Hi Mike. Really glad you mentioned the quoted rate, I forgot to mention that, as it is how I got better batteries in my glider - point being of course that it doesn't matter if SLA, NiMH or Li* - you have to check those discharge rates, and temperature curves. We recently sent equipment to Antarctica and learnt a heap about batteries then, but we are using primaries so no recharge to consider.
Also, the point about multiple NiMH is good too. Multiple batteries without equalisers is quite common in SLA and NiMH - which of course is not ideal. In Li* the risk of combustion when charging has encouraged better balanced chargers which is good news. As for life expectancy of battery, I am probably cheap, but my expectation is that the battery should last 5 years. 2 seasons (not 3 btw) for NiMH was not enough - but likely damaged for exactly the reason you said here. The NiMH batteries I put in were about $100 worth (to get the larger 9Ah size), while the new SLA I put in were under $50, and so far going well... Not that those numbers are in any way scientific or anything but anecdotal. I had not really thought of the weight saving behind CofG being a big issue, but you are right, it could impact take off weights for some glider/pilots - my glider has a huge cargo weight, which includes batteries and old oxygen cylinders. My modern O2 system is so much lighter that I have many kg to spare. Thanks Scott On 27/02/2013, at 11:52 AM, Mike Borgelt <[email protected]> wrote: > As I said yesterday, do proper engineering on your battery installation. > If you don't have real numbers for temperature limits, discharge curves at > various rates, charging characteristics etc etc you aren't doing engineering, > you're just guessing. > A battery designed to start a racing motorbike and then be recharged by the > alternator and floated at that voltage likely has quite different > characteristics, design and longevity from one designed for charging and deep > discharging over several hours then recharging. You can also just stop and > get off the bike when the battery catches fire. > Li batteries all need individual cell monitoring during charge and use or at > least when charging after mostly charged. > The problem with Ni MH batteries is the number of cells (10 for a nominal 12 > V system). You will have at least one weaker cell which will deep discharge > more than the others and will be undercharged or more likely the other cells > will overcharge resulting in reduced battery life. Individual cell monitoring > would help but with 10 cells vs 4 for Li it is a pain. > 3 years isn't bad for a NiMH battery pack. > Some people have a problem with max weight of non lifting parts and a few > kilos saved may make the difference between flying in or outside the weight > and balance envelope. For these LiFEPO4 may be worthwhile but use the correct > cells. > The cylindrical Tenergy cells sold by these people have engineering data and > are Underwriter Labs tested. The tests are published there too. Start > here:http://www.all-battery.com/lifepo4battery.aspx > I've dealt with them and they did what they said they would. > Also these people may be of interest: The batteries seem to be the same as > the Tenergy cells but with a different colour outer sleeve. They have battery > monitoring/cutoff circuitry available also. You must use a low voltage cutoff > at least. > http://lithbattoz.com.au > The old sealed lead acid batteries are OK. They are heavier although in many > installations that may not matter. The capacity is usually quoted at the 20 > hour rate. In modern gliders 1 amp continuous is not an unusual load so > that's the 7 hour rate. Likely it is a 5 A-H battery at this rate. Give it a > couple of dozen charge cycles and it is a 3 or 4 A-H battery and you begin to > have problems. Note also the number of cycles you get is non linear with > depth of discharge. Small % discharge you'll get lots of cycles. Large % > discharge many, many fewer cycles. > Size the battery to handle the longest flights and then use two batteries. > Use one routinely, keep the other charged then when the first battery dies > due to low capacity you have a known good battery. Put that one in the first > position then put a new one in the standby position. You should always have a > good battery available then. > If using some new type either learn enough to do an engineering analysis or > find someone willing to do one for you. Otherwise these things may get > needlessly banned or we have fires in gliders. It would be embarrassing to > explain to your insurance compny why you had to bail out of your burning > glider. > > Mike > > > _______________________________________________ > Aus-soaring mailing list > [email protected] > To check or change subscription details, visit: > http://lists.internode.on.net/mailman/listinfo/aus-soaring -- Scott Penrose [email protected] http://scott.dd.com.au/
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