I want to build a Lead acid battery charger. for that I want to use a small IC engine coupled to a dynamo and the dynamo I got is rated at 12V, 35A and I want to charge 12V 7.5 Ah Lead Acid battery. so here are the doubts which arose.

  1. Can I connect the dynamo output directly to the battery(with blocking diode of course) and expect it to charge since the battery's full charge voltage is around 14 V and dynamo is rated for 12V.
  2. And why is there such rating on dynamo, will it not produce more than 12V if it goes faster??
  3. Will I need some sort of current control while charging the battery or the battery will draw required current.?
  4. I just want a brute method which just works, I don't want to go for full charging algorithm stuff.

1 Answer 1

  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.

  • \$\begingroup\$ I received my dynamo today and as it turns out it's an alternator not a dynamo. So it's not as simple as connecting it to engine and reading the voltage. It does not have permanent magnets and a quick read up shows that when you connect the battery the rotor gets magnetized and then it starts generating electricity, i am very much confused about it's working and how would I achieve current limiting in this case.? You got any idea? \$\endgroup\$
    – Sajid
    Commented Mar 15, 2015 at 6:53
  • \$\begingroup\$ There should be a datasheet on the alternate telling you how much current it puts out, many self limit the current so when you are highway driving you don't over charge your battery from the higher RPM. I can help you much past that. \$\endgroup\$
    – Filek
    Commented Mar 15, 2015 at 22:45
  • \$\begingroup\$ The rating as said was 12V , 35A, and it was used in a car which had a 35Ah battery. And after opening it I found it has rectifier and voltage regulator. Well I'll just check things practically, using safe series resistor value. Thank you. \$\endgroup\$
    – Sajid
    Commented Mar 16, 2015 at 6:23
  • \$\begingroup\$ Sounds like a good plan. Good luck and make sure your resistors are of sucicie r power so they don't burn out right away. \$\endgroup\$
    – Filek
    Commented Mar 17, 2015 at 3:38

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