Hi Everyone

           Need some advise if possible ,we have just purchased 2  200 watt solar panels for the roof of our coaster as we want to power our fridge.The plan is 2 solar panels, battery charger regulater then 1   125 amp deep cycle battery.The fridge uses 5amps.We wish to also run off this unit about 12 watts of LED lighting.Will it work and remain operatioal,also will be secondly wired from our bank of house batteries with an isolated circuit

Thanks

Hi Ian

I would suggest you require a MINIMUM of 200amphr battery capacity to get you through a couple of bad days of solar input

You do have excess solar to battery storage capacity& should use that solar capacity for power storage

 Even 300amphrs would not be over doing it & would give you a bigger POWER bank for bad days of solar input

Also remembering the less a battery is dicharged the longer it's life

Your solar panels will AT THE BEST put in about 140amphrs per day

Depending on ambient temp, fridge thermo setting, opening of fridge door, & warm loading ,your fridge COULD  use between  30amphrs [actually running 25% of the time] & 120 amp hrs [running continuous]per day[24hrs]

But probably more like 40 < 90amp hrs per 24hrs

For long life it is advised that a lead acid battery not be taken below 50% of it's capacity

 A 200amp hr battery = 100amphrs safe discharge for long life 

Also remember any power taken from the battery requires about 25<30% more to replace

This applies to that power taken out when no solar input is available[from say 4pm to 9am]

From the above you could work out just what you believe YOUR situation may be

Peter

I see where you're coming from but I can't help but feel you're making life rather complicated. Why not just connect your solar to your existing system. Definitely install a voltage monitor so you can keep an eye on the consumption from the battery and take steps to avoid getting caught. I fail to see why you need an electric toaster and jug when you're dependant on battery power. A kettle and and stove top with a little camp toaster or a griller works fine.

As for powering a hot water system from a battery via an inverter???? why didn't you go for a gas system??? or diesel maybe??? Having sufficient battery capacity just to do that must have cost an arm and a leg! Without doing the arithmetic I guess if you live for a couple of centuries you'll eventually amortise this cost, provided you don't add in the opportunity cost.  I can't think of a less efficient way of providing hot water!

Rather than making an overly complex situation even more so, I feel that your existing house battery combined with the solar panels would be more than enough provided you don't use it up like there's no tomorrow.

Nichol wrote:

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I can see the benefit of having a separate battery here, it does mean that all the solar energy stored will go into storage to be used for the fridge, rather than being used for anything else.

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 Provided the two systems cannot be inadvertantly interconnected in the event of needing to fall back to the main house battery for the fridge.

This can be achieved by installing a marine type battery changeover switch and not using it selected in the "both" position.  These are available at any marine shop and I've seen them in caravan shops too.

With this fall-back position available maybe the 125ah would be enough.  Worth trying it out before committing to the expense of another battery.

PS: Would also suggest changing (with the switch) to the main house battery when travelling as there is no provision to recharge the fridge battery from engine, especially in conditions that are less than ideal for the solar panels.

Would also sugggest installing a separate battery charger to top up the fridge battery and power the fridge when mains power is available.  Your existing combo would not be suitable as it is already "spoken for" for recharging the main house battery.

Hi Ian

Re :" 6000 watt combi inverter charger,this runs our 240 volt gear,at present also runs fridge and lights.We run A/C toaster jug E/BBQ ---"'

I hope the Combi was installed by someone who knew the requirements of the standards & just how the Combi needs to be installed

Have you had ALL your power points tested for correct operation of the RCD[safety switch] ???

I DO NOT MEAN BY PRESSING THE TEST BUTTON

I am aware that some have not been installed correctly

Peter

Hi

Jim's points above are extremely important with high currents such as you have with THAT inverter [possibly in excess of 500amps}

It is not so important with an all series battery set up

BUT should be standards practise with ANY parallel battery set up to ensure equal load sharing & recharging

Best explained as the curent path using the same sized cable should be always the same length no matter from which battery the current is coming from or going to

Also with such high currents I would also suggest that all the interconnected batteries be the same amp hr rating & type

While Jim & I may have differing views on the pros & cons of series & parallel batteries, this is one type of service where parallel batteries do need great care

The more in parallel then the greater the care needed with the interconnections

That includes cleaness & tightness  of ALL joints

Peter

Hi Ian and Shaz,

Your head's probably "swimming" by now so I'll see if I can keep this simple.....

The more I think about your set-up the more I believe that you should give up on the idea of separate battery/solar for the fridge.

