Two inverters on one battery bank?

Tulumtam · September 2011

Is it possible to use two different inverters with the same battery bank? My client has a modified square wave inverter but wants to expand the system to power a refrigerator and washing machine. As modified square wave inverters are not recommended for use with these two appliances, I was wondering if I can add a pure sine wave inverter sized to these two appliances and leave the rest of the house on the inverter they have now. What potential problems could there be with this set-up?

They don't have space for chest-style DC refrigerators (they need both refrigerator and freezer, so would need 2) and the Sunfrost uprights are too expensive here (I am in Mexico).

If I can't use two inverters on the same battery bank, then I guess I will have to do a separate system for the new appliances. Unless there are any other ideas....?

mike95490 · September 2011

Re: Two inverters on one battery bank?

As long as the batteries and chargers can manage the extra load, not a problem

BB. · September 2011

Re: Two inverters on one battery bank?

Yes, you can have several inverters connected to one battery bank.

Ideally, you have a single battery bus (heavily connected to the battery bank with thick wiring) and then run sets of wires to each load/charging source (Home Run, or Star configured) not not Daisy Chain (one set of wires going from device to device).

Also, what is the battery bank size (voltage, current) and the inverter capacity (max load, surge current)...

For example, I like to use a 0.4*C as the maximum surge current. Say you have a freezer/fridge that starts with ~8 amps @ 120 VAC = 960 Watts, the minimum battery bank would be:

960 watts * 1/0.85 inverter eff * 1/12 volt battery bank * 1/0.4 surge rating = 235 AH minimum battery bank 12 volts

Note that this is really a recommendation for flooded cell battery banks, but also keeps the stress down for AGM's too (which can surge upwards of 4*C for some brands/models of AGMs).

Normally, to run an energy star refrigerator (plus a few lights and such), our host (NAWS) recommends a 1,500 watt to 2,000 watt inverter.

You would be right on the edge of deciding to do a 12 volt or 24 volt battery bank. 1,200 watt inverter would need to be wired for:

1,200w * 1/10.5 volt cutoff * 1/0.85 inverter eff * 1.25 NEC margin of safety = 168 amp minimum 12 volt branch circuit rating

-Bill

Cariboocoot · September 2011

Re: Two inverters on one battery bank?

Compare the size of the inverter needed for the refrigeration vs. the size now used for other purposes. There's a strong possibility that swapping the old MSW inverter for a PSW unit capable of running the 'frige will be all that's required. Normally an inverter large enough to run a refrigerator will have enough "surplus" capacity to run everything else as well.

Tulumtam · September 2011

Re: Two inverters on one battery bank?

They currently have a Conermex 24v 2400w MS inverter, 330 watts of PV, Steca PRS3030 (30A) charge controller, and 285 a/h in batteries, 2000w Honda generator. The system is a bit low on input as it is, so I hope to correct that in the system expansion. The system was installed in January of this year and has been fine for lights, music, computers, blender. They want to add a refrigerator, washing machine and 1/2 hp water pump (they have been using a generator for the pump). The pump can run on the MS inverter, but not the refrigerator and the washing machine.

The pump is run for one hour, twice a week. They want to use the washing machine (6A) twice a week. I plugged the fridge into a kill-a-watt meter for 48 hours while running and it comes out to 999w per day. What I don't know is the surge when the compressor starts up.

As usual, it comes down to cost. They have $2500-$3000 for the upgrade. The largest cheap PSW inverter I have access to is the Steca Solarix Pl 24v 1100w (3000w surge). I am not sure that this is quite large enough for the refrigerator and the washing machine, particularly should the fridge compressor come on while the washing machine is spinning.....

I can add another 285 a/h in batteries, 560w of PV and additional charge controller for $2200 installed. The Steca inverter runs $750, so keeps me within the budget (barely). My equipment selection is somewhat limited, but if the 1100w inverter is feasible, I think we can make this work.

BB. · September 2011

Re: Two inverters on one battery bank?

Glad you are already running 24 volt system... 1,100 watt inverter might be cutting it really close. The lowest I ever saw NAWS recommend was 1,200 watts--and later seem to round up to 1,500 watt minimum.

