UKH

Can I ask for advice from any electrical engineers or physics folk about an issue that’s spoiling my holiday?

I have a campervan with a professionally installed 120w solar panel with an MPPT controller going to a 110ah AGM battery, all fitted last summer. The campervan,a Hymercar, has a Thetford fridge which according to the instructions has an average power consumption of 0.40 kWh/24h - 0.30 kWh/24h (Night mode). The instructions also state in the FAQ section that the power consumption is -The T1090 refrigerator has an average use of 4 amps at 12 Volt in the day mode and 2 amps at 12 Volt in the night mode. 

The MPPT solar panel regulator has a Bluetooth connection to an app on my phone which indicates various measurements, but the main ones are the voltage of the battery and the amps going into the battery which vary between 0.9 when very dull but daytime and over 5 when in full sunshine. 
Problem I’ve had is the battery warning light has come on twice now saying flat battery, both times overnight when the fridge has been switched to night (energy saving)mode. I’m trying to understand whether I’ve asked too much of the system but I thought a solar panel would allow me to run the fridge for longer than a couple of days before flattening the battery. 
there are no other major energy using appliances in the van as the lighting is led and we have been using the campsite washing up facilities. 
We have also been driving the van daily sightseeing and this charges the leisure battery too. 
using the figures I’ve given is it possible to work out how long the battery should last before it is flat? 

In reply to didntcomelast:

Looking at the Fridge: at 3A (splitting 2 and 4A, day night) * 12V x 24Hr your fridge is using 0.85KWHr/day.

Split the difference between 1A and 5A into the battery for an average ish day and assume you get decent sun 11-6 this time of year: @12V you'd be getting 12V * (1+5)/2Amps * 7hr = 250WHr

In reality your days (3 season) might be a bit longer and it charges nearer 15V so say 300-350WHr as a first pass guestimate at how much energy you harvest per average day. Then you have the charging (and discharging) efficiency of lead acid to consider which isn't very good but at those low currents should be as good as it gets, say 90%, you're back to maybe 0.25-0.3KWHr harvested in an average 24Hr block.

You won't get that on a grey day of course and on a good day you maybe haven't got capacity to store all of what you'll get to build reserves (your battery is just over 1KWHr capacity so roughly 1 day of running the fridge from full to not quite flat).

From the fridge current numbers we can see the fridge uses broadly what the panel/charger makes in reasonable conditions, conditions which only persist for ~1/3 of the 24Hr day. Without making measurements and doing some more careful analysis I'd guestimate You need 3, maybe 4x as much solar panel for semi-reliable 3 season stationary off grid use. Even then you'd only be a dull day away from running the engine.

Get a terracotta plant pot, keep it wet and upturned in the shade, put your milk in that. Switch your fridge off and enjoy the rest of your holiday.

jk

In reply to didntcomelast:

If your battery is 110AH, then it will provide 1.32KWh (110A x 12v).

Your fridge will use this in about 4 days.

Leaving the battery flat at any time will destroy it's capacity.

In reply to didntcomelast:

I reckon that you'll get 3 days off the battery and 4-5 days with the solar cells based on what you've said.

But: if it's really hot, your fridge is badly ventilated and you are parked under a tree etc etc the battery will only last one day.

Chances are you have a mild energy deficit and after a few days the battery isn't fully charged and doesn't last.

But: this battery warning light you speak of is triggered by something unspecified and might not matter because leisure batteries remain happy for longer at lower charge levels than main batteries (which may be damaged by these conditions) so the warning light may simply be paranoid.

If you genuinely don't have enough juice to get you through 3 days and the conditions are reasonable and the fridge has ventilation etc etc then something is wrong, most likely the battery is shagged.

Classic problem is to box the fridge in so it spends all its time on.

Enjoy your trip, if a less than ideal fridge/solar charger/battery is the greatest of your worries consider yourself lucky!

In reply to jkarran: Thanks for a very comprehensive reply which as a layperson I understood barely any of apart from using the terracotta pot!! Sorry.

