Hi William;
One thing to consider is that you want to cycle the batteries at least
20% DOD. If you look at the cycle charts for regular Lead acid
batteries, the total KWH you can get from a battery for its lifetime is
fairly equal from around 20% DOD to 80% DOD. (basically you can get
twice the cycles at half the discharge rate) However this nice linear
relationship goes out at the ends (over 80% DOD, or less than 20% DOD)
You just don't get your money's worth cycling deep cycle batteries at
very shallow depths.
Another consideration is that it is very inefficient to charge a
battery completely with generator power. The last half of the
absorption phase is at fairly low current for fairly long time periods.
You should have the genny shut down when its efficiency gets low (might
be less than 25% of rated capacity, depends on the generator)
My basic design philosophy is to not cycle the bank more than 50%, but
have the generator shut off once the total watts (charging + loads -
PV ) drops below a certain threshold. Also having the generator kick in
when loads exceed a certain discharge rate (say C20) is important too.
If they discharge at higher rates, you won't have your design amp hours
because of the Puekert's exponent. (basically: batteries have less
capacity at high discharge rates)
Last point, I don' think its reasonable to expect the system to reach
full charge every day, I'd shoot for once a week, and save a bunch of
fuel. Deep cycle batteries can sit at less than full charge for a
couple of weeks before the lead sulfate starts crystallizing into a
non-disolvable form. In this case, you want a balance of fuel and
generator run time vs battery life.
I have a large system like this that the batteries have already outlived
the generator, so that's probably not exactly optimum. I've since
starting using smaller battery banks, also because batteries have gone
up in price.
For these types of designs, I find software like PV Design Pro to be
very useful. It can characterize the total % of energy from PV, as well
as model an hourly basis of the system operation over an average weather
year. You still have to ride herd on software simulations, as they can
turn out some pretty stupid designs if you're not careful.
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760
On 8/16/2013 11:53 AM, William Miller wrote:
Friends:
We have been receiving a flurry of requests for design of off-grid
systems where the loads are enormous, relative to normal off-grid
requirements. It is obvious that the size of PV and battery arrays to
power these loads is beyond what is practical. We have dubbed these
systems Generator/battery with PV assist, or PV assist for short.
These systems will cycle batteries multiple times per day. It is my
understanding that the extra battery cycels will shorten battery
life. Since generator run is expected, we are willing to increase
generator run time in order to prolong battery life.
The parameters I suggest are an aggressive load start and a very high
battery start parameter. This will prevent deep discharge of the
battery bank. I know shallow discharge is not the norm, but I don't
believe shallow cycling is a problem as long is it as at the top of
the voltage window, i.e. the batteries achieve absorption at least
once per day.
We know a load start ciuld result in an abbreviated run time,
curtailing absorption period, but we are assuming there will be a
battery votage start at least once per day allowing full absorption.
Has anyone else considered these issues, and what conclusions did you make
I am looking forward to a spirited discussion as usual. I
throuroughlyenjoy and beneift from them, even thought the advice is
usually conflilcting. Thanks in advance.
William Miller
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