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  How does High Impedance affect rechargeable batteries

How does High Impedance affect rechargeable batteries?
So what exactly is High Impedance and why should you care?

Newer, more advanced battery analyzers like the Maha MH-C9000 test more than just the battery capacity. If a battery is unable to maintain a load the Maha MH-C9000 returns a result "HIGH" which stands for High Impedance.

So what exactly is High Impedance and why should you care?

High Impedance is another term for "high internal resistance". Essentially what this means is with age or poor maintanance a rechargeable battery develops a high internal resistance causing the battery to collapse with heavy current demands. So while the battery may still show a 'full charge' on a volt meter and on some analyzers may also show an acceptable capacity level, when put to the test under load the battery will be unable to sustain the current draw the application is asking for and you will get the dreaded low battery indicator.

Are all rechargeable chemistries affected by high impedance?

The short answer is yes, but not in the same way.
The NiCad chemistry has the lowest starting internal resistance of the common rechargeable chemistries, and even after hundreds of uses it maintains a fairly low internal resistance.

NiMH batteries start with a higher internal resistance and develop higher resistance much quicker than NiCad batteries. This is why most NiMH batteries have fewer charges available than a NiCad battery.

Li-Ion batteries are somewhere in between NiCad and NiMH. Usage does not significantly contribute to high resistance. However, due to the chemical construction of Li-Ion cells, cell oxidation causes irreversible high resistance with age. Regardless of usage the typical life-span of most Li-Ion cells is 2 or 3 years.

Can high impedance be reversed?

Once high impedance reaches a critical level in a rechargeable cell it is nearly impossible to recover the cell to a highly useful state. However, with proper reconditioning NiCad batteries and in some cases Lead Acid batteries may be able to recover to a usable state again at least for a short time. Reconditioning of NiCad batteries can be done with chargers designed to provide a reconditioning cycle. Lead acid batteries may be improved by cycling or applying an equalizing or topping off charge. While this will not eliminate the grid corrosion that has already occured it may reduce the sulfication which inhibits the current of the cells.

For more information on high impedance see chapter 6 of "Batteries in a Portable World" by the President of Cadex Electronics.

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