I'm trying to make a high power UPS, I have a 13.6V PSU and an Inverter and a Lead acid battery. 

Everything would be connected this way: 

![enter image description here][1]

There's a problem with that circuit, if the battery is too discharged the initial charge current will be too high for it, so I need to limit the current while charging and not while discharging. 

The simplest solution would be this:

![enter image description here][2]

But that solution isn't very efficient. So now the next thing that comes to my mind is using a mosfet as diode, like the reverse polarity protection circuits. I only have N channel fets, so the end result should be this:

![enter image description here][3]

However according to LTspice that circuit doesn't work. As long as it is the default Nmos it does work, but if I try to replace it with with any Nmos in ltspice's catalog (Mosfets with a VGS above 20V), the MOSFET still conducts when it shouldn't. Why?

By the way, the MOSFET I want to use is an IRF3205. 


  [1]: https://i.sstatic.net/mmvB3.png
  [2]: https://i.sstatic.net/OAZRh.png
  [3]: https://i.sstatic.net/nO8Jc.png