Panel-to-battery sizing question

Plowman · July 2014

I have two Dmsolar 158W panels and two Trojan T-105 batteries. I'm using about 25 ah @ 12V per day. So I'm only running batteries down to ~90% SOC on a daily basis, often less than that.

Right now the panels are easily charging the batteries. It's July in Oregon, which means long days of full sun, few clouds, almost no rain. Based on hydrometer readings, batteries are at 100% by the afternoon every time I check them.

I had planned on getting 4 panels total, but I'm a bit confused about something. Trojan recommends a 10-13% charge rate. Imp of my panels is 8.47 amps, so 4 of them will produce 33.88 amps. Assuming 77% efficiency (a figure I often see here), that would give me 26.1 amps, or 11.6% charge rate. Right in the sweet spot.

On the other hand, I'm only drawing ~25 ah/day from the batteries. At 77% efficiency, I'd need to produce 32.5 ah/day. Which three panels will easily do in 1.9 sun hours (winter average in my area).

So which would be better, three panels or four?

BB. · July 2014

Re: Panel-to-battery sizing question

Longer term--Do you want to keep the battery bank you have (and possibly increase loads by 2x), or cut the bank size by 1/2 (which then makes the charging rate 2x higher)?

Both the 10-13% rate of charge and the WH/AH per day to restore battery back to >90% state of charge are important.

This is the "issue" of designing a balanced system. If the battery bank is "too large", it pushes the solar array larger than needed "to keep the battery bank happy". And you don't deep cycle the battery (below ~80% state of charge) every once in a while (for longer battery life).

Also another question--Do you use the system pretty much every day for 9+ months a year, or is a seasonal system (weekends/one or two seasons). You can usually get away with an array closer to the 5% rate of charge if lightly used.

If you keep the same bank size, use the bank most of the year, and looking forward to more loads--Then the extra panel(s) is not going to be problem (other than perhaps, for your wallet).

-Bill

Plowman · July 2014

Re: Panel-to-battery sizing question

BB, this is a year-round system. I use it to produce 100% of my electricity. I only have two 6V batteries, can't reduce the size. I just got them this spring, so hoping to use them for several years at least.

I'm actually a bit worried that I may not be cycling my batteries deeply enough. Some of that 25ah/day I'm drawing is when the batteries are full but panels are still producing full bore. So some (most?) days I probably don't even get down to 90%. This will change in winter, most likely.

I'm trying to size the system for winter conditions to minimize generator run time. If I could get away with 3 panels, I will, but if 4 is better, I'll bite the bullet.

BB. · July 2014

Re: Panel-to-battery sizing question

You can drop 1-2 days of charging every week (or once per month) to get the batteries cycled deeper.

During winter (depending if you are on the west or east side of Oregon), the extra panels will probably help a lot if you want to keep the system running through deep winter without much generator run time.

-Bill

vtmaps · July 2014

Re: Panel-to-battery sizing question

BB. wrote: »

You can drop 1-2 days of charging every week (or once per month) to get the batteries cycled deeper.

As panels are historically cheap, and batteries historically expensive, many folks are "over-panelling" their systems. Some european manufacturers, and Midnite in the US, have ways to automatically skip charging based on either number of days since last full charge, or based on SOC.

Problem with that approach is the controller may skip charging the day before an extended cloudy period. Better to listen to the weather forecast and manually skip charging. When I refer to "skip charging" you may want to just skip bulk and absorb and just do float charge for the day... this means that you daytime loads will not discharge the battery.

--vtMaps

Plowman · July 2014

Re: Panel-to-battery sizing question

BB. wrote: »

You can drop 1-2 days of charging every week (or once per month) to get the batteries cycled deeper.

During winter (depending if you are on the west or east side of Oregon), the extra panels will probably help a lot if you want to keep the system running through deep winter without much generator run time.

-Bill

I had originally planned on 4 panels based on Trojan's charge rate and the fact we get some long spells of clouds and liquid sunshine during the winter. I'm in the wet part of Oregon. Just wanted to make sure I wasn't "over-panelling."

I lived off a generator and a worn out old 150 ah AGM last winter. Had to cut my demand to the bone. Too much solar power is a welcome problem to have

How deeply should the batteries be cycled, and how often? Would drawing them down to ~75% SOC once a month be sufficient?

Plowman · July 2014

Re: Panel-to-battery sizing question

vtmaps wrote: »

Problem with that approach is the controller may skip charging the day before an extended cloudy period. Better to listen to the weather forecast and manually skip charging. When I refer to "skip charging" you may want to just skip bulk and absorb and just do float charge for the day... this means that you daytime loads will not discharge the battery.

--vtMaps

I don't think my charge controller can do that. Would prefer to do it manually anyway, easy enough to switch off the panels. I need to discharge the batteries, seems like keeping them on float might defeat the purpose.

BB. · July 2014

Re: Panel-to-battery sizing question

75% once a month should be fine--Just keep it simple.

Up to 13% rate of charge is easy. Little danger of thermal run away.

You can go upwards of 20-25% rate of charge, but you should have a remote battery temperature sensor to help prevent thermal run away (battery gets hot, charging voltage drops because of warming battery bank, charge controller things more charging needed, batteries get hotter...).

-Bill

Cariboocoot · July 2014

Re: Panel-to-battery sizing question

How about 75% SOC once a day.
Enough panel to recharge completely on a good, sunny day.
More if you can afford it to help out on those dim days, and set a current limit on the charge controller to avoid putting too much to batteries on good days.
Be prepared to fire up the gen to keep batteries above 75% SOC every couple of days when the weather does not co-operate.

stephendv · July 2014

Re: Panel-to-battery sizing question

Plowman wrote: »

I'm actually a bit worried that I may not be cycling my batteries deeply enough.

In my opinion this is not something to worry excessively about. If you look at any lead acid battery specs you get the longest battery life from the least discharge. If you were sizing the system from scratch then it would work out more economical for you to not have that much battery and to cycle deeper- but since you already have the battery you can enjoy shallower cycles and longer life.

The only place I've ever seen advice to deliberately deeply cycle a battery was from Rolls Surrette, so it's by no means a given that this is required for long battery life.

Again IMO, the biggest risk to battery life in your case is overcharging due to too long an absorb time. Not sure how programmable the TS-45 is, what I would do is set it up for a very short absorb during spring/summer, maybe only a few minutes daily to keep the batts between 80% and 95% charged, then every 2 weeks use the EQ setting to do a normal long absorb at 2.4V per cell to get them to 100%.
And of course do a normal high voltage (2.6V per cell) EQ periodically, perhaps every 2 months.

Regarding number of panels, you're still in the maximum current range with 4 panels so more is better.

vtmaps · July 2014

Re: Panel-to-battery sizing question

stephendv wrote: »

If you look at any lead acid battery specs you get the longest battery life from the least discharge.

from wind-sun FAQ:

a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film.

Other sources claim that daily cycling should be greater than 10% to avoid lead dioxide clumping.

--vtMaps

stephendv · July 2014

Re: Panel-to-battery sizing question

vtmaps wrote: »

from wind-sun FAQ:
Other sources claim that daily cycling should be greater than 10% to avoid lead dioxide clumping.

Good to know vtmaps! I think the OP is on the border with a daily 10% discharge, so I don't see much need to worry about daily cycling.

Panel-to-battery sizing question

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