Lithium vs NiMH Battery Packs

Do I really need the expense and the product risk of a lithium chemistry custom battery pack?

Custom rechargeable battery packs using lithium cells can provide both high voltage and excellent capacity, resulting in an extraordinary high-energy density. Many applications, such as cell phones, medical devices, electric vehicles, and others require such a high-energy density that only lithium chemistry will do. However, for many of the applications that we see, custom Nickel Metal Hydride (NiMH) battery packs are more cost effective to design and manufacture and do not come with all the potential hazards associated with lithium products.

NiMH batteries have been around since the early 1970s. The technology has been driven by its high-energy density (vs NiCd and Lead Acid), being environmentally friendly, good cycle life performance, and with having a good safety and reliability record. And while NiMH battery packs don't need the complex BMS (battery management system) essential with lithium batteries, we do design and manufacture BMS systems for NiMH packs that help the battery pack last long and communicate with the customer’s device in order to provide the same information as a complex lithium pack.


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What Are the Best Applications for NiMH Battery Packs?

Most applications where there is a high-energy consumption and demand, is where NiMH battery packs excel. They are also suitable in applications as backup power source, where the NiMH batteries have a BMS in order to control the charging and discharging to improve the lifetime of the battery pack.


Don’t NiMH Batteries Self-Discharge and Are Prone to the Memory Effect?

Yes, Nickel Metal Hydride (NiMH) batteries will self-discharge 1% per day if used in a low-energy consummation or stand-by device and the chemistry has been known to succumb to the memory effect. However, using very cost effective BMS technology from manufacturers such as Texas Instruments, Analog Devices, and On Semiconductor, we can develop a system that trickle charges (charge rate of less than 0.025 C (C/40) ) the battery in order to ensure that the pack is always at its maximum capacity and reduce the negative effects of overcharging.


Will NiMH Batteries Last as Long as a Lithium Battery?

If a NiMH battery is repeatedly overcharged, small crystals form on the electrodes, inhibiting their ability to fully charge, and as a result, the battery’s voltage drops. Undercharging can cause low service where overcharging can cause loss of cycle life. The way to address this is using our cost effective BMS that manages the charging process so that there will 100% never be a chance that the battery can be overcharged and damaged.

Most low-end chargers use rudimentary timers rather than actively monitoring the voltage to determine when to cut off the charge current. Typically, a moderate rate (2- to 3-hour) smart charger is preferred for NiMH batteries. Extremely fast charging (less than 1 hour) can impact battery cycle life and should be limited to an as needed basis.


Does an Enclosure for NiMH Custom Battery Pack Need Venting Similar to Lithium Chemistry?

The primary gases emitted from the nickel-metal hydride battery when subjected to excessive overcharge or over-discharge is hydrogen and oxygen. Battery enclosure should not be airtight and should be strategically vented. Isolation of the battery from heat-generating components and ventilation around the battery will also reduce thermal stress on the battery and ease design of appropriate charging systems.


Do I Really Need a BMS for NiMH Battery Pack?

No, you do not. However, NiMH battery packs are amongst the hardest batteries to charge accurately and because the BMS solutions are very cost effective we recommend having one in your design. The charging for NiMH batteries is based on forcing current through the battery whereas with lithium ion and lead acid batteries you can control overcharge by just setting a maximum charge voltage. Therefore, having an electronic safety in place will increase the overall life of the battery.


What is the Difference in NiMH vs Lithium in Overall Cost and Battery Pack Size?

This is where the two big differences are when it comes to these two chemistries:

  1. Cost: NiMH is less than 50% of a lithium battery in terms of the final battery pack production price, and it is less than 75% of a lithium battery in terms of the development of the product. While NiMH has some regulatory and other development steps, it is still significantly less than lithium.
  2. Size: Lithium cells are lighter and smaller than Nickel Metal Hydride (NiMH)cells, however lithium cells only have an average of 1500mAh compared to the 2200 mAH average of NiMH cells.

Key Design Features

  Ni-MH Li-ion
Cell Voltage (V) 1.2 3.6
Specific Energy (Wh/kg) 1 – 80 3 – 100
Specific Power (W/kg) <200 100 – 1000
Energy Density (kWh/m3) 70 – 100 80 – 200
Power Density (MW/m3) 1.5 – 4 0.4 – 2
Efficiency (%) 81 99
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