Panel amps required to charge batteries

ArizonaMTB

Hello all, another new guy here with a question that may have been answered many times but I can't seem to find a straight answer. Just looking into putting a solar system together for my 25' travel trailer. My question; with 2 US 2200XC's in series, each requiring a min of 18 amps (18-37amps) and 7.7V each totaling 15.5V to fully charge the batteries. Am I correctly deducing that I'll need enough panels to provide at least 18A's to fully charge those batteries? i.e. If each panel creates 250w at 8.15A I will require at least 3 panels to correctly charge the batteries? One panel will provide enough volts but the amperage appears to be the issue. (Assuming I can find a solar controller the will put out the 15.5V required.)

I guess I want to know what my options are to correctly and fully charge these batteries via solar panels. I don't what to do what I read so many are doing by not charging there batteries to 100%.

Thanks in advance

Cariboocoot

Re: Panel amps required to charge batteries

Welcome to the forum.

Not quite right: if you put two 6 Volt 232 Amp hour batteries in series you get the equivalent of on 12 Volt 232 Amp hour battery; the Volts go up but the Amp hour capacity remains the same.

The 5% minimum remains the same at 11.6 Amps and the 10% 'desired target' remains the same at 23.2 Amps. Only the Voltage changes.

You will have trouble with that 15.5 charging Voltage (where did that number come from?). The standard for a 12 Volt system is 14.8 maximum on a PWM controller. Expensive MPPT controllers can be programmed higher. But you may also have a problem with 12 Volt equipment shutting down or being damaged by this high Voltage: many 12 Volt inverters, for example, will scream and shut down from "over Voltage" at that point. This is much higher than a 12 Volt system typically operates at.

For use with a PWM controller check the panel's Imp rating. You can approximate the Wattage you'd be looking at by multiplying the desired current by typical "12 Volt" Vmp of 17.5. In this case that would be 23.2 * 17.5 = 406 Watts, so somewhere around 400 Watts total.

These battery specs are odd: I have US/East Penn PS2200 (where I picked 232 Amp hours) and they charge at "normal" Voltages.

Photowhit

Re: Panel amps required to charge batteries

ArizonaMTB wrote: »

... If each panel creates 250w at 8.15A ...

If 8.15 is the IMP then These will have a VMP of around 30 volts, which is really itching for a MPPT type charge controller (extra $$$) If your in 'Arizona' and close to Northern Arizona Wind and Sun, why not bike in for a short course on solar?

ArizonaMTB

Re: Panel amps required to charge batteries

Photowhit wrote: »

If 8.15 is the IMP then These will have a VMP of around 30 volts, which is really itching for a MPPT type charge controller (extra $$$) If your in 'Arizona' and close to Northern Arizona Wind and Sun, why not bike in for a short course on solar?

You are correct that I'm looking at 30.7 volts per panel and will need an MPPT controller. To be honest, I'm trying to avoid dealers if I can help it. Many understand solar very well but they are still just trying to make a buck by telling you when they want to sell you and not what will actually work better. Biggest example, "you can fit several panels up there. We'll put 4 solar batteries in back with 4 195W panels and some 12ga wire to a nice 60A MPPT controller. It should be around $3600. for the parts.". Now as a somewhat 12V literate guy I bluntly said "your nuts!". The 2 biggest mistakes he made was suggesting 4 batteries at $340 each and 12ga wire. Anything less than 8ga is crazy when it only costs pennies more and provides such a huge difference. The batteries would be fine if we were dry camping for more than 5 days with no sunlight to recharge them but still overkill. I hate sales people like that.

Photowhit

Re: Panel amps required to charge batteries

I'm many miles away, but hope to stop in some day. We've heard reports from people like those starting an off grid place in Oklahoma about them actually telling them not to buy more unless they find they need it. I personally would suggest buying golf cart batteries from Costco or Sams club for a small system. Unless you have a long run 8ga might be 'crazy'... Really looking for the amount of voltage drop rather than the gauge of wire.

If your talking an MPPT controller, I guess you understand your talking a bit of bananas here anywho, since it will down convert the voltage and produce more amperage, so your 250 watt panel which produce optimally 30 volts at 8 amps run through a MPPT type controller will produce around 15 amps at 15 volt?

