# charging two batteries simultaneousely

Discussion in 'General Electronics Chat' started by ibegbu_e, Dec 7, 2009.

1. ### ibegbu_e Thread Starter New Member

Nov 8, 2009
16
0
Can I charge two batteries from one dynamo of a generator

I need to use the second battery to power another device
The two batteries are of different Amp rating 65AH and 4AH but they are of the same volts 12V
How can I connect them to achieve simultaneouse charging
Thank you

2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
The charge current to the 4AH battery must be limited to 1A or less.

If the batteries are not the same voltage when they are connected together in parallel, the small battery may explode, as the large battery will try to charge or discharge the small battery. Very heavy current flow will result, and the small battery will get very hot, very fast.

If the small battery is at least 10.4 volts, you could charge it from a 14v source using a 4 Ohm resistor in series. The resistor should be rated for at least 10 Watts.

3. ### ibegbu_e Thread Starter New Member

Nov 8, 2009
16
0
Thanks Sgt
The two batteries are same volt.
I noticed that the charge voltage is about 13.5V when connected only tje big battery but when I connect the small one the voltage drops to 12.5V. Can this still charge the batteries well?

4. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
It sounds like there is something wrong with the small battery.

What is the current rating of the charger?

What is the voltage of each battery when disconnected from the charger, and left to sit for several hours?

5. ### ibegbu_e Thread Starter New Member

Nov 8, 2009
16
0
Well the current rating of charger is 68Amp

When the two batteries are standing alone for a short time(I have not been able to observe when syanding alone foe long time) the big battery reads about 12.8v and the small one 12.19v

6. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Did you really mean 68 Amperes??

Or did you mean 6.8 Amperes? (six point eight)

OK, the big battery reading of 12.8v seems to be normal for about 25°C.

However, the small battery is still in question. It could be simply about 1/2 discharged, or it could be reacting from a severe overcharge with a shorted cell. It would need to be measured after standing alone for 6 hours or so after charging, but before applying a load to it, to get an idea of its' condition.

Such a small battery should not be charged at a 6.8A rate, and certainly not at a 68A rate.

You should not charge the small 4AH battery at more than 1A current, or you will soon destroy it.

7. ### ibegbu_e Thread Starter New Member

Nov 8, 2009
16
0
Thanks this will be very helpful. So I should connect a resistor in series to limite the current to 1A.

But can I may not want the generator to draw current from the small battery even though I want it charged . Can I use something like diode to prevent that

8. ### ibegbu_e Thread Starter New Member

Nov 8, 2009
16
0
But I may not want the generator to draw current from the small batter even though I want it charged.
Can I use diode to prevent that.
Thanks

9. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
I tried to give you a simple solution. It was not perfect.

But the real solution is to design a charge controller for each battery, so that it won't get too much current, won't be overcharged, and won't be drained by the charger if it goes off.

You will need to do a lot of research on batteries and chargers.

Nov 8, 2009
16
0
Thanks
I appreciate