Solar panels to batteries ratio?

[Thunder Dan]

Say you have 400 watts of solar panels on top of your van. If I understand correctly, that's going to be 33.3 amps at 12 volts, right? So if I have a 12 volt, 200 amp hour battery, does that mean it would take 6 hours to charge the battery from depleted to full?

Or if I had two 12 volt, 200 amp hour batteries, wired in parallel, would it then take 12 hours to charge those batteries from depleted to full?

If you have 200 watts of solar panels and a 12 volt, 200 amp hour battery, would that mean 12 hours to charge it?

 

[29chico]

Say you have 400 watts of solar panels on top of your van. If I understand correctly, that's going to be 33.3 amps at 12 volts, right? So if I have a 12 volt, 200 amp hour battery, does that mean it would take 6 hours to charge the battery from depleted to full?

Or if I had two 12 volt, 200 amp hour batteries, wired in parallel, would it then take 12 hours to charge those batteries from depleted to full?

If you have 200 watts of solar panels and a 12 volt, 200 amp hour battery, would that mean 12 hours to charge it?

I would need more input:  Lead acid battery?  Flooded or agm? If flooded are we talking golf cart battery?  If agm conventional or pure lead premium (Odyssey, Northstar or Lifeline)? PWM or MPPT charge control?  Will the panels be mounted flat on the roof at all times?  Will the solar system be operated closer to the equator or the one of the poles?  How deeply are you going to discharge the battery bank on a regular basis?  If by depleted you mean fully discharged, battery murder is in the near future.

400 divided by 12 is 33.3    To properly charge a lead acid battery you are going to need 14.4-14.8v for an appropriate period of time.  For example, divide 400w by 14.8v and you get 27 amps w/o accounting for any inefficiency in the system.

If you want to get a fairly deep understanding of most of the issues involved in mobile solar electric you might consider the below link. 

https://vanlivingforum.com/Thread-Putting-together-system

Lots of savvy folks have helped a young man come up to speed on many solar power issues in the above thread.

 

[Van-Tramp]

Don't forget, a 200 ah battery has only 100 ah of usable power before you start causing some serious harm to the battery(s). So at worst, you *should* only have 100 ah to refill. Never, or at least rarely, should you go below 50% of your battery ah rating. If you run your battery to full depletion, you are likely buying a new battery very soon.

At 200 watts of panels (roughly 15 amps incoming power), then you will need just under 7 hours (100ah / 15amps) of perfectly placed sun (high noon in summer). But here is caveat; You actually need to burn 100ah the previous day/night to have to have that need. To put that into perspective, you need to have your standard laptop powered up and running 20+ straight hours (without any incoming solar power) to burn through that kind of power.

In my van, I have 200ah of AGM batteries and 300 watts of panels. I have two people running a laptop EACH for 8 hours a day and a 12v fridge. Plus cell phones, tablets, and other rechargeable devices, and never come anywhere near being 100ah down on the batteries. Biggest I have ever seen the batteries down is 54 amps. I am usually "floating" (all batteries full) by early afternoon the following day, which means any incoming solar is being wasted... so, I plug in any extra devices to capture that power to be used on a rainy day.

 

[akrvbob]

Van tramp is exactly right, power management is one of the most important parts of the equation and most people leave it out. With the 36 amps coming in you can run anything you want during the day and still charge your batteries. So when the sun goes down you minimize the amount of power you take out of it. As vantramp pointed out, the key to long battery life is to take as little out of them at night as you can. So you take out 30 amps at night and the next day you slam it with 36 ah at noon.

How do you do that?

1) Have all your devices fully charged at sunset. Then unplug them and use them on their internal battery
2) Have your fridge at 31 degrees at sunset and plenty of extra insulation so that it comes on as little as possible.
3) Use LED lights
4) Have a smaller, Energy Star TV/DVD player
5) Minimize water use at night.
6) Have a heater that doesn't draw electricity.

If you do all those things, how much do you actually need to pull out of your batteries at night? Hopefully not much. So the next day dawns with nearly full batteries how hard will it be to charge them? Not hard!! So with 400 watts during the day you use all the power you want for any reason you want and your batteries go into sunset full.

If you're willing to do that, I recommend a 1:1 ratio of batteries and watts. Why? Because with all that solar and good power management your batteries will always be full and last a long time. But when a storm comes, and everyone around you is out of power and running generators you'll be fine because you know how to conserve power and will have 400 ah of batteries.

This works very well for me with my 580 watts and 640 ah of battery.
Bob

 

[offgridder]

Van tramp is exactly right, power management is one of the most important parts of the equation and most people leave it out. With the 36 amps coming in you can run anything you want during the day and still charge your batteries. So when the sun goes down you minimize the amount of power you take out of it. As vantramp pointed out, the key to long battery life is to take as little out of them at night as you can. So you take out 30 amps at night and the next day you slam it with 36 ah at noon.

How do you do that?

