Panel vs. Battery sizing question

[kromboy]

Conventional wisdom on a lot of van conversion forums seem to be about 2-3x the solar for battery bank, i.e. 800-1200W of 12v panels mated to 400Ah (12v) battery bank.

I'm thinking that the skoolie gives me so much more space to install panels that I can drive down to santan solar and buy a dozen or so panels, totaling anywhere from 2500-4000W of panels. Should be cheap enough, but I do not want to spend $3000 on a battery bank to match that.

Given that my biggest draw would be AC, and presumably my need for AC during the night will be lower, hopefully non-existent, can I get away with a smaller battery bank compared to the panels?

Something like 2500W of panels with 400Ah of batteries. Has anyone done such a lopsided solar system and what is the verdict after having lived with such a set up?

 

[fo4imtippin]

currently i only have 4 of the 8 panels i own mounted. Just haven't had time to get up there and bolt the other 4 down.


the 4 up there give me 1500watts. I have a 14kwh battery in there. The current solar sustains the air conditioner. When the sun is shining, the heat is up and the A/C pulls more watts. I barely use any battery at night, the A/C turns down to about 200watts. I would fill the roof with panels and then you can easily plop another battery in if you aren't making it through the night.

 

[Malevolence4845]

14Kwh is what I figured would be a good bank size for me as well, did you use server rack style batteries or conventional type like battle born?

 

[cadillackid]

I would say put up as many panels as you can... the panels themselves help with your A/C load as they put a good portion of your bus roof in the shade with an air layer between the panels and the actual bus roof..

 

[nikitis]

I would say put up as many panels as you can... the panels themselves help with your A/C load as they put a good portion of your bus roof in the shade with an air layer between the panels and the actual bus roof..

Second this. It's better to go overkill. If you oversize your panels, it'll charge the batteries faster if you are undersized on the battery bank comparatively.

The only question becomes is will it get you through the night even if your bank is topped off. If it ends up being no, you'll need to ensure you have room to add more to your battery bank.

 

[kromboy]

the panels themselves help with your A/C load as they put a good portion of your bus roof in the shade with an air layer between the panels and the actual bus roof..

Huh that's an interesting take and makes perfect sense!

I was thinking of just getting some cheap panels from santan and covering about 2/3 of the roof. Probably gonna go with Poly because efficiency be damned and it'd still only cost like $500 for the panels, cheaper than the 800w of panels I got from renogy.

 

[fo4imtippin]

14Kwh is what I figured would be a good bank size for me as well, did you use server rack style batteries or conventional type like battle born?

I actually used an EG4 wallmount battery. Its a big 350lb mother.. Luckily i had a handicap lift to get it inside my bus garage. It comes with full outdoor rating and i really like the footprint for my setup. I wouldnt recommend for most skoolies as its a single point of failure, heavy to manage and move around, and probably not ideal for mounting under a bed or something. I suggest EG4 rackmount batteries to combat all of this.

 

[Iceni John]

Here's what the solar gurus on the NAWS forum have to say about this:
https://forum.solar-electric.com/di...edium=organic-search&utm_term=13%+charge+rate
They like to see a charge rate of between 5 and 13%. I think I have just under the top end of that range now, with 2040W of panels charging 840Ah of golfcart batteries.

One thing to consider is the ability of a bus's panels to actually produce usable power. NAWS assumes that a PV array is oriented towards the sun to maximize solar harvest: this is probably true with fixed ground-based arrays for homes, but all bets are off when the PV panels are on a bus's roof. Almost all such latter arrays are flat-mounted, something that's rarely (if ever) done with ground-based arrays for homes. Unless you happen to live between the Tropics of Cancer and Capricorn (i.e. within 23.5 degrees of the Equator), flat-mounted panels will not produce much power, especially in the winter and/or at higher latitudes. The only thing that really matters is how much actual power one can squeeze out of a panel, not what it's theoretically capable of producing under "ideal" (i.e. mostly unrealistic or unattainable) STC conditions. This is why I can tilt mine at up to 45 degrees from horizontal, to ensure I get a useful harvest even in the winter when the sun is low and insolation hours are short. When stowed for travel, they're all at 21 degrees down, which is fortuitously about the ideal fixed angle for SoCal. Yes, it's more work to mount them this way, but why waste potential power? Here are the numbers: https://www.solarpaneltilt.com/ In a nutshell, this article is saying "Don't mount them flat"!

