Questions...

System

I have a travel trailer that we use as a summer home in Utah (N37°46') fully off grid and will eventually be a homesite. I have 180w of combined solar and 2 Trojan T-105's (2 years old) with a newly purchased MorningStar PorStar 15 which I'm just using for PWM and no LVD.

The batteries have not been abused except perhaps a deep discharge once every other weekend and then only having a "dumb" charge controller to bring then back up.

QUESTION:

1.) Will this new PWM controller improve the condition of old batteries?
2.) With daily 50% discharge of 105Ah batteries in mid-summer, is 180w of solar enough to recharge my batteries?
3.) What (where) are the formulas for calculating how much solar one needs to replace energy used in a given size battery bank?
4.) If I have plenty of PV capacity for my battery set, can I add new batteries to an old existing battery set?

Many thanks in advance.

Paul...

crewzer

Re: Questions...

Paul,

1.) Will this new PWM controller improve the condition of old batteries?

Maybe. Work by Sandia labs has shown that PWM-type controllers can somewhat restore battery capacity. However, there’s s lsight disconnect between the the ProStar’s flooded-cell battery voltage spec (14.4 V at 77 F) and Trojan’s spec for their batteries (14.8 V at 80 F).

Suggestions:

(a) Make sure you connect the voltage sense leads between the battery terminals and the controller. This will force the controller to monitor voltage at the battery terminals and not at the controller’s main battery connections. This configuration will compensate for any voltage loss in the charging wiring between the controller and the batteries.
(b) Buy and install the option remote battery temperature sensor for your controller. This will make the controller adjust charging voltage in accordance with battery temperature. This should help with battery capacity as well longevity.
(c) Vent the batteries to the outside when they are being charged in absorb or EQ modes. This will exhaust hydrogen gas, which is potentially flammable/explosive in sufficient concentration, from the batteries to the outside.
See:http://www.solarseller.com/battery_box_power_vent_by_zephyr_industries.htm
(d) Familiarize yourself with Trojan’s Battery Maintenance Guide. See: http://www.trojanbattery.com/Tech-Support/BatteryMaintenance.aspx

2.) With daily 50% discharge of 105Ah batteries in mid-summer, is 180w of solar enough to recharge my batteries?

3.) What (where) are the formulas for calculating how much solar one needs to replace energy used in a given size battery bank?

Your two batteries are wired in series and are together rated at 12 V (nominal ) x 225 Ah. See: http://www.trojanbattery.com/Products/ProductSpec.aspx?Name=T-105 A good rule of thumb for 12 V battery systems is to have 1 W STC of PV power available for each Ah of battery capacity. Accordingly, current from a 180 W STC PV array is a tad on the low side for a 225 Ah battery bank, but should be OK if daytime loads are kept low.

The coulombic efficiency (Ah out/Ah in) of flooded-cell batteries is ~90%. Replacing 50% of the battery bank’s capacity will require generating 225 Ah x 50% / 90% = 125 Ah. The 180 W PV array’s maximum output current (Imp) spec is probably ~10.4 A. Accordingly, you’d need the equivalent of 125 Ah / 10.4 A = 12 hours of “full” Sun to recharge the battery bank. “Full” summer Sun on south-facing PV modules tilted towards the noon Sun in central Utah may be in the six- or seven hour/day (average) range, but not 12 hours.

See: http://rredc.nrel.gov/solar/pubs/redbook/PDFs/UT.PDF
And: http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/sum2/93129.txt

Assessment: You probably need to increase your PV array from 180 W to 370 W. However, this will require a larger controller (i.e., the ProStar 30, or, even better, the TriStar 45, because it has better battery voltage settings). The largest array the Prostar 15 can handle is ~240 W to 260 W STC.

4.) If I have plenty of PV capacity for my battery set, can I add new batteries to an old existing battery set?

Adding new batteries to old batteries is generally not a good idea. As the old batteries age further and decline in performance, they will adversely affect the new batteries.

HTH,
Jim / crewzer

icarus

Re: Questions...

