Electrical – Is it ok if lead acid battery charge with too low current

13V read on your multimeter is probably RMS value. There are probably relatively high voltage pulses.

I would just charge that battery and try to use it. If it does not hold charge - you should replace it.

I was experimenting a lot with desulphation and... for car battery it does not make sense. If there are big sulphur crystals - you have to use chemical method (instead of electrical method with fancy charger). If there is just small sulphur dust on electrodes - desulphating will not help a lot. It is waste of time.

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.