MPPT with Gel Batteries (was Gel Cycle Life) [RE-wrenches]

[Christopher Freitas - OutBack Power]

Travis is correct - Using a MPPT with sealed batteries for grid-tied PV 
systems is a good idea.  It easy for the MPPT to pay for itself by the 
increased performance - even in hot summer conditions. Especially since 
the batteries will be held at a relatively low voltage (float) and their 
temperature will most likely be over 25C,  result in actual battery 
voltages of 13.2 or so... 

As an example:

3 kW PV array at 48 vdc at a cost of PV array $5.00 per watt (retail)

If the MPPT improves the performance of the system even just 5% overall, 
 then that is worth at least $750 for the increased performance (3000W x 
5$/W x 5%). 

The actual amount of performance gain depends upon the difference in the 
PV array's maximum power point and the battery voltage.  Even very hot 
PV modules will have peak power points above 14.5 vdc.  If the battery 
float was 13.2 vdc (float with warm conditions) than the boost would be 
about 1.06% (14.5/13.2 * 98% efficiency). 

With the OutBack MX60 you can also wire the PV array for 5 in series (60 
vdc nominal) or even 6 in series (72 vdc nominal) to reduce the number 
of module strings and the required wire size - providing additional 
savings.  With a higher voltage PV array configuration and the MPPT 
controller doing step-down,  partial shading of the PV array will result 
in less loss of performance also...  

Christopher Freitas
OutBack Power Systems, Inc.
cfreitas at outbackpower.com  www.outbackpower.com
Tel 360 435 6030  Arlington WA USA

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