Another consideration: how the **** do people wash off all the inevitable dust/leaves/birdshit/etc from a roof-full of flat-mounted panels? One incidental benefit of angled panels is that rain will nicely wash then clean without pooling on the glass, and clean panels always produce more power. My panels are hinged to a central walkway between my two roof hatches, and I have two quick-connect water outlets up there so I can easily and safely wash the panels whenever I notice their power is declining because they're dirty. I also don't need to drag heavy buckets of water or hoses up there, just a small washdown brush with a few feet of curly hose, and the dirty water flows away from me instead of drenching me if I were on a step ladder below the panels. Easy!

Just because your bus's roof can theoretically fit X-thousand watts of panels doesn't automatically mean you should do so. Sometimes a smaller total wattage of panels can be more useful in real-world conditions if they're mounted right.

John

 

[Jono14]

If you tip the balance too far in panels over batteries you can run into a couple problems:
1) Maxing the charge rate of the batteries
For a 2500W solar grid, it might "actually" produce 2000W around noon on a sunny summer day. This is about 150A of 12V current into the batteries which may be faster than recommended depending on the battery brand. For example, LiTime recommends 0.2C meaning 20% of the Ah as the max amps which would be just 80Amps. 150A would be almost double that rate per battery. If you doubled the number of batteries up to 800Ah then 0.2C would be at about max of these panels. (Iceni John's link is from 2010 so I expect the 5-13% is lead acid? Not sure about those #s for Lithium).

2) Wasting harvested energy because of nowhere to store it.
In the 2500W example good sunny summer days can total up to about 5 "perfect hours" of sun so generate 12,500 Wh across the whole day. In the winter this might be 2.5 "hours" 6,250Wh based on the sun's angle. Yes as Iceni John states, tilt can help. particularly in the winter.
But 400Ah of battery (I am assuming LFP) can store only 5,120 Wh and so unless you start the day empty and use a lot of power during the day, you likely will not be able to fit all the Wh harvested by solar into the relatively small bank. On a regular day with a half-full battery to start you may be full by noon and waste the rest of the day's available energy. You may miss it when you have 3 cloudy days in a row.

Finally, an A/C unit can pull a lot of energy. Many rooftop A/Cs are not that efficient and may use 1400-1800W of energy. If the RV is already cool by night, maybe the compressor only runs 1/2 the time. But conservatively you can figure that each 100Ah of 12V battery only supports about 1 hour of A/C run time.

 

[cadillackid]

Using an efficient inverter AC doingvtestingcwjerecitcran on high the whole time pulling close to 1000 watts, I found your estimate of 1 hour per 100ah of batteries to be pretty spot on.. now at night that same inverter AC pulls on average 300 watts and sometimes cycled out completely. Which was 100 watts. This was on a warm summer night where it only dropped to 75 out and was humid.

As for overcharging batteries , doesn’t the BMS limit current going into the batteries? Excess solar isn’t “wasted” you just didn’t use it , with excess panel capacity you can also better handle cloudy days or times of year where it might still be warm but sun angle is lowering. Such as our most recent heatwave .. mid September where sun angle is much lower than the lay-July timeframe .. you still would need lots of AC but not as much sun to capture

 

[Jono14]

A BMS will not regulate current. It can only shut off charging MOSFETs when the voltage gets high. And most BMSs are capable of charging at 1C - it is just not recommended for LFP length of life. Current regulation can be done at the charging side through programming an design, so this is a very manageable issue.

Maybe 'wasted" is not quite right - but you are wasting an opportunity to store energy your panels could harvest. If you never would have needed it, then no harm.

I am not against this system setup - It just seems to me that with 2500W of panels, only 400Ah of batteries, and the need for A/C, you might quickly find that more batteries would be better. I'd just design the system to allow that phase 2 upgrade in the future.

