Can you explain then why my motorbike's battery charge drops to zero in January for the third year straight?
That's a lead acid or acid glas-mat battery, not a LiPo battery like in a MacBook Pro. Totally different things.
Actually, maintenance is the complete opposite between the two. With an SLA or AGM battery, you want it to ALWAYS be fully charged. If the battery drops below 12.4 volts, then sulphate crystals (from the sulphuric acid inside the battery) begin to form on the soft lead plates, impeding the batteries ability for electrons to flow. However, LIKE a LiPo battery, voltage will drop as the battery gets cold. That's because electrons move slower in the cold! That's why a battery tender is so important in the winter for vehicles that you don't drive at least every few days. As those sulphate crystals form, the battery will become useless. That's why some battery chargers will be listed as 'Desulphating', because they send short high-voltage pulses of electricity to attempt to dislodge some of the sulphate crystals from the plates.
If it's dropping to zero, it's toast. I wouldn't ride that bike with that battery lest you find yourself stranded. If it just won't start or turn on, chances are it's actually down in the low 11 volt range. If you hit it with a voltage tester that's probably what you'd find. However, 12 volt electronics (like your lights, dash, starter, etc.) won't work with less than 12 volts. Each time your battery drops below 12.4v, sulphation begins. A battery tender (not a trickle charger, an actual tender that is automatic and will shut off around 12.6 volts) is an excellent investment. I have one for all of my bikes. But I have no problem leaving them out in the cold, in fact, for lead-acid batteries, the cold is good for them. Just as long as the voltage doesn't dip!
The biggest concern with LiPo batteries (like in the MBP/MBA) is condensation. That's why Apple recommends a temperature range (though extreme heat can definitely kill it, or even cause it to catch fire or explode). Prolonged extreme cold could damage the battery, but once in a while being left in a car shouldn't hurt anything.
The biggest reason for Apples temperature recommendation (on the LOW end) is performance. The HIGH end (above 95F) can cause damage, but the LOW end, will just cause a really short battery life, or no charge at all. LiPo batteries are a chemical reaction, chemical reactions slow as it gets cold. I'm not at all surprised that the machine wouldn't power on at 4F (well below freezing) as the reaction had STOPPED completely. But, it actually hadn't lost any charge, the charge just couldn't 'move'. It's the same reason Apple recommends occasionally cycling the batteries (though it's not nearly as crucial on a LiPo), the reaction needs to happen and the electrons need to flow!