In reply to didntcomelast:

I wish I was as brainy as JK

In reply to gravy: The whole set up is in a Hymercar which is a specific campervan/motorhome manufacturer. The fridge is a factory fitted one without any modifications . The original battery was replaced when I had the solar panel fitted supposedly to five more days off grid use. Looks like that hasn’t worked  

The fridge isn’t running all the time it’s working as it always has done but clearly something is wrong. I’d like to think it’s just a battery issue which is still under warranty, though how you prove a warranty claim with a leisure battery failing in use is beyond me?

unfortunately your last paragraph wasn’t very helpful though as this holiday has been cut short by us having to return home early due to my wife having had a call from hospital to have a lump they found in her armpit biopsied, not good when she’s already had cancer twice, and my wife had read the post . Sorry to be blunt but we do therefore have far greater worries but this problem is just adding to them 

In reply to David Riley:

You'll kill a lead acid battery in very short order using more than 40%of its capacity. 

In reply to artif:

AGM is designed to go a bit lower than that, but yes, it's still not good to take it too low and for constant cycling you don't really want to be dropping below 50% SoC.

OP - I'm sorry to hear about your wife.  Best of luck with that.  To try to get this problem off your plate, I suspect the battery itself is duff.  When you say you were running the van every day - are we talking ten minutes down to the shops, or a good motorway speed blast?  If the latter then the alternator should have kept it topped up regardless of what the PV was doing.

What voltages are being reported by the app at various times?  Do you have a voltmeter you can confirm them with?

In reply to Jamie Wakeham: Thanks for the kind thoughts. We have been driving around for about 30-40 miles per day some at 60mph. 
The app would show the battery voltage at 12.8v up to 13.1v when in the sun with the solar panel working, when the fridge fires up that would drop to 12.6v or 12.4v but as soon as the solar panel stops the battery will sit at 12.6v until the fridge kicks in when the battery drops to 11.2v. I’m sure the battery has suffered a catastrophic failure for some reason and I’m resigned to sorting alternative power (mains) till we go home. I now need to think whether it was actually worth the outlay for the solar set up as it seems we are only getting a day or two more power than we did before we fitted the panel. I’m wondering whether fitting two slightly smaller lead acid batteries to replace the AGM battery would be a better option. I could get two lead acid for less than the single AGM 

In reply to didntcomelast:

That sounds a like like a sad battery to me.  The numbers suggest you're never returning the battery to a full state of charge and are often running it close to depletion.  This does not bode well for their longevity.  As the battery degrades ("sulfation"), its maximum charge rate slows down, which means you don't get as much benefit as you should from that 30-40 miles of driving a day.

In terms of future improvements, one to consider is adding a battery protector to the fridge circuit so that the battery isn't pulled down below about 70% SOC (maybe a bit lower for an AGM).   

You may nor may not be able to partially recover a sad AGM with a cheap device called a "desulfator" - opinions vary.

In terms of usable depth of discharged (without fast trashing the battery), a 12.8V, 50Ah lithium (LiFePO4) battery would probably do you better.  This would need adjustments to the settings on the solar MPPT and to whatever charges it from the main engine electrical system.  

No point questioning the past too much, it's set in stone.  You've got more power than you did have.  My suspicion is that the battery is knacked, and that a similar - but new - battery would do much better if you had a bit of daily driving, made sure it didn't get discharged too much during daily cycling, and kept it on a maintenance charge when not in use (the MPPT should suffice in the absence of vampire loads, if you park in the sun).  It's worth thinking about the time you run the engine as well to get the most out of both sources.

In reply to wintertree: I actually use the van as my daily transport when not away camping so the leisure battery gets lots of alternator charge and when it’s not being driven it’s parked in a sunny area so always gets light to thr solar panel  I did consider lithium initially but the high cost together with advice that as I was only ‘off grid’ for up to 5 days at a time I would not need it . The van came initially with a 95ah lead acid battery which was good for 3-4days use even without driving or plugging in  I thought the PV addition would extend that. Need to find out now whether the battery warranty will cover the battery failure, I suspect not 

In reply to didntcomelast:

my self conversion has a 270W solar panel and 2 x 140 amp hour gel Batteries. While I was working on the conversion in Sheffield the solar was keeping my Thetford T1100 fridge going and also kept the starter battery charged. I did invest in an expensive Victron combined mppt and charge controller which I believe maxImises what I can get out of the batteries. It may be that your solar panel and batteries are undersized for the use you are looking for? A good tip can be to set the thermostat on your Fridge not too cold?

