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Old 03-03-2013, 11:05 PM   #382
_cy_ OP
Beastly Adventurer
Joined: Sep 2011
Location: Tulsa, Oklahoma
Oddometer: 6,437
a clarification about importance of cells remaining in balance within a series/parallel cell battery.

it takes 4x LiFePO4 cells in series (4s) to make up correct voltage that mates to 12v system. prismatic cell batteries uses cells of different sizes to make up different AH batteries. number of prismatic cells always remain 4s for a 12v system.

cylindrical cell LiFePO4 batteries also use 4s configuration to achieve proper voltage to match 12v system. there are several sizes for cylindrical li-ion cells. for instance 18650 denotes 18mm x 650mm ... the most common size used for motorcycle batteries are 26650 or 26mm diameter x 650mm long.

A123 makes the highest quality cylindrical LiFePO4 cells... one 26650 = 2.3AH at 3.3v nominal ... so 4x 26650 in 4s = 13.2v @ 2.3AH ... an 8 cell 26650 uses two stacks of 4s cells in parallel to make a 4.6AH battery.

12 cell 26650 uses three stacks of 4s cells in parallel = 6.9AH battery... add 2.3AH for each additional 4s stack in parallel, so a 24 cell battery has 6x stacks of 4s cells in parallel.

if/when a single cell goes out of balance, that stack has a lower voltage than rest of stacks in parallel. which immediately will reverse flow to weaker stack. if out of balance cell is severe, due to lithium batteries low internal resistance, discharge to weaker stack will destroy battery.

if I understand correctly, Joel has purposely unbalanced LiFePO4 cells in 4s to approx 8% differences and watched cells achieve balance again towards end of charge.

will be recreating this scenario and many others during balance tests in progress ... more to come on this topic
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