I think it would be a good idea if the participants of this site test some of the 12 volt chargers that are available on the market and enter the results here so we can decide which charger would be best for our uses.
I think we agree that series charging batteries can damage the batteries if one or some of the batteries are not the same resistance as the others. The only way this charging method works is if the batteries are a perfect match. Small differences can be compensated for with battery balancers. The other method is to use separate 12 volt battery chargers and charge each battery individually. I have purchased 5 different 12 volt chargers and I will be testing them and entering the result here. To test the chargers I will use a Kill-A-Watt meter and volt meter. In some cases I will also subject the chargers to different temperatures to see if they have any voltage compensation for temperature changes.