Is 18 V enough to charge an infrequently used Roomba?

Mastiff · Joined: 20 Sep 2007 Posts: 135

I'm one PSU short here, since I have three Roomba's (2x535, 1x560) in different places, but only two 220 V to 110 V transformers. I have a rather good 18V 2.23 A PSU from an old HP multiprinter (which is long dead). Is that enough to charge the Roomba if it's not that often used (so it has a long time to charge), or won't it be of any use?

vic7767 · Joined: 14 Jan 2006 Posts: 3288 Location: Louisiana

You really should not use a PSU rated over 1.25 A, also you will probably discover that you will receive a charging ERR 5 when you attempt this charging plan. Post back what you experience.

Mastiff · Joined: 20 Sep 2007 Posts: 135

Is that what the error 5 is? I have one PSU (laptop) that charges the Roomba OK as long as it's not too far gone. But when it's really done a hard job, I get that "charging error 5" from the nice lady. I always thought that was because the PSU was to weak and couldn't cope with the demands of a drainede battery, not because it was too strong! Silly me...

Robotic · Joined: 03 Feb 2006 Posts: 63 Location: SF Bay Area

The iRobot power supplies put out 22V @ 1.25A
I'd think an 18V supply would not quite 'cut the mustard'.
...especially since a full battery is charged to somewhere over 17V.

Mastiff · Joined: 20 Sep 2007 Posts: 135

Yeah, I know about the output. But the batteries are only 14.6, if I remember correctly. So normally 18 V should be enough, only take a bit more time. Like a car battery, it's rated at 12V, and most cheap but still functional chargers only put out around 13-13.5 V.

Robotic · Joined: 03 Feb 2006 Posts: 63 Location: SF Bay Area

Mastiff wrote: Yeah, I know about the output. But the batteries are only 14.6, if I remember correctly. So normally 18 V should be enough, only take a bit more time. Like a car battery, it's rated at 12V, and most cheap but still functional chargers only put out around 13-13.5 V.
You must provide a voltage high enough to reverse the chemical reaction. If you do not, you will not be charging the battery.

Try your 18V PS. See if it works.
I doubt you'll be able to get a full charge out of it, though.

glo69 · Posted: July 3, 2008, 10:59 am Post subject:

Hello Mastiff

The 18V, 2.23A power supply should be enough to charge your battery. The problem you will eventually run into is that the Roomba power supply is limited to 1.25A. The U2 and U4 mosfets, which control charging in the Roomba, are barely adequate for the amount of heat built up in the circuit, and are notorious for failing.

When you try to run more amps through them they will get hotter, plus they throttle down the charging amperage above ~1.5A, which causes more stress on them. Don't be surprised if, in a reasonably short time, they fail while using that power supply.

Also be aware that, while the nominal voltage for the Roomba battery is 14.4V, the actual operating range with no load is ~14V-17V. In other words, a Roomba battery showing a no load voltage of ~14+V is actually fairly well drained.

Mastiff · Joined: 20 Sep 2007 Posts: 135

OK, that one made me scrap that plan! I really don't want to wreck anything inside the expensive piece of electronics. Thanks for saving me from myself! It goes to show how stupid I am: I always thought that power supplies can be as many amps as they want to be, since the powered thing will only use the power it needs. Wrong again. Gotta remove the rogue one I have on the ERR 5 Roomba as well!