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DRAFT

====== sizing a house battery bank ======



===== assumptions =====


  * Everything in this article assumes you have already assessed your [[electrical:12v:dailypowerrequirements|daily power requirements]] (DPR).  The article will stipulate a DPR of 1,000Wh (1kWh). This 1,000Wh will be the base of all our calculations below.  
  * We will think in terms of 12v systems and 100Ah batteries.  
  * We will assume lithium = the common LiFePO4 (12.8v)


===== factors =====


==== battery bank chemistry ====


=== usable capacity ===

  * lead-chemistry batteries (FLA, AGM, GEL) are typically [[electrical:nominal|nominal]] 12v and discharged to 50% [[electrical:depth_of_discharge|Depth of Discharge]].  A 100Ah lead battery holds ~1,200Wh (100Ah x 12v), and one discharged to 50% holds 600Wh (1,200Wh x 0.5).  It would take **166Ah of lead** battery to give us 1,000Wh usable. (1,000Wh / 600Wh = 1.66 100Ah batteries)
  * lithium batteries are typically nominal 12.8v and discharged to 80% DoD.  A 100Ah LFP battery holds 1,280Wh, of which 1,024Wh are usable (1,280Wh x 0.8).  It would take **98Ah of LFP** to give us 1,000Wh usable.  (1,000Wh / 1,024Wh = ~0.98 100Ah batteries)


== throughput ==

'Dwellers contemplating large currents relative to capacity ("[[electrical:12v:battery_capacity|C rates]]") might have to oversize the bank to get sufficient throughput, and/or choose a chemistry with lower resistance.  FLA are famously stingy with current, AGM good, and Lithium excellent.  


==== load timing ====

The simplest model assumes the bank will be charged then all the [[electrical:12v:loads|loads]] will be run from the bank. 

In real life **sometimes loads are running while the system is making power**.  

  * running a 400w when solar is producing 200w means a net load of 200w.
  * running a 100w while alternator charging is producing may mean a net load of 0w (a freebie, in terms of battery capacity)

You may want to add a section to your DPR spreadsheet to account for load use while solar is present, etc.  In extreme cases only a very small bank may be required if most loads are effectively run [[electrical:solar:offthepanel|off the panel]].


==== charging  ====


=== charging stability and predictability ===


Charging can be very predictable or highly unpredictable.  More predictable charging allows smaller battery banks, and less predictable charging may require bigger banks unaffected by [[electrical:12v:psoc|PSoC]] to cover the variability.

  * [[electrical:shore_power|shore power]] (plugged in somewhere) is extremely predictable.  It will make 15.20A, 30A, whatever, as long as you are plugged in.  
  * same for charging with [[electrical:generator|generator]];  as long as it's running you have predictable charging
  * alternator charging with DC-DC is relatively predictable;  it will charge at the DC-DC's rated output until the bank hit's absorption
  * alternator charging with combiner is less predictable;  it will tend to start off with higher current at lower SoC and taper off as the bank reaches the alternator voltage
  * solar charging is highly unpredictable, being greatly affected by weather and [[electrical:solar:output|other factors]].  Rigs with solar-only charging may have to plan for days of minimal harvest, or have the flexibility to reduce consumption when solar harvest is compromised. 

=== minimum current requirements ===

Lead chemistries typically have minimum charging current requirements to stay healthy

  * FLA 0.10C (10A for every 100Ah of capacity)
  * AGM 0.20C (20A for every 100Ah of capacity)

If we cannot meet minimums the bank should be downsized (or [[electrical:12v:alt_and_solar|charging increased]]).  

Note:  Lithium has no minimum charging current


=== maximum charging limits ===

Battery chemistries typically have maximum charging currents that can stress charging systems (particularly alternators via combiners):

  * FLA 0.20C (20A for every 100Ah of capacity)
  * AGM 0.33C (33A for every 100Ah of capacity)
  * LFP 1.0C (100A for every 100Ah of capacity)

If the alternator can only safely provide 50A of charging to the house bank this would limit us to a 166Ah AGM bank (166Ah x 0.33 = 50A). If the alternator has excess capacity then a larger bank can accept more current.  This doesn't mean the bank would get full any faster((it's larger, after all)) but you will gather more Ah/Wh on any given run.
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