# Max Lead Acid Discharge Current.

16 Posts
Jul 27, 2008 09:21 pm

I'm planning a solar power system for our weekend cabin and have a question regarding discharge current.  I have a ProSine 2.0 2000 watt inverter with a 12V supply.  The main load in cabin will be CFL lighting which I calculated to be about 425 watt hours/day.  The other loads would be small appliances at about 325 watt hours per day.

A set of 6V GC2 Golf cart batteries (225AH) could handle that load with about a 33% Discharge.  However, I'm having trouble finding what maximum current a lead acid battery can put out without hurting it. At 2000 watts the inverter is going to pull close to 200A.  At its 4500 watt surge its closer to 600A.  Can anyone provide feedback on determining how much continuous and surge current a particular set of batteries can take?

Thank you,

Jerry

Jul 28, 2008 06:00 am
Re: Max Lead Acid Discharge Current.

I can't answer your specific question with a specific answer, but I can tell you the only way to know the answer would be to experiment under highly controlled circumstances with safety first. But of course, this would ruin the batteries wouldn't it?

Try as I did to find generalized information regarding this, I could not. It would seem that the battery manufacture would know this information if they had their batteries tested by a consumer safety advocate.

I am having a little trouble with your math though.
2000 watts / 12 volts = 167 amps
4500 watts / 10.5 volts = 429 amps
(10.5 being a typical LVD)
Surges of this nature will only last for a few seconds, correct? More than likely your planning includes at least a set of 4/0 cables. 4/0 THHN has a rating of around 200 amps depending on certain factors.
What is the biggest size wire recommended by Xantrex for your ProSine 2.0 inverter, and what is the inverters terminal lugs temperature rating? More importantly, what size fuse or breaker is recommended?

It would seem implausible that one would "dump" a load of 600 amps on just 2 - 6 volt golf cart batteries wired in series for 12 volts.
Something would have to give.
Maybe a bank with more batteries, but common sense would dictate a higher voltage be used if amp loads of this nature where anticipated. Also, this is where proper sizing of and the type of fusing would protect personnel and property.

Even in the world of alternating current electricity, when higher amperages are anticipated, a move to higher voltages is the typical response.
Think about it, why pull the larger more expensive wire for a 800 amp service at 240 volts 3 phase when you can pull wire nearly half that size at 480 volts 3 phase.
I hope something here helps you.

16 Posts
Jul 28, 2008 09:29 am
Re: 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.

76 Posts
Jul 28, 2008 01:26 pm
Re: Max Lead Acid Discharge Current.

The manufacturer of the battery should be able to tell you how fast you can charge and discharge it.  The more power you push or pull, though, the bigger the wire you're going to need.  I run 400A max through my 12v floor machine batteries in my electric truck, and I use 2/0 cable to connect them.  Golf cart batteries are often used in EV conversions, typically putting them in series for 120v dc @ 400 or so amps.

351 Posts
Jul 28, 2008 03:52 pm
Re: Max Lead Acid Discharge Current.

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.

16 Posts
Jul 28, 2008 06:06 pm
Re: Max Lead Acid Discharge Current.

Ken,

I calculated my watthour/day usage from the wattage of the load times the number of hours I expected that load to be on.  That's where the 750 watt hours came from.  I don't expect to draw peak current during normal daily use.

I didn't do a good job of explaining why I'm asking about the high 200A+ currents.  My inverter is 2000W with a 4500W surge.  I rarely expect to draw 1500 or more watts.  However the inverter is capable of drawing 200A to 400A from the batteries if enough AC load is turned on.

I wanted to make sure I didn't damage the batteries if someone turned a microwave and toaster on at the same time, or plugged in a larger power tool.  I'd rather the batteries and DC cabling could handle an occasional high current instead of blowing a fuse or breaker or damaging a battery.

I looked through my inverter manual (Xantrex Prosine 2.0) and don't see a setting setting for max DC current draw.  I need to enter the battery type, but that appears to be for charging use not limiting maximum discharge current.

This is a weekend cabin and will generally only see load 2 days per week.  I planned on sizing the solar panel(s) to take 2 or 3 days to fully charge the batteries after a weekend of use.  I also have a 3000W generator, but would like to minimize its use.

Your comments on the discharging the batteries too much are well taken.  I was thinking I might be discharging the 2 GC2 batteries too much.  For a bigger bank, 4 golf cart batteries or 2 L16 batteries cost about the same.  I'm thinking 2 L16s are a better/simpler solution.

Any thoughts on sizing the batteries and DC supply system to take the loads of high unexpected AC usage?

Are 2 L-16s in series better than 4 GC2 in a series/parallel connection?

Any recommendations on battery manufactures?  Trojan and Crown seem to be popular.

Thank you for your patience! I'm new to off grid power.  I'm enjoying the learning and appreciate the help.

Jerry

578 Posts
Jul 28, 2008 06:41 pm
Re: 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.

------------

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

cheers,

james
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« Last Edit: Jul 28, 2008 06:49 pm by James Cormican »

351 Posts
Jul 29, 2008 03:39 pm
Re: Max Lead Acid Discharge Current.

I think the 4 GC2’s would be better for your use.  450 amp hours, vs 390 amp hours. Equally important, is at about 62 lbs apiece, they will be easier to lift than the 121 lbs, when you have to change them.  You will probably be changing them often. Until you learn a few “hard to learn” lessons.

