by Mario Bartolo, Ph.D.
Technical Consultant (Brain Products)
The actiPOWER is the rechargeable battery specially developed for the actiCHamp amplifier. It is the result of more than 10 years’ experience with the well-known PowerPack for the BrainAmp series. In order to maintain a reliable and enduring power source for the actiCHamp it is important to follow some easy maintenance considerations. In this article, I will present some tips on how to extend the lifetime of an actiPOWER battery and how to keep it at a maximum performance level for as long as possible.
The actiPOWER is a lead rechargeable battery with a typical discharge pattern when used with the actiCHamp. The advantages of using a battery to feed the actiCHamp are obvious: a line-operated power supply always needs costly engineering efforts to meet safety requirements without ever providing zero residual ripples as a galvanic power source can do. This is particularly important in shielded environments like a Faraday cage where mains-operated devices are out of the question. However, in order to ensure a reliable and enduring power source for the actiCHamp it is necessary to follow some basic maintenance procedures. But before presenting tips on how to extend the lifetime of an actiPOWER battery, I will firstly introduce its discharge pattern.
Figure 1 shows the typical discharging curve of the actiPOWER when the actiCHamp runs at 100 kHz sampling frequency.
Here, the battery voltage reaches the switch-off level (represented by the red line) after approximately 6 hours. Note that Figure 1 represents a special case in the EEG field i.e. recording with 100 kHz sampling frequency. When using typical sampling frequencies, the actiPOWER would supply the actiCHamp for a considerably longer time. For example, when using a 10 kHz sampling rate with 32 EEG channels, you can perform non-stop EEG recordings for more than 24 hours!
It is possible to keep up this performance level even after several hundred charge/discharge cycles, or in other words to acquire several hundreds of hours of EEG signals with the same battery.
Figure 2 shows the conditions under which you are able to record for several thousands of hours.
Here, it is shown how the capacity level of an actiPOWER battery depends on the number of cycles for different discharge depths. As a first example, consider the worst case scenario when the battery is deep discharged i.e. the EEG recordings are automatically terminated due to the lack of power. In this scenario, the battery capacity drops down to 60% after approximately 220 cycles. On the other hand, if the battery is always only 30% discharged during the EEG recordings, then the capacity only drops down to 60% after approximately 1100 cycles. The conclusion is that the number of cycles increases in a non-linear manner with the decrease of discharge depth, which means that it is better to discharge the battery twice down to 50% than once down to 100%.
But, what to do in between EEG recordings? The most important tip is to charge the actiPOWER immediately after use and always to store it fully charged. It is also important to note that one should not store an empty battery for a long period, because the battery will continue to self-discharge and would thus likely fall into a deep discharge state which is in fact the end of life of the battery. To simplify the whole charging and storage process, with the current FRIWO battery charger you can leave the actiPOWER always connected to the mains in between usages, as the battery cannot be over-charged. If you do not have the latest battery charger (FRIWO) please contact your local distributor for a free replacement.
Finally, I would like to use this opportunity to highlight that it is possible to monitor the current state of the battery and assess its performance. In BrainVision PyCorder 1.0.6 and later, you can save a log-file with the parameters of the actiPOWER (i.e. charge levels) and amplifier which is updated every minute.
You can then use this information to check the “health” of your battery. For more information about how to activate the logging tool, visit our actiCHamp/PyCorder Forum.
To sum-up, you can extend the lifetime of an actiPOWER battery if you:
- Always charge the battery immediately after use
- Do not wait to re-charge the battery until it is fully discharged
- Leave the battery connected to the mains
- Store the battery fully charged if a permanent connection to the mains is not an option
- Fully charge the battery at least once every 2 months if storage is needed for a long period of time (months)
Follow these simple guidelines and you can keep your actiPOWER in tip top condition for a long time, which translates into a hassle free and reliable operation for many EEG recordings.
©Brain Products GmbH 2014