Sizing battery for high peak VA and low daily energy

catkin

Hi all

I'm still designing for a washing machine, and still without a reliable peak VA figure for motor start. Best guesstimate is ~2 kVA based on motors being ~700 W. This for a 5kg dry laundry weight front loading automatic.

The daily energy consumption for 4 wash cycles a day is ~1.0 kWh. Calculating for a 48 V battery bank with 85% battery efficiency and 90% inverter efficiency gives ~50 Ah battery bank capacity.

The question is, will a ~50 Ah battery bank provide enough power to the inverter for the inverter to supply ~2 kVA? If not, how to calculate what size battery bank is required? Will it make any difference if the inverter is operating in overload? Should be OK to operate in overload, with the motor start being a short transient? Will battery efficiency be significantly affected when working close to peak power output capacity?

If the battery bank has to be upsized to provide enough power, presumably the PV array will have to be upsized to provide enough current to gas the batteries and de-stratify them ... ?

... and will this lead to a design where the load factors on battery and PV array are very low making it very expensive for each load Watt and Watt hour?

If I'm thinking right it does not make sense to put a washing machine on a standalone system -- or any other load with low daily energy and high peak VA such as a big pump doing little work. Far better to mix it with other loads, ideally running at different times of day?

mike95490

Re: Sizing battery for high peak VA and low daily energy

The peak only needs to be met for about 5 seconds each time the motor starts up. If the pack and cables (this is where a 48V system will outperform a 12V system) can transfer the power, it's up to the inverter to be able to supply the peak needed. It the pack and cables can't supply the peak needed, the inveretr can't do it's job.

Maybe some folks here have ideas of what inverters are best at starting a high peak load.

I suspect an inverter that is in "sleep" will not be able to wake up and start a large motor, it may sit and oscillate, trying to sleep and start. (This is just a wild guess on my part)

12v @ 2KW = 166A
48v @ 2KW = 41A

niel

Re: Sizing battery for high peak VA and low daily energy

catkin,
although i agree the inverter has to be able to supply this peak power along with the wires being rated for that extra power or the resistance will impede that peak power from being delivered, i believe you are more concerned with the battery end of things. looking at the battery with 50ah capacity at 48v would yield a total power of 2400wh in near shorted conditions (1 hour rate). this would most likely damage the batteries as they aren't usually rated at a discharge rate for 1 hour (even though it would go for seconds rather than an hour) and your peak power you've roughly figured upon would be only a few hundred wh below the 1 hr rating of the battery. now i'm not sure at what point it would be good to go and may even vary with different batteries, but doubling the capacity certainly would be minimum in my mind so that you would minimumly see double the wh capacity than the load would require, ie 4000wh. even if the 50ah battery would work i would see the need to expand battery capacity anyway as a constant rate of 700wh would deplete the battery capacity to less than 50% in less than 1.5 hours. you don't want to deplete your batteries to less than 50% or battery life suffers.
i'd like to hear other opinions on the deep discharge rate and naturally any battery upscaling would mean pv charge upscaling to match.

BB.

Re: Sizing battery for high peak VA and low daily energy

Are you planning on only running the washer when the sun is up and shining?

50 AH battery bank seems to be a bit small--only allowing for, what seems to be, a 1 day buffer of power... If we use the normal 3 day of no sun capacity and 50% max discharge:

1,000 WH * 3 days * 1/50% * 1/90% * 1/80% * 1/48 volt bank = 174 amp*hour (at 48 volts)

This allows the batteries to cycle less deeply and to operate without sun for up to three days (as well at night)--and last longer. And a larger bank will be better able to manage the higher currents...

If you plan is to only run when the sun it up and the solar panels charging--then you can get away with less battery capacity--because the solar charger/solar panels will be carrying the load (and keeping the batteries charged). Of course--the advantage here is keeping the battery bank much smaller and cheaper--if the limitation of only washing when the sun is up is worth it.

If the loads are washed when the sun is not out, the deep cycling of the batteries will limit their life by quite a bit... Check out number of cycles vs depth of discharge for the brand/model of batteries you are looking at.

-Bill

mike95490

Re: Sizing battery for high peak VA and low daily energy

Looking at the cheap Costco deep cycle batteries, 4 of which will make 48V, they are about 100AH rated, so they should be able to deliver the brief surge needed by the motor spinup.

You could improve the system surge, by adding a 4 pack of "super capacitors" like are sold for the "loud car radio" crowd. (to handle the bass thumping)
example: ( via www.crutchfield.com ) 5 farad, .002ohm ESR $190
http://www.crutchfield.com/App/Product/Item/Main.aspx?g=725&i=120PP1005D&search=capacitor&tp=2614

Put the caps as CLOSE to the inverter as possible, with as thick of wires as possible.

That would help launch the space shuttle (5F @ 48V) !

you may be able to make one from raw parts too, from any electronic supply shop

BB.