If you are going to routinely discharge the house battery to the point where this backup is warranted you are going to seriously shorten the life of those batteries. A much better option is to monitor the voltage of the battery and to take action to conserve battery power before this becomes critical.

By forgoing the separate battery you can use all your solar capacity to recharge the house battery and I think this is a much better use for those expensive solar panels.

While the batteries are quoted as having a capacity of 125ah (or whatever) the real world capacity of any battery is significantly less at the "mega" currents that your system would be drawing at times. The battery manufacturers aren't telling porkies, it's a function of the chemistry of the battery and the way they are rated. (usually based on a 20 hour discharge, 6 amps in the case of your batteries.)

If your hot water set-up contributes to the shortening of the life of the batteries it could make the cost of gas for a gas unit or a gas/electric type, common in caravans, look cheap.

hope this helps,

Jim

PS: If you wish to have some emergency lighting why not just purchase a couple of self contained LED camp lights, available at any camping shop.

 HI Jim

Totally agree with the above

Very well put

I do wonder just what the true remaing capacity of that battery bank may be!

 Another agreement, Gee what's happening???

Peter

Hi Ian

Re :'so we have fitted a VRCD which does."

Excellent .I AM impressed , was that something you asked for or did your electrician advice.

Mind you they can be used instead of the primary van RCD,provided they are wired in correcty with the combi  & mains power

Also effective with genny or inverter  simply plugged in via van power inlet socket

I believe when their mutli function use becomes better understood that is all you will see ,as the primary safety cutout, in ALL VANS MOTORHOMES ETC 

No more worries about isolated supplies developing earth faults & becoming potentialy unsafe

Peter

It all makes a lot more sense now that you have explained that it's a 24 volt system. I guess that the original query about a separate system for the fridge is to have a separate 12v system and that the fridge is a 12v unit (or 12v/240v), have I got this right? If so it all makes much more sense. I'm not familiar with bus wiring so I had assumed it was 12v. Obviously 24 volts does make a lot more sense but my mind reading skills are very limited so it does help to state these things.

Getting right back to your original question...obviously the solar/battery combination should work fine. Just what you mean by an isolating device is a bit vague but if it's something to specifically connect the 12v fridge to the 24v main battery I guess that should be ok.  You do have the option of running the fridge from your 240v ac supply as a fallback if you need to, assuming it is a 12v/240v unit.  Whether the 125 ah battery is sufficient, only time will tell.

That battery arrangement is a lot better as you are only operating the parallel batteries in pairs. The comments made by both Peter and myself about the need to ensure the load on the parallel connected batteries is balanced between them still applies but this can be achieved more simply without taking the individual batteries back to a common point as previous suggested, just check the positive lead to one of the batteries in the parallel bank is connected to the terminal of one battery and the negative lead to the terminal of the other battery.

I have heard that one about cycling deep cycle batteries to allegedly extend their life and must say I do wonder. Regardless, if you regularly deeply discharge them it will achieve the opposite result and yes this applies to deep cycle batteries too.

I don't have "hands on" with 24volt systems especially in the area of connecting 12v devices but I am aware that it is bad practice to just tap into the bottom 12 volts of a 24 volt bank. You would obviously be familiar with 24v/12v converters that are readily available and suitable for low power devices.   There are technologies for connecting higher power 12v circuits to a 24v battery bank and maintaining a balance between the two halves of the series connected bank but I'm out of my league here.

Hi Ian

Re:' spoe to an enginneer and his advise on prolonging the life on deep cycle batteries is to keep them constantly cycling,the inverter cuts out before damage is done as is with most inverters."""

I realy wonder what he meant.

All lead acid battery life is based on number of charge discharge cycles

They do have a limit

The biggest enemies of long battery live are:

[1]overdischarging

What consist of over discharging depends on battery type, but generally it is considered that a LA battery should not be discharged below about 60% capacity for max amphrs per dollar of battery life

The actual voltage for each type @ which that occurs is a little load dependent

With heavy loads it will aroung 11.5, light loads more like 11.8<12v

So your battery life will be very dependent on the actual drop out point of your inverter

[2]how long they lay discharged before being FULLY recharged

[3l the max total power for dollar over the life of such batteries is with frequent SHALLOW discharges ie taking out no more than 30<40% of their capacity, for a useful purpose ,before recharging

It is not advisable to simply discharge them for no useful purpose  

IF that is what cycling referred to .

It is an old wives tale             

THAT will not extend their life

[4] the rate @ which current is drawn. high rate current withdrawal gives reduced capacity[amphrs] & if excessive will do long tem damage to LA batteries

Again  different types have different acceptable discharge rates 

Your battery maker should be able to supply you with such data 

[5]Excessive overcharging will damage the battery particularly AGM types.

