Battery Charge Requirements

[Padlin]

Looking into a solar setup for my new trailer, first off is figuring out what I need for controller output to correctly charge my batteries. I'm planning on eventually having a pair of either 100w or 130w panels, that's all the space I have.

Batteries are 2 6v Interstate GC2-XHD, 232Ah.
Specs show per 6v
Bulk 7.2v
Absorption 7.75v
Float 6.6v
Equalization 7.8v

Do I read this right I need a 12v controller that will put out double these figures? Very few do, and those are user configurable to get to those levels (Blue Sky Solar Boost 3000i MPPT or Sun Chaser PWM, among a couple others).

[BFL13]

For Goodness sakes, all this voltage sensing temp comp stuff is such BS in real life!

Today, eg, we had sun, rain , heavy grey cloud, more sun , then more rain, sun, grey cloud- IAW a typical west coast day in April.

The solar was up and down, all over the place, all day. What good would a fancy temp comp, zillion dollar solar controller do better today than the average $20 eBay special (which I have and find to be perfectly good)

What a crock! Ignore the "controller snobs" and get real IMO! (Sorry about that controller snobs, but you are trying to make people spend extra money that could go to something more useful)

[pianotuna]

Hi red31,

Well said.

Amperage stays as high as it can until the voltage "pushing back" from the battery causes the controller to drop it down. 14.8 volts would be a good number for flooded bats.

[red31]

Almot wrote:
When voltage is rising, the current is dropping as the SOC is nearing Bulk setpoint (90%?).

What? see iv curve.

[red31]

PWM
The current from the panel follows the iv curve of the panel based on the sun light, the panel matches the battery voltage until the battery voltage gets high enough for the controller to limit current and maintain voltage (14.?v). If the battery ever gets fully charged based on the controller logic, the controller switches to floatof 13.?v.



The current basically stays constant (good sun) till the batt reaches the controller set point of 14.6 or whatever. Then voltage stays constant as current declines.

A PWM controller doesn't regulate current, it does regulate voltage when batt voltage gets in the mid to upper 14s and then the voltage is held constant which forces current to be reduced. If current was not reduced, batt voltage would increase beyond the controller's settings.

[Almot]

Padlin wrote:
Then the amperage/current from the panels via the controller slowly raise the battery amperage and as they do the voltage at the battery, which is the same as that felt at the controller output slowly rises?

Er... no.

I am not sure whether the amperage is rising first and then the voltage. When voltage is rising, the current is dropping as the SOC is nearing Bulk setpoint (90%?). Then both voltage and current drop when controller and battery go into Float stage. And, any time when it gets cloudy, the current drops too, of course.

And - no, the battery voltage is not "the same as that felt at the controller output". Cheap controller doesn't know that there exist voltage drop between the controller output and the battery. This drop can be significant when amps are high and cable to battery is long and thin. Better controllers like Rogue MPPT and Tristar MPPT have a separate "voltage sense wire" that tells the controller exact voltage on the battery contacts. With 130W I would get a short and beefy battery cable and ignored the voltage drop. With 260W I would consider voltage sense wire.

[Padlin]

Then the amperage/current from the panels via the controller slowly raise the battery amperage and as they do the voltage at the battery, which is the same as that felt at the controller output slowly rises?

[pianotuna]

Hi,

Let us suppose the battery voltage is 11.7 (god forbid). If you attach a PWM charge controller the voltage will appear to be 11.7, at least initially. As the battery charges the voltage will rise.

Once the battery bank is 90% charged then the voltage will be say (this is hypothetical and a guess) 13.2. Eventually when the bank is 99% that will rise. Probably the charge controller at that point will scale back the voltage (if it is a good one).

My original solar system on a previous RV had an old shunt type controller. When the voltage got high enough it would turn on and off.

MPPT works a little differently--depending on where you are in the state of charge.

260 watts of solar is going to be enough to do a GREAT job on the batteries--but don't "cheap out" on the controller. Buy a quality unit. PWM will do a good job for you.

At 130 watts I'd wish to see you use an MPPT controller. I'd also prefer to see 140 watts instead of 130.

On my first solar system I had just 30 watts to maintain 250 amp-hours of battery bank. It worked well, because it was several weeks between trips and the solar had enough time to "top charge" the battery bank. I did not buy it--but that 30 watt system cost $1400.00.

Padlin wrote:

pianotuna wrote:
Hi,

PWM controllers operate at the battery voltage.

Pianotuna, I don't understand the 2nd part of this response, please elaborate. Looked like you set the controller for the type of battery which then uses the applicable settings. On some you can choose custom setting and set them to whatever you want up to the units max.


