House battery and solar panel sizing

[windtrader]

Hey,

Looks like the "new" house batteries are fairly new in age but due to loss of trickle charging, they sat flat for too long, and need to go. Since I've got to replace them it makes sense to at least research adding solar to keep the new bank charged and supply some juice during the day.

Four 4D batteries, connected in series for 24v and parallel for capacity. It seems golf battery offer proven durability and deep discharge capacity and good fit for a boondocking bus with some solar help.

Most golf batteries are 6v, requiring four in series to get 24v. There are 12v golf batteries for more capacity but far less common and costlier.


Solar charging batteries during the day starts the evening with them fully charged and needing less capacity in the bank.

A Vanner 24-3600c inverter/charger and 8Kw generator are available to charge up or run any AC equipment if needed.

I'd like to break the replacement/upgrade into two phases due to cash flow starting with the batteries.

Typical draw during the day is quite low.  Refrig runs on LP, DC, and 110. TV and stereo don't draw much. Hot water runs on 110v, engine and webasto. Water pump draws a bit.

What size of golf battery bank seems reasonable, knowing there will be panels offering boost during the day, and generator/inverter/charger for backup when still running short.

Thanks
Don

[Geoff]

Solar??  An automatic start for your genset makes more sense.

--Geoff

[Iceni John]

Solar panels are more automatic than any auto-start generator!   They automatically start charging every day at sunrise, they automatically fully charge the batteries according to a precise 3-stage regimen tailored to any type of battery, and they automatically need zero maintenance, zero repairs, zero fuel, zero oil and coolant, and they produce zero noise, zero vibration, zero pollution and fumes, and best of all after they're installed they produce electricity for zero cost for two or more decades.   Show me a generator that does that.

If you have sufficient battery capacity to give you 3 days' autonomy without them being charged (how often is it so overcast that there's no solar production for three days, at least here in the US Southwest?), then you may not even need an emergency generator.   So saying, it's prudent to have a Plan B for electricity production, just as it is for heating water, for keeping warm, for staying cool, for starting the engine if the start batteries are dead, etc etc.   FLA batteries are best charged at a rate of between 5 and 13% of their 20-hour capacity, but for the usual less-than-ideal panel placement on a bus roof I would suggest having enough PV to charge at the higher end of that range.   That's why I have 2040 watts of panels to charge almost 900 amp/hours of  batteries  - that's almost a 13% rate, and my panels can tilt up to 45 degrees from horizontal for optimum winter insolation unlike most mobile installations.

If you have a 12V house system you will probably need two charge controllers for anything much over 1000 watts of PV, but that then allows you the luxury of splitting your entire PV and battery system into two completely separate halves.   By doing this you can have total redundancy (another Plan B), you can buy each bank of batteries at separate times (then they won't all need to be replaced at the same time), and the batteries don't even need to be all the same type or capacity.   And in case you're wondering how to keep two battery banks truly separate, I have 250 amp Schottky diodes between each bank and the combined DC load center so one bank cannot back-feed into the other.   Easy!

John     

[Brassman]

I'd be much interested in what you have installed John. Do you have pictures?

[windtrader]

Hi John,

Some of your old posts were ones I found while searching the topic. I'll need help with the math. My goal is to start with having PV charge the house batteries via the charge controller. The current house batteries, inverter, and balancer are wired for 24v, so plan to stick with that.

Shopping for new house battery bank, the T-105 golf battery appears to be a solid base. Connecting four 6V in series provides 24v and something like EDITED: 215 Ah (not 430Ah). If more stored energy is needed adding some more T-105 is possible.

I was hoping a single 200-300 PV panel would keep the house batteries topped off during the day with small draw for the refrig, TV, and water pump.

Based on your experience what do I need for PV charging and energy storage? The batteries only need to keep the above going and the occasional few minutes of microwave in the morning and a couple lights in the evening. Prolonged AC use will run off shore or gen power.

This is the link to the Trojan battery as an example
http://www.trojanbattery.com/pdf/datasheets/T105RE_TrojanRE_Data_Sheets.pdf

Based on the spec sheet, it seems each 6v battery delivers over 1 Kwh, so four of them should be more than enough for my needs unless I'm missing something. The footnote states these figures are for battery voltage remains above 5.25v (1.7v/cell)


thanks

[neoneddy]

The question: What size Battery Bank should I use for solar?  Great Question, I've been researching a while myself.  Here is what I know so far.

