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Tinker

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Joined: 07/11/2007
Location: Australia
Posts: 1904

Posted: 01:24pm 01 Jun 2011

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Somewhere I seem to have read that there is an optimal ratio of a battery bank's Ah and the connected solar panel array - but I can't recall where I'd seen this

Anyway, if anybody knows a table for this I would be interested.

At the moment I have 420W/ 24V of solar panels connected to a 500Ah/ 24V battery bank. Via a MPPT charge controller, panels are on a tracker.

This easily supplies around 3KWh/ day in summer, with the batteries getting full around lunch time. In winter its not so good and I reduced the load on the battery bank.
So, should I increase the panel Wattage or is it better to increase the battery bank capacity? Economics dictate only one of these, but which?

Klaus

VK4AYQ
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Joined: 02/12/2009
Location: Australia
Posts: 2539

Posted: 02:40am 02 Jun 2011

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Hi Tinker

In the old days I used the rule 10 amps charge to 100 amps batteries for a universal system, this was a recommended level in my old handbook. But as you note this isn't constant year round. I used to put extra panels if the charge allowed the battery capacity to go less than 60%.
Your situation sounds like a combination of more panel for winter and more batteries for summer. the more batteries the longer you can go in inclement weather but then you need more panels to recharge them a catch 22 situation.

Work out your daily usage in amp hours and your panel output in amp hours per day the ratio should be 1.33 power in to usage.

Battery capacity should be daily usage times 4 to avoid over discharging your batteries.

Hope this has confused you

All the best

Bob

Foolin Around

Tinker

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Joined: 07/11/2007
Location: Australia
Posts: 1904

Posted: 02:36pm 02 Jun 2011

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Thanks Bob, very useful info. I can easily check the Amp hour in with my Wattsup meter but the battery bank feeds an inverter that has one or two fridges as load (plus CFLights) and the startup current of the fridges is perhaps more than the wattsup meter can handle with the rather smallish wires it has fitted.
I'll try that measurement using a power consumption meter at the inverter which gives a W/hr reading.
I did record that in Summer the power into the bank was around 65 - 70 Ah before the MPPT decided the bank was full so I guess that would come close to what was drawn from it. That would indicate there is enough capacity for my summer usage.

Klaus

Rastus

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Joined: 29/10/2010
Location: Australia
Posts: 301

Posted: 04:16pm 02 Jun 2011

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Hi Bob,
I also found the post interesting!Your reference to a "universal system" suggests that the guide lines would apply equally well to a mill.Is this a fair conclusion?I hope Tinker doesn't mind my querie,I haven't been able to find this type of usefull info elsewhere.At times a lot of searching is done for no reward, which may be put down to the fact that I'm using the wrong description.If other guidelines are needed for mill battery banks I'm happy to start a new thread instead of sidetracking this one.All the best Tinker,Cheers Rastus

see Rastus graduate advise generously

VK4AYQ
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Joined: 02/12/2009
Location: Australia
Posts: 2539

Posted: 12:33am 03 Jun 2011

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Hi Rastus

Yours is a valid point and if I remember correctly Tinker has a wind mill as well to add to the total amps to the battery bank, it could also include a mains booster charger or power driven generator. The idea is of course as we go on we asses the power we use and match panels and mill to eliminate external high cost low efficiency power sources.

Our aim is to as independent as possible of the commercial system that in the future will become less and less cost and environmentally suitable.

Tinker and Rastus

On the high draw starting issue it makes little difference o the battery capacity providing the inverter can carry the short term load efficiently.
Run current 5 amps @ 24 volts 120 watts hr for 8 hours a day equals 960 watts a day for small fridge.
Start current 30 amps @24 volts equals 720 watts times 1 seconds equals 12 watts per start times 15 starts per day equals 180 watts per day.

Run 960 watts plus start 180 watts total-1,140 watts a day.

