Your FEH battery has already started to self-discharge. This is common as they age. The rate of discharge is highly variable per vehicle but it generally goes up with age and high usage (i.e. mileage). It has nothing to do directly with the 12V battery. Keeping the 12V on a tender will not stop the high voltage battery from discharging as part of it's normal life cycle (note that your vehicle is 10 plus years old so it's done well). The battery refresh may help to extend the life of your main. Or it may not. Its best to grab and laptop and load FORSCAN on it. With an OBD2 device, you can go into the Battery Control Module BCM and Powertrain Control Module (PCM) and look at how healthly your hybrid system is including SOC state of charge. Older FEH's should be driven a few times per week. Once you see on FORSCAN how quickly your battery is discharging when it sits, you can develop a plan that suits your circumstances. The only way to prevent self-discharge when sitting is to disconnect the negative on the (small) 12V battery in the engine compartment. Some owners have apparently put a switch in to facilitate this. That said, you may have a parasitic negative grounding issue somewhere. A Ford Escape Hybrid specialist could help you with this. Fortunately, this is unlikely. The FEH is a great vehicle and super fuel efficient but the newest ones are now 8 years old. if you want to keep your FEH, go on greenhybrid.com and learn all you can about their geriatric years. Your vehicle can possibly serve you well for many more years but you have to make at least a few minor changes in your relationship with it. On a final note, since your MY does not have a recharge button, ensure your 12V is high quality, fits properly and is never over three years old. Keep it charged by driving it regularly or remove (or have a knowledgeable person remove) the negative cable on the underhood (small) 12V battery. Anyone how touches your vehicle should use normal safety procedures and not do anything they have knowledge and experience in. The high voltage system is very dangerous unless you are a specialist so don't monkey with it. But do develop an understanding using FORSCAN from the drivers seat.
