I emailed S&B about the Mighty battery the other day and got this reply, hope this helps others.
Q:Hello,
I just got a Mighty and I have a few questions about using it while charging. Is it ok to use the mighty while it is plugged in and charging? When it is plugged in and charging and I turn it on does the unit use power from the battery or the wall outlet? Is it still charging the battery if I use the mighty while plugged in? Will it damage battery life to use the mighty plugged in charging with a full battery? Thank you if you can answer these questions, the bottom line is that I want to know what is the best option for the long term life of the batteries and unit: Drain batteries and recharge several times a day or mostly use it plugged in. Thank you again for any assistance you can provide.
A: thank you for your message.
When plugged in, the MIGHTY uses the power from the wall outlet without using the batteries.
We recommend to use the batteries from time to time. In order to recalibrate them, you should let them discharge completely and then fully charge them again.
We hope this reply is of assistance to you. If you have any further questions or concerns, please do not hesitate to contact us again.
Kind regards.
This contradicts what the guy in Oakland told me on the phone but he told me that he was not an engineer and I could try emailing S&B directly. Sounds like us heavy users can really stretch the long term life of our Mighty Batteries this way