[EVO 4G General] raiderinks:
Current status, 14 January 2011:

At the moment, I stand by my original claim that you shouldn't use SBC until we all know more, but I can't state that the kernel is unilaterally safe or unsafe at this point. There are legitimate claims in this thread that holding the battery at 4.2V can cause an eventual short in the battery, which could lead to a catastrophic failure. MS and I are in touch. Once I have more skilled EE's to help me, we are going to review everything and make some recommendations. This might involve upfront testing or it might not, as dictated by what we find in our first reviews. I will try to keep this up to date, but be aware it might not move as quickly as you like.

Original Post:

Let me preface this by saying I am a degreed electrical engineer and I specialized in integrated circuit design. I am slightly out of practice, but the fundamentals don't change much. I am not an expert on batteries, but I understand the basics of their operation. I will absolutely yield to the engineering experience of people that have devoted significant portions of their careers to the field of battery design and applications. However, I don't think we have many of those people running around here.

This is a long post, as the topic material requires. If you don't read it all, you won't understand what is going on.

I started out this adventure reading this thread:

http://xdaforums.com/showthread.php?t=876590

Quite frankly, this explanation shows a clear lack of understanding of battery fundamentals and perhaps even the basics of electricity. So I read the thread linked in the above post and got a clearer explanation of SBC kernels.

http://xdaforums.com/showthread.php?t=894880

Now, let me explain what a trickle charger actually does. Basically, at the end of the charging cycle, you change over to a constant current source (CCS) supply. This means in a true trickle charger, IT WILL CHANGE THE VOLTAGE TO (theoretically) ANY VALUE TO MAINTAIN A CONSTANT CURRENT! That is bad for a Li-ion battery because they react poorly to being overcharged (see the end of this post for an explanation of battery charging and voltage). If you increase the supply voltage beyond the battery voltage, you are guaranteed to overcharge the battery. Check it out:

http://www.mpoweruk.com/chargers.htm#trickle

What about the self-discharge of our Li-ion you say? Well, Li-ions have some of the lowest internal resistances out there, so they barely discharge themselves at all.

Well, why does my battery lose about 10% everytime I unplug it using the stock charging kernels? That seems like bad engineering! It isn't. It is an engineering decision.

http://www.mpoweruk.com/lithiumS.htm#charging

The experience of battery engineers and device designers has shown them that they can ignore the top ~10% of battery capacity and dramatically increase the cycle life of the battery. That means you can charge and discharge the battery many more times before the actual capacity (power) of the battery drops to 80% of its original maximum. For most people, this is a great tradeoff. But it is not bad engineering. At the very least, it is a bad design decision for some people.

Now, you might be asking yourself why it charges so quickly to ~70% then takes forever to get to the ~90% level. The explanation given by MS79723 makes so much sense you say. Let's debunk it a little bit.

http://www.mpoweruk.com/chargers.htm#cccv

The reason the charger switches from a constant current source (CCS) to a constant voltage source (CVS) is nestled in a few engineering details. Much like the opposite of a CCS, a CVS will (theoretically) provide any current required to maintain a constant voltage supply. Now, you can approximate the battery as if it were a simple capacitor. This is not true universally, but it works for this explanation. We have already established that Li-Ion batteries respond very poorly to overcharging. Overcharging means putting too many units of charge into the battery, which can be measured by an excessive voltage on the battery. If you have had engineering physics, you know that if you place a constant voltage in series with a capacitor, it is physically not possible for the voltage on that capacitor to ever exceed the voltage of the CVS, but it may take a long time for the capacitor to reach that voltage. So we have built in safety and much more precision, but it takes longer to charge this way. (If you want to know more about this, you should study up on the mobility of electrons and holes in solid state devices.)

By now, you should be starting to see some similarities to things you observe, but the explanations clearly do not match what others have offered. So, I have to propose a couple of conclusions. If someone doesn't want to sacrifice the last 10% of capacity on each charge cycle to increase their cycle life, they could conceivably modify the kernel to allow charging at the higher ~4.2V as a CVS. Maybe that is what MS79723 is doing, but I really highly doubt it considering how people are going for hours and only using the top 10% of their battery life. That smells like serious overcharging, which is what would occur in a true trickle charge system. So I would like some clarification from the people putting out the SBC kernels. What charging methods are they ACTUALLY using? How are they controlling them? Do they have an appreciation and deep understanding of the above principles? If they don't have good answers, I would highly recommend you discontinue use of their kernels immediately.

And one more thing folks. There are no panaceas in deployed hardware. Only engineering decisions.