Hi Allan;
This is an interesting thread, as all of us ( I think) have gone heavier
on the solar arrays and lighter on the battery banks based on
economics. I too have noticed higher gassing and more water
consumption, and I too have adjusted set points down slightly to account
for this. I think that it is really too early to tell what this will
mean for battery life. While the batteries will be reaching full charge
more often as you noted, they also will be cycled more deeply on a
regular basis. I also have concerns with acid loss, and over heating;
both of which lead to shorter battery life.
Over the years I've actually found that my systems with larger battery
banks lasted a long time because daily cycles were shallower, but as you
noted it depends on whether the user makes sure they reach full charge
regularly. Without proper management, heavy banks can be stuck in
Purgatory: never fully discharged, but never fully charged either.
Meanwhile small battery banks are being charged and discharged at much
higher rates, which is both less efficient and creates additional heat.
With this design trend to smaller battery banks, I think we all need to
revisit our thermal management strategies as well. The old days of
super insulating the battery box can actually lead to serious battery
over heating. Midnite Solar is looking at adding charge disconnect
programming based on the temp sensor. Basically, most batteries need to
be completely disconnected from a charge source at battery temps over
105F. Cycle life deteriorates rapidly at temperatures over 85F.
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760
On 5/21/2015 11:56 AM, Allan Sindelar wrote:
Wrenches,
In the last few years I have changed my approach to off grid system
sizing, primarily due to higher battery costs and lower module costs.
Rather than shooting for a traditional 80-90% of winter load profile
target, I will size a larger array and smaller battery bank, figuring
(especially here in the sunny Southwest) that batteries will recharge
more quickly, will spend more of their time full (and thus last
longer), and will be at least partially charged during cloudy weather.
This even allows for smaller battery banks that are cycled more deeply
(as long as there's a backup gennie in the mix); the lower cost of a
smaller size may allow for a set of higher-quality industrial cells.
I first began to rethink my design approach after reading Christopher
Freitas' Solar Pro article on large battery banks a few years ago.
Being able to refill in a day was part of his strategic approach to
battery bank design. I used and wrote about this approach in my tiny
house article in the current Home Power. However, I'm just beginning
to get owner feedback about greater water consumption and the need for
more frequent watering. This is especially the case when I add array
to an existing system, and the old watering schedule is suddenly not
enough.
When I design new systems I now encourage larger arrays and smaller
battery banks, as the batteries spend more of their time happily full
and recover more quickly after a discharge period. But I am thinking
that I will have to begin changing charging setpoints in response.
Flooded batteries have a range of recommended bulk setpoints, from
28.8 to 29.6 volts on a 24V system. When arrays were smaller we set to
the higher end of the range to take advantage of topping off when
available, and a two hour or longer absorption was considered
beneficial, as long as the CC could transition to float at 2% of
capacity or so. With a larger array a better approach may be to lower
the bulk/absorption setting, as well as reduce absorption time and
raise the float transition current ("done amps").
I'm writing to encourage a discussion among off grid Wrenches. Are you
changing the relative relationship between array and battery
capacities? If so, how are you adjusting charge controller setpoint
programming? For example, how would you program a Midnite Classic,
Outback FM or Schneider XW if you could achieve a C/10 or C/15 charge
rate?
Thanks, Allan
*Allan Sindelar*
[email protected] <mailto:[email protected]>
NABCEP Certified PV Installation Professional
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder (Retired), Positive Energy, Inc.
*505 780-2738 cell*
**
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