Electronic – Lead-acid battery charging fixed current stage specification

1. The dynamo will charge the battery as long as its voltage is higher than the battery's voltage. You are correct, as your battery approaches a full charge, it will be around 14 volts and the dynamo is rated at 12 volts, so it will not be charging in this situation if it is actually 12 volts. However, many commercial charging items rated at "12 volts" are actually giving an output higher than 12 volts specifically for the purpose of charging the 12 volt lead acid battery which is more like 13.5 - 14.1 volts depending on type of battery. Maybe your dynamo is built this way as well.

2. Without knowing the model of your dynamo it is impossible to answer specific questions about it. In general however, if you increase the RPM of a dynamo the voltage could rise, but my understanding is that most dynamos have voltage regulators that keep them to their specified voltage when RPM rises.

3. The battery will not "draw the required current". You will need to control the current going into the battery, especially since your battery has a relatively low capacity compared to the capabilities of the dynamo.

4. "I just want a brute method which just works, I don't want to go for full charging algorithm stuff." This is the best and fastest way to ruin your battery! Perhaps if you had a very large capacity battery you could do a "brute" charge method of just applying a current for a while and keeping the battery between 50-75% charged, but your battery is too small and it would be hard to not accidentally overcharge your battery. If you do go ahead and charge without a logarithm, it is generally recommended to charge a lead acid battery at 10 - 30% of its rated capacity, so in your case = 0.75amps - 2.25amps. Starter batteries even lower charge rate, and AGM could probably go higher.

The short answer is - NO! No need to waste power from the panels.

You are correct, the lead acid batteries are best charged on a CC-CV logarithm. However, constant current doesn't mean that the battery has to be fed an exact pre-determined current or the battery suffers. It would be better interpreted as a maximum current that the battery can be charged at.

In an ideal world, where lead acid batteries worked better, we would just apply the voltage at the level of the constant voltage and let the current flow in until it saturated. Unfortunately, if we did that with lead acid batteries currently being produced, the current flow at lower charge levels would be so high that they would fry the battery. As a solution, the charge starts at a constant current level that is lower until the battery starts to fill up to the point where you can apply the constant voltage voltage and the current at that point won't be too high because the battery is already fairly full.

So if that made any sense, you should be fine to charge the lead acid battery at a lower and uneven current in the constant current stage of charging as long as the current doesn't exceed what the manufacturer recommends for the battery that you are using.

Specifically to answer your question, charging at lower current doesn't hurt the battery (unless the current is so low that the battery never charges because self discharge is greater than the charge current!).