Hi @bscoventry ,
Documenting our conversation about battery life considerations for a chronic experiment in a pig model for public reference.
Overview
Under a supplemental award to the HORNET-COSMIIC grant (U41NS129436-02S4), you are attempting to operate a Power Module + 1xPG4 stimulator system for as long as possible in a setting that does not allow the typical external set up of Smart Charger and Cooling System for an extended amount of time. In response, we set up a script to stimulate at 1 channel of the PG4 at 1mA for 10 seconds ON, and 120 seconds OFF. During the OFF period, the network is turned off and the only power draw is the Power Module.
Battery Life Estimation
There are 3 x 200mAh battery cells in the Power Module. The charging maximum current is 100mA being delivered to all three cells. Thus, we estimate a full battery charge to take 2 hours. We recommend only using 70% of the battery capacity before recharging however. This reduces the charge time but also reduces the time of use.
We typically see the PM1C functioning for up to 22 hours. It has reached 40 hours before in a hardcore low-power mode that would not work in this case. These times do not the specifications you’re looking for when considering the frequency of recharging with animals given the difficult setup for the animal sling and holding an animal for extended period of time.
Charging Setup
Charging works optimally with 1/2’“ to 1" of gap between the PM and coil. The field is very unstable when the coil is very close. There are some ways to improve that, but operating with the coil in direct contact with the PM is not an intended mode of operation given that the PM will always be implanted at some depth.
Charging speed will be reduced significantly if not using active cooling. With cooling and a proper distance and alignment, charging should reach 100mA.
Possible improvements to battery life
The PM consumption is the most significant factor here. The best way to prolong the time between charges is to reduce the time the PM is on at all. If the experimental environment does not all for repeatedly turning the device on and off, there is not much to reduce the PM’s battery consumption. Tweaking stim parameters is not going to do much, maybe tweaking compliance voltage could help. That might help reducing network voltage.
Moving Forward
Your first chronic implant is now going to be used as the control experiment and will not include the continuous stimulation until we can find ways to increase the battery life. You are going to implant the device and interact with the device every few days. In these moments, it will be about 10-15 minutes of operating the device while charging; this coincides with putting a pig in a sling to eat to keep them occupied. You will interrogate the device with the external radio, run recordings, and use a MATLAB script to estimate the impedance at the electrode interface to understand the encapsulation response. Hopefully we can prepare the device to work for the continuous stimulation needs so that you can study the effect of stimulation on the encapsulation.
This discussion greatly helps us understands specifications of a chronic implant device for animal use, specifically pigs.