In reply to didntcomelast:

Yep, that's not right.  For whatever reason that's a fairly knackered battery.

As Wintertree says, you are where you are.  With hindsight you might have been better off not bothering with the PV and MPPT charger, and just fitting a second big AGM battery. Both charged from the alternator would give you several days of fridge before you needed to drive to recharge.  But given that you now have the panel and the charger you may as well get it working properly.  There is no way that an AGM battery in a custom designed system like this should be so badly depleted in, what, 14 months?  Go back to the installer and ask for a new battery.

No, you want AGM or Lithium for deep discharge.  If I were you I'd be going back tot he supplier, asking for a replacement for the AGM, and - once they've agreed to that - asking if you could add cash to get either a larger AGM or perhaps Lithium battery instead.

In reply to didntcomelast:

From your FAQ the fridge draws 4A in day mode, 2A in night mode so we can reasonably guess at about 3A average over a 24Hr cycle.

Your solar panel when it's working delivers between 0.9A and 5A to the battery according to your app, again let us take the average since there's no point just looking at the best case and worst case it's woefully under spec. On an ok day, we put 3A from the panel into the battery while the sun shines but only for about 1/3rd of the day, the 8 bright hours around noon, for the remaining 16Hrs 0A is a reasonable estimate. Overall we therefore only average 1A from the panel to the battery over a 24H cycle.

Over that same 24H cycle the fridge uses more like on 3A, you have an average deficit of about 2A, 1A in, 3A out. You therefore need 2 more panels to break even or you need to reduce the fridge's energy consumption or you need to run the engine regularly or you need to plug in.

Your battery alone, without the solar and/or the engine, is only sized to run the fridge for slightly more than 24Hrs. With the solar connected in ok sun conditions you might expect 2 full days of fridge running to dead flat (which isn't ideal!).

jk

In reply to Jamie Wakeham:

Or a resistive/loose connection somewhere.

jk

In reply to didntcomelast:

Then your fridge actually consumes significantly less than the datasheet/FAQ suggests. That's a good start!

jk

In reply to didntcomelast:

Best wishes to you and your wife!

In reply to didntcomelast:

Before changing the battery, unless you're going LiFePO4 I'd upgrade the solar panel to ensure that the new battery doesn't suffer the same fate - you can get a 340W domestic 40v panel for £100 these days (assuming your controller can handle it).

I bought a 100W LiFePO4 from China a couple of years ago now, and I reckon if you're doing a lot of heavy usage of the van it works out cheaper in the long run than buying multiple lead acid batteries as you wear them out.

AGM is a Lead Acid battery - it's a slightly better technology than a flooded battery, so it can take a bit more abuse for the same weight of lead, but at the end of the day it suffers from the same problems with being over-discharged as any other LA battery.... the only way to make a truly robust LA battery is to make it very heavy.

If buying two cheap flooded batteries means you don't discharge them as deeply, they probably will last a lot longer than the single AGM. 110Ah LA really isn't that big - you can only use about 45Ah of it if you want it to last, so 2 * 110Ah would give you about 90Ah of useable power. It will also be quite heavy.

On the other hand, lithium can be fully discharged, as long as the battery comes with a built in battery management system (BMS), which it certainly should. 

So one 100W LiFePO4 will give you more useably power than two 110 Ah lead acid, will be very light, should be good for being be reliably deep discharged a couple of thousand times, and won't be damaged if you run it down completely on a cloudy week.

You can buy direct from China for less than £300, although you should get customs charges on top (mine didn't attract any for some reason... I didn't see any markings on the box to say it was a gift or anything but they let it straight through)

The other think is you'd probably need a different MPPT controller, unless yours has a lithium mode.

So the cost of upgrading can be significant, but in the long run I think worthwhile for reliable and trouble free electrics.

In reply to jkarran:

The maths does seem a bit wonky.  The first post says it uses 0.4kWh over 24h (in day mode) and also that it draws 4A at 12V, which gives 1.15kWh for a whole day.  

I can only guess that that the 4A draw is not full time.  If the fridge is genuinely using 0.4kWh per 24h if run in day mode, and less than that if partly run in night mode, that sort of fits with the battery managing three or four days.

In reply to jkarran:

Yep.  Or the app getting it wrong, so do check these figures with a real world voltmeter!

