Battery bank not holding charge--Please help!!

techntrek · March 2011

Re: Battery bank not holding charge--Please help!!

Congrats, now you'll have fun building a GOOD system.

meghammock · March 2011

Re: Battery bank not holding charge--Please help!!

I do think we'll go with the MPPT controller.. From what I've heard/read, that's definitely the way to go. ..And I'll be super careful to get the right one!

We've actually got a place called Batteries Plus around here--They've got the same batteries the "company" sold us.. I think that's definitely a good idea to get a cheaper bank to begin with, so it doesn't hurt too much when we, as beginners, abuse it by accident :x
What do you mean about siphoning from a 5 gallon can? It'd "eat" from that can as well as it's own tank?? I think they've got smaller, quieter generators at Northern Tool.. I'll have to check those out... (would electric start be a better idea?)
Question about the controller/panel voltage--What makes the MPPT different from the PWM? I know the MPPT's better.. But why? What makes it better?
And are the lower voltage panels better than the higher? What would the optimum be, and why??

Thank you so much again.. It's actually fun now:)

Cariboocoot · March 2011

Re: Battery bank not holding charge--Please help!!

meghammock wrote: »

Question about the controller/panel voltage--What makes the MPPT different from the PWM? I know the MPPT's better.. But why? What makes it better?
And are the lower voltage panels better than the higher? What would the optimum be, and why??

Thank you so much again.. It's actually fun now:)

The handy glossary of terms: http://forum.solar-electric.com/showthread.php?t=6136

MPPT isn't necessarily better, just different. A PWM controller is essentially just a switch; connecting/disconnecting the panels according to preset Voltage levels. The MPPT controller can actually down-convert higher array Voltage to system Voltage. So whereas the PWM works fine with panels that have the same nominal Voltage as the system, they can not harvest the additional power available from higher Voltage panels. This is one of the problems with your now-gone system; the panels were above typical "12 Volt" levels and the controller was PWM. So you had the Wattage, but couldn't make use of it.

There's no major advantage/disadvantage to particular Voltage panels, as it all comes down to Watts. Before anyone starts the argument, a higher Voltage array used with an MPPT controller can give you charging in lower light levels that you would not get with the equivalent Wattage and lower Voltage. This is because you always need more Voltage to charge a battery than the battery has itself. Hence a "12 Volt" panel actually produces about 17.5 Volts at its maximum power point. A "24 Volt" panel is about 35 Vmp. Then there are the "odd" panels that have other Vmp's and require MPPT to be fully utilized. Generally these are meant for grid-tie applications, but it seems a lot of solar installers of a less reputable nature buy them by the pallet and foist them off on everybody without understanding the implications.

Pull-start generator? Depends on how big it is. Pull-starting a Honda EU1000i is real easy. Pull-starting a 5kW B&S rather more difficult.

What say we start a new thread for the new system? You could list your design needs, such as loads and system Voltage, and we can poke around from there. I think this was an RV install, no? So you'll probably want to stay with 12 Volt.

BB. · March 2011

Re: Battery bank not holding charge--Please help!!

meghammock wrote: »

What do you mean about siphoning from a 5 gallon can? It'd "eat" from that can as well as it's own tank?? I think they've got smaller, quieter generators at Northern Tool.. I'll have to check those out... (would electric start be a better idea?)

For the Honda (and I guess Yamaha), they have a small fuel pump that pulls fuel from the internal fuel tank... And you can get (or even make) a fuel cap with a fitting on top that you can add a fuel line too and drop it in a 5 gallon gas can. This saves you from having to fill the 1.1 gallon internal fuel tank every 4-10 hours of run time.

Basically, you fill the generator fuel tank, screw on the new fuel cap and drop the extra fuel line into a second gas can (obviously careful to keep fuel liquid and fumes from escaping and causing any fire hazards).

You start up the genset and it uses fuel from the internal tank. As the engine runs the pump pulls fuel from the internal tank and draws a vacuum. As the vacuum gets high enough, it pulls fuel from the external gas can to refill the generator internal fuel tank.

Even though you can run for a long time on the external tank, you still need to check the generator oil level and change the oil as recommended by the manufacturer.

Electric start vs manual start--That will be your choice... The little Honda's are pretty easy to start, but it may not be your cup of tea (especially during winter snow/rain when you need a generator the most). But if everything is running right, you should only need to start the generator every two or three days during bad weather, and much less during good weather.

Sizing the genset is second after you figure out your loads and battery bank size.

The big thing (from my point of view) is to size the generator so that it runs at 50% output load minimum... This helps ensure that your fuel use is efficient and you run the generator the minimum amount of time needed to recharge your battery bank.

You may end up with two generator anyway... You can use the small Honda eu1000i or eu2000i (~$750 or $920 delivered to your door) for recharging your battery bank and a larger, possibly electric start, genset for larger loads (running tools, once a week water pumping to cistern, etc.) and emergency backup.

There are also options for diesel and propane powered generators... Each has its advantages and disadvantages over a gasoline powered genset... And you may be able to find a very nice mid-sized electric start genset from an RV wrecking yard (like a Onan with oil pump and filter).

