Part 4: How do you protect the battery?
The goal of a charge circuit is to protect both the battery and your device. If you don’t have charge protection, you can damage the cells on a battery or cause a fire. Please use caution whenever charging batteries from solar power (or any other power source), they are not toys.
The activities mentioned here require special test equipment to show how the batteries behave under different conditions. If you don’t have the equipment, you’re more than welcome to follow along or perform simplified tests with a multimeter.
Failure to use a charge control circuit will adversely affect the performance of your battery over time in the best case scenario. In the worst case scenario, cells may explode, catch fire, or otherwise cause serious harm and damage to people and property. By avoiding these hazards, usable battery life can be preserved and extended. The best battery chargers will implement a range of protections and detection methods to rapidly and safely charge your batteries without damaging them.
Typical Protection offered by charge controllers and power management electronics:
Over Charge – Voltage and/or Current regulation is used to prevent the cells from taking on too much power, too rapidly, and overcharging. In simple circuits, linear devices such, as regulators, are used to limit voltage and/or current. In more advanced circuits, a microcontroller is used to monitor and control charge functions. The protection method varies according to cell chemistry: NiMH batteries require a circuit that detects a change in voltage or temperature, where Li-ion cells require a two-stage constant-current/constant-voltage charge method.
Over Discharge – Discharging the battery will cause the cell voltage to drop. The over-discharge protection monitors the cell voltage, turning off the output when the voltage drops below a preset “off” threshold. The output will typically turn back on once the cell voltage rises above a preset “on” threshold. Over Discharge protection can occur prematurely (before the cells are completely discharged) if the power consumed by the load is too high, but not high enough to trigger the short-circuit protection.
Short Circuit Protection – When a load draws too much current from the output of the battery, the output is turned off. Sometimes this feature will require a manual reset of the battery, including disconnecting and reconnecting the load.
Over Temperature – If cells are charged or discharged rapidly, the chemical reactions taking place inside may generate excessive heat. Over temperature protection in the form of either a thermal circuit breaker or a microcontroller reading the changes of a thermal resistor will disable the input and output of the battery until the temperature has returned to normal operating conditions. In the case of NiMH cells, a sharp increase in temperature is usually associated with over charging, in which case the charge rate should be drastically reduced or terminated.
Image may be NSFW.
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Image may be NSFW.
Clik here to view.
Here are images of Li-ion cell protection employed in our V15 USB Battery. On the left is a circuit board containing electronics that protect the cells from overcharging and over discharging. On the right is a thermal breaker. If the cell temp exceeds 80 degrees Celsius, the break opens the circuit, disconnecting the cells from the rest of the power management circuit. When the breaker cools, it automatically resets.
WARNING – The following activities are demonstrations of circuit protection implemented in Voltaic Systems batteries. It is not recommended to try these with other batteries as you may permanently damage them.
Activity 1 – Over Discharge and Short Circuit Protection of the V72
- What you’ll need:
- A programmable electronic load (one that will allow you to adjust the resistance OR the current drawn from the battery).
- A V72, V44, or V15 battery.
- Some way of connecting the output of the battery to the programmable load.
Watch what happens when resistance in the load is dropped simulating a short circuit.
Activity 2 – Testing Overcharge Protection
- What you’ll need:
- A programmable power supply (one that will allow you to adjust the voltage and limit the current).
- A V72, V44, or V15 battery.
- Some way of connecting the output of the battery to the programmable load.
Watch what happens to current flow into battery as the battery becomes full. (Note: Li-Ion batteries typically measure Voltage of the cells to determine when they are reaching full capacity. Other batteries such as NiMh will use different methods such as Delta-V.)