For running the washer vs the fridge--You may just want to wire up a transfer switch... Run fridge all the time and flip over for the hour or two for running the washer (need to automate with a timer so that the kids don't forget and leave the fridge turned off?).

If your proposed inverter will really do 3,000 watts for surge of a few seconds, then it should be good enough... Maybe. You will want some pretty hefty cabling to prevent the surge from dropping battery voltage too far.

-Bill

Cariboocoot · September 2011

Re: Two inverters on one battery bank?

Bill's innovative solution for the 1100 Watt inverter could work, providing people didn't open the refrigerator while it was turned off and let all the cold air pour out. It can get mighty hot in Mexico! Basically that is cutting it slim for inverter size, especially (as you know) if the 'frige and washer decide to pin up at the same time. They may have to resign themselves to running the washer off the generator.

I would definitely increase the PV. At 330 Watts it's about 1/3 what I'd like to see for 285 Amp hours @ 24 Volts. I expect panel temperatures can be a problem there too, so unless there's high elevation the derating is probably worse than the 77% we usually expect. If you've got lots of PV and can teach people to run loads while the sun shines and the batteries are full you can squeeze out more Watt hours than you might expect. Again the inverter size is a bit of a problem if it can't handle all those loads at once.

Doubling the battery size is probably not a good idea. 285 Amp hours @ 24 Volts ought to manage 3 kW hours around 50% DOD. How that fits with the over-all power usage and desired discharge maximum you'd know. Any other possible battery configurations available? Like around a 50% increase?

Of course low charging from PV can be made up with by generator power. If you have to run that for the washer any way it will be a bonus. Given the constraints I think you can make the system better, but not perfect.

PhilS · September 2011

Re: Two inverters on one battery bank?

I have three inverters connected to the battery bank. A benefit to this is when a washer kicks into spin or the water pump comes on, the more sensitive stuff (computer, etc.) doesn't get a quick brownout from the larger load starting.

I didn't originally intend it to be this way, it's just how things evolved over 30 years. Now, I like it this way.

I used an MSW inverter for our 1/2hp jet pump. It died an early death, but DID last at least 5 years.

Phil

Tulumtam · September 2011

Re: Two inverters on one battery bank?

I like the idea of the transfer switch, BB. They will definitely be using the washing machine in the morning to allow sufficient time for the solar clothes dryer to do its job and the batteries to charge back up after the draw. And they are trainable.

I am somewhat constrained as well by the equipment they currently have. The battery bank currently consists of 6 12v LTH Solar/Marine 95 a/h batteries. Obviously they already have 3 parallel strings and I don't like paralleling more than that. I can increase the size of the battery bank in increments of 95 a/h. The batteries are short (in stature) and have a relatively short lifespan projection to begin with, so I was going for a 5% recharge rate. Do you think it would be better to up the recharge rate to 7-8% and add less battery capacity? Seeing about 73% inefficiency down here on a good day.

I will switch them to a 24v pump as soon as their current one gives up the ghost. Thanks, PhilS for sharing your experience.

Can I just say, you all rock!

Cariboocoot · September 2011

Re: Two inverters on one battery bank?

How long have those RV/Marine batteries been in use? As you know adding new batteries to old isn't a good idea. This type of battery will "age" faster than a true deep cycle; if they're already 3 years old and/or get cycled to greater than 25% DOD it may be wiser to wait until they give up and replace them with some better batteries. You ought to be able to get "golf cart" type 6 Volts @ 220 Amp hours each: two parallel banks (eight total) would be 440 Amp hours @ 24 Volts and would, in my opinion, be a good size for the system over-all. I share your dislike of parallel banks beyond two. Four is the most I'd ever risk.

Tulumtam · September 2011

Re: Two inverters on one battery bank?

The batteries they have are only 9 months old. They know that what they had installed originally is flawed in many ways, but that is what they have and what we have to work with for the time being.

Most people in the business in these parts tell their clients that it is better to have more parallel strings because if one battery or one string fails, you can remove that string without losing the whole battery bank, cheaper to replace one string, etc. No even derates in their designs, which leads to lots of undersized systems, lots of batteries chronically undercharged, lots of people with little confidence in their systems. I spend a lot of my time trying to fix poorly designed and installed systems. I had a client last month who had 1200 a/h of batteries (2 different brands, 3 different capacities) at 12v with 250 watts of PV on them. They wondered why they were buying new batteries (and adding them on to the old ones) every year or two.

