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Topic Title: Off grid questions
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Created On: 09 June 2018 12:53 PM
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 09 June 2018 12:53 PM
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jonny705

Posts: 202
Joined: 26 September 2015

Okay so now I am thinking of turning my van into a camper van, and have inevitably, got into off grid systems.
Hopefully I can work out the basic stuff, like I need 60Ah of use to power a fridge rated at 2.5A 12v for 24 hours- hope this is correct!
Now there are so many variables as to the above example - ambient temp / Peukert effect cable size distance , the only way I am going to know is to connect my fridge a leave it on as such.
But if I just plugged it in and left it overnight I will simply discharge the cells to potentially zero, and learn nothing.

If I use an inverter (I have a decent 800w £300 one,) it will automatically shut down when it reaches a certain voltage and basically stops.
If I connect a known load to this and put an electronic timer on it when it turns off will I be able to work out via loads hours etc. the actual realistic A/H capacity?

Also if I have batteries of the same voltage, but different AH ratings and connected them in parallel, then tried to charge them using a suitable A/H rated charger would it work?
I have a feeling it wouldn't on the intelligent 240v chargers / solar MPPT chargers, both which I will buy, but don't want to make an expensive mistake.

Also if I use my 900 watt suitcase genie to top up batteries, could I theoretically get a 60 AH 12 volt charger, plug it in and run it for an hour to put the 60amps back in?
I have spent ages reading up on this, but wanted to know if I am reasonably getting there?

Fingers crossed someone on hear knows all about this
 09 June 2018 01:17 PM
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mapj1

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your thinking is along the right lines. Be aware that the fridge probably wont draw the same current all the time, it will depend on the temp reached, and the battery voltage - is it a gas-cycle one with a heater, or one with a peltier style heat pump? - the first is resistive, the second is more diode like and a quite non-linear load.
The good ones have a thermostat, and cut in and out when the temp is reached, the cheaper ones just get cold.
You could run a timer and an inverter, I suppose, but there are probably better ways, really you don't want to run the battery right out, so you need over specify capacity, usually by a large factor. a logger is cheaper than a new battery

-------------------------
regards Mike
 09 June 2018 02:05 PM
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jonny705

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Joined: 26 September 2015

That's why I am thinking of running it in a practical situation as such as it's the only way of truly knowing.
The fridge I have is the most efficient type, I done that research! It is an Engel MT17 and pulls maximum of 2.5a @12v or 1.25 @24v.
I did buy a cheap £10 watt meter type thing from eBay which was advised.

More not sure about miss matched, either A/H and manufacturers batteries , as no way of testing this other them to buy expensive charger /controller and find it don't work.
I just have a load of really decent ;( well 4 ) batteries as took my Landy off the road and it had a few for the winch so just all sitting there really.
 09 June 2018 02:30 PM
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AJJewsbury

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Hopefully I can work out the basic stuff, like I need 60Ah of use to power a fridge rated at 2.5A 12v for 24 hours- hope this is correct!

Basic numbers seem correct (but Ah is a bit of a variable feast, varying for a particular battery a bit depending on the discharge current and so on).

FWIW, if your fridge is like caravan ones, the 12V option is usually only used when the vehicle engine is running - at other times it's run on gas or even mains (230V) if it's a 3-way type and a hookup is available. The one we have is somewhat less powerful on 12V than on gas/mains - the manual says something along the lines of 12V will keep it cool, but won't get it cool - so you have to use gas or mains to initially get the temperature down.

Otherwise have a good read of section 721 - there are some quite significant differences in the requirements for (motor) caravans compared with normal buildings.

- Andy.
 09 June 2018 03:38 PM
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Zoomup

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12 Volt fridges are a good way of running a battery down. Most camper van people do not need one. Just live off fresh fruit, veg., tinned and dried stuff. Leisure batteries are best if you do use a 12 Volt fridge. Engine starting batteries are not made for constant charging and deep discharging as it lessens their overall life. Most cat batteries are kept pretty well fully charged if you think about it.

If charging batteries in parallel it is best if they are all the same capacity and make. If one is weak or faulty the others will try to discharge through it.

I saw a small camping fridge on fleabay that can run on 12 Volts, mains or gas. That may be an option. Just think about the weight and size of storage required to carry several 12 Volt batteries on board your vehicle. It will add to fuel consumption.

Z.

Edited: 09 June 2018 at 03:56 PM by Zoomup
 09 June 2018 04:13 PM
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broadgage

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The "3 way" caravan fridges do indeed have a substantial loading and a rather poor performance when on 12 volts.

However the O/P states that they have an Engle fridge, these use a high efficiency DC compressor and can be run on battery power for days, with a just about reasonably sized battery.
 09 June 2018 04:38 PM
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perspicacious

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could I theoretically get a 60 AH 12 volt charger, plug it in and run it for an hour to put the 60amps back in?

You could try but generally anything exceeding 10% of the Ah rating tends to buckle the plates so most lead acid batteries (even gel) recommend considerably less than the Ah rating.