[6]correct charger float/ maintenance voltage .too high a float voltage can shorten life

So it is essential your charging system is set for the actual battery type

Again battery maker should be able to supply their preferred figues

[7] then of course there are all the normal checks :

;fluid level if flooded wet cell

terminal clean & tight

 heat if AGMs . They do not take kindly to high temps

AS Jim has said it is bad practise to take 12v off a 24v bank unless via an equalizer

Realy now that we understand your reasons for wanting a dual ,set the picture changes significantly

Perhaps if you can post just what you wish to use on 12v& their current draw.it can be looked @ a little different

For instances it may be simplier to just keep your whole system 24v [using the new panels in series]

 & supply any 12v appliances from 24v to 12v converter

Peter

-- Edited by oldtrack123 on Saturday 3rd of December 2011 09:04:44 PM

Another point about overcharging AGMs is they do not like el cheapo chargers due to their high ripple voltage.

By the way Peter, a bit off topic I know, but my Optima Red Top that I use for my Aux battery in the Pajero is nine and a half years old! The Red Top is classed as a starting battery rather than a deep cycle. It still has plenty of amphour capacity and will recharge quickly from the alternator. I've measured a charging current of over 50 amps when charging from an (estimated) 50% discharged condition. The battery is mounted in the engine bay.

Hi Jim

That is good going

From what I have heard Optima users are happy users & they do accept high charge rates

Perhaps they should be "OPTIMUM"

Of course their construction is totally different to wet cell lead acids

Peter

Ian and Shaz wrote:

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Hi Everyone
Thank you Jimricho,time will tell,with L/A batteries they need to be used to stop sediment buildup between the plates,i am not saying to completly flatten batteries,my inverter drops out at about 11.8 volts,
Thanks
Ian and Shaz

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Hi Ian

The below is relevant to wet cell deep   cycle LA  batteries

If the sediment between the plates up is the reason for regular cycling , then I have serious doubts about your sources knowledge

Normal gassing when charging & vibration when travelling  will dislodge any sediment.

Bear in mind, that sediment is due to plate degradation that occurs during charge & discharge cycles

The more you charge & discharge the greater the degradation & LOSS OF ACTIVE MATERIAL

It is part of the life cycle!

the sediment rarely causes any problem unless it  becomes so deep in the bottom of the battery that it shorts out the plates

Undercharging & sulphation are the major causes of early battery failure

Actual shorting between plates can also occur, due mainly to plates buckling caused by excessively high discharge currents

Longest life wil be achieved by :

 [a] keeping the battery fully charged at the correct float /maintenace voltage [one that hold the battery around 13.2<13.6v] depending on makers reqs

 This is a voltage just below gassing

avoiding excessively deep discharges ,limit to about 50% at the most ,better to only use 30<40%

[c]Fully recharge ASAP after discharging

[d]Do not let stand in a partialy discharged state, even one week can lead to suphation occurring

Note

These remarks are relevant to wet cells they, do not  necessarily apply to AGMs which are more tolerant in many ways

Peter

I'm confused..... Isn't the system 24 volts....what does the 11.8 volts represent? Is that the voltage across each battery and the actual "trip" point of the low voltage protection 23.6 ? that certainly will prevent a deep discharge and AGMs would tolerate a lower trip voltage giving more effective capacity.

re the battery cycling... maybe your engineer mate is referring to UPS operation where the batteries sit for yonks in a stationary environment with a float charge connected and only get a discharge on the odd occasion of a mains outage. In some (possibly many) installations the UPS only has to supply power until a backup generator cuts in.

I also wonder whether the battery cycling rationale is relevant to AGMs at all. Either way your usage pattern is obviously one of regular cycling anyway.

As we've drifted into a more general discussion on batteries I'll drift further and throw in couple more..

When I started doing some 4WD Tag Alongs many moons ago our convoy leader at the time told us that he had found from his extensive experience that the best battery for an auxiliary in a dual battery system was Yuasa Overlander, (a 4WD starting battery) and he had better results out of that than a normal deep cycle. One of the advantages was its quicker recharge time from the alternator. This was before the days of widespread use of AGMs.

Another one to throw into a more general discussion....Gel batteries are usually not appropriate for RV use but I wonder whether this application (the solar for the fridge one, not the main system) might be an exception and their principal limitations, ambient temperature, and charge/discharge current limitations, do not apply here ?????

I don't have any "hands on" with gel batteries but perusing manufacturer's data they seem to have much greater longevity as expressed in charge/discharge cycles.