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
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