BTW - you've got to buy your stuff somewhere, you might not 'need' a 60 amp charge controller, but you might find it's the best value and NAWS sells the Midnite classic lite for $500 w/free shipping. 3 of those panels have a potential output of more than 45 amps at 12 volts. Don't think you'll find the Classic or Classic Lite sold cheaper. and I'd be glad to argue that it is the best value for a system with 3 250watt panels charging a 12 volt battery bank.

Photowhit

Re: Panel amps required to charge batteries

Back to the original post on needs for your system;

With out knowing what your using, the needs for charging can't be calculated. If your keeping them topped off, an array producing more than 5% of your batteries capacity is a waste. If your running a fridge, fans, stereo and lights while charging, well you might consider shooting for around 20% of your batteries capacity and having a backup generator.

Also needed is your location, if your in AZ then you have abundant sun and can count on not having more than a day of clouds... I think, I'm miles away.

So what are you planned daily loads?

ArizonaMTB

Re: Panel amps required to charge batteries

Cariboocoot wrote: »

Welcome to the forum.

Not quite right: if you put two 6 Volt 232 Amp hour batteries in series you get the equivalent of on 12 Volt 232 Amp hour battery; the Volts go up but the Amp hour capacity remains the same.

The 5% minimum remains the same at 11.6 Amps and the 10% 'desired target' remains the same at 23.2 Amps. Only the Voltage changes.

You will have trouble with that 15.5 charging Voltage (where did that number come from?). The standard for a 12 Volt system is 14.8 maximum on a PWM controller. Expensive MPPT controllers can be programmed higher. But you may also have a problem with 12 Volt equipment shutting down or being damaged by this high Voltage: many 12 Volt inverters, for example, will scream and shut down from "over Voltage" at that point. This is much higher than a 12 Volt system typically operates at.

For use with a PWM controller check the panel's Imp rating. You can approximate the Wattage you'd be looking at by multiplying the desired current by typical "12 Volt" Vmp of 17.5. In this case that would be 23.2 * 17.5 = 406 Watts, so somewhere around 400 Watts total.

These battery specs are odd: I have US/East Penn PS2200 (where I picked 232 Amp hours) and they charge at "normal" Voltages.

I do understand that my ah don't double like the voltage does. The amperage question was in regards to correctly charging the batteries. Will I need 3 8A panels totaling 24A's to correctly charge my batteries?

I spoke with a Golf Cart service technician who prefers and sells US Batteries, this is where I got the 7.7V charge rate per 6V battery. The golf cart chargers there using for the 2200's are supplying 7.7V to one 6V battery. Assuming 2 of these in series would double that charge rate I'd be looking at 15.4V to charge both.

westbranch

Re: Panel amps required to charge batteries

To be clear: no matter if you are putting batteries or solar panels in series the VOLTS are cumulative.
Putting panels or batteries in parallel, the AMPS are cumulative.

What setup do you plan on?
hth

Photowhit

Re: Panel amps required to charge batteries

ArizonaMTB wrote: »

I do understand that my ah don't double like the voltage does. The amperage question was in regards to correctly charging the batteries. Will I need 3 8A panels totaling 24A's to correctly charge my batteries?

I think you missed or didn't understand my post. You might be helped by reading through the FAQ's.

Lots of good information here.

On wiring and use of the proper gauge wire to avoid voltage loss.

On Charge Controllers, why 3 -250 watt panels will produce as much as 60 amps output with MPPT type Charge Controller charging a 12 volt battery bank. (CC)

On configuring and use of Batteries/battery banks.

ArizonaMTB

Re: Panel amps required to charge batteries

Photowhit wrote: »

Back to the original post on needs for your system;

With out knowing what your using, the needs for charging can't be calculated. If your keeping them topped off, an array producing more than 5% of your batteries capacity is a waste. If your running a fridge, fans, stereo and lights while charging, well you might consider shooting for around 20% of your batteries capacity and having a backup generator.