The 6v string of 4 to make a 24v bank @ 230 AH seem like the way to go. I think you doubled the AH in your post above.   AH only increases with paralleling the banks, to increase run time you'll need add 4 more batteries for a 2x4 setup and you'll have 460AH.  I have one in my bus now.  I might end up with 12 some day, or do LiFe (Lithium Iron).

Bank size starts with your needs.  For sake of argument let's assume you want to run  all your stuff minus AC for 24 hours.

Fridge - These sip power... .8kw per day on average, let's figure 1kwh.
TV - Assuming LCD /LED flat screen TV.  .3 to .5kwh per day. Let's say you binge watch and leave it on all day and round up to 1kwh to keep math simple.
Water Pump - This is going to be momentary, almost nothing, I'd say it's covered by our over estimation on the others.

Misc Other / Over head, whatever - Let's toss in another .5khw to cover other stuff we're not thinking about, thermostats, leds, parasitic losses, etc.

Total 2.5kwh ever day.

Your bank is 230ah @ 24v that equals (multiply amp hours by volts to get watt hours) 5,520 wh = 5.5kwh.   That's 100% depth of discharge, 50% is considered a good guideline, 20% or 30% will help the batteries last forever.

Will it work?  Looks like yes, 50% discharge would be 2.5kwh or so. , I was over zealous with your power usage estimates, in practice you might be more like 1.25kwh / day.

Now you need solar to put power back in.   In a perfect world, you can use that 200w panel for all 5 hours of usable sun hours (unless you use a solar tracker you won't get usable output more than this) that will be 1000 watts or at 1 hour 1kwh if everything is right.  That doesn't cover a light usage day, you'll need more panels.  From what I gather you want to have more solar than you think you'll need by a factor of 1.5 to 2.  So if 2 panels would cover you, get 3, then you have overhead for when they get covered in dust or partial shade / cloudy days, etc.  

3 panels at 200w each is 600w  for 5 hours a day is 3000w.  That covers your needs easily, but let's a clear sky, clean panels, etc.  Real world figure 1/2 that and you're at 1500w , that covers a light / normal day and banks some away for this higher usage days.

[Iceni John]

A few thoughts on this:

A typical generic 60-cell grid-tie panel produces about 250W at about 30V, with a maximum current of about 8.4A.   This is barely enough voltage to charge a 24V battery bank  -  it would work (just) by using a simple PWM charge controller, but not very efficiently.   (Trojan T-105s need slightly higher Absorb voltage than other FLA batteries.)   An MPPT charge controller will maximize solar harvest, but is hardly needed for just one panel:  MPPTs are most efficient when their array voltage is about twice the battery bank's voltage.   Just bear in mind the 5-13% suggested charge rate for FLA batteries  -  if the battery is not fully charged each day it is going to eventually sulfate due to prolonged deficit charging.   Any FLA battery should never be drawn below 50% SOC, and for maximum life should normally be discharged less than that.   Trojan's specifications about voltage remaining above 1.7V/cell seems like a surefire way to kill a battery!   At that voltage the battery is at only 10% SOC, effectively dead   50% SOC is about 6.05V (2.02V/cell).   Why does Trojan keep publishing those absurd specifications?

A single panel may be sufficient to keep a small battery bank topped off during the day, but will not be enough to fully recharge a discharged bank.   (An array of eight panels will fully recharge a 24V 430ah battery bank from a single 60A MPPT charge controller, but that's probably more than you want to do!)

The Northern Arizona Wind & Sun forum has a wealth of information about PV  -  it's well worth perusing the threads there to glean useful ideas.

HTH, John      

[windtrader]

@Shawn - Thanks so much for the reply. It helps a lot to follow the logic through all the computational considerations. I'm thinking I can always incrementally add/double up another bank if needed later.There is plenty of room in the storage area, now housing 4 4D.

The solar configuration should also be expandable as needed. Sizing and purchasing the correct sized charge controller seems to be the main consideration up front. Adding panels later should be easily accommodated. Additionally, I'm fine if some days solar generation does not top off the bank; I can always use the genset.

DOD might be fairly low especially during sunny days as the panels will add power to the bank. In fact, the specific max draw down will likely be at the end of the night. So the calc may be more like 100% full late in the day and computing usage from that point to the end of the evening. Who knows, DOD just might be only 20-30%, ensuring very long life.