1) Have all your devices fully charged at sunset. Then unplug them and use them on their internal battery
2) Have your fridge at 31 degrees at sunset and plenty of extra insulation so that it comes on as little as possible.
3) Use LED lights
4) Have a smaller, Energy Star TV/DVD player
5) Minimize water use at night.
6) Have a  heater that doesn't draw electricity.

If you do all those things, how much do you actually need to pull out of your batteries at night? Hopefully not much. So the next day dawns with nearly full batteries how hard will it be to charge them? Not hard!! So with 400 watts during the day you use all the power you want for any reason you want and your batteries go into sunset full.

If you're willing to do that, I recommend a 1:1 ratio of batteries and watts. Why? Because with all that solar and good power management your batteries will always  be full and last a long time. But when a storm comes, and everyone around you is out of power and running generators you'll be fine because you know how to conserve power and will have 400 ah of batteries.

This works very well for me with my 580 watts and 640 ah of battery.
Bob

What if I have a 100w Lithium-ion battery and 300 watts of solar on top of the van? Is 300w too much for a 100ah battery? While Lith-ion batteries can be safely discharged to almost 0%, I only intend to discharge it down to 40-50%. I have a 30A MPPT solar charge controller, 600w inverter (pure-sine) powering: Shurflo water pump, 30L dometic fridge, 6 LED lights, laptop, external flat 17" monitor, and phone charger, and I will be adding an induction cooktop, locatioin southern CA and my panels can tilt but will be flat 95% of the time as I plan to city dwell. Is this a good setup or should I get another 100ah Li-ion battery? Currently the batteries charge pretty fast (I have not timed how fast yet) and last @ 2 days without sun. I heard that fast charging is not good for batteries. Right now the power goes out (charge controller resets(?)) every couple hours and turns right back on again each time without fail. I suspect I have too much solar for that size battery. Is 100ah battery enough or should I get a 2nd lith-ion battery?

 

[tripper]

What if I have a 100w Lithium-ion battery and 300 watts of solar on top of the van? Is 300w too much for a 100ah battery? While Lith-ion batteries can be safely discharged to almost 0%, I only intend to discharge it down to 40-50%. I have a 30A MPPT solar charge controller, 600w inverter (pure-sine) powering: Shurflo water pump, 30L dometic fridge, 6 LED lights, laptop, external flat 17" monitor, and phone charger, and I will be adding an induction cooktop, locatioin southern CA and my panels can tilt but will be flat 95% of the time as I plan to city dwell. Is this a good setup or should I get another 100ah Li-ion battery? Currently the batteries charge pretty fast (I have not timed how fast yet) and last @ 2 days without sun. I heard that fast charging is not good for batteries. Right now the power goes out (charge controller resets(?)) every couple hours and turns right back on again each time without fail. I suspect I have too much solar for that size battery. Is 100ah battery enough or should I get a 2nd lith-ion battery?

The lithium can take a high charge rate. The amps from the solar controller start off slow in the morning as the sun is low anyways. 300 watts is not too much.  100 ah will work if you use minimal power at night (see Bob's tips above), if at night you like to watch a lot of TV, surf on the laptop for hours, and have the dometic cranked down to 0°F, you will want 200 ah. With the items you listed as using at night 400 watts of solar would better for winter/northern USA climates.

I found with a lot of solar it is even usable on a smaller lead acid battery as the first few hours of daylight the panels aren't producing near max power, by the time the sun is overhead at noon the controller is normally at float or tapering way down on current in absorb mode. It would be hard to have too much solar for a battery. Some solar controllers allow you to limit the amount of current they put out, for example a 100 ah battery might be specified by the manufacturer to be charged at a rate of no higher than 20 amps, just set the controller to 20 amps output, even if your system is capable of putting out 30 amps. The extra solar capacity will still help a lot in the morning and late evening when the angle of the sun is low.

 

[Weight]

I favor having more storage battery ah than solar watts. The solar watts should be more than you use from the battery.

 

[John TF]

You have the info , we do not , so here is some info to go with what you have ,
https://electronics.stackexchange.c...e-time-of-charging-and-discharging-of-battery
You need some metering to confirm what is going in & out of each item - batteries , solar , charger controller .

 

[jimindenver]

There is only one ratio that makes any difference out here, that is the ability to have more power coming down from the panels than you need to take out. 1 to 1 is fine as long as you get more in than you are taking out. This means in the winter months when low sun and shorter days effectively reduce your over all solar haul by half with a flat mounted panel.

1 to 1 is the ratio needed to equalize at a set temperature. 2 to 1 is the ratio to keep a bank happy in the winter if you are pulling from it heavily. 3 to 1 gives you the ability to bring your bank up in a day after a few days of bad weather. NONE of these matter as long as you are pulling in more power than you take out. NONE of these are enough solar if you are pulling more power out than the solar is putting in.

I have dealt with dozens of banks dead or nearly dead with 1 to 1 ratios out here. Some were fine in the summer months, others were never doing well. It isn't the 1 to 1 ratio that is giving them grief, it is the ratio of power out to power produced. This includes marine batteries, 6 volts, AGM's and even lithium. They all die if you take more than you give.

