Yes, 6 volt batteries should be of be charged at 12 volts as a 12 volt charger is probably putting out 14 volts plus. Short term the battery will probably be fine, but definitely not good long term.
You really ought to get a 6 volt charger to charge a 6 volt battery long term.
John
Haflinger 703AP LWB 1973 - (Once owned by Lady Sutherland & Sons.) Now called "Lurch" !
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Battery will still work, but it's working lifetime will be reduced. It will not hold charge as long as it used to and probably won't be able to supply as many amps as it used to. The problem is that it is likely that the lead plates inside will have gotten hot and buckled. Bucked plates can touch each other and thereby reduce the batteries capabilities.
You might be lucky and it will work sufficiently well for the use you have for it for a long time, but I would be preparing to have a replacement ready for when it no longer works the way you want it to!
John
Haflinger 703AP LWB 1973 - (Once owned by Lady Sutherland & Sons.) Now called "Lurch" !
Have you hit the "DONATE" button at the bottom of the page after reading this post? Many thanks if you have!!