Electronics > Projects, Designs, and Technical Stuff

Inverter power rating?

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iXod:
I need to run a lithium battery charger for power tool from an inverter in my vehicle.

The charger is rated at 120VAC, 2.75A max current. I don't know the PF or efficiency of the charger, but I think these are usually around .8-.85 efficient. It is an SMPS type charger.

How can I ballpark the required wattage of an inverter I'll need for this?

Thanks.

jbb:
Well, efficiency doesn’t come into it because the 120V 2.75A already includes efficiency.

120 * 2.75 = 330VA
Do a 350W inverter should do the trick, even if the charger has power factor correction.

However, the charger may have poor power factor - maybe 0.8 - in which case an inverter rated for 250 W at 0.8 power factor may do the trick.

Finally, you have to consider whether you need a true sine wave inverter (recommended) or can use a cheaper ‘modified sine wave’ inverter (which will be cheaper but may make some electronics very angry).

iXod:
Thanks for your informative reply.

I've read that surge could be a problem when using inverters with some equipment. Specifically, SMPS input caps could cause large surge which would shut down (protect) the inverter. I would like to choose an inverter with over-kill watt rating (recommendation of some inverter manufacturers is 3x), but of course I'd like to still choose something economical. So:

1. is SMPS a reason to de-rate inverters?

2. how much to de-rate?

Yeah, already decided on true sine. Most adverts for inverters say (in fine print) that power tool battery chargers may be damaged by their mod-sin models.

Cheers.

coromonadalix:
In my case, some of my laptop chargers wont work with some pseudo sinusoidal (step) inverters

In my case i always consider an inverter to run at max of 75% of its full stated power,  had many china models who wont even meet 60% of the written specs.


Unless its a model who specifically says  EX: 1500 max power  2000 watts surge, i only use pure sine  xantrex or american made models.

james_s:
Many years ago I blew up a cordless drill charger by plugging it into a modified sine (square wave with dead time) inverter. This is only a single data point and was around 25 years ago so I'd imagine charger tech has improved since then but it's something to keep in mind.

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