Schumacher is the brand that comes to mind. They have been around for ages. I've owned a couple of them and they have been working perfectly for the past 15 years or so. If you just need a slow charge, an inexpensive 1.5 amp charger will do the trick.

Compare the cost of the device with the cost of a new battery with a 3-5 year warranty. You may also want to call a local auto repair shop for a price. It's likely less expensive than a charger and you may find that the battery has outlived it's useful life and wil need to buy one anyway.

I have had the same old Schumacher for nearly 30 years, and it still gets the job done. I have another Schumacher that is about 3 years old that I use to keep the battery up on my Lotus, since I do not drive it much, and it also does a great job. That one cost under $20, but don't look for it to charge a battery that is serious down in an hour or two. One other comment on Schumacher: you will find most repair shops also use their products.