to be practical.)

Of course if you're trying to measure or calculate efficiency of the storage element rather than how charged up it is you'll want to look at charging and discharging voltage.  And if you're trying to figure out what size batteries you need to power a particular load you might need to compensate for the voltage shift due to operating temperature.

So you really need to tell us why you want to "calculate watts" to get an answer applicable to your situation.

[madlabs]

I can see I wasn't clear in my question.

I am working on a multi purpose gizmo that will log all power in and out, as well as drive my lawnmower generator. It's mostly soldered up (waiting to get a couple of parts)and now I am working on the programming.

So, my question was (which I think has been answered): Should I use the temp compensted voltage for anything other than charging? For calculating watt hours (I'm going to use watt hours rather than amp hours) I should use the measured voltage.

I'm having fun with this project. I'll be able to tell my generator how many watt hours to charge, log all my solar input, keep track of run hours on the genny (total hours, hours today, hours since oil change). All data will be logged to a thumb drive for later review on a PC. I hope to get the unit installed and deployed today, but then I am often overly optomistic.

Thnaks for the help!

Jonathan

[Ungrounded Lightning Rod]

I'd use uncompensated amps for everything related to charge and discharge.  (Why do you want watts for an R.E. system?  Fuel is free, charge is what matters.)

I might additionally have a report of watts at the loads, for comparison to what running them with line power would cost.  That would use actual volts times actual amps for any DC loads.  For things driven by the inverter I'd like to measure actual watts after the inverter losses, though I might instead report actual volts times amps at the inverter's DC input and derate it with a fudge-factor for inverter efficiency if I couldn't get a true RMS measurement interface for the output side.  (Dealing with reactive power isn't simple.)

[madlabs]

OK, now I am confused. Is one supposed to temp.compensate AMPS? I thought hte thing was temp compensating voltage when charging.

Jonathan