There are a couple of ways to get around this, to see if your battery will recover and take some charge. It still may have to be desulfated later. First, a non-intelligent charger, like you had in your garage 25 years ago, may be used with supervision, to cram some current into the battery, which will raise battery voltage. This is another case for having a decent digital voltmeter around the shop/house. You can keep checking the battery until it has enough voltage to register on the 'smart' charger, then let the smart charger take over. If you have golf cart type 6 volt batteries, you can series a pair (positive to negative) to look like a 12 volt battery, and use a 12 volt charger as above. Second method, if you don't have a 'dumb' charger: parallel a second battery (positive to positive, negative to negative) that you know to be recognized by the charger, with the bad battery, and attach the charger. After the charger has a while to put some current back, disconnect the good battery. It may take a time or two to get the charger to stay running. What you don't want to do is leave both batteries connected for a long time, as the good battery will end up overcharged. So, both methods require supervision.