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--- 12v:bank-sizing [2023/05/17 13:26]
frater_secessus created
+++ 12v:bank-sizing [2023/12/29 09:57] (current)
frater_secessus [charging] chargig
@@ Line 1: / Line 1: @@
+DRAFT
 ====== sizing a house battery bank ======
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-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).
+  * 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.
-This 1,000Wh will be the base of all our calculations below.
+  * We will assume lithium = the common LiFePO4 (12.8v)
-We will assume lithium = the common LiFePO4 and not any other form of lithium chemistry.
@@ Line 21: / Line 21: @@
 === 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.
+  * 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.
+  * 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)
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+==== 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
-Finally we get to size something!  We will divide the daily power requirement by the maximum [[electrical:depth_of_discharge|depth of discharge]] preferable for each battery chemistry to get required battery capacity in Wh.   Deep cycle batteries are traditionally rated in Ah (amp-hours), so we will divide Wh capacity by the battery chemistry's nominal voltage.   See below.
+=== maximum charging limits ===
+Battery chemistries typically have maximum charging currents that can stress charging systems (particularly alternators via combiners):
-Lead is typically discharged to 50% DoD as a good balance between performance and longevity.
+  * 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)
-,084Wh / 0.50 DoD = 4,168Wh  \\
+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.
-,168Wh / 12.0v nominal voltage = **347Ah** of lead-chemistry required.
-Also see the [[opinion:solar:sizing.walkthrough#minimum_charging_current|section on minimum charging current]] below.
-Lithium is typically discharged to 80% DoD as a good balance between performance and longevity.
-,084Wh / 0.80 DoD = 2,605Wh \\
-,605Wh / 12.8v nominal voltage = **204Ah** of lithium required.

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