I'm not too familiar with the reo grande. Isn't that a box mod with separate batteries? Regardless , get a meter. Even a cheap $25 dollar digital meter from ebay is better than guesswork. I have one and it's surprisingly accurate for ecig uses. I use mine mostly for measuring resistance of atomizers and cartomizers to avoid blowing a LR atty on my 5v passthrough. They are also indespensible to find out the overall condition of your atty or carto and to check the switch on your PV if it becomes temperamental, as mine sometimes does. I have a buttload of batteries and a separate charger, so I don't use it so often for batteries, but for attys and cartos, it has become mandatory.