[RE-wrenches] Charge Control in AC coupled system

[Howie Michaelson]

Hi Ken,

I think I must be not seeing the forest or the trees.  Maybe I'm missing
something, but what I'm trying to do is get the most I can from the AC
coupled array (2.25 kW) while still protecting the batteries.  I can use
the relays in the inverters or in the FNDC to operate a contactor which
shuts down the SB using either voltage or SOC% readings and then leave the
DC coupled array (1kW) going through an MX60 to finish the charge.  I am
concerned that if I set the SunnyBoy to shut down at too low of a setting
then I will be throwing away a lot of useful power, but if I set if the AC
shut-off too hi then every time the battery bank drops below that high set
point the AC coupled array continues to charge the batteries whether they
need it or not. If this happens over an extended period of time, we run
the risk of overcharging the batteries.  Even if I have 3 diversion loads
set to kick on at successively higher voltages this does not appear to be
a fail-safe way of guarding against the chronic over charging of the
batteries.  Maybe I shouldn't worry about those unlikely scenarios, but
I'm wondering if there is an elegant way of mimicking a 3 stage Controller
which goes to some kind of "float" after the batteries are full.

I'm now thinking perhaps my best option is to utilize the SOC% relay
setting in the FNDC, and as long as the reading is accurate, I can set the
AC coupled array to shut off near 100%.  Does anyone have a more
bomb-proof way of utilizing the AC coupled array to it's fullest potential
while still keeping the battery bank healthy?

Thanks,
Howie
-- 
Howie Michaelson
NABCEP Certified Solar PV Installer™

Sun Catcher, LLC
Renewable Energy Systems Sales and Service
VT Solar & Wind Incentive Program Partner
http://www.SunCatcherVT.com
(cell) 802-272-0004
(home) 802-439-6096


On Thu, October 28, 2010 11:25 pm, Kent Osterberg wrote:
> Howie,
>
> In an ac coupled system with Outback inverters you could use a diversion
> controller such as a Morningstar Tristar 60 on the dc side.  That would
> allow using an ac coupled system and a nicely controlled finish charge
> for the batteries too.  Since there is already a MX60 to do the finish
> charge in the system you are considering, a voltage controlled switch,
> like the Morningstar relay driver, could simply shut down the SB3000.
>
> Kent Osterberg
> Blue Mountain Solar
>
>
> Howie Michaelson wrote:
>> Hi All,
>>
>> I have done several AC coupled off-grid systems using Sunny Islands and
>> Sunny Boys, but I am now doing one with a dual stacked OB VFX3648 (with
>> a
>> PSX240 on the Inverter out for load balancing) and a SB3000.  There is
>> also an existing array that is running through an MX60.  What I am
>> wondering about is how folks have set up AC coupled string inverters to
>> best utilize the solar gain while not toasting the batteries.  I'm a big
>> fan of the tapering control offered by SMA through the frequency shift
>> of
>> the SI's, but that's not available here.  What relay/contactor setup
>> have
>> folks found most efficient and safe?  Thanks in advance,
>>
>> Howie
>>
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