# battery charging current?

Discussion in 'General Electronics Chat' started by cjdelphi, Aug 14, 2011.

1. ### cjdelphi Thread Starter New Member

Mar 26, 2009
272
2
what determines the current, so eg, a 6v battery it reads 5.5v if i apply 5.6v and let's say it drew 100ma.... Would it also draw 100ma if i applied 6.1vs to the same battery reading 6.0v ? Sure i could use a resistor but is there some kind of rule?

2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,729
One usually charges a battery using a constant current or current limited source up to a threshold voltage, then change to a constant voltage charge until the current drops below another threshold, and then change to a "float charge" voltage.

A resistor won't provide you with a constant current from a fixed voltage supply; the current will decrease as the battery charges.

You really should look at the manufacturer's datasheet to determine the recommended charge rates, as it varies with battery construction. Gel cells have to be charged more slowly than AGM-type batteries, or bubbles will form in the gel that will permanently reduce battery capacity.

3. ### wmodavis Well-Known Member

Oct 23, 2010
737
150
Wookie is right! And from his answer there is a much wider principal that is one of the most valuable lessons I learned in my electronics career. "Know component characteristics in all the detail you can find." That includes batteries, resistors, diodes and the list goes on. If you don't know - find out. Manufacturers are one of the best sources of how the components they make function. If you know a component's characteristics you will be beter able to troubleshoot and/or design.

4. ### kcyrf New Member

Aug 15, 2011
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Thank you Wookie for explaining how a battery charger work, but that is not what Delphi is asking.

To answer Delphi's question, the amount of current sink into a battery is determined by the applied voltage (5.6V) and open-circuit cell voltage (5.5V) divided by the internal resistance. I = (5.6V-5.5V)/Rint.

Battery internal resistance vary over the battery operating voltage range. For example, internal resistance may vary from 0.1 ohm to 0.15 ohm from 4V to 6.5V. Also internal resistance varies over temperture and age. Battery resistance is higher as the battery is aged. That is why an old battery takes longer to charge than a new battery.

5. ### #12 Expert

Nov 30, 2010
16,705
7,358
This variation that kcyrf described is the reason that constant current is used for the first stage of charging. You just can't trust a battery to be linear.