Allen, does your 1512 have the starter and GP upgrade yet? It makes a big difference when it is done. I've had good luck with Deka batteries and as the others have said, get the largest CCA you can. I keep a battery maintainer on mine too. Here is one I like from Hazard Freight and I've seen them go on sale for about $25.
https://www.harborfreight.com/4-amp-...ner-63350.html
Your shed probably stays cold this time of year so as Sam has suggested a block heater of some type would be a big +.