1140 watts divided by 24 hours is average 47.5 watts per hour equals average current at 2 amps per hour. This is to run fridge not allowing for system losses.

Allowing for system losses 2 amps times 1.33 equals 2.63 amp hours.

System losses are the inverter loss wire loss and battery cycle losses, on most piratical systems this equals 1.33 as a multinational factor.

The four day rule applies unless you have alternate power back up such as generator or mains power boosting so battery capacity needed at 50% discharge would be 500 ah

This assumption is based on measurements on a DC fridge I have tested, running a AC power fridge will be a bit higher due the lower motor efficiency on AC.

My two door AC fridge uses 1700 watts a day as compared to the DC unit @ 1140 watts a day. This situation is repeated by everything we use, and once you get your head around it it is quite easy to work out.

The input is a different kettle of fish as so many variables apply hours of sunlight a day or hours of wind a day, so have fun with that one.

All the best

Bob
Edited by VK4AYQ 2011-06-04

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Rastus

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Joined: 29/10/2010
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Posts: 301

Posted: 02:21pm 03 Jun 2011

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Hi Bob,
I don't have a wattsup meter like Tinker has,so my sums will be theoreticle and I'll use the example above to gauge battery bank size.The only thing that would use additional start up power is the small water pressure pump, and if that increases the bank size by to much,I'll limit its use to the petrol gen, which is already past an economically viable point.Its not likely to be the case, going by the small increase the fridges indicate.A few simple sums can provide the logic needed,Thanks for the extra details,Cheers Rastus

see Rastus graduate advise generously

VK6MRG

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Joined: 08/06/2011
Location: Australia
Posts: 347

Posted: 08:27am 26 Jun 2011

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I'm no expert, but if you added an extra panel, the battery would charge up a little faster and once charged, the excess current from the panels would co-power the inverter. Helping to reduce the load on the battery during the day and saving capacity night-time use.

Its easier to ask forgiveness than to seek permission!

............VK6MRG.............VK3MGR............

VK6MRG

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Joined: 08/06/2011
Location: Australia
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Posted: 08:41am 26 Jun 2011

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I have a remote APRS weather station, VHF radio, weather station controller, batteries, solar charger/load controller, and solar panel. I still require a solar panel to finish the project and at this stage I�m using a modified battery charger at act as the solar panel so that I can get some power usage stats to help determine the solar panel size required. This testing is being done at this time of year as the day light is shorter and helps to give a worst case type calculation. In another week or so i should have some useful voltage and current numbers to determine the power rating of the panel that i need. And for good measure I�ll chuck in an extra 50% on top to cover a move back to the other end of the state in a few years. (I'm in Northern WA) Any excess power that the system has once the batteries have charged will go to co-powering the system this way if I do have a few weeks of very overcast weather the system will be able to cope and the batteries will survive.

Battery charger is on a timer and turns on about 30 mins after sun up, and turns off about 30 mins before sun set. The 30 mins business is because the charger outputs 100% all the time, but in real live conditions the solar panel would ramp up to 100% after a few hours in the morning and ramp down a few hours before sun down.Edited by VK6MRG 2011-06-27

Its easier to ask forgiveness than to seek permission!

............VK6MRG.............VK3MGR............

VK4AYQ
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Joined: 02/12/2009
Location: Australia
Posts: 2539

Posted: 12:53am 27 Jun 2011

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Hi VK6MGR

Sounds like an interesting system to play with, but if you are prone to long periods of overcast weather at your QTH it would need to have extra battery capacity to prevent over discharging the battery, you could try a solar input to switch your battery charger off when solar is available and leave it on to boost when it's overcast.

I have a boost charge battery charger set up through a solar panel controller to avoid over charging the battery, it is a cheap 10 amp controller but it works fine to float at 27.5 volts as it just unloads the charger at that voltage but kicks back in as the volts drop to 26.5. It also has a low voltage cut out control to disconnect non essential loads should volts drop to low.

All the best

Bob

Foolin Around

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