The popular four-button-chargers support charging of different battery types. This article examines the different charging strategies applied for NiMH and LiPo batteries.
6-cell NiMH battery
Using the Eagle Tree Systems eLogger we measured the charging behaviour of a fully discharged Ace RC branded 6-cell 4500 mAh NiMH pack.
While superseded by better performing LiPo packs, we still run NiMH in our scale and trailing trucks, and in Tamiya vehicles where the battery compartment is unable to fit rectangular LiPo packs.
NiMH are charged at a constant current of 1C, where C is a measure for the rated capacity, which is in this case 4.5A. Every 80 second or so the charger briefly interrupts charging, presumably to perform some sort of check.
When NiMH batteries are fully charged two things happen:
- The voltage actually drops
- The battery temperature rises sharply
The typical 4-button chargers use the voltage drop as detection when the battery is full.
Notice that the voltage drops even further after charging has finished. At the same time the temperature of the battery remains high for a very long time.
2S LiPo battery
Lithium Polymer batteries are the battery type of choice for today's RC models. They store a lot of energy for their size and weight, allow for very high discharge currents and have a very flat voltage drop during discharge.
This diagram shows charging of a 4600 mAh 2S LiPo from its storage charge to fully charged.
A short side-note on storage charge: LiPo batteries should be kept in a storage charge (3.8 V per cell, approx 40% of their nominal charge) when not in use to prolong their life. For a detailed explanation on this please refer to the article How to Prolong Lithium-based Batteries on the Battery University web site.
At first the LiPo battery is charged at 1C constant current. After the battery voltage reaches 4.2V per cell the charger switches into a constant voltage charging mode. The charge current drops continuously until it reaches approximately 0.1C, where the battery is deemed being full.
In contrast to the NiMH battery, the voltage on the LiPo battery does not drop after the charge current terminates. LiPo batteries also do not generate heat during charging.
Conclusion
Different battery types require different charge methods. Using the wrong charge method can damage your precious batteries and can be very dangerous.
All articles in this series:
Part 1 - Tenth scale 4WD Buggy power consumption
Part 2 - Battery charging
Part 3 - Tamiya CC01 power consumption
Part 4 - Full metal Scale Land Rover
Part 5 - Drift car power consumption
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