24 volt battery bank load

gactrx · September 2014

Hi, I wonder if someone could tell me if I have a 400AH 24 volt battery bank and had a load of 30 amps draw on it, how long would it take to decrease the battery bank capacity by 20%. I think 20% is the recommended maximum to discharge batteries and still have a reasonable life span. I expect if I doubled the AH of the bank I could double the time.
Thanks,

westbranch · September 2014

Re: 24 volt battery bank load

By dead reckoning, 30 amps @ 24v discharge would be about 1.3 hrs for ~ 10% of total capacity, so ~ 2.5 -2.6 hrs for 20% of total, but you should only deplete to a max of 50% so 20% of the 50% is back to ~1.3 hrs...
Yes if you double the bank capacity you would double the time...

Photowhit · September 2014

Re: 24 volt battery bank load

Likely around 2 hours, but this will vary depending on the type of battery.

The battery is rated at 400 amp hours, but this rating is based on a discharge of 1/20th of the batteries capacity or 20 amps. If the battery is discharged faster than that it has effectively less capacity. This varies depending on the type of battery. I suspect most lead acid batteries will come in around 2 hours of discharge at 30 amps at 24 volts(nominal) Please note that you may experience some voltage sag at this high discharge rate, so maintaining the high wattage load may drop below 24 volts and actually draw more amperage...

gactrx · September 2014

Re: 24 volt battery bank load

Hi, thanks for your replies which will be very useful. I was afraid 30 amps might be a bit much to regularly draw. My next option is to fit a dump load controller which would only supply the 30amps when the batteries are full therefore when my 2kw PV array is supplying power which would subsidies the batteries. The total array current is about 17 amps. The one I'm looking at is the SEA440. I want to supply from the battery banks so to keep the voltage around 24 volts.
Cheers,

CALLD · September 2014

Re: 24 volt battery bank load

Welcome to the endless learning world of batteries! I would say about 2 hours, but that is likely going to be the least of your worries! Trying to maintain your batteries in the top 20% is the most inefficient thing you can possibly do with solar power! You are probably going to be wasting more than half your PV production trying to accomplish that. Batteries are most energy efficient operating between 25 and 75% DOD but the trade off is the onset of permanent sulfation. You may keep them on average above 50% SOC and occasionally take them down to 25% SOC but ensure they get a full charge every week or so. Voltage under load means little but can give you some indication of SOC. I've been running my 24v 400AH leisure battery (semi-deep cycle) bank down to 23.5v regularly over the last 4 months and they are still doing fine. Under a 5minute heavy load of over 100Amps the voltage can drop down to just above 22volts in the early morning after a night of discharging slowly powering a light load of about 10Amps on average. To recharge them I usually need about 130% of the AH removed from the battery, but if I was trying to keep the battery in the top 20% SOC it may be around 200% in vs 100% out...

Photowhit · September 2014

Re: 24 volt battery bank load

gactrx wrote: »

Hi, thanks for your replies which will be very useful. I was afraid 30 amps might be a bit much to regularly draw. My next option is to fit a dump load controller which would only supply the 30amps when the batteries are full therefore when my 2kw PV array is supplying power which would subsidies the batteries.

Not sure what your trying to do? If you are trying to run opportunity loads once you batteries are full there are charge controllers that will start them for you, look at the Midnite Classics. Solar does NOT need a dump load, the charge controller will simply turn off the current running to the batteries, Wind and Hydro NEED a dump load.

gactrx wrote: »

The total array current is about 17 amps. The one I'm looking at is the SEA440. I want to supply from the battery banks so to keep the voltage around 24 volts.

17 amps into a 400 amp battery bank, is a pretty slow charge rate of around 4%, 5-13% is typically the desired range. Not sure what you mean by keeping the voltage around 24 volts. 24 Volts is a nominal rating. The voltage will vary depending on the state of charge and loads and charging current. A full '24' volt battery at rest will be around 25.4 volts, while charging it will need to be higher, some batteries need to run around 30 volts when charging, at night when discharging(running loads) it may fall below 24 volts while still being above 50% of capacity.

Cariboocoot · September 2014

Re: 24 volt battery bank load

Technical note which seems to have been missed or not made clear:

A "400 Amp hour" rating is based on 20 hour (usually) discharge: steady current of 20 Amps over 20 hours. With a 30 Amp draw the real capacity of the battery is reduced. It is not a simple percentage or linear adjustment but depends on the actual battery used. It is therefor difficult to say how long a 400 Amp hour battery will last against a constant 30 Amp draw.

