#2 Charging and battery options - Upgrade strategy

Based on the parameters identified in the previous post, 150-200ah usable is a reasonable house bank capacity for Windstar's typical use, in the absence of any additional charge source.  While this leaves little safety margin, it also assumes no motoring or marina time.  Today Windstar has approximately 120ah capacity.  (best case) Supplemental solar generation would provide extended range and a needed buffer.

Which Batteries are Most Appropriate?

See below for a comparison of available battery types that meet this requirement within Windstar's existing battery footprint.  (2ea group 31)  Note:   I have not updated this table with accurate pricing for each.  

The green-shaded cells indicate that without any supplemental charging, and at full rated capacity, only the lithium and largest deep cycle batteries can exceed 150ah capacity with the DOD as specified.   This is the low end of the range identified for Windstar's use, and does not consider the gradual reduction in capacity as the batteries age.  The table does not consider the possibility of an expanded footprint or of mixing battery group sizes to "get to a number", which is a reasonable approach.  The cost and weight per available amp hour is consistent by battery type, so extrapolations are not difficult.    

The costs are only estimates (not very accurate it turns out) and include only the batteries themselves.   (i.e. purchasing an appropriate charger for lithium or up-sizing the charger, or replacing the flooded start battery with AGM to match the house bank might also be required)     I have eliminated several columns of data from the original spreadsheet, however my early conclusions are:

• Without any supplemental charging, and at full rated capacity, only the lithium and a pair of largest deep cycle batteries (GC2H) can meet the 150ah capacity with the DOD as specified.  Lead acid/agm can only just meet the 150ah threshold without adding additional weight and possibly having to modify the boat to increase the space available.  Lithium can easily meet the objective, but at a considerable up-front cost.  
• Group 8d lead acid batteries were ruled out due to weight.
• Lithium batteries are as cost-effective as lead acid over their service life provided they (and I) last for their rated 10 year life.  ***  note i have since found lithium batteries at significantly lower prices than anticipated.
• The 3-year cost is included as I may move up to a larger boat within this time frame.  
• Flooded batteries must be installed upright, limiting options as there is less than 12" clear  beneath the quarterberth.   Fitting smaller lead-acid batteries alongside the larger ones is a possibility.  AGM batteries, which can be installed on their sides, opens a number of possibilities and is a better choice for other reasons as well.
• While the lithium technology is very appealing, the cost of the batteries is high, a new or additional shore power charger might be required.   If I were contemplating net-zero solar in a larger 10-year boat for extended cruising, Lithium would be the clear #1 choice.   This is discussed and resolved in  a subsequent posts.
• It is a challenge but not an impossibility to install a 200 a/h (usable) lead-acid battery bank in a boat of this size and type. 

 2ea GC2h AGM batteries provide capacity in the target range, and would not require additional space or add much incremental weight.  An additional  12v AGM battery could easily be added later if required.    AGM batteries cost more than flooded batteries, but tolerate deeper discharge, charge more rapidly, and have a longer service life

.  (* update - high capacity GC2H AGM batteries are not as widely available or as competitively priced as the commonly used  GC2 batteries.   The price delta between GC2H AGM and lithium sourced through industrial suppliers is not nearly as great as estimated initially) 

Which Solar Panels are appropriate?

The table below shows some of the form factors and outputs of the flexible monocrystalline solar panels available today.   Those shaded  are the maximium output power that will fit on Windstar's bimini, the highest output of these claims 175w.  Solbian panels are extremely costly - at least 5X the price of Renogy.   

• 3 ea 50w panels could also fit on the bimini with the advantage of having independent controllers.
• A 50w panel could be installed on Windstar's dodger if needed.

Ideally, a single 175w solar panel would suffice.   

What are others doing?

To validate all the preceding calculations and theory, I asked the fine people on the C&C owners list about their experience with both solar generation and battery bank size.

• A reasonable rule of thumb for my cruising area is 3 hours of rated wattage per sunny day.  This equates to 24ah/100w panel.  ** note that an article in Practical Sailor suggests that 5hrs while sailing and 7hrs while at anchor is a useful rule of thumb.  This is not consistent with the experience of C&C list members, however, the three hour per day figure may be very conservative, especially in mid-summer.
• Increasing either solar generation wattage  or house bank capacity reduces typical depth of discharge in use, and therefore increases battery life.   
• 200w/200ah (usable) seems to be the threshold for a self-sustaining solar system aboard sailboats of this approximate size and type, provided a MPPT controller is used.   
• Boats that can comfortably accommodate lead acid banks and solar panels of this size are typically larger than Windstar.  Many of the installations I've seen locally use additional bracketry for the panels, and are somewhat inelegant.   I wouldn't do that to Windstar!

These examples validate most of the previous estimates and assumptions, and suggest that 175w of generation will extend Windstar's range, but may not be adequate to self-sustain.   

So, what is an appropriate strategy for Windstar?

• Using only the available bimini top, Windstar can add a 175w Renogy flex panel or other of a similar form factor.    This would suffice to extend Windstar's "no charge" range by the two days as intended, but she would likely require charging from other sources in some hopefully rare situations.
• Removable panels could (theoretically) be incorporated when required.
• While less than ideal due to shading, it is physically possible to install a 50w panel on the dodger.  This should be enough for Windstar to become independent of external charging.
• Given the 175w generation constraint above, energy storage capacity should be 150ah at a bare minimum, however more should be considered.   
•   Lithium is therefore worth considering.
• So, it is most practical to start with a 175 watt panel(s) on the bimini and use the existing battery footprint.  If this is unsatisfactory:
• Any charging or control hardware should be lithium-compatible for possible future upgrades.
• Storage capacity can easily be increased if required. 
• 50w incremental solar capacity can be added to the dodger if required. 
• while the alternator could be upgraded to one better able to quickly charge the house bank, there are many reasons why this is not a good approach.

What does the proposed setup offer in use?

Here's a simple estimate of how the various batter configurations and types would be impacted by 3 days at anchor with only one sunny day.

Same cruise, 3 sunny days.

Conclusions on Generation and Storage:

• 200/200 would take Windstar off the grid.    Less storage or less generation capacity would not achieve this under all conditions, but should do so most of the time.  Very encouraging.   175/150-200  should comfortably meet the original objective of extended cruising in the Great Lakes.
• Windstar's primary constraint is the space available to install solar panels.  Generation is limited to 175w.  (225w with the addition of a second panel.)   
• Generation and battery capacity are a tradeoff with respect to depth of discharge and carefree cruising.
• Therefore, adding additional battery capacity is the most appropriate path to keeping depth of discharge reasonable.
• This makes using 2X100ah lithium batteries even appealing as it provides 180ah of storage within the existing footprint at less than half the current weight.   (54lb vs 120lb).  The GC2H option offers 156ah at over 150lb.  

The next post deals with determining how to charge and manage this.  

To read more on this project click the label/filterwith the tag ‘solar and batteries’ on the right.