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--- lifestyle:faq_solar_generator [2023/09/24 20:47]
frater_secessus [capacity]
+++ lifestyle:faq_solar_generator [2023/09/24 20:54]
frater_secessus [capacity]
@@ Line 58: / Line 58: @@
 ==== capacity ====
-Ah are often expressed as mAh.  Which is more impressive, 33Ah or 33,000mAh?  They are the same capacity expressed in different ways.
+Capacity is most often listed in Wh (watt-hours), which makes comparison quite easy.  Some (especially lead cells) retain Ah ratings.  For lead batts, 12v x the Ah rating = Wh.  Sometimes Ah are expressed as mAh, or 1/000th of an Amp.  Which is more impressive, 33Ah or 33,000mAh?  They are the same capacity expressed in different ways.
+Nefarious marketers sometimes multiply each cell's Ah rating times the number of cells, resulting in a 3x inflation of Ah rating.
-The 33Ah **capacity** lead-acid battery in the example above is stated as 400Wh.((volts x Amp-hours = watt-hours))  This is technically correct (and more useful as lithium cells increase in popularity) but mixes units in a way that consumers may not understand.
-Consumers may also not realize that lead-acid chemistries are usually only drawn down to 50% [[electrical:depth_of_discharge|depth of discharge]], giving an actual usable capacity of 16.5Ah.  In addition, lead-acid battery [[electrical:12v:battery_capacity|capacities]] are measured over 20hrs.  With our example this means a **10w** continuous DC load in this case, or 9w from the built-in inverter.  Loads greater than those will decrease usable capacity due to the [[https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=2ahUKEwjNo6ayjbXgAhXJwVQKHZ7xAmAQFjAAegQIChAB&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FPeukert%2527s_law&usg=AOvVaw0guPoMgsy0smO6ISiV1t32|peukert effect]].
+=== usable capacity ===
-Nefarious marketers sometimes multiply each cell's Ah rating times the number of cells, resulting in a 3x inflation of Ah rating.  \\
+Most people know that only about 50% of lead batt capacity should be used in order to ensure a long life.  Lithium also has a recommended safety margin, 20%.  This means 80% of the rated capacity can be used and still hit the manufacturer's cycle life claims.  400Wh x 0.8 = 320Wh usable.
-On the upside, they have almost no peukert effect and therefore can support heavier loads (at the expense of running time).  Also, higher end units run 4S or higher voltages then downconvert them for a ~13v output much closer to what nominal 12v devices expect.
+With 320Wh usable we could run the a theoretical 400w inverter at max load for 48minutes.  (320Wh / 400W x 60 minutes)
-You could run run [[electrical:12v:loads|a load]] at the **inverter's 300W normal rating** for ~36 minutes.((including 10% efficiency loss on inversion, but not including peukert effect))  Some manufacturers will list the inverter's peak output (600w in this case) in the title as if it were the amount of power the unit could deliver over time.  New folk sometimes read this as "I can run 600w of appliances off the unit forever!", forgetting this is a //peak// load and that the unit has a finite capacity.
 ==== cell chemistry ====

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