Check with the battery supplier for maximum charge rates.

Also, the Ah rating is based on typically a 20 hr rate so your 60 Ah battery should in theory be able to supply 3 A for 20 hours before being discharged to a declared value.

Regards

BOD
 09 June 2018 04:43 PM
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perspicacious

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Also, standard car battery (as opposed to a leisure battery) has very limited charge discharge cycles (possibly as few as 20) and even the leisure batteries can offer as few as 100 cycles.....

Check manf data.

Regards

BOD
 09 June 2018 04:49 PM
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HarryJMacdonald

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My maths says that 2.5 times 12 times 24 is rather more than 60! I suggest your battery will be flat in 2 hours not 24.
Yes, the 12V input is recommended for use when driving not from the battery.
Regarding parallelling batteries, if they are the same type and in roughly the same condition then you can parallel, but I suspect the one in the poorest condition will pull the others down so probably not a good idea unless you are sutre they are all in reasonably good condition.
 09 June 2018 05:02 PM
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jonny705

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When I first looked into doing this, I thought fridge and heater is the thing that will make your life easier, Webesto diesel heater will cover that and Fridge was only concern.
The 3 way fridges are basically terrible as not designed for one thing or other, you cool them at home on 240- run them on 12v while driving to site and then wack them on gas or hook up - the 12v will last about 12 hours if you are lucky.

The Engel is probably the best of its type and well made - it has one moving part- I also intend to build an enclosure for it and insulate that and maybe put a small 6/12v pc fan with its own little battery and solar panel to remove the heat round the compressor and make it more efficient.

I have read since O/P that unless the batteries are all the same age and spec (at least with solar banks they will shorten the life out of all your battery's so not good idea).
But interesting to find out that 2 say 100 amp in parallel is more efficient than one 200 amp- hence I guess the battery banks you get in solar systems.
TBH I am under no illusion that all this is down to the quality and cost of the battery's ,my spare ones are car batts so not ideal- but free as such and will let me play around hopefully even if they last just this summer

As for topping up unless I spend loads of money on a marine charger that can be any amps really, I will get a 25a ctek 240v and run it from my honda geny - there is no way the fridge will consume the 2.5a for 24 hours ,but am getting it next week so will have a play then.

Once I work out that, the solar panel spec will be available to try and work out-but have found this off grid stuff really interesting TBH.

Just seen other posts - thank you - I also have just found out that you couldn't put 60a in the battery's as like you say they will overheat plates and shorten life dramatically-the marine chargers have thermal probes to prevent this, conventional ones apparently would only give maximum amps at full discharge but tail off amps wise rapidly as the thing charges up I think 10% of the rated A/h is the optimal recommend for long life unless using an intelligent type charger.
 09 June 2018 05:09 PM
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broadgage

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The current used is said to be 2.5 amps, if that is accurate then in 24 hours, 60 ampere hours have been used.

A 60 ampere hour battery would therefore last about 24 hours IN THEORY, but in practice such a deep discharge would be most unwise.

A 200 amp hour battery would give just over 3 days if fully discharged, or 24 hours with a generous margin.

Clarification is however needed as to the current used by the fridge. Is it 2.5 amps continually ? or is it 2.5 amps only when called for by a thermostat. If the later, then we need to know the likely duty cycle.

I suspect that it might be 2.5 amps only when called for at a fairly low duty cycle.
Engle fridges are very efficient. Most can be used as a fridge or as a freezer, and the average energy input would be less when used as a fridge.
More info needed.
 09 June 2018 05:46 PM
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HarryJMacdonald

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Oops, quite right Broadgage.

Do you know if the fridge is an absorption type. Most multifuel ones are. If so, it is likely to be 2.5 Amps continuous with no thermostat. You would really be better off running it on gas except when you have a car engine running to supply the electricity.
 09 June 2018 06:36 PM
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chrispearson

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Originally posted by: jonny705

The 3 way fridges are basically terrible as not designed for one thing or other, you cool them at home on 240- run them on 12v while driving to site and then wack them on gas or hook up - the 12v will last about 12 hours if you are lucky.

The Engel is probably the best of its type and well made ...


But not cheap!

... it has one moving part- I also intend to build an enclosure for it and insulate that and maybe put a small 6/12v pc fan with its own little battery and solar panel to remove the heat round the compressor and make it more efficient.


V. sensible. In my youth I used a 3-way fridge when touring in France. I hooked it up to the mains in the hotel (just possibly gas when camping) and plugged into the car when on the move. It may have been more effective not to have plugged it into the car because a boot full of gear didn't really allow the heat to dissipate.
 09 June 2018 06:55 PM
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broadgage

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Originally posted by: HarryJMacdonald

Oops, quite right Broadgage.



Do you know if the fridge is an absorption type. Most multifuel ones are. If so, it is likely to be 2.5 Amps continuous with no thermostat. You would really be better off running it on gas except when you have a car engine running to supply the electricity.