In fact - is it possible that the physical connections that the app is using to measure voltage are dodgy, so the app is sending warnings for what's actually a perfectly healthy battery?

In reply to Ciro:Unfortunately there is not enough space on the roof to accommodate anything bigger than the panel I’ve got. I did initially consider lithium but the price was off putting.

I will look into the costs once we get home, fussing over this trivial matter is good for taking my mind off far more serious clouds on the horizon  

In reply to Jamie Wakeham:No the fridge isn’t running continuously, the compressor only starts up when the thermostat tells it the fridge temp is to warm I guess. That’s one of the reasons why I’m baffled how the solar panel charger seems so inefficient 

In reply to CantClimbTom:Thank you. You’d think that at the third time we would be used to this stress but sadly this time we are more worried than ever, we can’t be lucky every time can we? 

In reply to didntcomelast:

Re: battery testing - I'm not sure but the last dodgy battery under warranty I had was declared knackered by the garage that sold it to me and replaced no quibble foc.

Good luck with the hospital - thats crappy new and apologies for the insensitive dismount.

In reply to gravy:Apology accepted. It’s not the best time at the moment and we ran away from the problem a bit when we came in holiday. It was our way of taking some time out from the stress that failed when the battery failed. You weren’t to know. 
it’s never simple though as we are currently in Sussex, live in Newcastle and the place that supplied the battery is in north Wales. 
 

In reply to jkarran:Thank you for a simplified explanation which I do now understand. I would hope the solar panel with the MPPT controller would push out more than a minimum 3a over the course of a day but I can see how it is a close thing. Sadly I cannot increase the solar panel size but I could look at lithium as an alternative storage system which seems as though it can take regular discharges. 

In reply to didntcomelast:

It really doesn't sound like an "inefficient solar panel charger". 4A on day mode (as other have suggested) will not be full duty cycle. Might be a lot easier if you could supply the model numbers of the fridge & MPPT.

We've got a 100w panel and 110Ah battery. Last year I managed a week at fairhead with a borrowed compressor cool box (good one, 24l or so). We drove 20 minutes twice during the week, and every man and his dog was also charging their phones etc. We kept good charge all week. A battery dropping to 11.6V on load sounds a lot like a bad battery or poor connection somewhere.

We did have that once. Whenever we put mobiles on to charge the battery dropped right off - loose earth connection! 

In reply to didntcomelast:

Does your solar controller give you a reading of total energy generated? Mine does via the Bluetooth app. Checking the current at random times doesn't give you a a particularly good estimate of how much energy you're actually getting.

For comparison I have a 130W panel and a 110Ah sealed battery that I think is pretty much knackered. Even so it will run my fridge indefinitely in summer, as long as I have the panel facing in a sensible direction and not shaded. This is a 15 year old Waeco fridge that is rated to use more power than yours, and we'll be working it hard,  making ice, putting warm beers in etc. But if I park with the panel even slightly shaded it drains the battery in a day or so. So it all seems to be down to how much actual sun it's getting, and the only way to know this is with the total energy figure.

In reply to Ciro:

Where did you get your LiFePo from? I've been reading up on these and I'm very tempted to get one to replace my old LA battery. £300 is a very good price for a 100Ah one.

In reply to didntcomelast:

Adding more battery or a switching to Lithium (allowing deeper discharge for the same rated capacity) won't get around the basic issue of the panel likely being undersize for all season off grid use but since it sounds like you drive it quite a bit that maybe doesn't matter. A bigger battery will get you through grey days or last longer between runs in better weather.

On the upside it sounds from your earlier experience like your fridge energy use figures (0.35ish kWh/day) are much closer to reality than the current consumption data and actually not far off what the panel should produce in ok weather.

I'd have someone give it a once over for battery health and loose wires but it may be working just fine.

One thought is any partial shade on basic solar panel ruins the output of the whole thing.

Jk

In reply to didntcomelast:

Maybe not every time, but definitely only 3rd time is still plenty good reason to stay positive. I've only had 1 scare myself (which thankfully came to nothing). Sure for you guys it will work out fine.

Take care

In reply to CantClimbTom: Thank you. Two cancers in the past ten years and another lump found is a little wearing to say the least. We have learned though that the unknown is often worse than the reality we sincerely hope it’s the case this time again. 