@John - Thanks for the insights. You're so right - 8 full size panels is way more than I'm wanting; not like your design. Obviously, more research required on my end for a proper solar design is baked and ready to go.

Batteries are the current pain point. House battery bank is so worn out, can not even keep a charge for a few hours. The GC2 6 volt deep cycle golf battery form factor seems to be an acceptable choice. Made for bumping around rather than stationary, deep cycle and heavy use on a frequent basis, thicker lead plates, and respectable costs.

Shopped around last night, checking specs and prices, and wondering if there are reasons not to buy at a discount retailer like Costco, Sam's Club, or Walmart. The prices are hard to beat and looking at $400 for a set of 4 GC batteries should get the bus boondocking with a smile again, pronto. Any reason to pay twice as much for some brands? Any sage advice on warranty differences?

Thanks
Don

[Iceni John]

Costco has Interstate golfcart batteries for $80-something each, but I think their core charge is $24 per battery.   I don't know about Walmart / Sams Club because I refuse to step in any of their stores, ever!   Good ol' golfcart batteries are still the best bang for the buck ($ per Watt/hour), and if you check their SG and water them every month they can last you a good long time, especially if solar is keeping them fully charged all the time.

John   

[Iceni John]

I'd be much interested in what you have installed John. Do you have pictures?

Sorry, I'm photographically challenged, so no pictures!   There's a YouTube video of my bus leaving one of our annual get-togethers for a jaunt into town, and it shows my panels' support frames before I installed the panels themselves  -  I think the clip is titled Buses Gone Wild VII, or something like that.   My panels are hinged to a central walkway, and each can be positioned from 21 degrees down to 45 degrees up depending on season and latitude.

John

[neoneddy]

https://youtu.be/d-1nMvB8aYw?t=27

Video in question above.

[bobofthenorth]

I'm sorry to interject a note of reality but ............

Solar is a lifestyle choice, not a power source.  If you are willing to drastically alter your lifestyle you MAY be able to put sufficient solar panels on your coach to live off them but I stress the word "MAY".  If you are just wanting to keep the batteries topped off while the coach is unused then no worries, solar will work. 

If you are serious about solar then its all about the watts.  Figure out your power usage.  There's no magic.  800 or 1000 watts of solar on a coach is huge but that's the kind of numbers you will need and in my never to be humble opinion you'll still need a generator.  There's no point obsessing about battery capacity if you don't have enough watts coming out of the panels to recharge the batteries.  And you've only got about 6 hours of truly useful solar input each day.

Been there done that.  Solar can drastically reduce your generator run time and keep your batteries floated but most people can't live without a generator.

[Iceni John]

Bob, 800 or 1000 watts really isn't that much.   I've got 2kW, and I would have more if I had roof space for it.   With solar now down to not much over 50 cents a watt (I paid 79 cents, and that's now high by current pricing) it's almost cheaper to carpet the roof with PV panels than to build a tropical roof over it for insulation!   To complement my PV I'll also have two solar water heating panels  -  heck, if I'm getting my electricity for free, why not also heat my water for free?

With enough PV power I really don't feel I'll need to compromise my lifestyle.   I'll have a 12K minisplit in the front and a small window A/C in the bedroom, I'll run my tools and toys just like now, I can use a microwave and an induction cooktop, so it will work for me.   I'm not an energy hog even at home  -  I lived for many years in a little old house with only a 20 amp feed (yes, twenty!), and two of us managed just fine with it.   I intend to not be anywhere stinking hot in the summer or bloody cold in the winter  -  that's why buses have wheels!   No more brass monkey weather for me.

John

[neoneddy]

I've thought that the PV panels would act as good roof shading as well.

Happen to have any photos of your roof?

[Iceni John]

I've thought that the PV panels would act as good roof shading as well.

Happen to have any photos of your roof?

Sorry (see above!).

My interior is significantly cooler now that the sunlight doesn't even reach most of the roof.   The temperature difference between the ceiling under the panels and the ceiling elsewhere is a lot.   My bus's interior now doesn't much ever exceed ambient air temperature  -  there's no heat soak into the roof any longer.   There a few inches of air gap under the panels to allow air to freely circulate under them.   Essentially I now have a big tropical roof that makes free power all the time.   Yeah!   

John