I am planning on just one panel to start and will see how it works out, we're not heavy consumers so hopefully one is enough. One panel that fits (narrow), would be either 100 or 130w's. Still have to measure for length.

[wa8yxm]

One comment I will make.. I know this sounds confusing but I'll try to clear it up.

You do not have six volt batteries in your RV.. Oh yes, there are two things there that look like six volt batteries, And in fact they were six volt batteries when they were sitting on the shelf in the store.

But in your motor home they are two halves of one 12 volt battery.

Like this

-{Bat}t{ery}+12V

Think of the two of them as a single 12 volt battery.. NOTE also this answers many questions.

For example:

Can the two six volt be different makes, models, sizes, ages, whatever?

Would you buy a 12 volt battery that was half one thing and half another?

Same answer

See how easy that was.

Mine used to be Interstates.. They are now DEKAS. Just got new ones after 9 years.

[Almot]

Padlin wrote:
Looked like you set the controller for the type of battery which then uses the applicable settings. On some you can choose custom setting and set them to whatever you want up to the units max.

I am planning on just one panel to start and will see how it works out, we're not heavy consumers so hopefully one is enough.

1) Yes, you set it to "Flooded" or whatever it's called on controller. More expensive controllers will have adjustable setpoints, this is better. What Don meant (I think) is that PWM operates at the panel input voltage, which is the same as output voltage, more-less. MPPT takes "any" voltage and downconverts it to the required output voltage, upconverting the amps accordingly. Having adjustable setpoints (rather than choosing one of 3 fixed settings when each one differs from your battery specs), is same important as a temp compensation.

2) Unless it's sunny 99% of time, it's difficult to predict what is "enough" because you can't predict the weather. 130W flat will harvest 40-60 AH on a good day, and maybe 10 AH in overcast. Can you live off 10 AH? If yes, then alright.

[mena661]

MEXICOWANDERER wrote:
I recommend a FRANCIS FREAS. BUT BUT BUT, by doing the timer battery bubbling twice a week or at least once a week you will reduce the need for using a hydrometer by 80% and the need for equalizing 90%.

I don't do this once a week but the occasional bubbling has all but eliminated my batteries requirement for equalizing. It's been about a year and a half since the last equalize. I'll probably need to do sometime in the next couple of months after I water them though.

[doughere]

BFL13 X3

To make life easier, you can use a twist timer as MEXICOWANDERER suggests.

I have 120 watts fixed on roof through controller; with good sun, that's all we need. I have a 100 watt portable panel that I connect directly to batteries when needed.

My $15 e-bay POS PWM controller has no problem just being bypassed.

Regards,
Doug

[MEXICOWANDERER]

Just remember, unless you want to become a PLATINUM GRADE BATTERY SHOP CUSTOMER...

Charge those batteries long enough to allow a few of the cells (meaning all) to bubble 1 or 3 bubbles per second per cell PER WEEK. Meaning once a week. I like to do it twice a week but then I hate buying batteries. Nothing on the face of the earth can substitute looking in a cell and verifying the electrolyte is starting to bubble. By doing this, and then looking at the voltmeter and ammeter and CALIBRATING what happens when, you can eliminate the visual check after a few trial runs.

It is Damned tough to beat an INTERMATIC spring wound 2-hour timer to bypass and over-ride the solar controller. But you have to start with the batteries almost charged and 2 hours of good daylight left. Wind up the timer and let it have at it. Don't let the voltage creep past 15 volts doing this. When you are ready to equalize, shut off the coach power switch and wind up the timer two or three times after the batteries have charged as much as they can.

A hydrometer, that is a real one, not a toy, works excellent to verify battery charge. It is the supreme court of verifying state of charge in a battery. I recommend a FRANCIS FREAS. BUT BUT BUT, by doing the timer battery bubbling twice a week or at least once a week you will reduce the need for using a hydrometer by 80% and the need for equalizing 90%.

NOW, you MPPT and PWM Freaks, out there, will BYPASSING the controller BUT NOT ISOLATING IT damage the controller?

[Padlin]

pianotuna wrote:
Hi,

12 volt panels are nominally 17 volts. PWM controllers operate at the battery voltage.

Pianotuna, I don't understand the 2nd part of this response, please elaborate. Looked like you set the controller for the type of battery which then uses the applicable settings. On some you can choose custom setting and set them to whatever you want up to the units max.


I am planning on just one panel to start and will see how it works out, we're not heavy consumers so hopefully one is enough. One panel that fits (narrow), would be either 100 or 130w's. Still have to measure for length.

[time2roll]

Most any controller with a float voltage will charge and maintain those batteries just fine.