In reply to Alasdair Fulton: The fridge is a Thetford D1090 which is a 90l compressor fridge, so a fair size. The MPPT is a PV logic 15A. I have been in contact with the battery supplier and am collating info over the next couple of days to see if they think the battery is at fault. 

In reply to Hooo:

Think I used someone on DHGate - felt like a bit of a punt at the time but worked out well. Just made sure I used someone with a reasonably long trading history and high positive feedback, rather than going with the cheapest option.

In reply to didntcomelast:

I think that's going to be fairly limiting for UK use then - a general rule of thumb is you want 1W of solar per Ah of battery capacity with Lead Acid, but I think in the UK you ideally want 1.5 W per Ah. I suspect you're going to want at least 200Ah of LA (or 100Ah of LiFePO4) to run off grid for any length of time with a fridge, lights, mobile and laptop charging, etc. - which means you'd want a minimum of 200W of panel in southern Europe and 300w in the UK.

I've got a much smaller fridge than you, and my 170W panel works fine off grid in Spain, but I've found isn't really enough in the UK. The only thing stopping me from upgrading the panel at the moment is the thought of trying to un-bond the ABS mounting brackets from the roof - that and the fact that with LiFePO4 I don't have to worry about killing the battery. 

You could also look into a portable wind generator to stick up when you're parked up - they are getting cheaper but I believe they're still quite noisy.

I hope the fussing over trivial matters works, and the serious clouds turn out OK.

In reply to Ciro:

I've never heard of DHgate before. Definitely looks like a bit of a punt venturing on there. They have a 12 100Ah LiFePo4 battery for £178 with free delivery. Part of me wants to give it a go, the other part is telling me not to be so gullible.

In reply to didntcomelast:

I'd say, without even getting the calculator out, that a single 12V 110ah battery is not going to be enough battery to reliably run your fridge while charging from solar. Part of the problem is in how lead acid batteries charge — it takes forever to return them to full charge as the charge current tails off towards the top end of charge, and if you don't do it regularly the battery will quickly deteriorate. An hour drive might get you to 80-90%, for example, but it'll take several hours to get from there to 100%.

The solar is undoubtedly a good thing though; because of the above effect, you'll struggle to return your battery (or batteries) to full charge by driving, unless you happen to be regularly driving all day. The solar will be able to supply that long period of low current to complete the charge without you running the engine for hours every other day.

What you need to do is balance your consumption with your charging. You need an accurate idea of how much power you consume on average each day, and then work out how best to replenish it. Bimble Solar have a good calculator that will help you do an audit: https://www.bimblesolar.com/solarcalc

There's also the possibility that the charge voltage from the solar controller is not set correctly, and so your solar energy is being squandered by not charging the battery efficiently.

The ultimate solution is lithium polymer, because they charge at a constant current at all states of charge, and they don't care about being left partially charged like lead acid batteries do. This means both that they charge more efficiently, and that you can abuse them (and you will abuse them, no matter how careful you are) without causing expensive headaches. About the worst thing that will happen is that your fridge will go off if you haven't charged them enough. But they are expensive.

The fridge will still go off though, unless you balance your charging with your consumption such that there isn't a charging deficit.

In reply to tehmarks:

Lead acid batteries have the advantage that they DO charge quickly on a short drive. They will not suffer from being left at 80% for a few days. The failure modes are shorting growths and sulfation which occur slowly, allowing deep discharge for short periods.
Car batteries have to deliver high starting currents and so have thinner plates which are more prone to damage than leisure batteries.
The fridge uses less than 0.4KWh each day. So should run for more than 3 days (the first thing to check). The system seems reasonable to me. Maybe the battery is damaged.  But changing to lithium won't increase the charging currents.
A maintenance charger is vital when not in use.

In reply to David Riley:

I've allowed my reply to be coloured by living permanently off-grid, where a maintenance charger simply isn't a option and the batteries are used and charged every day, all year round. I lost sight of the fact that the OP can and will go home and plug a charger in after a week.

I'll come back to this later — having to type on my phone, and it's infuriating compared to a real keyboard when you want to write more than a handful of words!

