Charging time for 6V Batteries with solar panels

[hurrican33g]

I have 4 x 240AH 6V Batteries that I charge with 2 x 200w solar panels. In full sun I am getting 12A charge so I wanted to check my math...
In order to fully charge my batteries, I am trying to figure out the time it would take.
240AH x 4 = 960 Ah / 12 Ah = 80hrs to fully charge the batteries.

This seems like a lot, and it would mean that for a full day of sun 10hrs I would only charge back about 12% and it doesn't seem right.
I seem to consume about 40% of the batteries on an average day using a all house inverter, TVs etc and I was hoping 400w of solar panels would be enough to cover that usage.

Is my math correct? Can anyone share their experience for that matter?

 

[Lou Schneider]

Welcome to The RV Forum!

Batteries in series add voltage, not amp-hours. So (4) 6 volt, 240 amp-hour batteries give 12 volts at 480 amp-hours, not 960.

I'll let others with more solar experience chime in, I just wanted to welcome you to the Forum and correct this error up front.

 

[Contributor]

Batteries in series add voltage, not amp-hours. So (4) 6 volt, 240 amp-hour batteries give 12 volts at 480 amp-hours, not 960.

This is correct, 480 mot 960

with 2 x 200w solar panels. In full sun I am getting 12A charge

That seems low, you should be getting around 20A

a full day of sun 10hrs

You will get about 6 hours of good quality sun

So no shade, 6 hours at 20A, then you can figure about 100 to 120Ah per day

I seem to consume about 40% of the batteries on an average day

40% 0f 480Ah is 192Ah, and you are harvesting about 120Ah. You are running a deficit of 72Ah per day so if you start out with fully charged batteries at 480Ah, and you want to be careful and only go down 50%, then you have 240Ah to use up. At your current deficit of 72Ah per day you will be at 50% charge in 3.3 days (240/72).

One last thing is that the Batteries will only accept so much charge when the get to about 90% full. But because your battery bank is big at 480Ah, the 20A charge rate will be mostly accepted between the 4 batteries as you will not be above 90% after the first day deficit.

 

[Ray-IN]

Stop thinking about 6V; you have 2 12V batteries. Amps do not multiply, so the two separate section making 1 12V battery have 240AH total. Only the voltage multiplies.

This website should be quite informative: The 12V Side of Life. parts 1 and 2
pg2 also covers solar installations.

 

[Ex-Calif]

The rule of thumb I have always used for solar is = 6 hours a day collection = this allows for the differing angle of the sun throughout the day and 80% efficiency on collection that allows for partial shading clouds etc.

I am curious how you are measuring you solar output.

Anyway 200W X 6 hours X .80 = 960W / 12V = 80 Amps - You might expect a little better collection time in California in the summer.

So I agree with Pedro that you solar output seems low - even though we might differ on the expected harvest.

How do you know you are consuming 40% of your 480 A/H available?

Again Pedro is correct you are running a deficit. Also 192 amps per day is a pretty high consumption. Well not actually high per se but you are practically doing no conservation here.

You say you have whole house inverter and that's a clue. You could be using high amp appliances without realizing it. Do you run the microwave, coffee pot or any other small appliances?

For comparison a couple using lights, tv, phone/tablet/PC charge and the 12V control panels on tings like the fridge might run 80 amps a day.

This all ties in with my experience on liveaboard boats. To run independent from the grid you generally need a minimum of 400W of solar and 400a/h+ of battery.

 

[hurrican33g]

Thank you so much for the feedback, right on point! This is very helpful. So if my solar output seems low, I should probably check that the 2 panels are both working properly and then maybe check the solar charger?

 

[Ksouers]

First off, you need to get an accurate accounting of your usage. This will help greatly:
Victron smart shunt. It's bluetooth connected and they have a phone app to monitor your usage. It will show how much you've depleted your stores and when you have replenished what you've used. It will also report the net input/consumption based on power coming in from your charge controller and battery outflow due to load usage. For example: 20 amps in from solar with 10 amps out due to loads (lights, appliances) equals a net 10 amp replenishment. The converse is also true, 10 amps in from the panels with 20 amps out due to loads is a 10 amp deficit.

