Battery acid does NOT evaporate. If the fluid level is low, it's because the water that maintains the acidity at the proper PH level has evaporated. Refill with water. Good quality tap water is OK, bottled water is also OK, but distilled water is best.
I've topped off my car batteries with tap water for donkey's years and have never had a battery failure due to contamination, which may be a tribute to the quality of water around here more than anything else.
The very few failures I've had were back in the 1950s, when battery technology was pretty primitive. This is one area of tecnology which has benefitied greatly from the NASA space program. I was amazed, when I bought my Honda electric start lawnmower 12 years ago, that the starter batteries for the 215cc engine were essentially 6 rechargeable D-cells. They're still going strong sfter 11 seasons and the engine-driven generator keeps them charged. I've only had to plug the thing in once and I've never had to use the recoil starter
I guess you take your choice based on whether you want to pay the money.
Frank Damp -Anacortes, WA,(DW- Eileen)
ex-pat Brits (1968) and now ex-RVers, as of 08 Dec 14.