Max Lead Acid Discharge Current.

okay, i have to jump in at this point, i cant resist.

for practical purposes of safety we will assume the max current available from the battery will be infinity - for human purposes.  shorting straight across the battery bank can be lethal.

here are the common questions related that I often answer to the best of my knowledge.  i am by no means any ultimate authority, so forum faithful, please check in to factcheck, lest i lead anyone astray.

if the question is actually:  my inverter has a surge of X, will surging it to X hurt my batteries?

then the answer is usually no.  a surge load, typically an inductive load is by nature something that may "surge" for a SHORT (less than 1 sec usually) period of time as it gets going.  motors, compressors, are the usual culprits.  remember the difference between power and energy.  watts x time = energy.  so 4000w x < 1 second = lots of power, not very much energy.  batteries store energy, and will not be affected by this short burst.  were this burst to last for any actual time, then it would deplete the batteries.

if the question is more like:  my inverter has a surge of Y, will surging it to Y hurt my inverter?

well no.  but this is like the price is right.  just dont go over.  and if you are not SURE of  the surge on your inductive load, dont run it, unless you are willing to sacrifice your inverter to find out.  many inductive loads can surge from 3 to 7 TIMES normal running wattage upon startup. 

lastly the question may be:  my inverter has a surge of Z, will surging it to Z be too much for the wires i have connecting the battery bank to the inverter?

good question.  surprisingly, the code does not care about the surge of an off-grid inverter.  if the goal is to protect the wires, mainly from heat and fire, then even the surge limit if double the normal ampacity that they are used to carrying only lasts for a few short seconds, likely nothing bad will happen as a result.  the equation for sizing battery to inverter overcurrent protection device/ disconnect is as follows from NEC 690.8:

battery to inverter disconnect rating

inverter continuous wattage output / lowest operating voltage x inverter efficency (at that condition)

conductors must be sized to handle a higher ampacity than the overcurrent device applied from the equation above.  see NEC section 310.16 or 310.17.

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hope this helps with the discussion and the system in question.

cheers,

james
Alt-E staff

Thank you for the feedback.

I like the idea of 1 outlet to limit what can be plugged in.

My philosophy is to make an educated estimate on a system that will provide our off grid power needs.  The support from the community here is definitely helping me become more educated. I know that as I learn I may need to add batteries or more solar/wind charging.  Instead of giving my batteries an early death, I'd rather use the generator to fill in the power shortage until I evolve the system to meet our needs.

Along those lines, I planned on doing the following:

*Educating the family on minimizing power use and hope some of it sticks.

*Monitoring the DOD on the batteries and either shedding load or running the generator to prevent > 30% DOD.

*Recharging the batteries ASAP.  Either solar or generator.

*Limiting the max current the batteries get hit with.

*Checking water and equalizing when needed.

Will doing those things help get more years out of my batteries?

My inverter calls for a 300A fuse/breaker to handle surge loads.  Should I be looking at a 200A breaker, which will limit hitting my smaller battery bank with unexpected surge loads?

Does anyone have any thoughts on whether my process of putting in a estimated system, and filling in with the generator until any shortfalls in the system design are corrected?