given that the battery will take charge from anything that can push a higher voltage than whats in the battery.
I suggest starting with an N70 type battery or one thats the same as the one thats in your vehicle.
head to somewhere like super cheap and pick up somethign thats between N55 (just a mite smaller than the orignal D1 battery) and an N70 (the original series/defender battery)
hell head to a wreckers and try your luck their too if you want to do it on the cheap.
Just make sure that the first thing you do when you turn up and the last thing you do before you leave is to make sure that the cells are topped up and that the battery isnt copping direct sunlight or sitting in the damp and its top is clean.