Re: Sizing battery for high peak VA and low daily energy

For super caps--at that price it would probably just better to get another storage battery...

-Bill

catkin

Re: Sizing battery for high peak VA and low daily energy

Thanks all

mike90045 wrote: »

The peak only needs to be met for about 5 seconds each time the motor starts up.

That's a useful design guideline, given that the manufaturer does not publish the information and I haven't been able to measure.

mike90045 wrote: »

Maybe some folks here have ideas of what inverters are best at starting a high peak load.

The installation will be in India and importation is impractical so inverter choice is confined to what's sold in India.

mike90045 wrote: »

I suspect an inverter that is in "sleep" will not be able to wake up and start a large motor, it may sit and oscillate, trying to sleep and start.

It looks as if there will be spare daily energy so no need to use sleep if it is problematic.

niel wrote: »

... battery with 50ah capacity at 48v would yield a total power of 2400wh in near shorted conditions (1 hour rate). this would most likely damage the batteries as they aren't usually rated at a discharge rate for 1 hour (even though it would go for seconds rather than an hour) and your peak power you've roughly figured upon would be only a few hundred wh below the 1 hr rating of the battery.

That's a helpful way to get a handle on the figures. Would the thermal cycling be damaging? Even though the one hour rate current would only be drawn for a few seconds it would be repeated regularly. Would rapidly changing currents produce high magnetic fields and, if so, would these be damaging?

niel wrote: »

... now i'm not sure at what point it would be good to go and may even vary with different batteries, but doubling the capacity certainly would be minimum in my mind so that you would minimumly see double the wh capacity than the load would require, ie 4000wh.

OK, at least doubling. The manufacturer advises 0.15 to 0.23 kWh per wash cycle when the heater is not used. Planning for 4 washes per day, Ive rounded that up to 1 kWh. OK?

niel wrote: »

... even if the 50ah battery would work i would see the need to expand battery capacity anyway as a constant rate of 700wh would deplete the battery capacity to less than 50% in less than 1.5 hours

700 W is the motor rating. The motor does not run constantly (one reason why front loaders use ~60% less electricity than top loaders?)

BB. wrote: »

Are you planning on only running the washer when the sun is up and shining?

50 AH battery bank seems to be a bit small--only allowing for, what seems to be, a 1 day buffer of power... If we use the normal 3 day of no sun capacity and 50% max discharge:

1,000 WH * 3 days * 1/50% * 1/90% * 1/80% * 1/48 volt bank = 174 amp*hour (at 48 volts)

We plan to wash in the mornings so mostly the sun will be shining and the laundry can dry in the afternoon sun (passive solar!). The latest dawn is only ~1 hour later then the earliest dawn.

Our weather patterns and electricity supply expectations permit designing for only 2 days of no sun. We are on the east coast and it has been rare, even at the height of the monsoon, to go more than two days without effective sun. We are tolerant of electricity shortages, enjoying gathering in a single room at night to save lighting usage when necessary. We do the laundry manually now so plan revert to that and use the washing machine only for spinning during the dark days.

mike90045 wrote: »

Looking at the cheap Costco deep cycle batteries, 4 of which will make 48V, they are about 100AH rated, so they should be able to deliver the brief surge needed by the motor spinup.

You could improve the system surge, by adding a 4 pack of "super capacitors" ...

Capacitors are an interesting idea. High power car audio is almost unknown here so they would probably have to be imported. At USD 199 each plus shipping and bank charges for the 5 Farad (wow!) kit, we could buy a set of four 150 Ah batteries for less (we recently bought 12 V 150 Ah deep discharge flooded cell lead-acid batteries for Rs.7,500, USD 175, each). Maybe the capacitors would last a lot longer though and all those flashing blue LEDs would look neat in the power room!

Can I sumarise the thread so far as "would have to double the battery bank capacity and upsize the PV to provide minmimum charging current to suit the battery bank"? And does that mean "it doesn't make sense to put the washing machine on its own system"? We have an existing system with 14 kW of panels that is underloaded, underoptimised, and mostly loaded in the afternoons and evenings. It has a 48 V, 300 Ah battery bank.

System2

Re: Sizing battery for high peak VA and low daily energy

Just a thought on the "700 watt motor rating".
Lately I've been noticing, at least here in Canada, motor manufacturers are tending away from listing the output power of motors in the grossly inflated HP ratings they have been using, and moving to "watts". Which would mean that, for example, an induction motor listed as 700 watts, could have an efficiency of 50%, so actually consume 1400 watts. I know this is an extreme example, but it does get the point across.
Hopefully this is not what you're facing, but it is something to perhaps verify under the circumstances.
Cheers
Wayne

crewzer

Re: Sizing battery for high peak VA and low daily energy

I'm still designing for a washing machine, and still without a reliable peak VA figure for motor start. Best guesstimate is ~2 kVA based on motors being ~700 W. This for a 5kg dry laundry weight front loading automatic.

