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connecting different batteries in series

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Yasserbn

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Ha

I have connected two batteries in series. The first one is 12v / 7AH and fully charged. The second one is 4 volt/1.2AH and it is 10% charged - almost not charged.
I have two questions.
First, what is the max current I can obtain and what's the AH?
Second, how can I know the age of a battery in general. I have heard that I can measure the voltage while the display is on DC current. I tried that but does not understand the readings.

Regards
 
Bad Idea!

The Ah will be that of the uncharged battery.

If you must series two batteries, they must be of the same type, charge state, and age.

Determining the capacity of a battery is done by fully charging it, and then discharging it into a known load resistance (or using a constant discharge current) while timing how long it takes for the battery voltage to drop to the cutoff voltage (typically 11V for a 12V battery).
 
Ha

I have connected two batteries in series. The first one is 12v / 7AH and fully charged. The second one is 4 volt/1.2AH and it is 10% charged - almost not charged.
I have two questions.
First, what is the max current I can obtain and what's the AH?
Second, how can I know the age of a battery in general. I have heard that I can measure the voltage while the display is on DC current. I tried that but does not understand the readings.

DON'T EVER DO IT!!!

If you're connecting batteries in either series or parallel it's ESSENTIAL that they are the exact same types, and in the exact same condition (so sensibly must be brand new).
 
The Ah will be that of the uncharged battery.
Does the uncharged battery resist passing current from the charged battery. Why does not allow passing the current if it comes from the charged battery?
Shouldn't work as a wire?

Regards,
 
Does the uncharged battery resist passing current from the charged battery. Why does not allow passing the current if it comes from the charged battery?
Shouldn't work as a wire?

No - see my previous post - DON'T DO IT

Once the battery goes flat the other battery reverse charges it, and if going totally flat doesn't destroy the battery, reverse charging it will.
 
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Does the uncharged battery resist passing current from the charged battery. Why does not allow passing the current if it comes from the charged battery?
Shouldn't work as a wire?

Regards,

Like any other series circuit, all current has to go through all elements in the loop. So the larger charged battery will force current through the smaller discharged battery causing it to go to zero volts, and then to a negative voltage. Which is a Very Bad Thing.
 
Like any other series circuit, all current has to go through all elements in the loop. So the larger charged battery will force current through the smaller discharged battery causing it to go to zero volts, and then to a negative voltage. Which is a Very Bad Thing.

I tried it. Practically, the uncharged battery (4v) did not go to zero volts. It still has 1.5v with no load. Probably, near to zero volt when running.
But why does it give negative volts. I did not understand that, could u explain it more?
 
I tried it. Practically, the uncharged battery (4v) did not go to zero volts. It still has 1.5v with no load. Probably, near to zero volt when running.
But why does it give negative volts. I did not understand that, could u explain it more?

Internally a battery tries to maintain its ("positive electrical polarity" redox ion charge separation reactions) chemical energy equilibrium to match the electrical energy being used from it. So when it's totally discharged (little or no remaining charge separation) and current still being is forced in and out of it, the forced redox reaction from the electrical energy swaps the ion polarity movements as the cell now has no positive polarity energy to supply and starts to charge backwards. So what happens is the charge separation inside the battery is reversed causing negative voltage. Usually this is very inefficient and generates a lot more heat than stored energy and is a "Very Bad Thing".
 
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I tried it. Practically, the uncharged battery (4v) did not go to zero volts. It still has 1.5v with no load. Probably, near to zero volt when running.
But why does it give negative volts. I did not understand that, could u explain it more?
Nigel told you already:
Once the battery goes flat the other battery reverse charges it, and if going totally flat doesn't destroy the battery, reverse charging it will.

Are you aware that if you allow a SLA battery to discharge below 1.8V/cell (3.6V for a two cell battery) you are seriously damaging it, and it will not recover to a full charge? So why would you want to connect a badly discharged battery in series with a higher capacity one, knowing that when you draw load current, the lower capacity battery will be forced to a lower terminal voltage (eventually reversed voltage) by the load current flowing through it?
 
Have a read(Figure) over here **broken link removed**
It shows you all about your Question.Figure 1 series battery
Figure 2 Faulty Battery
Figure 3 parallel BatteryFigure 4 parallel faulty battery
Thanks
 
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