My battery pack for the onboard glow (4-cell, sub-C) is mounted directly behind the engine and in front of the firewall. My Rx pack (5-cell, sub-C) is directly behind the firewall. Fortunately the big Enya v-twin is heavy enough that I don't have any lead hanging anywhere on the plane. When I first had it powered with the Saito 1.70 radial it needed 26 ounces of lead mounted to the firewall. I'm now a firm believer in only adding weight that contributes to turning the prop