Series and Parallel

fechter

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Frequenly we need to make a battery by connecting a bunch of smaller batteries together.

If you want a higher voltage, the batteries are connected in SERIES. This means they are connected + to - between cells. The voltage of the individual cells add in this case, but the capacity (amp-hours) stays the same. The maximum current for the string is the same as a single cell also.
 

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When you put batteries in PARALLEL, the voltage stays the same, but the capacity and maximum current add. This is good when you want more range.

Nimh and Nicd batteries generally cannot be charged safely in parallel. They can be discharged in parallel.

When placing batteries in parallel, they must be the same voltage and chemistry. Generally, there's no limit to how many cells can be put in parallel.
 
Putting batteries in series increases the voltage. Putting batteries in parallel increases the capacity.

What if we want to do both things at the same time?

Then it's called a SERIES-PARALLEL connection.
There are a few variations on how to do this. You could make long series strings, then put the strings in parallel only at the ends. This is good for Nimh and Nicd batteries where the strings must be separated during charging.

Conversely, you could make clusters of parallel cells, then put the clusters in series. This is also known as 'buddy pairing'. This works well for lead-acid and lithium chemistries, since it forces more of the cells to be balanced.
 

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There are times when you might want to switch between series and parallel configurations. This is ususally to allow charging with a single charger that is made for half the pack voltage.

In this case, the switch must be rated for the maximum current draw of the controller. The wiring of the switch must be done in such a way as to prevent any short circuits if one side of the switch makes contact before the other side.

This switching scheme can also be used to switch motors in a dual brushed motor setup.
 
Many lithium battery packs contain an integrated battery management system (BMS) that controls charging, protects against overdischarge (low voltage), and protects against over current during discharge. "Duct tape", Ping, DeWalt and many others have a battery management system.

When attempting to put packs containing a BMS in series, there is a potential problem. If the BMS ever trips and goes open due to an overcurrent, the transistor switch in the BMS will be exposed to the full voltage of all the packs combined, possibly taking over the rated voltage for the part and causing destruction.

The way to avoid this problem is to place diodes across each pack's output terminals to keep the voltage from going higher than the BMS was designed for.

If one pack trips, the diode will conduct, allowing voltage from the other pack to pass to the controller. The tripped pack will not "see" more than its own voltage, plus the voltage drop in the diode.

For a typical 36v pack, you'd want to use something like a 50 volt, 50amp rated diode.

Since the diode is only working when a pack trips, low losses are not very important and you can use just about any diode with a high enough rating. Note the diodes are placed across the battery in the normally non-conducting direction.

BMS in series config 2.jpg
 
Woooow
Ahhhh!
I got prob with this kinda making two 36 bat pack in series to got 72 v total !
After 2 years i send my pack for repair repeatedly and i am sorry for the pack designer to do my pack without going according to this simple and cheap principle 😑☹️☹️☹️
Just two diods !!!!!
 
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