A Discussion about Batteries. I'd like to talk about the fundamentals of batteries in parallel.

But overall, this discussion is about Batteries, mainly in parallel.

P1 : Batteries in Parallel...
P1-Q1 : Is it true that batteries in Parallel will share the load equaly (Amps)?
P1-Q2 : Is it true that the batteries voltage will be the same?
P1-Q3 : What happens if the batteries have different Voltage? (V)
P1-Q4 : What happens if the batteries have different capacity? (mAh or Ah)

 

Discussion is meaningless unless you talk about real batteries where it is impossible for batteries to have the same exact Voltage and the same exact Capacity.

 

Discussion is meaningless unless you talk about real batteries where it is impossible for batteries to have the same exact Voltage and the same exact Capacity.

Real Batteries huh..

Ok buddy.. How about these?



I actually just bought these.. I tested they're voltage using a UNI-T UT61D True RMS 6,000 Count (Resolution) Digital Multimeter (DMM). The Voltages were exactly the same, and I'm going to take a stab at this, and I'm going to say the capacity is pretty much the same as well.

But I guess your probably right, that there's absolutely no way in hells creation, that this could be possible.

 

the condition with batteries in parallel will END with the batteries having the same voltage. When first connected together current will flow from the one with higher to the one with lower voltage. one will charge the other. Doesn't matter how great your DVM is, or that it shows equal readings, there will ALWAYS be a voltage difference before you parallel the cells.

 

Two Batteries having the same voltage at some point in time (within the resolution of a meter) is not surprising. It would be very surprising if they had identical discharge curves...

 

the condition with batteries in parallel will END with the batteries having the same voltage. When first connected together current will flow from the one with higher to the one with lower voltage. one will charge the other. Doesn't matter how great your DVM is, or that it shows equal readings, there will ALWAYS be a voltage difference before you parallel the cells.

How about this..? Those things on the left are SB240-E3/54 Schottky Diodes. They have a 0.5v forward voltage drop.
Would this prevent them from charging one another?

 

How about this..? Same type of diodes.

 

the small current exchange between two parallel batteries is not a bad thing just a fact of life. the duration of the exchange is brief and the result is increased capacity of the battery supply.
your first pic with each battery having a diode is correct. be aware that if connected this way they will not be able to be charged unless they are removed from the circuit.
that is the usual way to connect a primary and secondary power supply to a circuit.

 

The former connection using two diodes is the correct way to prevent one battery from charging the other.

The latter connection is fairly useless, as one battery does all the work until fairly discharged, then the other may take up sopme slack but will also try to keep the first one charged.

In the real works two things are never equal so you can't count on identicle batteries sharing equally. They may share "just good enough" to work in some application if a share on the order of 70/20 is acceptable.

 

The problem with a circuit that has two or more batteries directly in parallel is that you can't guarantee that they always will be replaced with identically fresh batteries. Adding the diodes prevent cross conduction at the expense of overall efficiency, but they are worth it if you are using lithium batteries. Those puppies get HOT when mismatched.

ak