Paul,

As usual, Jim has it about right. My concern would be the loads that lead to a 50% discharge every use cycle. While in some measure that is "acceptable" depth of discharge for a "deep cycle" battery, in my experiance it is not optimal. The trade off between depth of discharge and longevity of the battery bank is a personal one, I prefer not to let my bank get below 80%.

Having said that, if you have need for the loads to discharge as you do, instead of adding to your bank, you could perhaps add a second stand alone bank so that each bank would be drawn down ~25% each cycle and then recharged. If you are planning on building a bigger system in the future, perhaps this could fit into your planning.

While it is not considered "good practice" to mix battery sizes and ages, there is NO technical reason you can. The consiquence of doing so will be to age the new set to basic age of the older ones. With 2 year old batteries I might consider doing it, if I could reduce the % of drawdown enough to make it "pay".

I suggest that you read the following links: http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

http://www.batteryfaq.org/

Good luck,

Icarus

BB.

Re: Questions...

Just to clarify on adding batteries... Yes, you can add them and nothing will "blow up"--however, the new batteries will probably not last much longer than the existing batteries in the bank.

So, if you are willing to accept the new batteries may die 2 years sooner than they would have otherwise--then go for it (with added PV panels because of the added capacity).

The question is how much life is left in your existing batteries... If they have had a "rough life", then they may only last 3-5 years. If they have been well maintained, they may last 5-7+ years. Really hard to tell from here. But over discharging/under charging is a common form of "battery abuse".

Also, you should use a good quality hydrometer to monitor your batteries. Short of using a battery monitor, this is the best way to accurately ascertain your battery's state of charge.

-Bill

System2

Re: Questions...

This is all great information and I first want to say thanks for all the input!! I plan on following all the links that you have posted.

I think I am in better shape than I wrote about in the first post. As far as battery condition, I have always given them TLC (maintenance, distilled water etc.) The 50% discharge rate is really a worst case senerio. I bet that during the summer, when it is warm and it stays light late, we will use very little 12vdc. When the need arise for more demand, I fire up a genset.

The other isue is that these are 6v batteries in parallel so I am at 12v~105Ah. Using the formulas provided and the hours of available sunlight at this latitude, I'm gussing that I will be OK...really OK if I keep my dischage to say, 25%.

Let me know if I'm wrong here and THANKS again.

Paul...


CORRECTION: I intended to say SERIES...My mistake.

BB.

Re: Questions...

50% discharge in itself is not really the problem with battery life--it is getting a flooded cell battery quickly recharged (or at least above 75% state of charge) that is most important (ideally within a day or so).

When a FC lead acid battery remains below ~75% SOC for more than a few hours, that is when sulfates begin to harden (which is, a more or less, irreversible process).

-Bill

System2

Re: Questions...

plongson wrote: »

The other isue is that these are 6v batteries in parallel so I am at 12v~105Ah. Using the formulas provided and the hours of available sunlight at this latitude, I'm gussing that I will be OK...really OK if I keep my dischage to say, 25%.

Your T-105's are wired in series, you have 225A at 12V.

Cheers,

Bad Apple

System2

Re: Questions...

Well, its been a few days...Happy New Year!

I got a chance to look over all the links and data you folks provided and I have a lot better idea understanding of the situation...

One place where I really goofed up was in the Ah of the T-105's...DUH...For some reason I had it in my head that they were 105 Ah instead of 225 like you guys corrected me on. Oh well, I'll take my flogging and move on...

I do have another question though. In the tables at http://rredc.nrel.gov/solar/pubs/redbook/PDFs/UT.PDF they list the solar radiation as kWh/m2/day but reading threads on this site it appears that you are refering to these numbers as the hours of sunshine/day.

What am I missing here?

Thanks, Paul...

BB.

Re: Questions...

Full sun at noon is about 1kW/m2... So even though there may be 10+ hours of sunlight on your array--it is a close enough approximation to say that 5kW/m2*day spread over 10+ hours is equivalent to 5 hours of noon-time sun.

That is why we say 5 hours of sun per day...

You will notice that with changing the tilt/tracking arrays, when the sun hits the surface more squarely over the whole day, you collect more power over the day--so it is equivalent of more more hours of full 1kW/m2*day of sun.

-Bill