When we size batteries for systems based on daily Watt hour usage there is a fudge factor in that the current demand is not constant but averaged over the 20 'no charging' hours of the day. As such sometimes adjustments must be made to make sure the bank can withstand the demands of any particular high loads if they are other than momentary.

CALLD · September 2014

Re: 24 volt battery bank load

Cariboocoot wrote: »

Technical note which seems to have been missed or not made clear:

A "400 Amp hour" rating is based on 20 hour (usually) discharge: steady current of 20 Amps over 20 hours. With a 30 Amp draw the real capacity of the battery is reduced. It is not a simple percentage or linear adjustment but depends on the actual battery used. It is therefor difficult to say how long a 400 Amp hour battery will last against a constant 30 Amp draw.

When we size batteries for systems based on daily Watt hour usage there is a fudge factor in that the current demand is not constant but averaged over the 20 'no charging' hours of the day. As such sometimes adjustments must be made to make sure the bank can withstand the demands of any particular high loads if they are other than momentary.

One thing that has always bugged me with regards to peukert's law is intermittent loads. For example if I were to place a C/10 load on the battery for 10 minutes on and 10 minutes off repeatedly would I get the same capacity as a perfectly steady C/20 load? It's an important question for me because I know for a fact that my inverter does NOT present a smooth load to my batteries. The load presented is a pulsed load that mirrors what one would get out of unfiltered bridge rectified AC. This means that at my inverters frequency of 50hz the load on the battery would be moving from zero to a peak RMS value 100 times per second. This peak current draw is substantially more than what it would be if for example I had an unfeasibly large capacitor across the input terminals. The pulsing nature of the load is so significant that when I'm running my inverter with a moderate to heavy load I can actually feel the pulsing in the parallel input cables due to the magnetic field generated!

Cariboocoot · September 2014

Re: 24 volt battery bank load

CALLD wrote: »

One thing that has always bugged me with regards to peukert's law is intermittent loads. For example if I were to place a C/10 load on the battery for 10 minutes on and 10 minutes off repeatedly would I get the same capacity as a perfectly steady C/20 load? It's an important question for me because I know for a fact that my inverter does NOT present a smooth load to my batteries. The load presented is a pulsed load that mirrors what one would get out of unfiltered bridge rectified AC. This means that at my inverters frequency of 50hz the load on the battery would be moving from zero to a peak RMS value 100 times per second. This peak current draw is substantially more than what it would be if for example I had an unfeasibly large capacitor across the input terminals. The pulsing nature of the load is so significant that when I'm running my inverter with a moderate to heavy load I can actually feel the pulsing in the parallel input cables due to the magnetic field generated!

The current variation caused by the frequency cycle is insignificant to the batteries. They don't even react that fast and your inverter already has electrolytic caps inside (just watch the spark on start-up if you touch-connect the wires) sufficient to smooth it out. Adding additional capacitors externally will change nothing.

The larger and unpredictable effect comes from varying load intensities which cause temporary fluctuations in the real battery capacity. How much and for how long changes moment to moment with the changes in load. That's what will make a difference in Amp hours used and battery lifespan.

gactrx · September 2014

Re: 24 volt battery bank load

Thanks, sounds like it may be difficult to achieve. I will just get what batteries that will suit my new panels. The Midnite online calculation tool says for 24 volt battery bank I will have array power of 1920 watts, current 26.01 amps, battery charging current @ 28.8 volts of 66.7 amps. What AH's should I be looking for to match this system? Would you need to know the sunlight amount for my area to calculate this?
Oh and is says in this configuration I need 0.8 Classics required. Would this current be running the controller a bit hard? eg too hot for long term reliability?
Thanks,

Cariboocoot · September 2014

Re: 24 volt battery bank load

That's a bit cart-before-horse. Usually you figure out how much battery you need for the intended loads and then size the array to recharge it.

By my reckoning a 1920 Watt array will produce 61 Amps peak on a 24 Volt system. There is always some real-world variation on these calculations. Staying within the charge percentage guidelines it's right for around 600 Amp hours: as little as 450 perhaps and as much as 1220 although I wouldn't go over 800 if possible.

