I've been using a (small) Schumacher automatic charger (1.5 amp) for awhile with no problem. It is keeping the battery @ 12.30. Received as a gift, couple of years ago. Model XM1-5-CA
I DO NOT recommend a Yuasa 900 SmartShot which I've used on motorcycles because when tried on the Crossfire, it did not want to switch over to "storage" for some reason & could overcharge.
Yuasa output: 12Vdc; 900mAdc; perhaps too low, causing the malfunction.