The daily energy consumption for 4 wash cycles a day is ~1.0 kWh. Calculating for a 48 V battery bank with 85% battery efficiency and 90% inverter efficiency gives ~50 Ah battery bank capacity.

The question is, will a ~50 Ah battery bank provide enough power to the inverter for the inverter to supply ~2 kVA? If not, how to calculate what size battery bank is required?

Catkin,

Applying a load to a battery causes the battery voltage to drop quickly and then somewhat recover. This battery behaviour is called the “coup de fouet”, or “crack of the whip”.

Applying a large load to a small battery bank may cause the voltage to briefly drop below the inverter’s low battery voltage spec, which would disable the inverter. The low voltage cutoff value is probably in the 40 V to 42 V range for a nominal 48 V inverter.

I’ve previously estimated that a healthy and fully-charged battery bank’s capacity in (in Wh) should be ~6 times the expected start up load (in W or VA). If you’re expecting a 2 kVA start-up load, then the battery bank’s capacity should be ~12 kVA, or 250 Ah for a nominal 48 V battery bank.

FWIW, my 24 V x 400 Ah (9.6 kW) battery bank can comfortably start and operate my front-loading washing machine. I don’t know the startup power requirement.

I suspect that a start-up load of 2 kVa is a bit on the high side. I believe that a healthy and well-maintained 48 V x 200 Ah battery bank would reliably meet your stated requirements.

HTH,
Jim / crewzer

mike95490

Re: Sizing battery for high peak VA and low daily energy

note
I'm assuming that your INVERTER will be a PURE sine wave, not a modified sine.

They are EXPENSIVE compared to the mod-sine inverters, but the pure sinewave is critical for long motor life.

If you do have an existing system, just adding the sinewave inverter will quickly let you know how the battery bank behaves, or if it is too strong of a load to start the motor.

catkin

Re: Sizing battery for high peak VA and low daily energy

Wayne from NS Cana wrote: »

... an induction motor listed as 700 watts, could have an efficiency of 50%, so actually consume 1400 watts ... it is something to perhaps verify under the circumstances

Thanks, Wayne. Unfortunately I have neither a suitable meter nor a washing machine to measure! But the 2 kW figure given for the heater is likely to be correct and that the maximum practicable draw from the 220 V AC supply will be around 3 kW ... it seems likely that 700 W is somewhere near.

crewzer wrote: »

... I’ve previously estimated that a healthy and fully-charged battery bank’s capacity in (in Wh) should be ~6 times the expected start up load (in W or VA) ...

FWIW, my 24 V x 400 Ah (9.6 kW) battery bank can comfortably start and operate my front-loading washing machine.

I suspect that a start-up load of 2 kVa is a bit on the high side. ...

Thanks Jim. That "factor of 6" calculation is exactly what's needed for design calculations. Do you mind if I ask how you estimated it? Your own battery and washing machine experience is reassuring. I suspect 2 kVA is high, too. The designers probably use "soft start"now control electronics are relatively cheap and worth the benefits in quietness, longevity and not making the lights flicker so much. In the absence of dependable data I'm being cautiously conservative, though.

mike90045 wrote: »

I'm assuming that your INVERTER will be a PURE sine wave, not a modified sine.

They are EXPENSIVE compared to the mod-sine inverters, but the pure sinewave is critical for long motor life.

If you do have an existing system, just adding the sinewave inverter will quickly let you know how the battery bank behaves, or if it is too strong of a load to start the motor.

Thanks again, Mike. All our existing inverters are pure sine wave and we would buy nothing else. We have inverters that could be used for testing but no washing machine yet.

A community member has strongly suggested that the washing machine should go on a dedicated PV system. Thanks to the answers given here I can now show that would be the wrong approach and why. It would take the load factor on the battery banks down from our usual 100% (in the dark days) to 17% and on the PV array from our usual 100% (again in the dark days) to 43%. Those are poor usage metrics for expensive equipment, leading to a very high cost per useful Wh.

My recommendation will be to put the washing machine on our "pool" of existing co-located systems with 2.5 or 3.0 kVA inverters and 300 or 400 Ah 48 V battery banks. The only investment that may then be required would be to increase the capacity of one battery bank that is anyway coming up for replacement.

crewzer

Re: Sizing battery for high peak VA and low daily energy

Charles,

The "factor of six" guideline is based limiting a load (in Amps) to ~16% of a battery bank's capacity (in Ah). Here's a link to the complete discussion: http://www.outbackpower.com/forum/viewtopic.php?f=9&t=2312&hilit=coup+de+fouet

HTH,
Jim / crewzer

niel

Re: Sizing battery for high peak VA and low daily energy

catkin,
i agree with your decision to integrate it into the present system provided the age of the batteries are not too different and are of the same model and manufacturer as this will help to cushion that extra draw upon the batteries that would've drawn the voltage down too low. that was a good point i overlooked jim.