Electronic – Stopping criterion of lead-acid battery charging operation with SMPS

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.

You will probably need an intelligent charger (micro controller based with a custom program to handle inputs from V&A meters, solar charge controller, & a power regulator).

Common methods of determining the SOC of a 12V(nominal) battery aren't very accurate. Most often people use a generic table that shows V & SOC %. The problem with that data set is that it applies to a new/good battery and as a battery ages & deteriorates the internal battery resistance increases, charge capacity decreases, & charge rate decreases (i.e. the charge rate of an aged battery is slower at a specific voltage than a new battery)--so you can't rely on it as a reliable method to charge a battery with solar energy while the engine is off & power is being drawn from the battery.

If you use a micro controller based smart charger with a good program, you should be able to do the following to determine the maximum safe voltage to apply to the battery while the engine is off: 1. measure the battery V 2. measure the A draw of the system 3. To maintain the battery's current SOC, your power system must produce as much power as the system is consuming. Your program would calculate the optimal charge A & V to provide sufficient charge current to offset the power consumption rate. 4. To add a net charge increase in the battery, your computer program will have to provide a higher charge rate than the charge consumption rate (you can decide that rate or make it a user configurable parameter).

Note: The regulated charging system of a running vehicle provides sufficient power to charge the battery & maintain other electrical loads. Your smart charger will have to emulate the same power availability while the engine is off (which might mean you'll need quite a bit of PV surface area (depending on loads).