A very basic 12 volt PV system for emergency lighting.

[MaggieM]

How long does a deep cycle battery last? I don't mean in terms of being drained in use, I understand how to work that out, but in terms of expected life of the battery itself. I have been following a couple of blogs by people who live aboard narrowboats and they seem to have to replace their battery banks surprisingly often. It has always seemed to me that the need for a battery is a weak point of off grid systems.

[kenneal - lagger]

It depends on how much you discharge the battery, Maggie. If you discharge to 90% Depth of discharge (DOD) of its capacity, say you use 90 Ahr of a 100 Ahr battery it will only last a year if you are lucky. If you discharge to only 50 or 60% DOD, using 50 or 60 Ahr of 100 Ahr, it will last about 7 years. You need twice as many batteries but they last 7 times longer. When you set up your charge controller or inverter they should have a setting either for DOD or battery voltage at which they cut out.

There are tables available which give the life at various DODs and the most economic DOD to use in various situations.

[adam2]

How long does a deep cycle battery last? I don't mean in terms of being drained in use, I understand how to work that out, but in terms of expected life of the battery itself. I have been following a couple of blogs by people who live aboard narrowboats and they seem to have to replace their battery banks surprisingly often. It has always seemed to me that the need for a battery is a weak point of off grid systems.

The need for a battery is indeed the weak point of an off grid system.

Most types of battery do not fail suddenly like say a light bulb, but gradually deteriorate, therefore to a certain extent the date of failure is a value judgment by the user.

As a rough guide, the cheaper type of deep cycle battery as used in boats or caravans, has a standby life of about 4/5* years if correctly treated.
If the battery is discharged only infrequently for emergency lighting then it should last for the 4/5* year standby life.

If the battery is discharged regularly then life depends on the number and the depth of the discharges.
If deeply discharged every day, then it might only last a year*, or even less. If the depth of discharge is limited to say 20% then the battery should last several years, maybe 4/5*.
*As an average, may last longer but dont count on it

In an emergency if a deep cycle battery is not available, or fails unexpectedly, then a vehicle SLI battery (starting, lighting and ignition) could be used, such batteries are not suited to regular discharge and will have a very short life though.

If one wishes to plan for the longer term, top qaulity batteries are available with a design life of over 20 years, very expensive though.

(in the still longer term, lead acid batteries are low tech and could be manufactured locally useing Victorian technolgy. They would last a lot longer than most modern batteries but would be much bulkier and heavier for a given capacity)

[adam2]

SUGGESTED SUPPLIERS AND PRICES

http://www.windandsun.co.uk/
Are a well established reputable supplier, and I suggest buying the PV module and charge controller from them

They have a 50 watt module for £240, or an 80 watt one for £357
http://www.windandsun.co.uk/

The basic charge controller at the top of this page is sufficient and costs only £20http://www.windandsun.co.uk/prices_char ... ollers.htm
If expansion is likely then it may be worth buying a 10 amp controller, lower down the same page.

A suitable deep cycle battery should be about 70 to 100 amp/hours, since such are not easily transported it would be best purchased locally.
Any motor spares or car battery stockist should be able to order one if not held in stock. Cost would be about £50 to £80.
The battery supplier should also be able to supply battery terminals and the in line fuseholder.

The light switches and cable are standard items widely available.
When buying the light switches be sure to get the deep mounting boxes as used for sockets, the shallow ones wont have room for the cables.
http://www.screwfix.com/ are cheap

http://www.screwfix.com/prods/92326/Ele ... -1-5mm-50M for cable
http://www.screwfix.com/prods/30386/Ele ... X-1G-1W-Sw for switches
http://www.screwfix.com/prods/74062/Ele ... urface-Box for mounting boxes.

For 12 volt light fittings, wind and sun, link above have a large selection, of good qaulity if rather expensive 12 volt lights.

Cheaper articles may be obtained from any supplier of caravaning and boating equipment, avoid incandescent lamps except perhaps for lamps only used briefly such as toilets.

Alternativly, if prefered, normal domestic type pendant lamps may be instaled, lampholders and ceiling roses from screwfix direct.
Fit 12 volt low energy lamps, available from wind and sun, these are available in 5w 7w and 11w, roughly equivalent in light output to 25 w, 40 w and 60w incandescent lamps.
This is probably the best solution but may be a little more expensive.
The use of 12 volt compact flourescent lamps has the advantage of no fittings to fail since the control gear is built into the lamp.
5 watt lamps should be enough in toilets, bathrooms, halls,landings, bedrooms, etc.
The 11 watt lamps being used in kitchens or living rooms if reading or fine work is required.
http://www.windandsun.co.uk/lighting_fluorescent.htm

(edited to fix typos caused by change of server)

[RenewableCandy]

Brilliant, thanks Adam!

