Solar panel and lithium battery advice pls

[Brody]

Solar, b2b, lithium:

Ok guys , just wondering if someone knowledgable can give me some advice , I am suffering information overload at the moment!

I currently have a 100ah standard leisure battery , I run a waeco cr50 fridge , 4 x led spotlights , 2m led tape, and a propex heater, also charge the odd smart phone .

I am planning on going off grid more , so looking at solar panels (but can’t figure out best size to get ) I currently have a 20amp ctek d250se (which has mppt built in but max ocv of 23v which is restrictive for panels) also have a victron battery charger for when on hook up .

I am looking to get a lithium battery and a solar panel , plus probably a victron smart solar mppt.

It looks like my budget will only stretch to a 100ah lithium battery , will this be sufficient for off grid for say 72hrs ? What would be the best solar panel size ?

Many thanks , i know the info is out there , but I’m a bit bamboozled if I’m honest !

 

[VanDamMan]

Solar, b2b, lithium:

Ok guys , just wondering if someone knowledgable can give me some advice , I am suffering information overload at the moment!

I currently have a 100ah standard leisure battery , I run a waeco cr50 fridge , 4 x led spotlights , 2m led tape, and a propex heater, also charge the odd smart phone .

I am planning on going off grid more , so looking at solar panels (but can’t figure out best size to get ) I currently have a 20amp ctek d250se (which has mppt built in but max ocv of 23v which is restrictive for panels) also have a victron battery charger for when on hook up .

I am looking to get a lithium battery and a solar panel , plus probably a victron smart solar mppt.

It looks like my budget will only stretch to a 100ah lithium battery , will this be sufficient for off grid for say 72hrs ? What would be the best solar panel size ?

Many thanks , i know the info is out there , but I’m a bit bamboozled if I’m honest !

The maximum solar panel input is 300w ( Ctek) ..but in my case i‘ve brought two 150w panels to keep within the 23 v (voc ) and on a good day can produce 16amps ...it suits my needs but I also brought a 100amp battery in a box to back up when needed...the Ctek can be restrictive but I work within its limits...



edit Better post below

 

[Phil_G]

You're bamboozled because you're looking for a definitive answer that doesnt exist!
There are so many variables its impossible to say. Weather, ambient temperature, sunshine hours, fridge thermostat, heater thermostat, humidity, lights usage, etc etc etc
All you can really do is to maximise your chances for the worst case, ie no solar at all. Your heaviest drain is probably the fridge.
The fridge takes about 3A when its running, but most of the time its not, its 'cruising' between compressor cycles. To measure the consumption of a fridge you need to monitor it over a long period of many thermostat-controlled on/off cycles. The on/off ratio depends on thermostat setting, outside temperature, how often you open it, how full is the fridge etc but you can guess around 6:1 off/on time. So your 3A becomes an average half an amp over a long period - this is why fridges are rated in the weird unit of "kWh per annum".
72 hours x 0.5A is 36Ah.
The heater fan similarly changes speed to control the temperature so its current also varies. On ignition theres also plug current. Say an average of 2A and its on every evening for say four hours:
4hours x 3days x 2A is 24Ah. Thats a total of 60Ah so far.
LED spots are quite frugal, say an amp for four hours every evening, and the strip will take an amp or so:
4h x 3days x 2A is 24Ah. Total so far 84Ah.
Phones - mostly about 3-4Ah batteries. This is per charge from flat. Say you charge it from flat every evening, if its a switching charger thats about half at 12v, say 2Ah per charge
3 days x 2Ah is 6Ah, thats 90Ah total so far.
Anything else you plug in will be on top of the 90Ah.
Is 100Ah enough? who knows. Worst case, its very close. But there will be some solar, even in winter, so probably. Why go to the expense of lithium though, especially if you're unsure if it will meet your needs? If you find the 100Ah lithium isnt enough, and you want to parallel another, you're compelled to go lithium again. Huge bucks!
A larger AGM will be much cheaper and with less potential for disaster. Work on half the stated capacity, so around 200Ah. Maybe two smaller ones in parallel, still much cheaper than Lithium. On a purely personal note, I've worked with lithium, I've experienced the fires, I wont have one in my van, but thats not a recommendation its my own personal choice.
/2c
Phil
PS

What would be the best solar panel size ?

the best solar panel size is the largest that will fit in the available space! 100w absolute minimum. 250 is popular. Its not about maximum power, its about maintaining some output in crap weather.

 

[Brody]

The maximum solar panel input is 300w ( Ctek) ..but in my case i‘ve brought two 150w panels to keep within the 23 v (voc ) and on a good day can produce 16amps ...it suits my needs but I also brought a 100amp battery in a box to back up when needed...the Ctek can be restrictive but I work within its limits...


