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

[Richard A. Smith]

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