CrazyEngineers Archive
Old, but evergreen and popular discussions on CrazyEngineers, presented to you in read-only mode.
@mibawad • 12 Sep, 2011
Hello,
I want to use a DC permanent motor as a generator in order to use it to charge a battery. This generator will be rotated by hand. The speed is varying as it rotates by hand. I would like to know what is the circuit that I could use it to charge 12 V chargeable batter?
Best wishes,
@mibawad • 13 Sep, 2011 Actually, I would like to know if there is a special circuit to charge a battery as the output voltage is varying according the rotation speed as it rotates by hand, and I think you need constant voltage to charge the battery. Do you have an idea about the charging circuit in this case?
Many thanks
@Ramani Aswath • 13 Sep, 2011 The output voltage is an open circuit voltage. The battery is a low internal resistance load. When you connect the generator to a battery all that happens at different RPMs is that the charging current varies, which does not matter. No extra circuit is needed.
@mibawad • 13 Sep, 2011 Many thanks for your help. I would like to know if the varying in charging current has any effect on battery life time. I mean it is better to get constant charging current or it doesn't matter. Many thanks.
@lal • 13 Sep, 2011 But still, dont we need a circuit to limit the charging rate to be within the maximum value?
Also, it is undesireable to overcharge the battery. The case of overcharging may be left behind cosidering the fact that it takes a long time to fully charge the battery and hand-cranking all this time is practically impossible.
@Ramani Aswath • 13 Sep, 2011
mibawad
Many thanks for your help. I would like to know if the varying in charging current has any effect on battery life time. I mean it is better to get constant charging current or it doesn't matter. Many thanks.
The speed range of hand cranked generators cannot be more than a factor of two (typically 60 strokes/min). This is well within the capacity of batteries to handle.
@mibawad • 14 Sep, 2011 Does it mean that I could charge 12V rechargeable battery at 5V for example? And, it will charge, but it might takes long time for charging. Is that true? Moreover, is that a circuit to avoid overcharging for the battery?
Many thanks.
@Ramani Aswath • 14 Sep, 2011
mibawad
Does it mean that I could charge 12V rechargeable battery at 5V for example? And, it will charge, but it might takes long time for charging. Is that true? Moreover, is that a circuit to avoid overcharging for the battery?
Many thanks.
I am afraid not. While the internal resitance is small, there is a back EMF. Unless the charger voltage is higher than the battery voltage no charging current wiil flow. What is worse, you may even drain the battery further if a lower voltage source is connected.

If the discussion is about hand cranked generators, why would any one overcharge? Literally a pain to do it. Just check for the battery status occasionally.
@mibawad • 14 Sep, 2011 What do you mean by "What is worse, you may even drain the battery further if a lower voltage source is connected"? Moreover, during charging process do I connect battery +ve to the +ve terminal of the generator and battery -ve to the -ve terminal of the -ve generator terminal. Or, I have to connect +ve to -ve terminals. What do you think?
@Ramani Aswath • 14 Sep, 2011
mibawad
What do you mean by "What is worse, you may even drain the battery further if a lower voltage source is connected"? Moreover, during charging process do I connect battery +ve to the +ve terminal of the generator and battery -ve to the -ve terminal of the -ve generator terminal. Or, I have to connect +ve to -ve terminals. What do you think?
The hand cranked generator will have an impedance. If the battery voltage is higher than the output voltage, the battery will drive a current through the generator is what was mentioned.

Charging is always done by connecting the +ve of source to +ve of the battery. You are putting current into the battery somewhat like pouring water into an empty tank.

If you connect +ve (Source) to -ve battery you are putting the two in series. No charging takes place. The voltages will get added.

This gives you a load of info. However, most of it may not apply to a hand cranked generator.

https://www.mpoweruk.com/chargers.htm
@mibawad • 14 Sep, 2011 Many thanks, for those useful information. I have another question what will happen if I charged 12V battery by 40V source or generator. Does the battery charge faster or it will be destroyed?
Many thanks
@Ramani Aswath • 14 Sep, 2011
mibawad
Many thanks, for those useful information. I have another question what will happen if I charged 12V battery by 40V source or generator. Does the battery charge faster or it will be destroyed?
Many thanks
What charges a battery is the current, which is why the capacity is given in ampere hours. However, the voltage is needed to drive the current into the battery. If the source is at 40 volts and it has the capacity to give any amount of current the 12 V battery will get damaged. If the voltage is open circuit voltage and the current delivered is limited the battery may not be affected. Unless actual parameters are known one cannot predict results. While this is not a problem with hand cranked generators, it may be necessary to add a current limiting circuit for other charging supplies.

However, the usual charging current is 10% of amp hour capacity. Rapid charging can be done at three times the normal charging current. It is not advisable to charge faster than this.
@mibawad • 15 Sep, 2011 What do you mean by "If the voltage is open circuit voltage and the current delivered is limited the battery may not be affected."? Is it possible to charge a battery in open-circuit mode?
@Ramani Aswath • 15 Sep, 2011
mibawad
What do you mean by "If the voltage is open circuit voltage and the current delivered is limited the battery may not be affected."? Is it possible to charge a battery in open-circuit mode?
It may help to refresh on batteries.
This is quite informative and can answer most of your questions.
https://itacanet.org/eng/elec/battery/battery.pdf

A
battery has an internal resistance of less than 0.1 Ohm. Take the differential voltage between source and the battery. Divide by the total resistance of your connection circuit. THis will give you the charging current. See that this is below 30% of the ampere hour capacity of the battery. If it is much higher add additional resistance in series to bring the current within limits.
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