An off grid system has to be sized for it’s intended use. It also has to be balanced, batteries, inverter, generation, and USAGE. You are putting a larger inverter on small batteries, with smaller generation, and expecting not to hurt anything when you ask it to do a bigger job than it was sized for.

The fact that you are even considering the possibility of a microwave and a toaster being operated at the same time on this size of battery, suggests that you are in for a “learning experience”.  The best solution would be don’t have either one in the cabin. Second best would be a single outlet, forcing people to switch plugs and preventing simultaneous operation.  Put the refrigerator (assuming its electric) on a switched outlet and turn it off while preparing meals (or other peak periods), to prevent it from kicking on and further peaking the load. Don’t have a convenient plug for that large power tool. Drag out the generator for large loads.

Everyone using the cabin has to be load conscious. Otherwise, they will act like they are at home with an unlimited source.  When you put 2 or 3kWh on a system designed for 1kWh, your batteries will have a short life.

You might want to purchase a Kill-A-Watt meter and put it on some of your appliances and get some hard data as to how much you really use them. Beginners tend to underestimate how much use they actually get. You also learn such things as a 1000W microwave draws close to 1500W. (The 1000W is the output, not the electric draw)

If you size your system for 1kWh, you have to learn to live within that allowance. If living within that allowance is unacceptable, you have to increase the allowance.

If you want more power, do a worse possible estimate. Cabin full of people, extensive microwave, toaster, blow-dryer, and large power tool usage. Then size your system for that.  It will cost more money up front, but changing batteries every 1-2 years is not cheap.

Don’t look at tripping breakers as a nuisance. Tripping breakers in an off grid system is a gentle reminder for everyone, that someone is exceeding your predetermined limits.

16 Posts
Jul 29, 2008 05:11 pm
Re: Max Lead Acid Discharge Current.

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?

578 Posts
Jul 29, 2008 05:56 pm
Re: Max Lead Acid Discharge Current.

remember the equation? nec 690.8

continuous inverter output / lowest acceptable dc input voltage x inverter efficiency

in your case, xantrex prosine 2.0,

2000 / 10vdc x full load efficiency 87% (.87)

2000 / 8.7 = 230a --- 250a circuit breaker, -- 300a fuse

here are possible dc load centers that could work

cheers,

james
Alt-E staff

AltE
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http://www.altEstore.com/

Tel: 877.878.4060 x107  or +1.978.562.5858 x107
Fax: 877.242.6718  or +1.978.562.5854

578 Posts
Jul 29, 2008 06:01 pm
Re: Max Lead Acid Discharge Current.

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

AltE
"Making Renewable Do-able"
http://www.altEstore.com/

Tel: 877.878.4060 x107  or +1.978.562.5858 x107
Fax: 877.242.6718  or +1.978.562.5854

351 Posts
Jul 30, 2008 04:28 pm
Re: Max Lead Acid Discharge Current.

Jerry

Your list is a good one. Following it, will prolong the battery life. In daily use, the batteries would have a 3-5 year life expectancy. You can get 4-8 if they do not get abused. Abuse them on a regular basis, and they will only last a year or two.

You asked again about limiting surge loads.  Surge loads are very short duration, lasting from a few cycles, to several seconds. They are most commonly caused by motor startup. While they may draw large currents, the short duration of them will not normally harm the battery.

Although the inverter makers call anything above their rating to be surge capability, most of this is really peak loads of limited duration. So, are you asking about surges ?  Or are you asking about peak loads of short duration ?  (eg 1-5 minutes of microwave, 5-10 minutes of blowdryer)  These are the ones that I worry about controlling.

The DC fuse is there to protect the inverter. While it would also provide some level of protection for a large battery bank, it doesn’t provide any significant protection for your small bank.  Downsizing the DC fuse to 200amps would not be worthwhile. It would just limit the inverter, without adding significant protection for your battery. A sustained 200 amp load will do almost as much damage to your bank, as a 300 amp load.

IIRC, the Prosine uses a 300 amp class T fuse. It takes about 100 seconds at 600 amps to blow it, or about 1,000 seconds at 450 amps.  That is why you can peak and/or surge the inverter above 3600 watts.  And the inverter is supposed to load limit itself, before blowing the fuse.
A 300 amp breaker is supposed to kick out within a few cycles of exceeding the 300A. It would effectively limit the inverter to 3600 watts of surge/peak.

What is your current wiring situation in the cabin. What is the main breaker ? How many 15 amp circuits do you have/plan ? And how is the generator wired in, if it is ?

ken

Aug 3, 2008 08:09 am
Re: Max Lead Acid Discharge Current.

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.

3 Posts
Aug 18, 2008 03:55 pm
Re: Max Lead Acid Discharge Current.

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.

T.A.S.,

Just read some of the papers and still reading others. Very, very helpful. And so far not too technical as I would think coming from Sandia Labs, etc.Thanks for posting the links.

Tim F.

1 Posts
Aug 24, 2008 02:52 pm
Re: Max Lead Acid Discharge Current.

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.

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