sizing a house battery bank

• Everything in this article assumes you have already assessed your 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)

• lead-chemistry batteries (FLA, AGM, GEL) are typically nominal 12v and discharged to 50% 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)

'Dwellers contemplating large currents relative to 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.

The simplest model assumes the bank will be charged then all the 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 off the panel.

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 PSoC to cover the variability.

• 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 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 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.

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 charging increased).

Note: Lithium has no minimum charging current

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 faster¹⁾ but you will gather more Ah/Wh on any given run.