SIZING LITHIUM BATTERIES

Author: Bruce Loxton   Date Posted:18 April 2018 

 

SAFIERY UNIVERSITY
SIZING ENERGY SYSTEMS
for RV's (Boats follow)

120Ahrs quality Lithium Battery costs about the same as a quality solar system to produce 100Ahrs a day: 

  • 100Ahrs a day requires 500W quality solar+ MPPT controller on caravan roof.
  • 500W Lightweight Sunpower Solar plus MPPT controller is less than $2,000.
  • 125Ahr good quality Lithium Battery is a little over $2,000.

1. SIZE YOUR SOLAR REQUIREMENTS FIRST

  • You should have Solar in place to replenish the energy you use first;
  • Even on a poor solar day, you will receive 20-30% energy from a good solar system.
  • Purchase a portable system that can be directed to the sun as a backup.
     This adds energy when parked in the shade or on a poor solar day. Adding a 150W portable system will cost less than $600 and when angled to the sun is twice as efficient as the solar on your roof!

2. ASSESS HOW MUCH ENERGY YOU CONSUMER PER DAY

    From 15 years assessing and talking with caravan travellers:

  • Frugal couples use 60 Ahrs/ day with a compressor fridge
  • Moderate couples +1 use 80-100Ahrs/ day with compressor fridge
  • Bold users consume 120Ahrs/day with compressor fridge and laptop charging

        If you have a 3 way gas fridge then take 40-50 Ahrs/day off this number.

3. Sizing for Lithium:

  • Take your daily usage and multiply by 2 for 2 days requirement with zero solar;
  • For Lithium, multiple this by 1.2 ) to allow for 80% depth of discharge);

     Examples

  • Moderate user, gas fridge:
    90 - 50 = 40 Ahrs/Day; x 2 days = 80Ahrs;  x 1.2 =100 Ahrs
  • Bold user, compressor fridge: 
    120 Ahrs/Day; x 2 days = 240Ahrs;  x 1.2 =268 Ahrs

4. Remember

  • Round the sizing. The 264 Ahrs above could be matched with 250Ahrs.
  • Solar will add significantly to the 2 days when available;
  • Long drives will top your battery right up;
  • Lithium absorbs the most solar energy because of high charging efficiency;
  • The critical time point is 8am in the morning. Overnight the batteries have dropped in capacity with use and the solar is yet to kick in;
  • If you want to stay for a week or more "off-grid", then if the solar replenishment matches you daily usage, this sizing will give you a good overnight buffer.

SIZING ENERGY SYSTEMS FOR BOATS

1. Assess how much energy you consume per day.

    From several years assessing and working motor yachts:

  • Frugal couples use 200 Ahrs/ day with one main fridge
  • Bold users consume 400Ahrs/day+: Inverter with multi-media, laptop charging

2.  Sizing for Lithium:

     Assuming the genset is running till sunset...

  • One Days consumption with give you capacity
  • For Lithium, multiple this by 1.2 ) to allow for 80% depth of discharge);

 

Assuming there is no genset on your yacht and you reply on wind generator and/or solar. Follow the caravan guidelines above assessing your

  • Solar capacity + Wind Generator +small outboard alternator if you have one.

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