As others observed charging at a low rate is usually quite safe and actually can be a good idea to help lessen the strain on the battery in cold weather. Do check the electrolyte level once in a while though because it can cause a slightly higher rate of evaporation.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold