Lithium Batteries

Last Reviewed: Mon, 27/Mar/2017

A battery engineer must consider a number of variables in lithium battery design. He has to know the capabilities of the cells that make up the battery and the load it is powering.
Current Delivery - If the battery cannot deliver the required current at the load's requirement, it will wear out quickly. Capacity - The battery must store enough energy to last over a certain period of time or miles.
Size & Weight - The battery must be small and light enough for the device or vehicle it is powering.
Cost - A great battery isn't a great battery if it's too expensive for most people. A cheap battery isn't a good deal if it doesn't supply enough current for your application.

Given a requirement of 50 amps (power) and 15 amp hours (capacity/range), we could choose 15 amp hour cells, but they may not deliver 50 amps. If one 15 amp hour cell can deliver 10 amps, it would take 5 cells in parallel (5 x 10A = 50A) to meet the current requirement, but the resulting capacity (5 x 15AH = 75AH) would be far in excess of what we need, making the battery too large and expensive. If we choose a 1.5 amp hour cell that can deliver 5A, we would need 10 cells in parallel to make 15AH, and 10 cells would deliver 50 amps. This would make the battery the optimal size, but the downside is that it would also be run at 100% of current delivery capability, so it would have shorter cycle life than a battery run at less than peak. There are always trade-offs, and the best balance can only be achieved by luck or by considering all the variables.

ElectricRider considers all these things and more when engineering a battery. We make safe batteries that store gobs of energy and can deliver that power in excess of the load requirement. We do not use cell chemistries that explode, catch fire, or shoot sparks if punctured or over-stressed, and we do not use chemistries with short cycle life. Our chargers, BMSs, and automatic reset circuit breakers offer multiple layers of protection to you and the battery.

Lithium Iron Phosphate (LiFePO4) Characteristics (High Level)

  • Most popular e-bike and general purpose battery due to lower cost
  • Very stable, but requires a BMS to manage balancing
  • Cylindrical cells can produce high current
  • Good storage density
  • High recharge cycle life

Lithium Manganese Dioxide Characteristics (High Level)

  • State of the art in rechargeable lithium batteries.
  • Used for power tools and high current loads
  • High storage density for the amount of current they can deliver
  • Battery packs can be small because of the high current output
  • Extremely stable

Recharge Time
The amount of time it takes to recharge a battery pack can be roughly calculated by dividing the capacity of the pack in amp hours by the current supplied by the charger. A 12AH pack being charged by a 3 amp charger would take 12 / 3 = 4 hours to recharge. This assumes a "dead" pack. Recharge time will be shorter if the pack is not exhausted.

Power Tool Batteries
Power tool batteries use high current cells and it takes several in parallel to store enough energy to run Phoenix for useful distances. Depending on the quality of the cells, you can make a very good, long-lasting battery from these cells. Most power tool batteries require a BMS.

Low Voltage Cutoff (LVC)
When most types of batteries are at rest, their voltage will climb to nearly their fully charged level. However, when a load is applied, the voltage will drop substantially if the battery is discharged. Our controllers and BMSs have a circuit called "low voltage cutoff" (LVC). When your batteries are discharged to their limit, the controller will limit the current draw to the battery's capability to maintain output above its LVC. This gives a "limp-in" mode that gives you a low speed way to get where you're going. If you do not use the controller's LVC, a battery that uses a BMS will cut off immediately with no further output. All lithium batteries MUST be shut off when any cell reaches its LVC or permanent damage will occur.

Battery Management System (BMS)
Most lithium battery chemistries are not stable enough to maintain equilibrium on their own. The use of a BMS is required to keep all the cells in sync electrically. The BMS monitors cell voltage and cuts off the battery output before damage occurs. The BMS limits current output to a specified number of amps. The battery should be able to supply more current than the BMS will allow to flow. This will enhance the battery's cycle life.

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