View attachment 91117
edit Better post below

 

[Brody]

So if you get two panels the ocv doesn’t increase at all ? Eg 2 panels at 22v ocv , stays the same ?
that is a pretty good idea having an additional battery in a box , one of my issues is that my leisure battery is under the drivers seat , so i am very pushed for space , as much as I would like 2 batteries in place permanently, I just don’t have the room for it . Thanks for the info

 

[Pete C]

So if you get two panels the ocv doesn’t increase at all ? Eg 2 panels at 22v ocv , stays the same ?

Yes, as long as they are wired in parallel.

Pete

 

[Brody]

You're bamboozled because you're looking for a definitive answer that doesnt exist!
There are so many variables its impossible to say. Weather, ambient temperature, sunshine hours, fridge thermostat, heater thermostat, humidity, lights usage, etc etc etc
All you can really do is to maximise your chances for the worst case, ie no solar at all. Your heaviest drain is probably the fridge.
The fridge takes about 3A when its running, but most of the time its not, its 'cruising' between compressor cycles. To measure the consumption of a fridge you need to monitor it over a long period of many thermostat-controlled on/off cycles. The on/off ratio depends on thermostat setting, outside temperature, how often you open it, how full is the fridge etc but you can guess around 6:1 off/on time. So your 3A becomes an average half an amp over a long period - this is why fridges are rated in the weird unit of "kWh per annum".
72 hours x 0.5A is 36Ah.
The heater fan similarly changes speed to control the temperature so its current also varies. On ignition theres also plug current. Say an average of 2A and its on every evening for say four hours:
4hours x 3days x 2A is 24Ah. Thats a total of 60Ah so far.
LED spots are quite frugal, say an amp for four hours every evening, and the strip will take an amp or so:
4h x 3days x 2A is 24Ah. Total so far 84Ah.
Phones - mostly about 3-4Ah batteries. This is per charge from flat. Say you charge it from flat every evening, if its a switching charger thats about half at 12v, say 2Ah per charge
3 days x 2Ah is 6Ah, thats 90Ah total so far.
Anything else you plug in will be on top of the 90Ah.
Is 100Ah enough? who knows. Worst case, its very close. But there will be some solar, even in winter, so probably. Why go to the expense of lithium though, especially if you're unsure if it will meet your needs? If you find the 100Ah lithium isnt enough, and you want to parallel another, you're compelled to go lithium again. Huge bucks!
A larger AGM will be much cheaper and with less potential for disaster. Work on half the stated capacity, so around 200Ah. Maybe two smaller ones in parallel, still much cheaper than Lithium. On a purely personal note, I've worked with lithium, I've experienced the fires, I wont have one in my van, but thats not a recommendation its my own personal choice.
/2c
Phil
PS
the best solar panel size is the largest that will fit in the available space! 100w absolute minimum. 250 is popular. Its not about maximum power, its about maintaining some output in crap weather.

Thanks so much for such a detailed reply , whilst i like the look of the lithium , the initial cost is incredibly high , and I really am struggling to justify that, I know theoretically over the life they are cheaper , but what if they fail prematurely and I am unable to claim on warranty?
I think what you are saying makes sense for me , a larger ah agm might be more sensible , then spend a little more on panels . I have a lwb t5.1 with sca roof , so I was leaning towards a large solid panel , but will increase the roof height by about 60mm, which will put me over the limit for most 2.0m car park barriers , looking at etfe flat panels , but worried about premature failure . Think i will go with flat etfe panels and fix them to the active rails , should only increase by about 30mm once fixed to brackets, and if they fail easier to remove than if they are bonded.

 

[t0mb0]

As @Phil_G says, too many variables for a definitive answer. I have the factory 75Ah AGM battery, a 250w solar panel and a DC-DC charger. This does us off-grid indefinitely in the summer months when we’re travelling most, EHU lead lives in the garage. When the battery packs up, I’ll probably replace it with a 130Ah AGM unless lithium have got much cheaper in that time.

 

[Unimog]

I have one 100@mp lithium battery which I charge via the CTek. My 150w solar panel charges the battery also via the solar connection on the CTek. Once the leisure battery is charged it will then if necessary charge you vehicle starter battery. I monitor the charge with a Victron 700 which as well as the wall gauge has blue tooth so I can check on my phone the charging status.
Remember you can discharge a lot more out of a lithium battery than Other batteries.
Mains hook up is via a Victron charger which is wired direct to my fuse board at the rear of the van, and charges the battery via the supply cable from the fuse board back to the battery.

I find the van isn’t parked for more than a couple of days on sites as we tend to venture out, which charges the battery.

With the fridge running, charging phones , led lights, start up on the Webasto heater we still find we have more than enough spare capacity in the battery afte a couple of days.