The Classic will have no trouble handling this current. It's rated for 80 Amps, and the current will not be constant but rather a peak under best sun conditions and tapering off as the batteries go through Absorb.

What you need to know about local insolation is how many hours of good sun you can expect, and what other atmospheric conditions may affect this. Quite a few factors affect the maximum output of an array, but usually they are not significant enough to warrant concern. Getting enough hours of charging in a day is: shorter daylight = less time to complete the charge, and that can be a big problem. This is why most calculations are based on 10% peak current, 4 hours equivalent good sun, and 25% average daily discharge. That gets you 'in the ballpark', but then you have to adjust to specific location needs.

gactrx · September 2014

Re: 24 volt battery bank load

Thanks, we have an existing 12 volt 880AH battery bank and three solar panels when we bought the property. I purchased 6 x 320w panels but have now decided to get a second battery bank for these and run it at 24 volts. The 12 volt bank is getting old and we will retire it in a year or so. The reason for picking 6 x 320 watt panels was it was the most wattage for the best price at the time. We can survive quite well on the existing 12 volt system but I want to experiment with PV water heating for summer only. It's not practical for us to use thermal panels.
I'll go with a 450-500 AH 24 volt bank as this fits our budget. I'll try to get around feeding directly from the batteries if I can using the Midnite aux output. I am looking at the OPzS 2 volt batteries to hopefully give a longer lifespan if we can afford them.
Cheers,

westbranch · September 2014

Re: 24 volt battery bank load

But you still have not come up with a solution to handle your 30A load unless you are going to somehow split your loads onto the 2 battery banks... Rewire into 2 Distribution boxes (load centers)?

gactrx · September 2014

Re: 24 volt battery bank load

westbranch wrote: »

But you still have not come up with a solution to handle your 30A load unless you are going to somehow split your loads onto the 2 battery banks... Rewire into 2 Distribution boxes (load centers)?

No, if that's not an acceptable current to draw regularly from a battery bank of the size I intend and will decrease it's life span, I'll have to feed current directly from the panels when the bank is charged. This creates a problem with the variable voltage from the panels and a fixed resistance element.

One option may be is to have a very small A/H 24 volt bank (say 2 x 12v batteries in series) mainly to run the Midnite controller and this would nearly always be fully charged and feed the element from these batteries only when the controller can supply the equivalent charging current. eg when there is good sun. The banks would just be acting to stablise and fix the voltage for the element.

westbranch · September 2014

Re: 24 volt battery bank load

Why do you want to power a DHW heater from the batteries when you could do it during the day from excess PV power once the batteries are in FLOAT?
On way is to use the Aux function on the Classic for this...

gactrx · September 2014

Re: 24 volt battery bank load

westbranch wrote: »

Why do you want to power a DHW heater from the batteries when you could do it during the day from excess PV power once the batteries are in FLOAT?
On way is to use the Aux function on the Classic for this...

This would be great if it works but what voltage/wattage element would I use? Also, to get maximum usable power from the panels, wouldn't it be better to have the MPPT function somehow incorporated into the element supply?

gww1 · September 2014

Re: 24 volt battery bank load

g
I have an outback controller and use its aux. to turn on an ssr that turns on my pre- hot water heater with 2000watt elements in it. The water is heated with 240 volts through inverter. My cc setting is to have it start to kick on at 3 volts lower then absorb or float. The guy that helped me understand used a midnight classic in waste not mode. You need a good charge rate for the batteries to divert power from them.
gww

ps The mppt will still be maxamizing the solar output

gactrx · September 2014

Re: 24 volt battery bank load

gww1 wrote: »

g
I have an outback controller and use its aux. to turn on an ssr that turns on my pre- hot water heater with 2000watt elements in it. The water is heated with 240 volts through inverter. My cc setting is to have it start to kick on at 3 volts lower then absorb or float. The guy that helped me understand used a midnight classic in waste not mode. You need a good charge rate for the batteries to divert power from them.
gww

ps The mppt will still be maxamizing the solar output

Ok thanks for that information. Can you tell me how much your inverter draws for the batteries and what capacity bank you have?
Cheers,

