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Converting a mains device to battery power (Li-Pol in series and charging).
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castingflame:
Hi all.

I have a mains powered 'Device' that I wish to run from battery power so that it is not tethered by cable. The specs on the back of the unit say that it is rated at 200VA from a supply of 240 volts.

200VA @240 volts (if I did it correctly) translates to just under 840mA current draw.  If I wish to bypass its built-in PSU and run the device directly off of the required 24 volts, is this just a straight equation like  VA/Supply (ie 24) still?,  ... making it just under 8.34 Amps draw from a 24 volt DC source?

Apologies if this is a naive question.


Cheers
mariush:
It would be best to measure the average current draw and all that using a multimeter (at the very least). 
That 200VA rating could be just the peak current the device will take when it starts up (like inrush current charging capacitors inside, creating the magnetic flux inside the transformer etc) but the average power consumption could be much lower than 200VA.
The power consumption could also vary with the other things like how bright the lcd screen is (if the device has any etc)

Depending on the power supply the device has, it may be only around 85-90% efficient when converting 240v ac to 24v DC (or whatever your device uses), so if you measure 200w taken from the mains socket, the device may use only 180w or something like that, while 20w would be waste.

castingflame:
Thank you mariush for you reply. You raise some very good points.

The unit has one of those stock looking Switch Mode PSUs.

As this is really a feasibility study and not my unit I am limited in using the unit in a fully functional way at the moment. Could I use my multimeter to measure the current between the output of the PSU and the PCB?


mariush:
It's hard with a regular multimeter but it would be possible to get a reasonably close measurement of the current using a clampmeter (which can also work as a regular multimeter, but for the price you'd pay for a clampmeter you would get a more specialized multimeter, so think of the regular multimeter part as a bonus.

You would need a multimeter that can also measure DC current. A cheap and more than good enough clampmeter would be Uni-T UT210 :  https://amzn.to/2ISWmCE



So basically what you would do is start the clampmeter in DC current mode, then insert the +24v wire(s) between the psu and the board (or whatever) in the center of your clamp, and the meter will tell you the amount of current going through the wire.  The measurement will vary slightly with the position of the wire inside the clamp, so it's best to be consistent if you make multiple measurements and keep the wire(s) as close as possible to the center.

If the current flowing through the wire is too small, you can practically make a loop through the clamp, for every loop the meter will show double the actual current value or something like that.

That's pretty much it... you just have to separate the positive and negative wires and only insert one type through the clamp otherwise you won't get a good measurement.

castingflame:
Great advice mariush, I have just ordered one.  :-+

Hopefully I will be able to get the instructions to get the unit fully working soon and I will take a measurement when the clamp arrives.

I really appreciate your help mariush.


I am just doing this a a favor to my father and it is really a bit of a stretch for me. Once I have an idea of the requirements I will need to look at a suitable battery solution and charger. Not sure if a 'pack' made from Li-Poly would be suitable at this stage ...

I will update in a few days.
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