If you have issues with your battery tender there might be a quick and innexpensive alternative to a new charger. I use mine regularly and recently began having issues with the unit bringing the batteries to full charge (green light at the end of the charge scale). It would flash on the lowest indication or get stuck halfway through the charge and never finish. Turns out the problem was caused by the MagCode connector, PN M2K211-1AE and available from DigiKey at digikey.com/en/products/detail/rosenberger/M2K211-1AE/19477488?s=N4IgTCBcDaILJgNJgIwoLQoIIFEQF0BfIA. Both male pins are spring loaded to ensure solid connection on the boot, but the + side failed on mine. All you need is a phillips P1 screwdriver to take out three screws. Both connector and wires (at least my wires) are labeled +/- so it is pretty much idiot proof.

Next up: Hood gas strut replacement.

 

BTW, it's important to clean the connectors - particularly the one on the car since they can get covered in road grime over time and just a drop of WD-40 will keep the springs working.