 

[cadillackid]

A BMS will not regulate current. It can only shut off charging MOSFETs when the voltage gets high. And most BMSs are capable of charging at 1C - it is just not recommended for LFP length of life. Current regulation can be done at the charging side through programming an design, so this is a very manageable issue.

Maybe 'wasted" is not quite right - but you are wasting an opportunity to store energy your panels could harvest. If you never would have needed it, then no harm.

I am not against this system setup - It just seems to me that with 2500W of panels, only 400Ah of batteries, and the need for A/C, you might quickly find that more batteries would be better. I'd just design the system to allow that phase 2 upgrade in the future.

I never knew a BMS didnt regulate current flow... I always figured it regulated how the battery was charged...



me personally I also think they should have mnore batteries than 400AH when the plan is to run A/C.. like I mention in my tests, you only get far on batteries if your A/C is running at a fraction of its capability.


a minisplit is worlds more efficvient than a standard rooftop RV unit.. RV units are easy to install and great if running on generator or shore power where manging every watt hour isnt necessary..

 

[nikitis]

A BMS will not regulate current. It can only shut off charging MOSFETs when the voltage gets high. And most BMSs are capable of charging at 1C - it is just not recommended for LFP length of life. Current regulation can be done at the charging side through programming an design, so this is a very manageable issue.

Maybe 'wasted" is not quite right - but you are wasting an opportunity to store energy your panels could harvest. If you never would have needed it, then no harm.

I am not against this system setup - It just seems to me that with 2500W of panels, only 400Ah of batteries, and the need for A/C, you might quickly find that more batteries would be better. I'd just design the system to allow that phase 2 upgrade in the future.

Your thinking on this is incorrect. You're only considering the charging aspect as if the system was not being utilized. You have to account for usage + charging. You want extra wattage so that your batteries can charge while also in use during the day, and by the end of the day your batteries still be near full to last you the night. DC is direct current, so if you are producing more direct current than the batteries are taking, then it can basically go right past your batteries into your active devices in use.

To achieve this, the rule of thumb to maintain batteries at 100% while you are using devices is typically 1.5 times the wattage from panels as you have in battery capacity, and that's just to maintain at 100%. If you are using things heavily you'll want more than 1.5 times battery capacity.

 

[nikitis]

I never knew a BMS didnt regulate current flow... I always figured it regulated how the battery was charged...



me personally I also think they should have mnore batteries than 400AH when the plan is to run A/C.. like I mention in my tests, you only get far on batteries if your A/C is running at a fraction of its capability.


a minisplit is worlds more efficvient than a standard rooftop RV unit.. RV units are easy to install and great if running on generator or shore power where manging every watt hour isnt necessary..

BMS's monitor voltages of each cell, and when it reaches it's programmed voltage it cuts off charging to it, if it starts to drain, i.e. is being used by devices, and voltage drops it switches those lines to the cell back on.

Also that's what the extra P in MPPT is for. MPT is Pulse Width Modulation and not as sophisicated. MPPT is a bit more sophisticated and acts like a BMS.

Key differences:

Complexity: MPPT controllers are more complex and have more electronics inside, making them more expensive.

Power handling: MPPT controllers can handle higher-power systems, while MPT controllers are limited to smaller systems.

Voltage conversion: MPPT controllers can convert excess voltage from the solar panel to additional current, while MPT controllers do not. (This is key. If you have MPPT you can use the direct current of the panels bypassing the batteries when they are 100% charged)

System cost: MPPT controllers require smaller cable cross-sectional areas due to the decoupling of array and battery voltages, resulting in lower overall system costs.

 

[Iceni John]

Also that's what the extra P in MPPT is for. MPT is Pulse Width Modulation and not as sophisicated. MPPT is a bit more sophisticated and acts like a BMS.

I've not heard of the 'MPT' designation. I think of charge controllers either as PWM or MPPT (Maximum Power Point Tracking): PWMs simply chop off (i.e. waste) solar power that cannot be directly used to charge batteries, making them wasteful compared to MPPTs that convert excess solar voltage into extra charging current, thus using every last bit of solar power to charge batteries. I see absolutely no reason whatsoever to use PWM charge controllers in a mobile installation when one's solar power is necessarily limited by roof space; they're simply throwing away potentially-usable solar power.

