1 pointSome suggestions to "When to replace a Battery ?": a) Start the dive trip with new or fully charged batteries, as Pavel suggests. Use the best batteries available, they pay for themselves. b) Use a digital voltmeter to measure the battery's voltage: If it is significantly below the battery's nominal voltage, replace the battery. One learns quite fast when to replace. Digital VOM's are quite small, light weight, inexpensive and readily available on eBay. Carry one on trips away from home, they can be given as a nice gift at the end of the trip. c) It is a good idea to carry spare batteries for the lamps, strobes, camera, vacuum leak detector... Don't forget the chargers. If you do not need them, a fellow diver will - great way to make new friends!
1 pointDifficult to say for sure, because batteries on the market have different quality. Some of them keep high pulse power (for LEDs) good enough till the end of battery life, some batteries are rather bad in that. In common case, better change them significantly before they are go dead. I have a few housings in my own usage. Mainly, i change batteries 1 time per year in them. But i shoot not very often. Anyway, if i go to an important dive trip, i change all batteries in my equipment before the trip, just for insurance.
1 pointPower saving time 10 sec is just a recommendation for maximum battery saving. Because TTL-Converter switches Off when camera goes to standby mode. As i know, many users set 1 minute, it is acceptable time for normal shooting. I agree, we have to write 1 minute in instruction. If you need 2 min or 5 min active camera screen between two shots, no problem to set it. Battery life will be some reduced, but not significantly for real dives. Sometimes you can keep your camera active the whole dive, if it is an important dive, no problem.