Again, figuring out your loads and battery bank size first--then look at the genset options.

Question about the controller/panel voltage--What makes the MPPT different from the PWM? I know the MPPT's better.. But why? What makes it better?

Better is a relative term...

From a simple point of view, a PWM charge controller is simply the electronic version of a On/Off switch between the solar array and the battery bank. Turn the switch on, the battery charges. Turn the switch off, the battery stops charging.... Turn the switch on and off really fast, the battery charges at 50% average current (pulse width modulation--The more % on time, the higher the average current, etc.).

A MPPT controller is, sort of, the equivalent of an AC transformer, it matches the solar panel voltage to the battery voltage (sort of like a wall transformer takes 120 VAC and outputs 12 VAC for your battery charger).

In the end, if you have Vmp=17.6 volt (approx.) solar panels, a PWM controller will be just about as efficient as a MPPT transformer overall... You make get 10% more from the MPPT controller during freezing weather, but it is not always worth the extra money for that reason (10% more power during poor sun is still not much extra power).

Very roughly, for systems under 200 watt solar arrays--a PWM controller is a nice fit (cheap and relatively efficient).

For arrays over 400 watts, many times, a MPPT controller is a better choice... Not specifically for the ~10% extra power in cold weather, but because you can run the solar array at a higher voltage (run solar panels in series for Vmp>>Vbatt). This allows you put the solar array farther away from your battery bank and run higher voltage / lower current wiring (use 1/2 or 1/4 the amount of copper/price of wiring).

This gets back to the math of Power = Voltage * Current...

Your battery bank is ~12 VDC. A solar array with Vmp~96 VDC would be ~8x your battery voltage and 1/8th the amount of wire needed (or you can place the solar array ~8-16x farther away, for example in better sun, than with a PWM controller).

And are the lower voltage panels better than the higher? What would the optimum be, and why??

And this gets into the real reason a MPPT controller is frequently the better choice... For the most part, solar panels >100 watts tend to be designed for Grid Tied Solar applications which are not tied to a 12 volt (or 24/48 volt) battery bank and PWM controller (Vmp must be ~ 17.6 volts per 12 volt bank voltage).

Instead, they make these panels at very high volume (and low costs) for the GT inverter market at the optimum voltage/current for GT setups (which are typically Vmp of 200-600 VDC).

A MPPT type can take these "oddball" voltage solar panels and efficiently down convert the High Voltage / Low Current from the solar array to the Low Voltage / High Current needed to recharge the battery bank.

MPPT controllers tend to be a small bit "more fragile" (be careful connecting them to a battery bank--follow instructions and don't hook them up backwards) and 2-3+x more expensive than PWM.

So, to get your "optimum system" (meets your needs without emptying your bank account):

Measure/Understand your daily loads (DC and AC needs)
Size the battery bank to meet your needs.
Size the solar array to meet your bank and energy needs
Size the AC charge controller to your bank needs
Size the AC Genset (or gensets) to your charger/other AC needs
Pick the fuel/manual/electric start genset(s) to meet your needs

As I said before, it is possible that you are limited in your battery space requirements...

Can you build an external, insulated/vented, box for your battery bank?
Must the battery bank be internal to the RV because you will be moving / traveling at times?

If that is the case, I would simply measure the space up and put a comfortable amount of "inexpensive flooded cell" batteries in there for now and see how they keep you going for the next couple years... Then size the next bank/system for your lifestyle based on your experience.

-Bill

meghammock · March 2011

Re: Battery bank not holding charge--Please help!!

We are not moving the camper/battery bank/anything else as of now, so we can make the bank as big as we need it to be, space-wise, and yes, we can make a vented box for it.. Are rubber totes a bad idea? Would a flatter surface, possibly a wooden platform with a wooden box, be better?
Could I "oversize" the system safely?? I'm not sure what the wattage and everything is off the top of my head, but I was thinking about making the array 'more' than we needed (so we'll already have a good bit of incoming wattage when the time comes to expand).. Would that be a bad idea?
And how can I figure out how many batteries we'll need to 'fit' the camper's needs?

I was just browsing online for panels--I saw something like a 230w panel, 16v for somewhere in the $500-$600 range.. So I could use the PWM controller with those, correct? (not saying that's what I WILL do.. Just trying to figure out the best options:) ) I'd only need the MPPT if the panels were a higher voltage??

That's pretty neat about the genset siphon setup! MAN that got annoying having to wake up just to fill the thing up!! What a great idea!!!

Cariboocoot · March 2011

Re: Battery bank not holding charge--Please help!!

Working backwards through the questions ...

It would depend on what that "16 Volt" rating was in reference to. If Vmp, it's actually a tad low for a 12 Volt system. This is because some of the Voltage inevitably "disappears" by the time it gets to the battery. On an RV set up where wire lengths are probably kept short it may not be an issue. If that "16" is the Voc, it won't work. If it's nominal ... that's one weird panel!

Battery bank size is based on how many Watt hours of power you need daily. The basic formula:
Watt hours / system Voltage = Amp hours (multiply by 2 for 50% maximum DOD, 4 for 25%).
So 1200 Watt hours daily @ 12 Volts is 100 Amp hours * 2 = 200 Amp hour battery bank.
Okay I used those numbers just to simplify the math, and there's no efficiency loss allowance either. But that's the basic idea.