John

 

[nikitis]

I've not heard of the 'MPT' designation. I think of charge controllers either as PWM or MPPT (Maximum Power Point Tracking): PWMs simply chop off (i.e. waste) solar power that cannot be directly used to charge batteries, making them wasteful compared to MPPTs that convert excess solar voltage into extra charging current, thus using every last bit of solar power to charge batteries. I see absolutely no reason whatsoever to use PWM charge controllers in a mobile installation when one's solar power is necessarily limited by roof space; they're simply throwing away potentially-usable solar power.

John

MPT is PWM, MPT is just another term some folks use instead of PWM outside of the USA.

PWM's or MPT's are meant for small applications like a toy hauler or smaller. MPPT's are meant for homes or larger solar array setups.

 

[kromboy]

Thanks for all the input so far.

I too think 400Ah (4.8kW right?) is cutting it a little close, but considering I already have a 200Ah 12v battery, and would like to run them in series for 24V, buying a third 200Ah battery to get them to 600Ah just isn't an option, and 800Ah seems a bit.. much? It would be $2k+ for the batteries alone anyway, and I don't know if I'll ever need that much power storage.

Maybe I should just run them in parallel so I can go with an odd number of batteries.
I think the sweet spot for me would be right around 600Ah.

 

[nikitis]

My advice would be start with 400-600ah.

Leave room to go higher if you find it's not enough.

A lot of people get along with 400 AH, but if you are a heavy user it won't be enough. But on the road most people use laptops vs Desktops etc. LED lights help, lighter versions of electronics help. Every little bit helps.

Using items with a heating element will kill your power fast. No ELECTRIC Stoves as they are huge power drains. We switched our electric stove in our house to gas and saved $100 a month on the electric bill, and the gas bill was only like $25.

 

[kromboy]

Ahhh yes, I also plan on having a generator for the quick, high draw items like cooking appliances and will be installing a diesel heater for heat. So really the only thing I'll be running on the solar/battery bank setup that draws a lot of power will be AC, it's also more of a "since I'd already have so much solar produced by the panels, why not use it?" approach, and in a pinch I'd just run the generator for AC.

Other than that, things I'll be constantly drawing from would be the fridge, internet, wall plugs for charging and probably a TV if I decide to get one.

My advice would be start with 400-600ah.

Leave room to go higher if you find it's not enough.

A lot of people get along with 400 AH, but if you are a heavy user it won't be enough. But on the road most people use laptops vs Desktops etc. LED lights help, lighter versions of electronics help. Every little bit helps.

Using items with a heating element will kill your power fast. No ELECTRIC Stoves as they are huge power drains. We switched our electric stove in our house to gas and saved $100 a month on the electric bill, and the gas bill was only like $25.

 

[AlphaHare]

Power Budget - what do you actually expect to need?

Rather than guessing, I'd take the time to write up a power budget on graph paper or Excel. Write down all the loads you plan to have times the amount of time you expect to run them per day. Also suggest you use Watt Hours where Watts = Volts x Amps. Most manufacturers will give the power needed in Watts, so you don't have to calculate (make mistakes with) 12vdc, 24vdc, 120vac, etc.

So a small A/C fridge might be 175 Watts - that's for every hour (24) but with maybe a 50% duty cycle, so 175x24x.5 call it 2100 W/H (maybe less if you keep the door closed!) A hair dryer is 1500 Watts, but only for 10 minutes, so 250 W/H. An AC unit might be 1000 Watts, but you might decide to make some loads A/C shore power or generator only (many AC units and microwaves are this way in RVs). Laptops, phone charging, internet equipment, cell phone booster, tablets, music, water pump, diesel heater....

Add up all your Watt Hours, multiply by say 1.25 (conversion inefficiency/buffer) and that's how much power you will consume in a day from your batteries. Add more for cloudy days or decide what to run less.

Now take your Watt Hours and divide by your planned battery voltage (12, 24, 48 ) and you'll have how many Amp Hours of battery you need at that voltage. That is the primary driver of everything else that follows....