Also needed is your location, if your in AZ then you have abundant sun and can count on not having more than a day of clouds... I think, I'm miles away.

So what are you planned daily loads?

To be honest, our loads are undermined. We haven't even attempted dry camping yet as one night using the 12v furnace kills our current interstate battery. The biggest draw will be the 12V fan I sleep with each night and our inverters, one 400W and one 1200W. The 400W is used for charging the laptop, phones, and TV. The 1200W wouldn't be used much except for coffee, vacuum, etc... The inside lights and water pump will be next. We do live in AZ so sun shouldn't be a problem. I don't want to purchase a generator. With the proper solar setup there should never be a need unless your in shade or volcanic ash blackens the sky.

ArizonaMTB

Re: Panel amps required to charge batteries

If I can help it I'd prefer to avoid the cost of an MPPT controller. The only reason I'm considering a MPPT controller is to make sure I have sufficient amperage to fully charge my batteries.

ArizonaMTB

Re: Panel amps required to charge batteries

Ok, I found what I was looking for. I was not previously looking at 12V panels as they seemed to all have a low amperage along with them. Doing a bit more searching I see there are plenty of options out there for 12V panels that have plenty of amps per panel but the wattage is cut in half by comparison to non 12V panels (at the same price). So get more panel for the $ (watts, volts) and pay for an MPPT controller or stay 12V (less wattage, more panels) with a standard controller?

Photowhit

Re: Panel amps required to charge batteries

ArizonaMTB wrote: »

...With the proper solar setup there should never be a need unless your in shade or volcanic ash blackens the sky.

Making statements like this are fine if you know what your needs and understand how to meet them. In truth I find them insulting to those trying to help someone who doesn't understand the basics. FWIW - inverters are loads but very tiny 10-30 watts normally, what you run through them can be quite a different matter.

12v furnace? AACCCKKK, like you mean fan for the furnace...

I'll send you a link to likely the best deal currently on 12 volt panels. NAWS who sponsors this forum sells some as well, so I don't want to be rude.

I encourage you to read and learn so you can speak intelligently about the subjects.

ArizonaMTB

Re: Panel amps required to charge batteries

Photowhit wrote: »

Making statements like this are fine if you know what your needs and understand how to meet them. In truth I find them insulting to those trying to help someone who doesn't understand the basics. FWIW - inverters are loads but very tiny 10-30 watts normally, what you run through them can be quite a different matter.

12v furnace? AACCCKKK, like you mean fan for the furnace...

I'll send you a link to likely the best deal currently on 12 volt panels. NAWS who sponsors this forum sells some as well, so I don't want to be rude.

I encourage you to read and learn so you can speak intelligently about the subjects.

I do apologize so coming off rude, not my intention. I was only speaking from what I have seen and learned from people I've met and setups I've seen. I have met several full time boon-dockers all with solar systems under 400W that are living completely off solar. Only one of the 4 even had a generator which he hadn't used since a bad snow storm last year. They were all using more power than the wife and I ever could (they had larger TV's, central vac, fans, 2200W inverter, Freezers, etc..). The first point they shared was "Keep the big things small and little things big" meaning there was no need for 600W's worth of panels when less will suffice. Spend more on the charge controller and wire so you can provide your batteries with what they need.

I think I get a little defensive because to many sales people keep telling me what I need when I see and know it can be done better for half the cost.

My apologizes

Cariboocoot

Re: Panel amps required to charge batteries

ArizonaMTB wrote: »

I think I get a little defensive because to many sales people keep telling me what I need when I see and know it can be done better for half the cost.

Couple of things to mention here:

1). None of us are salesmen. Even we moderators are not in any way connected with the forum host NAWS. They nicely do not tell us what to do other than some general guidelines. None of that "we have a lot of these, push 'em" stuff. As such the info you get here will be as unbiased and accurate as possible and includes the experience of many real-world users from, well, around the world in fact.

2). Don't be so sure you can do it better and/or at half the cost. Do be sure you can do it right if you listen to the collective experience here. I quite agree that too often salesmen are selling stuff and have zero knowledge of what it is or how it works. I do not believe you'd get that experience if you were talking to NAWS personnel however.