The O/P states that it is an Engel fridge, AFAIK they only make compressor fridges.
The stated input current of 2.5amps is improbably low for an absorption fridge, they are normally about 10 amps.
 09 June 2018 08:04 PM
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Weirdbeard2

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Hi Jonny how long are you planning to go off grid for? I can get 3 days cold beer and food from a couple of polystyrene boxes that dry ice got delivered by post to a theatre in, it takes a bit of planning cooling the stuff good and cold before setting out, and freezing drinking water in 2 litre bottles to act as cold packs, but at least there there is no hum of the fridge at night, and you can get a 48 hour silent recharge for a quid for a bag of ice cubes at the nearest shop with a freezer isle
 10 June 2018 12:03 PM
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jonny705

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Morning, and thanks for the replies.

Weirdbeard the only reason I am going down this route is the appeal of being in the middle of nowhere as such, the freezer would be used for meat / egg/cheese type stuff.
Also am thinking in the highlands you don't get many shops if you venture out - you defiantly have to plan fuel etc., Water from freshwater lochs via Osmosis filter, and catchy some fishy
TBH have no idea how long you would get before hunger sets in!

Woke up in the middle of the night with a Heath Robeson moment - thought about bonding a continuous length of copper pipe internally inside roof, (black van) feeding to a black catch tank, on roof pumping it with a small solar pump, and heating water, some sort of stat to cut it off when temp drops, went on utube and seen lots of other people have done similar type things, But was my idea till I got up at least

Just steep learning curve regarding solar -chargers and batteries are the thing you need to learn about more than anything.
Will get fridge next week and do some real time testing, in sun with temp probe -to many variables to try and guess it.
 10 June 2018 12:40 PM
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mapj1

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Do not try and charge a flat battery at faster than the 10 hour rate (i.e. capacity in AH, divided by ten, amps ), and ideally a bit slower, if you want it to last, and allow 12 hours for a C/10 charge.

-------------------------
regards Mike
 10 June 2018 01:42 PM
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broadgage

Posts: 2749
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Solar charging is well worth while in these situations.
Practical experiences shows that in good sunlight, that a PV will put about one amp into a 12 volt battery for every 20 watts of nominal capacity.
So a 200 watt module will in good conditions put about 10 amps into the battery. If the module is fitted flat on a van roof I would expect very roughly 10 ampere hours a day in winter and 50 ampere hours a day in summer.

I would proceed as follows.
Fit to the van roof the largest reasonably available PV module and connect this permanently to the leisure battery via a suitable charge controller.
Fit to the van roof a second smaller module, and connect this via suitable charge controller to the vehicle starter battery. This will replace any small or incidental discharge from the starter battery.
Consider purchasing a second large module in addition to the one on the roof. Store this inside the van whilst in transit, and when camping lay this extra module on the ground and connect it via a suitable charge controller to the leisure battery. Connect the charge controller permanently to the battery and use a weather resistant plug and socket to connect the PV module.
 10 June 2018 02:32 PM
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jonny705

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Well strange you say that as was likely to start with a 200 watt panel.
Also was going use some BR aluminium profile 30x30 t slot type stuff to mount to roof.
Will do something so I can slide panel and lock in in place when on site, don't want on my roof permanently as will get bust I know it, also will mount on some hinge type device so can angle it.

I have a honda EU10i which I will take just in case I flatten car battery as is Automatic so no bump starting, but I think some of the decent solar charge controllers allow you to tap of to supply car battery as well to keep topped up.

Can any tell me if it is loads less efficient to run a used house panel at @36volts , or a 17v one more suited to 12v applications. - I will get a decent mppt regulator, but wondered how they step down voltage to @14 volts for battery charge - ie they have heatsinks, will it lose efficiency?

X2 of these are tempting me

https://www.ebay.co.uk/itm/NEW-120W-12v-Energy-Solar-Panel-MONO-crystalline-MC4-Cables-TUV-ISO-UK/253592453083?hash=item3b0b49bfdb:g:TSEAAOxyeZNTQBMB
 10 June 2018 03:15 PM
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broadgage

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It depends upon the type of charge controller used.

The better, but more costly sort are MPPT controllers, these can actually increase the current whilst reducing the voltage.
So for example a module with an output of 10 amps at 30 volts can put perhaps 19 amps at 15 volts into the battery.
If the battery is low at only 12 volts, it might put 23 amps into the battery.
If using a module primarily intended for use as part of a grid tied array, then you need a MPPT. The PV module can be any voltage from about 18 volts up to the maximum the controller can handle, often as much as 100 volts.

The cheaper and simpler type of controller simply connects the battery to the PV module and monitors the battery voltage.
When the battery is nearly full at say 14 volts, the controller either short circuits the module, or open circuits it, for a fraction of a second at a time.
The net charge into the battery is controlled by altering the mark/space ratio.
For this to work efficiently the module must have a higher voltage than the battery, but not too much higher. About 17 or 18 volts is usual.
IET » Wiring and the regulations » Off grid questions

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