Let’s have a glance at the positive attributes of NiMH battery first:
· Accelerated charging is possible
· Wider operational temperature ranges (-20C to +60C)
· Physically robust and often able to withstand physical fracture without harmful spill effects.
· Non-toxic when compared to other chemistries
· Raw building materials that are relatively abundant and can be recycled
· Low internal resistance
· Deep cycling is possible under some scenarios and conditions
· Relatively high energy density when compared to other chemistries
· Relatively flat discharge curve (which often demands greater accuracy in measuring the SoC since the changes are very minute).
· Steady lowering of costs as production volumes increase and manufacturing quality ramps up.
· Relatively mature and predictable manufacturing processes which help minimize lower yields and quality rejections.
NiMH battery Negative attributes:
· While refurbishing a battery pack is possible, the results may vary often making such attempts less cost effective and even discouraged.
· Small charge rates (trickle rates) is not usually supported since slow overcharging can occur and lead to premature failure of the battery (typically through loss of capacity). That is why timer circuits are usually built into the charger circuitry to ensure limited exposure to overcharge.
· Self discharge rate is very high often dwarfing all other competing chemistries. That is why hybrid cars should be powered up at least once every 30-60 days to prevent a full and crippling charge depletion. The SoC indicators ARE NOT a reliable indicator of health when a vehicle is in storage.
· Can be conditionally stored in a fully charged or fully discharged state.
· Like NiCad, it too has a memory effect requiring battery management routines to contain it.
· Battery venting for the release of gases must be designed into the package.
· Requires cell reconditioning if SoC is kept at a steady level for longer periods of time.
· With the allowed packaging constraints, battery packs must be made from many individual cells – each at 1.2V – in order to produce the required voltage levels. This compounds the probability of failure since each cell is now a potential point of failure in the entire chain of cells.
What can be done to ensure a longer NiMH battery pack life?
· Cause a deeper discharge periodically to nullify the inherent memory effect. This is automatically done by the governance and battery management systems.
· Ensure that the amplitude of your regenerations and electric assists are as small as possible.
· Use some electric assist once in a while. Please remember a static SoC is just as bad as excessive use.
Keep your battery pack cool. While there are many things we can do some do not involve hardware modifications at all.