Oh and BTW, sometimes the "GT panels" + MPPT charge controller combination works out cheaper than "standard panels" + PWM controller for the same power requirement due to the lower cost per Watt of the GT panels making up for the additional cost of the MPPT controller. This is especially true in systems with over 400 Watt arrays.

niel

Re: Panel amps required to charge batteries

chiming in a tad late, i think you need to know your load requirements better before settling in on any battery setup. some of those loads if run long enough can destroy a better battery bank than your proposed bank. once the loads are determined then you can use that info to size a battery bank capacity and do note that you should not be taking any battery bank below 50% as it destroys their lifespan.

now the interstate batteries do require a higher voltage as i recall and many controllers can meet that need. where you have to watch is that the inverters you have may not like how high that voltage is going in the latter stages of bulk charging and the absorb stage. many inverters can literally fail around 16v. this begets the question of should you go with interstate or something like costco? either is good as a cheap battery to make mistakes on, but do you want to sacrifice the inverters too?

anyway, come up with better figures on your daily wh requirements to size the bank is the first step. from there the choices of batteries, controllers, and pvs can be addressed. don't always assume that you can run anything with a minimal pv system because somebody else is bragging that they can. some do extreme sacrificing to achieve low consumption enabling minimal systems to be utilized. it can be done, but not with big ticket items making huge draws over long time periods. the laws of electricity and physics are not being broken as you can't get more out than you put in.

westbranch

Re: Panel amps required to charge batteries

ArizonaMTB wrote: »

The first point they shared was "Keep the big things small and little things big" meaning there was no need for 600W's worth of panels when less will suffice. Spend more on the charge controller and wire so you can provide your batteries with what they need.

they are right on the money ...

ArizonaMTB wrote: »

I have met several full time boon-dockers all with solar systems under 400W that are living completely off solar. Only one of the 4 even had a generator which he hadn't used since a bad snow storm last year. They were all using more power than the wife and I ever could (they had larger TV's, central vac, fans, 2200W inverter, Freezers, etc..).

There is something missing here...not sure what it is but there is a problem with, for example, running a 2000W inverter to power a fridge all day and supplying ~ 280W (400w derated to 70%) per hr x 4 avg sun hrs = ~1,120 Whr to the batteries, This is the average amount of running a full size fridge and doesn't leave much left for other things.. Not sure what but ....???

hth

Cariboocoot

Re: Panel amps required to charge batteries

westbranch wrote: »

There is something missing here...not sure what it is but there is a problem with, for example, running a 2000W inverter to power a fridge all day and supplying ~ 280W (400w derated to 70%) per hr x 4 avg sun hrs = ~1,120 Whr to the batteries, This is the average amount of running a full size fridge and doesn't leave much left for other things.. Not sure what but ....???

hth

Physics.
It is completely impossible to supply large amounts of kW hours with small amounts of panels.
Deficit charging does not show up tomorrow if you've got big enough batteries.
For years panels were very expensive and batteries very cheap so the plan was "use a small panel; it will recharge those big batteries over time" and so what if you replaced the batteries every two years?
Now it's the other way around and the sensible off-gridder limits his DOD, makes sure he's got plenty of panel, and always has a back-up plan.

How do I know? Because much of what I do is salvage systems that were designed the wrong way.

As I always say, no one has to listen to me; it's not my money you're spending.

westend

Re: Panel amps required to charge batteries

Downloadable spread sheet for calculating use: http://www.quartzfest.org/php/main.php?RV_Solar_Battery_bank_and_Solar_systems_by_N8GS

Example: My daily use is calculated to be 60-80ah's. I'm able to achieve that by having no parasitic loads like alarms, circuit boards, etc. and am using a non-electric vented propane heater. All lighting is LED. I have 320 AH of battery capacity. My solar system is one 235w panel and a Morningstar MPPT15 charge controller. The Morningstar controller has an output limit of 15 Amps. The controller's setpoints for charging are fully programmable through the RS232-->USB interface and the MSView software. I'm assuming that two or three days of volcanic eclipse (or snow, rain) are going to move my hand to the generator rope. I'm hoping that isn't necessary.