I think that Roman is right, you should consider using relays for
direction change and low Rdson FETs for the ON/OFF function. This way the
relays will never spark and last a very long time. Automotive DPDT relays
with >20A contact ratings are available for little money. A 50A VFET with
0.045 ohms Rdson is also inexpensive and will not require a serious
heatsink at 5A, so you can use insulated ones (f.ex. BUK445-50 which is
what I have on the table now ;-). Since they are cheap you can use the
same kind to drive the relays too and use the same level shifter for the 5
of them (you need to shift 5V logic level to 12V gate voltage using simple
trasistor buffers for example).

You can even brake by switching the relay with the FET off. This will do
regenerative braking by feeding current back into the battery through the
reverse bulk diode of the FET. Now you can charge the battery by pushing
the mower in front of you ;-).

Peter

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