The individual cells in a battery pack are rarely an exact duplicate of all the cells in the battery pack.
 
In actual practice, when a pack of batteries reaches a specified charge cutoff voltage then the charger will shut off. If one cell in a pack reaches that voltage before other cells reach a full charge it will cause the charger to shut off - prematurely denying the balance of the pack a full charge.
 
Conversely when the weakest cell in a pack reaches its discharge cutoff voltage it will show as the discharge point and shut down the pack prematurely - thereby denying the user the balance of energy the other cells have stored.
 
These battery equalizers prevent this from happening. This yields up to 30% more power and will extend the life of the battery pack.
 
Benefits:
Quick and easy installation
Keeps your batteries voltage difference within 10mV!
May increase the lifespan of your battery pack by 2-3 times by preventing the batteries from being overcharged or over discharged
 
Features:
§  High frequency pulse, dynamic battery voltage-amperage balancer
§  Simple wiring and high efficiency
§  Works with all charger amperage and voltage inputs
§  All 1S Single Balancers come with the interconnecting and battery wiring
§  0.5 m/1.5 ft. maximum suggested wire length for all the multiple series balancers. (35m 4S, 5S, 6S, 8S)
§  Idle Current Draw is <50mW
§  Series modules
·Modules may work with 3, 4, 5, 6, or 8 batteries in series (AKA 3S, 4S, 5S, 6S, or 8S)
·Does not come with the wiring to connect to the batteries
Example:
A 144v pack of 48 cells,
20 in the front and 28 in the back
can use 8S+8S+4S in front
and 8S+8S+8S+4S in the back.

Sizes (Length * Width * Height):
§  1S single cell:
o   3.1 * 1.5 * 0.8 inch - 80 * 38 * 21 mm
§  3S Module:
o   4.7 * 2.7 * 1 inch - 120 * 70 * 25 mm
§  4S Module:
o   5.9 * 2.7 * 1 inch - 150 * 70 * 25 mm
§  5S Module:
o   7.5 * 2.7 * 1 inch - 190 * 70 * 25 mm
§  6S Module:
o   8.7 * 2.7 * 1 inch - 220 * 70 * 25 mm
§  8S Module:
o   11 * 2.7 * 1 inch - 280 * 70 * 25 mm

 Here are several ways to make sure they are installed correctly:
1.   Measure the voltage of the batteries with a multi-meter to see if all the batteries voltage is normal.
2.   Listen. If the balancers are connected correctly, they will produce a high-frequency sound. Because the sound is quiet, you need to get close to the balancers. The more balanced the batteries are, the smaller the balance current it will have, and the softer sound they will make and vice versa. If they are connected the wrong way, you won't be able to hear the sound.