battery sizing and maintenance with grid tie

mikeo · March 2008

Hi all, I have searched the archives and haven't found a topic covering sizing and maintaining batteries used in a grid tie hybrid system. My 1800 watt PV system is located about 300 ft from the house in my barn and my ideal system would provide emergency backup power to run lights, fridge, freezer and well pump for up to 3 days when grid power is out before running a 10kw PTO generator to recharge batteries. From experience I have found my house to draw about 360 watts on average (8.6 kwh/day) when operating in emergency backup mode. When the sun is shining my MX60 shows that I generate around 1200 watts on average during peak sun hours (50 amps, 24 volts), but worse case would be no sun when grid is down. Power outages with my utility occur about 3 times a year lasting 11 days worst case during ice storm of 2003 to 1 day more typically. So far this year the power was out 6 days after the Feb. tornadoes, and 4 days after the snow storm. Amazingly it stayed up during last weeks flood. Basically my questions boil down to this:

1) What types of batteries are better suited to being cycled about 3 times a year, or are all about the same?

2) What different maintenance procedures may be required since the battery's are floating only when the sun is shining and otherwise used so little?

3) Given my usage pattern, what size battery bank should I shoot for?

crewzer · March 2008

Re: battery sizing and maintenance with grid tie

provide emergency backup power to run lights, fridge, freezer and well pump for up to 3 days… 8.6 kwh/day when operating in emergency backup mode. When the sun is shining my MX60 shows that I generate around 1200 watts on average during peak sun hours (50 amps, 24 volts), but worse case would be no sun when grid is down.

Mike,

1) What types of batteries are better suited to being cycled about 3 times a year, or are all about the same?

VRLA batteries (AGM or gel) are very popular for standby applications because they require virtually zero maintenance and don’t need to be vented. Here’s what Ross (Surrette) has to say about their flooded-cell batteries and grid-tie applications:

Grid tied systems-Back up battery banks
Normally these systems see very little cycle service and, at most, are cycled
once a month. If cycled, the banks should be charge for 3 hrs at the mean
voltage setting. After charging, the water level should be checked and a specific
gravity reading taken. If the specific gravities are not 1.265 the bank should be
further charged.

2) What different maintenance procedures may be required since the battery's are floating only when the sun is shining and otherwise used so little?

Voltage checks, temperature checks, hardware torque checks, and visual inspection are required for VRLA batteries. All of the above, plus the usual watering along with periodic recharge and EQ are required for flooded-cell batteries.

3)Given my usage pattern, what size battery bank should I shoot for?

For three days of backup and limited battery bank discharge to 60% of capacity (40% remaining), you’d need a battery bank rated at 8.6 kWh/day x 3 days / 60% = 43 kWh. For a 24 V bank, that would be ~1,800 Ah.

HTH,
Jim / crewzer

niel · March 2008

Re: battery sizing and maintenance with grid tie

mikeo,
i was going to answer your questions yesterday, but i had connectivity problems and i see that jim has answered. i will add to what jim has said in that you can use any battery except of course the starting batteries. as far as maintenance goes there would be nothing different using your own wording. the only batteries that seem to be designed for and typically allow for deeper discharges would be the agms and they are practically maintenance free with a higher cost. most standard lead acid batteries should not be depleted beyond 50% depth of discharge (dod) as it takes away some of the life of those batteries and the only way those batteries could be used would be to increase the total ah of capacity or conserve more with the ultimate sacrifice being less backup time with the standard lead acids.
this also brings up an important note here that you site getting 50amps from pv and you must keep in mind what your charge rate percentages are. 50/1800=2.78% and is a bit on the low side. this could be improved by splitting the battery bank in 2 and charging the 2 sections individually and remembering to charge both of them, but also to be cautious that for your loads are accomodated properly and evenly from the entire battery bank. you don't want 1 bank depleted and the other 100%.
the obvious cure for that may be more pvs, but you can also go with a good charger made to plug into utility power that can take on the task of the majority of the charging needs and this would allow for the generator to charge your batteries too while at the same time supplying your needed 360w. this would be a supplemental amount of current in addition to that supplied by the pvs to bring your charging rates up and keep costs down. the feature of charging from the generator is important when your outages could exceed the 3 days you designed the backup for. remember the pv charge rate is low so it could take weeks to recharge a large battery bank like that from the pvs alone.