These are homes and businesses that are off grid because the grid is not available. Because of their poor experiences with solar, they will all jump right onto the grid when/if it becomes available. Which is too bad....

Cariboocoot · September 2011

Re: Two inverters on one battery bank?

Tulumtam wrote: »

The batteries they have are only 9 months old. They know that what they had installed originally is flawed in many ways, but that is what they have and what we have to work with for the time being.

Lightly cycled batteries only 9 months old you could add to without worries in my opinion.

Most people in the business in these parts tell their clients that it is better to have more parallel strings because if one battery or one string fails, you can remove that string without losing the whole battery bank, cheaper to replace one string, etc.

The irony here is that such a set-up is more likely to result in failed batteries as keeping them all in balance becomes more difficult!

No even derates in their designs, which leads to lots of undersized systems, lots of batteries chronically undercharged, lots of people with little confidence in their systems. I spend a lot of my time trying to fix poorly designed and installed systems. I had a client last month who had 1200 a/h of batteries (2 different brands, 3 different capacities) at 12v with 250 watts of PV on them. They wondered why they were buying new batteries (and adding them on to the old ones) every year or two.

I spend a lot of my time doing much the same thing. That old "lots of batteries in parallel and a small panel will recharge them in enough time" plan just doesn't work, does it?

These are homes and businesses that are off grid because the grid is not available. Because of their poor experiences with solar, they will all jump right onto the grid when/if it becomes available. Which is too bad....

How good is the grid? If it's cheap and reliable that's okay. If it costs a lot of money to hook up to something that is energized only sporadically ... Why bother?

Look on the bright side: these systems you're encountering have been designed and constructed so poorly that anything you do can not help but be an improvement. You'll be a hero!

BB. · September 2011

Re: Two inverters on one battery bank?

I always several different battery sizing calculations...

First the standard 1-3 days of "no sun" and a maximum of 50% battery discharge... So, for 2 days of "no sun" and 50% maximum discharge would be:

2 days * 1/0.50 max discharge = 4x average daily load

The other is maximum surge current... the battery bank capacity (especially for flooded cell type) should be at least 2.5 times the maximum surge current.

Say you have a 1,200 watt inverter with 2,400 watt surge (say an AC well pump) running on a 12 volt battery bank:

2,400 watts * 1/0.85 inverter efficiency * 1/12 volts * 2.5 surge = 588 AH @ 12 battery bank minimum

Next you need to do the normal solar panel sizing calculations twice. Once for the minimum power needs (hours of sun needed to recharge the battery bank assuming 52% system efficiency) and a second calculation of panel size assuming 5-13% maximum rate of charge (assuming 77% derating).

You can go with a maximum of ~25% rate of charge--but that is a hugely expensive solar array for most people. 10% is a pretty healthy sized solar array for most people.

-Bill

GreenGen XVIII · January 2015

Re: Two inverters on one battery bank?

I was thinking of connecting a couple of inverters using 24V dc input, what should be the nominal battery bank voltage?

waynefromnscanada · January 2015

Re: Two inverters on one battery bank?

GreenGen XVIII wrote: »

I was thinking of connecting a couple of inverters using 24V dc input, what should be the nominal battery bank voltage?

Is this a serious question?

GreenGen XVIII · January 2015

Re: Two inverters on one battery bank?

yes wayne, i'm afraid it is, i'm experimenting with designs and ideas in designing a solar power system

Photowhit · January 2015

Re: Two inverters on one battery bank?

GreenGen XVIII wrote: »

yes wayne, i'm afraid it is, i'm experimenting with designs and ideas in designing a solar power system

I think he was suggesting you could look at the question for the answer.

...but since I run a 24 volt system, I might suggest if you plan on using 24 volt flooded lead acid, battery bank, you look for inverters which can handle a range of @22-31 volts. Some inverters top out at @30 volts, usually less expensive ones. If an inverter can't handle voltages above 30 volts, it wll shut down when you are equalizing your battery bank.

Two inverters on one battery bank?

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