There is no battery-operated heater you can realistically use. They draw WAY WAY too much current for a battery to be useful.
There are propane (etc) type heaters, but there is a significant chance of burning down the barn, plus they are not cheap to buy, and I really would not suggest using them in your situation.
Be very, very careful with this business of puttting up plastic. If there was significant air movement going on in your coop (draftiness) then you do need to fix that of course, and it may be partly the cause of your frostbite; BUT another very common contributor to frostbite is HUMIDITY. And the more you plastic-wrap a coop, the more the humidity builds up.
How damp is your coop? Is the floor earthen with rising dampness, has snow/rain come in anywhere, are there leaks (waterer spills, roof drips) anywhere, how deep and how old is the bedding? Do you use a droppigns board, and if so, how often is it cleaned?
I would suggest that if you are *forced* to keep them in this coop for now, your best bet would be to vaseline their combs (be very gentle with the frostbit portions), do a total litter change-out to new dry pine shavings, fix/prevent any and all sources of incoming dampness, and install a droppings board under the roost that you sprinkle with shavings and then clean every morning without fail (removing the poo from the coop entirely). This should get your humidity down somewhat and give you more "margin for error" in terms of cold air temperatures.
I sure wouldn't put 7 wk old chicks in an unheated open drafty barn without electricity in weather getting down into the single digits, personally. If you *have* to, I guess give them a little plywood hover to get into, pile shavings over top of it, and maybe put a jug or two of very hot water in there with them that you replace a couple times a day. But it is not a good situation...
Good luck,
Pat
There are propane (etc) type heaters, but there is a significant chance of burning down the barn, plus they are not cheap to buy, and I really would not suggest using them in your situation.
Be very, very careful with this business of puttting up plastic. If there was significant air movement going on in your coop (draftiness) then you do need to fix that of course, and it may be partly the cause of your frostbite; BUT another very common contributor to frostbite is HUMIDITY. And the more you plastic-wrap a coop, the more the humidity builds up.
How damp is your coop? Is the floor earthen with rising dampness, has snow/rain come in anywhere, are there leaks (waterer spills, roof drips) anywhere, how deep and how old is the bedding? Do you use a droppigns board, and if so, how often is it cleaned?
I would suggest that if you are *forced* to keep them in this coop for now, your best bet would be to vaseline their combs (be very gentle with the frostbit portions), do a total litter change-out to new dry pine shavings, fix/prevent any and all sources of incoming dampness, and install a droppings board under the roost that you sprinkle with shavings and then clean every morning without fail (removing the poo from the coop entirely). This should get your humidity down somewhat and give you more "margin for error" in terms of cold air temperatures.
I sure wouldn't put 7 wk old chicks in an unheated open drafty barn without electricity in weather getting down into the single digits, personally. If you *have* to, I guess give them a little plywood hover to get into, pile shavings over top of it, and maybe put a jug or two of very hot water in there with them that you replace a couple times a day. But it is not a good situation...
Good luck,
Pat