[RE-wrenches] FLA battery setpoints with larger arrays

[Ray Walters]

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*
> allan at sindelarsolar.com <mailto:allan at sindelarsolar.com>
> 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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