Depending on the battery tender/trickle charger, the good ones won't overcharge, the cheap ones will. I had two HF cheapo trickle -charge units that went in the trash - no overcharge protection and yes, they overcharged (evidenced by fluid use/dry out). HF seems to have better units now than 10 years ago, so can't speak to those.
I currently run a couple Schumacher SEM1562A units that are fully automatic maintainers, and will auto select for 6V or 12V, microprocessor controlled. Tractor Supply routinely discounts to about $25, although that promotional doesn't seem to be in effect at the moment. Battery Tender is the Cadillac of equivalent maintainers, and can be found for around $30.
Hope this helps!
