Hi I've used for this Centech automatic battery float charger Automatic Battery Charger - 12V
for last two years especialy during the winter storage. I purchased this at harbor freight, some times on sale for $6.99 + shipping. This brand is Centech automatic battery float charger . I have 4- 6 volt trojan t109 batteries. Two banks in series and then hooked up together for 12volt output . Like I said this unit is always on them when its in storage. I do have a 12v positive battery disconect to prevent power dranage from the batteries. This unit works to keep the batteries topped off. and not boiled out! You should still watch the water levels! I chec them with a battery float every couple of months.