Max Lead Acid Discharge Current.

Thomas,

My estimate on current draw included inverter efficiency.
 
2000 watts/ 12volts = 167A.  167A / .85 % Eff = 196A.

The 600A was what the inverter manual said was its maximum current draw.

As you said, I don't think a set of golf cart batteries are going to put out 600A.  I'm looking for guidelines on what is the max current draw that a battery can handle.  Which, as you said, battery makers don't seem to publish.

Your correct about the surges only lasting a couple seconds. I only plan on having 1 appliance (microwave, toaster, etc) plus a lighting load on at any one time. 

Now that I'm seeing the size of wire used for a 12 volt system, I do see the logic in running higher battery voltages.  However, this is a weekend cabin with power for lights and a few appliances, so I think the 12 volt system will work fine.  I may look into bigger L16 batteries and check my inverter programming to see if it has a current limit setting.

Does anyone else have any more thoughts on the max current draw on a lead acid battery(s)

Thank you,

Jerry.

Your question is confusing.  You say that your loads are 750 watthours or 882Wh at 85% efficiency. That is 73.5 amp hours.  Then you are talking about peaking at around 200 amps.  It shouldn’t happen.
If we pack most of that into 5 peak hours of usage, that would indicate a typical load of about 15 amps. Your peak loads probably won’t be more than 30-40 amps, unless there is something you haven’t told us about.

Batteries discharge rates (and charge rates) are discussed at C rates. Your 225 amphours is most likely a C20 rate, 20 hours to discharge or about 11.25 amps.  When you discharge it at a higher rate, you will not have the same capacity. You will get about 180-185 amphours out of it at a C5 rate (45 amps) or about 105ah if you use a C1 rate (225amps). A dead battery in less than ½ hour.

Your 73.5 AH gives you a state of charge (SOC) of about 67% on a daily basis. I would give some serious thought to doubling your battery bank. If you make it a 450 AH bank, that would prolong your battery bank life. It will give you about an 83% SOC on a daily basis, and 2 days power before you hit the 67% SOC.  It would also give you a 22.5 amp C20 rate. 

Without hearing about the charging side (panels, generator?) we really can’t talk fusing. But if your peak load or charge will actually be 40 amps or less, I would be tempted to fuse the system at about 50-60 amps and consider anything above that value to be a dead short.

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?

putting in a 200a fuse and 175a breaker would to limit your inverter output to 2100-2400, depending on the duration of the surge.  you could do this as an artificial barrier, but usually it is easier to know your loads than to replace fuses. 

there is nothing wrong with starting small and using the generator.  the only real problem is once batteries are used for 6 months to a year (depending on how they are cycled) they generally do not take well to new batteries added to the mix, and self discharging occurs.  other than the battery issue it is often wise to start small.

i would recommend getting beefy balance of system equipment that can stay the same as the system grows; things like combiner boxes and load centers that can accept growth and limit re-running of wires and conduit.

james
Alt-E staff

Well, after reading the other postings on this thread, I can more clearly understand your questions and, I would like to clarify, I was not suggesting that you, move to a higher nominal voltage, just that, generally speaking, sustained higher amperages might necessitate higher nominal voltage systems. Also, the calculations I performed are/were not intended for sizing any part of a PV battery charging/inverter system,  it was just an effort to deduct where you got your "numbers" from. Enough of that.
If you're up for it, here is some, more or less technical articles you might find enlightening in regards to your system.
http://photovoltaics.sandia.gov/main.html
In particular,
http://photovoltaics.sandia.gov/docs/BattIntro.htm
and,
http://photovoltaics.sandia.gov/docs/John_Wiles_Code_Corner.htm
Personally speaking, when I size a battery bank for an off grid PV battery charging system, after calculating the number of amp hours of capacity anticipated per 24 hour day/night cycle (all things, such as inefficiencies, included), I multiply that by a factor of 5. Then I size the PV array to replace that amount used in a 24 hour day/night cycle in one sunny day. This has allowed me, in my region, to not have need of a generator. Of course this, genrally speaking, doesn't always fall within a prospective budget, so sometimes improvises are made.

I have a similar small system that I have been slowly
trying to improve over the last 20 years.

Golf cart batteries have a moderately high internal
resistance when new, and the resistance goes up fairly
quickly with use and age.  This means that the batteries
are somewhat self-protecting when used with a high-power
inverter - the battery voltage drops dramatically at
high currents, and the inverter shuts off.

For the AC power mentioned, 2 GC batteries are much
too small, and I would also go to 24 volts - something
I wish I had done on my system from the beginning (most
DC devices were only 12V back then, though).

Since there is no way to size a system for never having
a power loss, most off-gridders around here assume to
use their generators often, keeping the PV system only
moderately sized.  For most of us, the cost and complexity
of a near-perfect system is not worth it.