Photovoltaic (or Solar) panels

[TaxiDriver]

I'm looking to install a leisure battery in my transit minibus to provide an overnight 240v mains supply via a 300 watt inverter, hopefully for 6-8hrs overnight.

The appliance is a cpap machine and I don't suppose it runs anywhere near 300w.

 

I was going to go down the route of a split charge relay and it's been suggested I might have to upgrade my alternator to a higher output.

I would also possibly have to run the engine to charge the battery ready for each night, which would be a pain.

 

Another option suggested to me was to install a PV cell (or two?)

How do I calculate,

A. How big a leisure battery do I need

B. How big a PV cell do I need to charge it during the day ready for use the next night ?

 

Apologies if the above seems longwinded.

Edited March 17, 2015 by TaxiDriver

[bobt]

6to 8 hours at 300w, is a huge amount to expect ftom a solar panel and battery,

not to mention losses with the inverter and storage,

 

its a non starter Im afraid,

 

someone will be along in a bit with the maths Im sure,

[TaxiDriver]

6to 8 hours at 300w, is a huge amount to expect ftom a solar panel and battery,

not to mention losses with the inverter and storage,

 

its a non starter Im afraid,

 

someone will be along in a bit with the maths Im sure,

Thanx,

Have edited my OP as it was badly worded.

I have run the same cpap machine/inverter off a 110ah leisure battery but only lasted about 6-7hrs

[grrclark]

Here are a couple of links that might give you some information http://www.batterystuff.com/kb/articles/solar-articles/solar-info.html and http://www.battery-force.co.uk/chargetime.html

 

If your 110ah battery was flat then to charge it up over 1 hour would need a 110A charging source in theory, assuming no charging losses. To charge it over 8 hours then it is 110/8 = 13.75A charging circuit.

 

So the value that you need to establish for any solar panel is what the power output is and then from that you can workout the amperage. Watt = Volts x Amps, so Amps = Watt/volts. So a 165 watt output panel gives you 13.75A output based on 12v. That one panel would take 8 hours to charge your battery from flat to full capacity, again assuming no losses.

 

To work out the capacity of your battery you need to know what power is being drawn from your inverter & cpap machine together. If your 110ah battery was flat after 6 hours, according to your 2nd post, then assume that they are drawing 18.3ah or so.

 

To get a full 8hr run then it is 8 x 18.3 = 146, so you would probably need to go up to 220ah battery. I don't think there is a common leisure battery between the 140ah and 220ah sizes.

 

Assuming that you need to recharge 146ah of battery use and you want to charge that over 12 hours then you will need solar cells that produce around 146watts of output at 12v. If you want to charge over 8 hours then you will need a 220w output panel.

 

All that is very approximate of course, but should give you an indication.

Edited March 17, 2015 by grrclark

[aris]

Many CPAP machines run off 12v - using a transformer to get the 12v from the mains. Might you be able to power the device directly from the battery without going the inverter route? Might just need something which regulates the power from the leisure battery to make sure it stays steady at 12v throughout the evening.

[TaxiDriver]

The machines manufacturers do a 12v lead but being Australian the + and - are the wrong way around for UK usage,

They don't do a UK version,

Did try and get a lead from Maplins and shorted or damaged the machine and had to get it repaired which was awkward.

 

I guess with regards the leisure battery and PV panel I'll just end up going completely overkill size wise.

[bubble]

hi taxi driver

 

You need two 110ah batteries, and 200w of solar, with a solar controller

 

you have a PM for the rest

 

N

[TimeDrags]

See this link for Resmed who do vehicle converters for their machines - might work with yours

 

http://www.resmed.com/us/en/consumer/support/treatment-and-maintenance/traveling-with-your-equipment.html

 

You can always get a rectifier to run from the vehicle cigarette lighter socket to give 240v to power your machine as though it was at home

[oliver90owner]

Bubble is close to the mark, but all this assumes bright sunshine and correct panel alignment, etc, etc.

 

You give no indication of vehicle driving time each day. Most newer diesels have an alternator capable of 850W or more and would return a good starting battery to full charge in a very few minutes. That could leave oodles of time to charge a second battery.

 

Do remember that even the usual leisure batteries will not do so many cycles if depth of discharge is sufficient to exhaust it. The inverter may well have a relatively high cut-out supply voltage for battery protection against excessive discharge.

 

The usual 300W inverters are likely no better than 80% conversion efficiency at lower than max output, so a direct 12V feed would save a deal of power, but the depth of discharge protection may not be sorted for you.

 

With a good starting battery, you might get some extra running time (at your risk, as well as needing to change it over duri g the night) after the leisure battery expires.

 

Belt and braces? Both split charge from alternator and a large PV panel with a good charge controller. Remember, too, your battery must be ventilated to the outside - to avoid the risk of hydrogen gas escaping into the habited area.....

 

RAB

[V8 90]

Have a look at Optima Gel batteries

[TaxiDriver]

Thank you to everyone who's offered advice/assistance so far.

I spoke to Resmed today (it's their machine I have)

 

It seems that a Converter is the way to go, more efficient than using an inverter

A 110AH leisure battery should in theory give me 3 - 4 nights usage providing I don't use the humidifier as it almost doubles the power consumption.

 

NOW, all I have to do is decide how big a PV cell to fit on the roof of my van

Ohh, and by the best type of deep cycle leisure battery I can afford.