2 Batteries, Same Chemistry, Different AH - In Parallel. Please advise.

Thread Starter

jimbo69ny

Joined Feb 13, 2018
4
Hello Everyone,

I am new here. I have been scouring the web for an answer to this question. I really hope you can help me!

If I have 2 batteries, both use the same lithium ion cell, one has a higher capacity than the other, both use the same voltage, discharge and charging curve. Why is it bad?

In a nutshell, I have an electric car. The OEM pack was made by Tesla and uses 18650 cells in a 92s48p configuration. The pack uses 2600 mah cells. The pack is about 3-4 years old.

I want to add an additional pack to be run in parallel with the OEM pack. My pack will also use 18650 cells although I want to go with 3400 mah cells. It will be in a 92s31p configuration.

I keep hearing that it cant be done and I am going to set my car on fire, my house on fire and kill my family..... After reading thread after thread all day today I cant find a reason this cannot be done, just hyperbole. Please help!
 

nsaspook

Joined Aug 27, 2009
7,752
It's like running mismatched horses in a parallel harness.
Unless you know exactly what's happening it's a recipe for something bad..
 

Ramussons

Joined May 3, 2013
892
Hello Everyone,

I am new here. I have been scouring the web for an answer to this question. I really hope you can help me!

If I have 2 batteries, both use the same lithium ion cell, one has a higher capacity than the other, both use the same voltage, discharge and charging curve. Why is it bad?

In a nutshell, I have an electric car. The OEM pack was made by Tesla and uses 18650 cells in a 92s48p configuration. The pack uses 2600 mah cells. The pack is about 3-4 years old.

I want to add an additional pack to be run in parallel with the OEM pack. My pack will also use 18650 cells although I want to go with 3400 mah cells. It will be in a 92s31p configuration.

I keep hearing that it cant be done and I am going to set my car on fire, my house on fire and kill my family..... After reading thread after thread all day today I cant find a reason this cannot be done, just hyperbole. Please help!
The little I know about Li Ion cells tells me that Paralleling 2 Same Voltage Different VA capacity should really do no harm as long as they are Discharging.
But Charging 2 different capacity batteries in Parallel is a different case UNLESS each battery current is individually monitored and controlled charging is done.
Else, the lower capacity battery will overcharge and may lead to overheating and explosion.
 

Thread Starter

jimbo69ny

Joined Feb 13, 2018
4
The little I know about Li Ion cells tells me that Paralleling 2 Same Voltage Different VA capacity should really do no harm as long as they are Discharging.
But Charging 2 different capacity batteries in Parallel is a different case UNLESS each battery current is individually monitored and controlled charging is done.
Else, the lower capacity battery will overcharge and may lead to overheating and explosion.
This is exactly what I suspect. They both have the same discharge curve.

As for recharging, both will be monitored and balanced using 2 BMS systems.
 
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