Over-sizing an array isn't a problem, except for the bank account. The charge controller does just that, so you shouldn't have any trouble there (unless you buy a bad controller - "BZ" comes to mind). You also have the option of mounting more panels than you need now, and just not hooking them up until later. Although why anyone would want to ... I guess it comes down to how the cash flow works and what amount of effort is required to put the panels in.

As for battery containment, everything has its advantages and disadvantages. Wood is easy to work with, but will allow any spilled acid to soak through. Plastic can be easier to work with (to a point) and will be better at containing acid spills (some plastics will dissolve). Metal is probably the worst choice, as it is harder to work with and offers an almost absolute guarantee of short circuits.

BB. · March 2011

Re: Battery bank not holding charge--Please help!!

Megan,

I don't want to ignore your questions, but I don't want to confuse you with multiple subjects distracting from your goal of reliable power at your home.

A "big" off grid system would be around 100kwh per month or 3.3 kwh per day.

In 12 volt land:

o 3,300 wh / 12 volts = 275 ah per day

A smaller off grid system would be about 1/3rd the size.

Both can work well--But one will be 1/3 the cost of the other.

Personally, I would describe a 3.3 kwh per day system as a normal small home where everyone works very hard at conservation. Includes an efficient electric fridge/fill time life style and minimizing propane / genset use.

A1 kwh per day system would be a remote cabin life style with only enough electricity for lights, radio, laptop computer, shallow well pump. And using a purpose fridge/part time home.

I do not live off grid, so others probably disagree with my characterization, but placing your home on this curve, where do you fit?

-Bill

meghammock · April 2011

Re: Battery bank not holding charge--Please help!!

Hey, all! Just an update since I've been pretty rare lately..

We now have a 2kW generator (a nice, QUIET Honda).. That and other things we had to do around the farm broke the bank, so we're having to re-save up for our panels and all. We may be planning for the house before the camper again. Haha!
I did get some more reading material sent to me--THANK YOU!! I will read it once I'm home, so I am not distracted and uh... Working.

I hope everyone's doing well...

BB. · April 2011

Re: Battery bank not holding charge--Please help!!

Glad to hear you guys are doing OK...

If you want to try and build out your system as money becomes available...

Get a smaller battery bank and inverter that will meet your needs, and charge with the genset. Get the solar panels+solar charge controller(s) later as funds become available.

Run the generator when your loads are higher and use the battery / inverter during off-peak times. Would allow you to have some peace and quiet evenings/night yet still have power during the time you do the laundry, vacuum, etc.

See if a 300 watt inverter (MorningStar True Sine Wave 12 volt) would be useful (few lights, tv, computer, cell phone charger), a smaller AC battery charger, and 2-4 "golf cart" sized batteries.

A nice sized system would be 4x 6 volt golf cart batteries (225 AH @ 6 volts) arranged in series parallel like your previous bank. A total of 450 AH @ 12 volts. Also, because it is flooded cell, you can get a hydrometer and monitor the specific gravity.

And a ~45 amp 12 volt battery charger. Iota DLS-45 12 volt 45 amp regulated battery charger or one of these Meanwell 12 volt 60 amp chargers.

I would also recommend thinking about getting a Tri-Metric (or better) Battery Monitor--Not only to take care of your battery bank, but so you can measure and understand your loads for sizing your eventual "permanent" off-grid system.

This thread goes into the electrical details and the search that that "SteveK" went through to find the "optimum" battery charger for his eu2000i (he found the Meanwell above). Not a cheap charger, but one of the better designs for using with a generator.

The rough rule of thumb for charging a flooded cell battery bank would be ~5-13% rate of charge:

450 AH * 14.5 volts charging * 1/0.77 derating * 0.05 = 424 watts minimum solar array
450 AH * 14.5 volts charging * 1/0.77 derating * 0.05 = 1,102 watts maximum cost effective solar array

Solar Charge controller wise, a ~30 amp to 60 (or pair of 45 amp) rated units could be used (we have to work out the details). Either PWM (less expensive) or MPPT (more expensive) charge controller.

I think such a system would keep you pretty happy for a while without breaking the bank. And you always have the genset for when you need more power.

~$1,000 for the batteries+inverter+AC battery charger.
~$152 for battery monitor
~$1,100 for 450 watts of solar panel (note: shipping can be very expensive)
~$2,500 for ~1,100 watts of solar panels
~$200-$700+ for solar charge controllers
+$xxxx for solar mounts, wiring, fuses, switches, etc...

You could also get a larger MSW inverter for running a few random power and kitchen tools if you want from your battery bank.

Also get a Kill-a-Watt meter or equivalent to run on your generator. You can measure how much 120 VAC power you currently use and estimate that size of battery system you can start with. Note the basic Kill-a-Watt forgets the measurements when the AC power is turned off... Read the unit before shutting down your genset.

Hopefully with the new, smaller, genset, your fuel usage is way down from before.

Take care,
-Bill

Battery bank not holding charge--Please help!!

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