Inverter power rating? - Page 1
Forum section: electronics / projects

author: iXod
author_rank: Frequent Contributor
timestamp: March 24, 2019, 08:30:02 pm
content: 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.

author: jbb
author_rank: Super Contributor
timestamp: March 24, 2019, 10:12:44 pm
content: Well, efficiency doesn’t come into it because the 120V 2.75A already includes efficiency.120 * 2.75 = 330VADo 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).

author: iXod
author_rank: Frequent Contributor
timestamp: March 24, 2019, 10:24:12 pm
content: 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.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 24, 2019, 10:38:09 pm
content: In my case, some of my laptop chargers wont work with some pseudo sinusoidal (step) invertersIn 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.

author: james_s
author_rank: Super Contributor
timestamp: March 24, 2019, 10:53:13 pm
content: 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.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 25, 2019, 01:11:52 am
content: Get a smart plug with power monitoring to check the actual power usage.

author: james_s
author_rank: Super Contributor
timestamp: March 25, 2019, 01:55:30 am
content: No need for anything fancy, those Kill A Watt devices are cheap and the couple I've tested compared very closely with a real benchtop power analyzer.

author: iXod
author_rank: Frequent Contributor
timestamp: March 25, 2019, 02:17:30 am
content: The Kill-A-Watt will measure instantaneous power, so won’t show me peak surge. That’s what I need to know, I think, in order to determine how large the inverter should be.Cheers.

author: james_s
author_rank: Super Contributor
timestamp: March 25, 2019, 02:26:47 am
content: I don't think a smart plug will either will it?You could use a multimeter that has min/max mode, it's unlikely you'd need anything faster than that. If a load draws too much current from the inverter the thing will shut down.

author: iXod
author_rank: Frequent Contributor
timestamp: March 25, 2019, 02:34:32 am
content: Yeah, I know it will shut down to protect. I don’t yet have an inverter and I’m trying to decide how to calculate the required wattage of the inverter I need to purchase so it won’t shut down when powering the battery charger.Cheers.

author: james_s
author_rank: Super Contributor
timestamp: March 25, 2019, 04:05:32 am
content: I don't think you need to worry about the instantaneous draw, battery charger is going to be a fairly well behaved load. I would be surprised if the draw is more than 300W but you could get a 400 or 500W inverter to make sure you have plenty of overhead in case you need to power other things.

author: soldar
author_rank: Super Contributor
timestamp: March 25, 2019, 08:00:51 am
content: Quote from: iXod on March 24, 2019, 08:30:02 pmSeveral thoughts:The charger may be nominally rated for 2.75 A but it may use much less in normal use or a bit more at startup. The only way to know for sure is to measure it. That will give you much better information than relying on the label.I doubt a small battery for a hand tool would recharge at such rate. If indeed it uses so much power be careful you do not discharge the car's battery to the point where it cannot start.Rather than convert 12Vdc to 120Vac you would be much better off getting a charger than ran directly off 12 Vdc. Efficiency and reliability will be much better.I would not plug something so powerful into the car's lighter because it will probably blow the fuse. I would connect directly to the battery terminals.

author: iXod
author_rank: Frequent Contributor
timestamp: March 25, 2019, 03:43:04 pm
content: Thanks for your great reply.Manufacturer of these tools doesn’t make a rapid charger (which I need) with 12vdc input, unfortunately.I’ll have to see, empirically—with my planned discharge/charge pattern for the tool batteries—if I need to be concerned about the car’s battery. I have a lithium “jump starter” pack in case car battery goes low.Inverter will be connected to battery, not lighter socket.Cheers.

author: soldar
author_rank: Super Contributor
timestamp: March 25, 2019, 09:24:01 pm
content: If this is for professional work, where you are going to be doing this day in and day out, I would consider getting more batteries so you do not need to recharge as often or as fast. Fast charging kills batteries fast. Slow charging extends their life. If you have several packs you can recharge them overnight and if you have a totally dead one you can give it a slow charge from the car. This is cheaper, simpler and healthier for the batteries.What is the voltage of these batteries?

author: iXod
author_rank: Frequent Contributor
timestamp: March 25, 2019, 09:42:42 pm
content: Mobile car detailing using battery powered 18v buffer.Appreciate your advice to fully charge batteries overnight. But to cover that plan I'd need several more batteries and 4-6 chargers to charge overnight (or setting the alarm every 90 minutes to get up and swap batteries).Taking to heart the information about quick=short life; slow=long life. My initial plan does primarily revolve around needing the in-car rapid charger only if I find business especially good one day and run out of pre-charged batteries.Cheers.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 25, 2019, 11:14:27 pm
content: Have you looked at 12V input R/C chargers? Limit the charge voltage to 4.1V/cell and the charge current to 1C or less to increase battery life.

author: iXod
author_rank: Frequent Contributor
timestamp: March 26, 2019, 01:56:32 am
content: The battery is not balance-friendly (externally), but made specifically for these power tools and its proprietary charger that communicates with the battery’s on-board monitor.Not really up for the challenge of reverse-engineering the factory charger’s communication protocol, etc.Cheers.

author: iXod
author_rank: Frequent Contributor
timestamp: March 26, 2019, 04:36:54 am
content: The rapid charger is now drawing 1.8 A on 120 VAC while charging one of the (almost empty) batteries.What current might this translate to at the 12 VDC input to an inverter?Any guestimates?Cheers.

author: iXod
author_rank: Frequent Contributor
timestamp: March 26, 2019, 05:37:45 am
content: Thanks for the well-calculated guestimate.I'll take some measurements when I have the inverter in-hand.Cheers.