A kill-a-watt meter will also help determine the exact usage, instantaneous and over time, of the power that your individual 120V appliances use. Measure each appliance over a 24 hour period (or several days and average out the daily usage) and keep record of it in a spreadsheet, add up all the amp-hours used daily. Virtually all modern appliances have some parasitic load all the time that will work against total energy consumption.

Do you know what kind of charge controller you have? If it's PWM, at your panel wattage you may likely be losing efficiency because of voltage clipping from the panels. PWM controllers are basically pass-through controllers but they do monitor the charging and adjust output power by clipping input voltage and modifying charging pulses.

An MPPT controller, on the other hand, uses all the power output by the panels and adjusts the charging amperage output going to the batteries. At your wattage you should be using an MPPT controller for max efficiency.

Your panel output will vary greatly depending on the time of day, transparency of the atmosphere (humidity, dust, pollution, clouds), the angle of the panels relative to the sun, the temperature of the panels and the time of year. The light has to pass through much more atmosphere during the low angles of winter than it does in summer.

Since you have FLA batteries, you should try to never deplete them below 50% state of charge, it will shorten the life of the batteries considerably.

This is by no means a comprehensive manual on solar power but should get you started on your research and on the right path to getting the most usage out of your solar and storage capacity. Check out some of the documentation on Arizona Wind & Sun. They've been at this a very long time.

Kevin

 

[Contributor]

I'm basing the 20A on my experience. I only have 200W of solar and a PWM controller and 2 6V GC2 batteries. With my shunt I measure about 10A max during peak sun. So at 6 hours I'm getting 60Ah max, more like 50Ah, hence my estimate of 100Ah per day.

Since you have twice the solar, then I would expect twice my numbers.

 

[Ex-Calif]

I'm basing the 20A on my experience. I only have 200W of solar and a PWM controller and 2 6V GC2 batteries. With my shunt I measure about 10A max during peak sun. So at 6 hours I'm getting 60Ah max, more like 50Ah, hence my estimate of 100Ah per day.

Since you have twice the solar, then I would expect twice my numbers.

In Sunny Cal I would expect a little higher than average. Your 50 amps per day makes sense as should 80-100 for the OP.

He should also do a little better if he has an MPPT controller vs. PWM.

@hurrican33g - Still wondering how you are measuring your solar output. Is the solar being measured by your solar controller via a monitor and shunt?

 

[hurrican33g]

Agreed on all points, I have a 30amp Go Power solar controller and I am looking now to upgrade to an MPPT one. I have also started to look at each appliance consumption but I have a lot more research to do.

 

[Ex-Calif]

Agreed on all points, I have a 30amp Go Power solar controller and I am looking now to upgrade to an MPPT one. I have also started to look at each appliance consumption but I have a lot more research to do.

Other than being close to the limit for 400W worth of panels that doesn't seem like a bad controller.

 

[Contributor]

I have two Renogy 100W panels I acquired at separate times. One is rated at Optimum Operating Voltage of 20.4 and the other 18.6 (older panel). So the one panel at 20.4 is rated at Optimum Operating Current of 4.9A and the 18.6 is at 5.37A for a total of 10.3A when using PWM. I have them in parallel.

These "12V panels" work well with PWM because their voltage output is close to the battery charging voltage plus the controller voltage drop across it's circuity. If the charging voltage is 14.6V plus 2V for the circuity (14.6V + 2V = 16.6V), then 3.8V (20.4V - 16.6V) are not being used (only during Optimum Conditions - lab testing). Under nominal conditions, out camping, the 20.4V panel may only be putting out 18V, so in this case only about 1.4V is not utilized. With the MPP, the 3.8V or the 1.4V would be harvested and that is the advantage over the PWM.

So using PWM I'm getting about 71.5% (4.9A x 14.6V = 71.5W) of the 100W panel. This 71.5W is steady from Optimal conditions output of 20.4V all the way down to non optimum panel output of 16.6V.

I'm guesstimating that an additional 18.6W (4.9A x 3.8V = 18.6W) would be available with a PPM (under optimum operating conditions). So with MPP I would get 71.54W + 18.6W for a sum of 90.14W or 90% of 100W.

Note that the difference is this big only during max solar production and Optimum conditions. Under less than optimum, the PWM will keep providing the 71.5W from panel voltage of 20.4V all the way down to 16.6V.

If the 100W panel is only putting out 16.6V, then both PWM and MPP will provide basically the same amount of charging power.