Hello all It just occurred to me that I have omitted to thank John Parncutt for his research and his willingness to share the findings with us. Of course, in this context Mike Borgelt's professional advice must also be mentioned. Both contributions are extremely useful to many of my gliding friends including myself. Many thanks to both of you!
Believe it or not, the last set of SLA batteries powering the engine circuit in my ASH 25 lasted for 10 years. At the time I opted for the most expensive SLA batteries I could get my hands on and now it appears that the old saying holds indeed true. You only get what you ........ Kind regards to all. Bernard Eckey -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Mike Borgelt Sent: Wednesday, 27 February 2013 11:22 AM To: Discussion of issues relating to Soaring in Australia. Subject: Re: [Aus-soaring] LiFePo4 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 _______________________________________________ Aus-soaring mailing list [email protected] To check or change subscription details, visit: http://lists.internode.on.net/mailman/listinfo/aus-soaring