gww1 · September 2014

Re: 24 volt battery bank load

g
Depends, when the voltage just reaches the set point to start heating water it oscolates (turns on and off really fast) as voltage raised the percentage of on time raises. When I reach absorb it is sending 2000 watts to the heater. I don't know what the convertion losses are but do know that the solar produces about 10 kwhs more daily with it then when the battery cuts prodution of solar off due to not accepting the power fast enough. I have a 48 volt 800 ah battery bank.
gww

jcheil · September 2014

Re: 24 volt battery bank load

gww1 wrote: »

g
I have an outback controller and use its aux. to turn on an ssr that turns on my pre- hot water heater with 2000watt elements in it. The water is heated with 240 volts through inverter. My cc setting is to have it start to kick on at 3 volts lower then absorb or float. The guy that helped me understand used a midnight classic in waste not mode. You need a good charge rate for the batteries to divert power from them.

That sounds pretty cool. So you have the water heater connected to your inverter and use the SSR to basically enable or disable the AC circuit (both poles I am guessing) feeding power to those elements? What are the settings/voltage points that you use on the Outback AUX? I'm interested because I am going to do "something" like this with my excess power when I get all this new stuff installed. Was thinking of a some window AC unit in my storage container and one in my laundry room that would only come on when my production was in excess. I'd never size my system to always cool them, but if I have the extra it would be nice to have a cool workshop/laundry room sometimes.

gactrx · September 2014

Re: 24 volt battery bank load

gww1 wrote: »

g
Depends, when the voltage just reaches the set point to start heating water it oscolates (turns on and off really fast) as voltage raised the percentage of on time raises. When I reach absorb it is sending 2000 watts to the heater. I don't know what the convertion losses are but do know that the solar produces about 10 kwhs more daily with it then when the battery cuts prodution of solar off due to not accepting the power fast enough. I have a 48 volt 800 ah battery bank.
gww

You system is so much larger than mine will be there isn't much relationship between what each one can achieve.
I'm inclined to just give things a try starting from the cheapest and simplest.
Cheers,

gww1 · September 2014

Re: 24 volt battery bank load

g
I agree, I was just throwing Ideals on cc use based on one post. I admit I didn't read the whole post. Many poeple here are smarter then me, I just wrote one thing I was doing.

j
I am not saying that an ac unit could not be used. You could not use my set points to do it. A heating element can be turned on and off very fast. An air conditioner due to motors and compressers have to be either on at full power or off. The settings in the charge controller would allow you to do this but it would take some expermentation to make sure you are charging and not just draining the battery. Some folks do do it though, Its just not quite as simple.
Good luck
gww

vtmaps · September 2014

Re: 24 volt battery bank load

gactrx wrote: »

I'm inclined to just give things a try starting from the cheapest and simplest.

The method that gww uses ('waste not' mode on a Midnite Classic) is probably the cheapest and simplest, if you have a Classic. "Waste Not" is a mainstream, supported function on the Classic. There's lots of "waste not" info on their website and their forum.

--vtMaps

gactrx · September 2014

Re: 24 volt battery bank load

As soon as I have my panels up in a couple of weeks I'll get a Classic and make a start.
We will use PV water heating from September to March when the weather is warm and we don’t have our wood burner running. We will always boost the hot water with the wood burner as even though those months we use it for cooking and all our baking. When in use the water is being heated. At the moment we heat the cylinder by the wood burner every three days which gives us enough hot water when we are there. I have just installed a new cylinder with much better insulation. The existing cylinder is so old it’s asbestos insulated. During those months according to NIWA the government weather agency our position collects between 4.49 and 5.33 kWH/sqm per day so I guess times 12 sqm of solar panels equals 58.92kWH/day average. Not sure if I have to take into account panel efficiency. That seems a high figure so maybe I do. That’s 16.77% so I think that will be 9.88kWH/day or 4+ hours running a 2kW element.
Does this sound correct?
Cheers,

gww1 · September 2014

Re: 24 volt battery bank load

g

I will let some one else check your math. I always go to pv watts website to get a general Ideal of what will happen. Also there are convertion losses, inverter tare loss and keeping the batteries happy. My veiw is battery first and then rather then having the power lost due to the battery filling at a certain rate I try to salvage the extra for something usefull. I do this 365 days a year and can't say I am perticularly successful due to having massive other loads as well. One way to look at it is if I get all my hot water then the hot water shuts off just like the battery and I start losing solar I could have made.
Cheers
gww

24 volt battery bank load

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