Peter,
Go to Walmart and in the automotive section look for a 1.5 amp "On-Board, Automatic Battery Maintainer." They used to carry the Schumacher brand which you can find on
www.batterychargers.com, but you are more likely to find their "house" Everstart brand, which is also OK and sells for about $20.
www.sportsmanguide.com has on close-out the Vector brand of a very similar unit for $14.97!
These units have two LEDs, a red one and a green one. The green LED is on when the unit is charging. The red LED alone being on on the Schmacher unit means that you have reversed polarity, it has a built-in protection circuit. On smaller batteries such as your Yuasa NP2-12 when the battery approaches full charge the green light will stay on, and the red light will "blink," which is cpmpletely normal and OK. It will blink slowly at first, then the red light will stay on longer between short blinks until full charged is reached and both lights stay on, indicating that charging is completed. When the battery is fully charged both the green and red LEDs will stay on. On small batteries I would then disconnect the unit. I recommend connecting them to you equipment, checking voltage drop under full transmitter load and recharging monthly.
Considering the low cost these are great devices. Every amateur should have one for their "Go Kit." They compare the battery voltage against a programmed reset point on the PCB and begin charging when battery voltage drops below 12.6 volts. The charger shuts off automatically either when it gets warm, as it will when working hard to charge a larger discharged battery, and starts up again when it has cooled off, and will continue until it reaches about 13.8V and shuts off. The units have built in reverse polarity, over temperature and over-voltge protection and most are suitable either for gel cell or flooded batteries.
I have these connected to all of my vehicle batteries so that I can keep the battery warm in cold weather and to prevent discharge in recreational vehicles which are only used occasionally. These units will safely recharge a gel cell as small as 2ah if you monitor time and temperature and ensure that the battery doesn't get more than slightly warm to the touch. On a battery larger than 10ah, such as for a garden tractor, motorcycle, boat or RV you can leave the maintainer connected all the time and not worry.