In reply to tehmarks:

Yes, I looked at your profile after I posted and it suggests you live on a boat. So thought that was where you were coming from. Completely understand.  I think you can mistreat leisure batteries for a week or so at a time, and the short vehicle charges are probably more reliable than the solar.

In reply to didntcomelast: Thanks for all the helpful replies and good wishes. We have had the van & battery on mains for 12hrs and just finished a 5hr drive so the battery is showing over 13v. As an experiment I have left the fridge in night setting (we rarely use it anyway, preferring to store drag stuff in there until a specific meal time then quickly remove everything we need so as not to raise the internal temp too much. See how we get on over the next couple of nights before heading back to reality. I’m recording all the battery readings so the place we bought the battery from can evaluate whether it’s a faulty battery or my over expectations of performance. 

In reply to Hooo:

That price is not totally unreasonable though for direct from a chinese manufacturer, however I'd be surprised if the free delivery holds up when you make an order. Think it's more likely to costs around £50 - £70 for shipping, which would probably take it up to nearer £300 in total by the time it clears customs - you should get 20% VAT on the total including the shipping charges, plus 4% tarrif, so if it costs £50 for shipping it'll come out around £282.

Still cheaper than buying here - you're looking at £450 for the same battery.

I guess the main problem is if you do get a faulty item it's going to be hard to return it to china... for a single item you're taking a risk, if you buy direct more regularly then as long as the success rate is above 2 in 3 you're in profit.

For me, as long as you can afford to take the hit if it's a dud, it's worth a punt.

In reply to didntcomelast: There is a Facebook group Campervan Electrics that may be of help

In reply to David Riley:

Maintenance charger or a decent DC-DC charger that runs off your solar. Our split charge relay died a while back, so we used that as an opportunity to get solar and a CTEK D250SE DC-DC charge AND solar MPPT charge controller. Really handy bit of kit that also keeps the engine battery topped up if you're leaving it for long periods.

https://www.ctek.com/uk/battery-chargers-12v-24v/d250se

In reply to Jamie Wakeham:

The 4amp figure is the peak current draw which you need to know so you can fit the correct fuse in the supply circuit.

To the OP - what is the threshold for the low battery alarm? Have you checked the accuracy of the app with a multimeter?

In reply to Toerag:

That would be my assumption; in post #2 JK uses it as a constant draw and hence overestimates the fridge's energy use per day.

In reply to Alasdair Fulton:

Handy bit of kit indeed, but it only takes up to 25v input - you'll get more efficiency from a split charge relay, 40v panel and separate MPPT controller.

In reply to Toerag:

It would appear there isn’t an audible alarm on the battery, something I’m seeking to take up with Hymer. I am going to use a multimeter to properly check the both the app readings and the vans own meter which I have to admit I’ve had little faith in since day one.

i did contact Thetford to ask whether it would cause a problem using the fridge on the night setting all the time the van was parked off grid and using solar power only, they stated that other than taking longer to cool the fridge down to operating temperature, there would be no issue. Testing that this weekend. 

In reply to Ciro:

I'm intrigued as to why it would be more efficient?  Not doubting, just interested. Quick google didn't throw up much. 

For us, having the battery well managed with a proper dc charger AND being sure the engine battery also gets topped up (often a month between drives for us, with a 22 year old van) were key.  We get about 0.2 - 0.3v more charge at the top end compared to charging off the alternator. 

If we'd gone for MPPT, we'd have needed a DC-DC charger also, and something to replace the dud VSR.

In reply to Alasdair Fulton:

Higher voltage means lower current, and lower current means less losses due to resistance in cabling, and also means thinner cables can be specified for a given power (this is why we transport electricity through high voltage power lines, then bring it down to domestic voltage).

Fair enough - I suspect the efficiency losses won't be all that great, ventricle you have a use case that justifies it 🙂

In reply to Ciro:

Just saw this, thanks. Ok, so it's just for the I^2R losses. With the typical cable lengths in a van I'd be surprised if that made a huge difference?  

Quick calc assuming a 300W panel connect via a 3m run of cable.

24v system, 4mm^2 cable: cable losses would be 4W or 1.3%.

54V system, 2.5mm^2 cable: cable losses would be 1.27W or 0.4%. Better, but not worth fussing over really.

On a house install (on even more importantly in a field with a long run) it really would add up (both copper costs and losses).