Warning for using XO 1.0 power adapters with B2 XO 1.5s

John Watlington wad at laptop.org
Tue Nov 10 22:45:09 EST 2009

More details at:


On Nov 10, 2009, at 6:07 PM, Richard A. Smith wrote:

> Some testing in Uruguay has uncovered that the 25W power limit in  
> B2's internal DC/DC converter is not functioning correctly.
> The result is that if you _heavily_ load the machine _and_ throw in  
> a completely discharged battery then the B2 XO 1.5 will pull close  
> to 34W.
> If you try this with on 17W XO-1 adapter bad things happen. At best  
> the overload shutoff kicks in and turns off the adapter but at  
> worst it seems to be possible to run the XO-1 adapter hard for long  
> enough that it fails.
> The black prototype power supplies that were shipped with the B2's  
> are plenty beefy enough to handle the XO 1.5 B2's so no worries there.
> We also have mods that restore the 25W limit.  (most) B3's will  
> have these mods.
> The obvious question that will arise is: What about in the field  
> when you plug up a XO-1 17W adapter to a XO-1.5 that can draw 25W?
> Part of the reason this bug occurred is due to some changes wad and  
> I made to allow the XO to do MPPT (Maximum Power Point Tracking).  
> This allows the EC to detect that the input power has been exceeded  
> and will dial back the power limit of the charger.  This helps when  
> operating on solar panels.
> The voltage regulators that run all the operational voltages have  
> much lower impedance than the battery so the power limit will  
> affect the battery charging.
> When the load draw exceeds the source the MPPT should detect this  
> and throttle back.  I'm working on the code now and should have  
> something in the next EC release.
> To function properly it will require hardware mods so non-modified  
> B2's won't ever be safe but >= B3's should be.
> -- 
> Richard A. Smith  <richard at laptop.org>
> One Laptop per Child
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