Maxi Sys 3.7 V Internal batteries should be able to accept charge from any DC power source from 5V -24V (whether or not you use the AC adapter or not, because there is a voltage regulator inside between the charge port and the battery. You can velcro and hook an external  12 volt portable lith battery to the back of tablet and connect to AC/DC adapter port in the mean time. I think Autel is just in the planning stages of shipping batteries from China here to the USA. I do not believe they have any established repair centers to change parts and I wouldn't hold my breath they will become available or develop a universal plan. Like trying to buy replacement drawer guides for a Chinese tool cart from harbor freight. If it is not covered under the 1 year warranty or an amended Total Care update ,repair deductible $ you will have to pay during a subscription , I don't believe Autel will ever make genuine battery replacements available to us to  do it ourselves cheaply.You jury rig it and try to use it as is or replace it with a newer unit, or hope another Chinese company makes a good direct fit  bootleg aftermarket battery you can buy from a warehouse in the USA for less than $50, LMFAO

 So I guess a feature in upcoming Autel devices might be a removable and rechargeable battery? This way busy Auto shops can have one on charge and one in device. Good for Autel too selling extra batteries and chargers $$$ Though Witsend your idea of piggybacking is creative genius!!! They've done something similar with mobile phone cases that are battery extenders, can't see why Autel couldn't develop the same thing for their older devices that require batteries, in clip on convenience while you wait for the internal battery to be "replaceable".