I've been looking for a product that offers what the OptiMate TM-500 does for over 6 months and I'm delighted to have finally found it, and report that it delivers.Why would you need this?You have a vehicle that sits for extended periods of time resulting in the car battery becoming discharged. For whatever reason you cannot reach an AC power outlet, and as a result a typical smart battery charger cannot be used.We realized that we had this problem with the car my son had with him away at college at New Mexico Tech. First we bought a combo jump start battery / air compressor unit (Stanley), which kept him from being stranded with a low battery. The jump start battery supplies a lot of current all at once to allow your vehicle to start. That's great, but because my son doesn't drive the car a lot, the car battery never got fully charged. Using a jump start battery to directly recharge a car battery fully is likely dangerous compared to a smart charger with an algorithm that monitors the charging process and supports a float / maintenance mode.Next we tried a 10W solar panel with an integrated smart charger. This would have been fine for maintaining a charge, but at just over 400mA of supplied current, delivered only when the sun was shining, was not sufficient to bring the car battery back from a deep discharge over the 3 weeks we tried this.Using Optimate, and the jump start battery as a supply source, solves the problem. We connected the Optimate’s charging port to the car battery and then the input cables to the jump start battery. This delivers a monitored 2A charge and will automatically switch to float mode to prevent damage to the car battery.The unit also claims to bring back damaged batteries that have as low as one volt left, which I have not tested.As a bonus, the cables have what I infer is an SAE spec connector for the input and output connections that allow the use of some of the Deltran Battery Tender adapters such as the OBD port charging adapter.Awesome product!