 

[Mrcap]

Last year I had 2 x 100w solar panels with 2 x 100ah of battery and it did well in winter because I was in the desert. This year I have upgraded my controller to MPPT and added a third panel. The panel are flat and as Jimindenver explains having a little more on the panel side gave me decent usage in rainy Washington in the fall. Now back in the desert and my batteries are topped off before noon and I don’t spend much time wondering if my batteries will reach full charge like I did in the past. I am not discharging my batteries more than about 25-30 % and expect them to last longer than when I was pulling up to 50%.

As Bob mentioned I have also learned to shift as much device charging and heavy use during the day while the bulk of the power is coming in rather than at night as I did in the past.

This forum has contributed hugely to understanding my hardware and best practices of how to use it. Thank you to all the contributors and all the videos Bob!!!




Sent from my iPhone using Tapatalk

 

[offgridder]

There is only one ratio that makes any difference out here, that is the ability to have more power coming down from the panels than you need to take out. 1 to 1 is fine as long as you get more in than you are taking out. This means in the winter months when low sun and shorter days effectively reduce your over all solar haul by half with a flat mounted panel.

1 to 1 is the ratio needed to equalize at a set temperature. 2 to 1 is the ratio to keep a bank happy in the winter if you are pulling from it heavily. 3 to 1 gives you the ability to bring your bank up in a day after a few days of bad weather. NONE of these matter as long as you are pulling in more power than you take out. NONE of these are enough solar if you are pulling more power out than the solar is putting in.

I have dealt with dozens of banks dead or nearly dead with 1 to 1 ratios out here. Some were fine in the summer months, others were never doing well. It isn't the 1 to 1 ratio that is giving them grief, it is the ratio of power out to power produced. This includes marine batteries, 6 volts, AGM's and even lithium. They all die if you take more than you give.

I see. My main concern, which I forgot to mention, was that with 300w solar to 100AH battery I would be overloading the battery. I guess this is not an issue since the charge controller and battery both have mechanisms built-in to prevent overcharging. And even though the van has been sitting outside unused for a few months (I have yet to move in) without any load on the solar system, the extra power didn't do any harm to any of the components. I better hurry up and move in...what a waste

 

[jimindenver]

With your van sitting you could likely set the controller to just float the battery until you are ready to use it.

 

[RVTravel]

100 ah battery and 100 watts solar is always an effective 2 to 1 ratio (solar to battery), if not using more than 50% battery amp hours?

 

[John61CT]

The real bottom line is to use less than you can put back in.

There is never a problem with "too much" solar or any other *potential* input, as long as charging voltage is properly regulated, the batts only accept what they need.

Just means you will be more likely to get enough when conditions are overcast a few days in a row.

Outside of always-sunny areas, 400w of solar per 200AH, 2:1 makes more sense than 1:1.

 

[Ticklebellly]

Say you have 400 watts of solar panels on top of your van. If I understand correctly, that's going to be 33.3 amps at 12 volts, right? 

Theoretically, and in full sun.   Peak Sun Hours (PSH) only happen for about 4 hours a day, as a rule of thumb.

Best to start with some idea of how much power you will use in a day and put enough solar in to have your battery charged by mid to late morning.    Most consistent advice I see used to come up with a ratio is twice the watts in the panel as Ah in the battery.

What do you consider to be a fully depleted battery.   I used to regularly cycle a Deep Cycle AGM from full to 11.8 Volts, then recharge.   What stuffs a battery is not recharging as soon as possible.   Sulphation and capacity loss in a Lead Acid battery starts to occur when any level of discharge is present.   The deep cycle battery I referred to earlier was just under 8 years old when I gifted to someone who had stuffed their 2 year old battery by leaving it in a discharged state for most of the time, hence the advice to have the battery up to full charge my mid morning or thereabouts.

 

[offgridder]

I'm using a 100AH BattleBorn Lithium-ion battery. I have 300w solar on the roof of the van. I park in the city, often in between 3-4 story buildings so it gets quite a bit of shade bits it's sunny everyday here in CA. I try to conserve energy but nighttime is when I wash dishes, clean my face, brush teeth so I use the water quite a bit. In the morning also, brush teeth and wash face, clean fruit for cereal. I recently purchased an EUC (Electric Unicycle) which is 800 watt-hours and charges @ 1.5 amps. It drained my Battle Born battery when I had it plugged in overnight. I need to figure out how much of a drain on the battery the EUC will be. I'm not familiar with watt hours (mostly with amp), it's a new concept for me but that's a topic for a different thread. I plan to get another 100ah BattleBorn battery in the future and upgrade to a larger inverter. Right now I have 600w pure sine wave inverter by Samlex. I want to charge the wheel and plan to get an induction cook top.

Sent from my LG-H910 using Tapatalk

 

[highdesertranger]

keep in mind you are losing at least 10% running anything through the inverter. some of your lower quality inverters can have losses in the 30% range.

still that doesn't seem right draining your battery overnight. was the battery fully charged? better get to the bottom of that issue. is a 12v charger available?

highdesertranger