[MaggieM]

Thank you, Adam2 and kenneal, for the answer to something I have wondered about for ages.

[emordnilap]

Another question, a bit simpler than Mitch's I hope.

I have a lantern with a rechargeable battery. 9 watt cfl, the input states 12 v dc, .75A.

What size and power panel would be needed to recharge it fully in, say, 8 hours of daylight?

[adam2]

Another question, a bit simpler than Mitch's I hope.

I have a lantern with a rechargeable battery. 9 watt cfl, the input states 12 v dc, .75A.

What size and power panel would be needed to recharge it fully in, say, 8 hours of daylight?

We would need to know the capacity of the internal battery in A/H and also do you mean 8 hours of direct sun (unlikely to be acheived in the UK) or 8 hours of diffuse daylight?

[emordnilap]

Another question, a bit simpler than Mitch's I hope.

I have a lantern with a rechargeable battery. 9 watt cfl, the input states 12 v dc, .75A.

What size and power panel would be needed to recharge it fully in, say, 8 hours of daylight?

We would need to know the capacity of the internal battery in A/H and also do you mean 8 hours of direct sun (unlikely to be acheived in the UK) or 8 hours of diffuse daylight?

Diffuse, I suppose. As for the capacity, leave it for now - I'll have to check it when I remember and I'm off on my jollies today!

[emordnilap]

Another question, a bit simpler than Mitch's I hope.

I have a lantern with a rechargeable battery. 9 watt cfl, the input states 12 v dc, .75A.

What size and power panel would be needed to recharge it fully in, say, 8 hours of daylight?

We would need to know the capacity of the internal battery in A/H and also do you mean 8 hours of direct sun (unlikely to be acheived in the UK) or 8 hours of diffuse daylight?

Both, I suppose. The battery states "sealed lead acid battery 6V 4Ah(20hr)"

[adam2]

Another question, a bit simpler than Mitch's I hope.

I have a lantern with a rechargeable battery. 9 watt cfl, the input states 12 v dc, .75A.

What size and power panel would be needed to recharge it fully in, say, 8 hours of daylight?

We would need to know the capacity of the internal battery in A/H and also do you mean 8 hours of direct sun (unlikely to be acheived in the UK) or 8 hours of diffuse daylight?

Both, I suppose. The battery states "sealed lead acid battery 6V 4Ah(20hr)"

Since the lantern has a 12 volt input and a 6 volt battery it SHOULD contain a voltage regulator in order to provide a constant 6.9 volts to the battery from an external 12 volt source.
I bet it does not though! The charging circuit in all these cheap lanterns (and spotlights) consists of nothing more sophisticated then a blocking diode and a dropper resistor to limit the current to some arbitary value such as 0.5A.
This is not the correct way to charge lead acid batteries and a short life is probable since correct charging is most unlikely.
Charging from a PV module will be even worse since the charging current will be unknown and variable.

In theory to fully charge a 4 A/H battery will require about 4.4 A/H. Therefore a 10 watt module with an output of about 0.5 amp should charge the battery in about 9 hours of sunlight,or a few days of daylight.
Its a bit "hit and miss" though without a proper charging circuit.

If the battery fails, consider replacing it with a 5 cell (6 volt) NiCd battery since these are more tolerant of overcharging and could be left on indefinate charge from a suitable module.

[emordnilap]

Question for you, adam2, from this person-struggling-to-understand-electricity:

I have a solar battery charger which supplies all my battery charging needs in summer. I also have a small mains battery charger as a spare.

Now the daylight is getting scarcer, I would like to use that mains battery charger more often but powered by my 12v leisure battery which is being charged by a small solar panel.

The wall adapter for the charger states 9v, 400mh.

What do I need to run that charger directly from the battery? Just being able to do this simple task may well enhance my basic understanding and I'd be grateful for your help.

[adam2]

Purchase from an RE dealer or auto parts store a step down converter that will step down the 12/15 volts from your solar charged system to the 9 volts required by the small battery charger.
Make certain that the convertor is able to supply enough current, although the load is stated to be 400ma I would buy a converter rated for at least 500ma.
Most such converters come with a cigar lighter plug for in car use, you will therefore need to fit a matching car type socket to your 12 volt system.

Alternativly purchase a new charger, intended for in car use and ready fitted with a cigar lighter plug.

Take care not to overdischarge your main 12 volt system, some of those chargers for small cells are not very efficient and use a suprising number of A/H.

[emordnilap]

Thanks. That's great, you're very kind.

[emordnilap]

Further question: would a step down converter require an in-line fuse?

TIA