Battery weight

Some things to consider...at 40AH's you will only have 20 that you can actually use. Otherwise you will ruin the battery in no time. Also if you have no means of recharging you will also ruin the battery quickly. You can't discharge to 50% and then leave it that way for long. It will just carry less and less charge till it's useless. The lithiums look pricey, but you can use all the AH's without damage so a 20AH lithium is equivalent to a 40AH lead. And I'm not sure but I think lithiums survive being left discharged far better. A 20 AH lithium will cost you close to $300 vs $50 for yours but it will last you many years while yours will be lucky to survive a trip or two. And a 20AH lithium weighs less than 10 pounds I think. I have a minn kota endura 30 and, according to minn kota, at medium speed you will get 2.5 hours from a 40AH battery. They also state "Failure to recharge lead-acid batteries within 12-24 hours is the leading cause of battery failure." I don't know how long your trips are but if you cannot recharge within that time frame I would not use an expensive lead-acid battery. You are likely to ruin whatever you use so that $50 Exide is probably a good choice. I guess your only option is to find room for a solar panel.