marks · April 2008

Re: battery sizing and maintenance with grid tie

The system you desire is similar to the one I built, 36 Sharp 208s, 3 MX60s, 2 3648 inverters, and 8 Rolls 4000 series lead acid batteries. Being that I too need to run a well pump, pressure pump, and other crucial motors the sizing of my system was designed to meet these needs.

The initial drive behind the battery back-up was to allow the panels to operate during times when the grid is down. However, in general terms the battery bank is wired to several lighting circuits, and in the event of an extended outage all other demand units would be manually and individually transfered. This allows me to manage the heavy start-up amperage draws relative to the current power of the system.

After doing the numbers, lead acid provided the most value for my needs. Yes there is more maintenance (the water miser caps are helpful), yet overall I'm the hands-on type and like to monitor the system. My system is only four months old, yet I project that total maintenance will require one or two hours per year.

mikeo · April 2008

Re: battery sizing and maintenance with grid tie

Thanks to all who responded! I was hoping that I could discharge the batteries more deeply then 60% as long as they were recharged quickly with the PTO generator and not left in a depleted state more than a day. The SW4024 has a 120 amp charger built in. It is likely that an acquaintance who works for GNB is going to fix me up with some newly reconditioned traction batteries. What is the maximum size battery of this type that can be efficiently recharged with a 120 amp charger in say 6 hours which is about the maximum run time I would want to put on the tractor at a time? I suspect it would be 120 amps * 6 hours * .80 battery efficiency= 576 amp hours. If I run a battery down to 40% SOC than I am guessing that 576 /.6 = 960 amp hours is about the biggest battery my system will support weather charging from the generator or the grid with my current equipment. I will have 576 amp hours available * 24 volts = 13.834 KW hours/ 8.6 KW daily usage = 1.6 days of backup. Does this logic sound correct, please correct me if I'm wrong?

niel · April 2008

Re: battery sizing and maintenance with grid tie

what you propose there looks like it may work. it will be within a good range for the pvs to charge as well as for your 120amp charger from ac sources, but those percentages are based for a bit over 5% (20hrs) and 10% (8hrs) which do not equate to a 6hr charge timeperiod. many batteries may be able to take the extra current from charging beyond 13% (ask the manufacturer), but at a cost of higher maintenance and shorter lifespan. i believe trojan does not advise beyond 13% on their batteries, but on the other extreme agm concorde sunxtenders are rated for 100%.

System2 · May 2008

Re: battery sizing and maintenance with grid tie

I know this is kind of an old thread, but I feel like adding something anyway. IN case someone else stumbles across this thread.

As of recent reading / investigation It appears that for straight battery life and high discharge levels a large forklift battery can be well suited. They are designed to be discharged up to about 80%. So this may reduce the size of the battery bank some. Also the batteries are also designed to take a fairly fast charge. Meaning generator charging can be done fairly fast.

Like everything else there are trade offs. I have read that they use water faster than other lead type batteries, meaning slightly more maintainence.

Also the with out a load just sitting idle they will discharge faster as well. I believe I read they can discharge at a rate up to 1-2% per week. So more of a trickle charging needs to happen.

Anyway think that was all correct. If I am wrong someone let me know. New to all of this, but have done a fair amount of reading.

niel · May 2008

Re: battery sizing and maintenance with grid tie

yes, it could very well be as you say in general for fork lift batteries, but as i said it varies per manufacturer as to the amount of charge/discharge they designed their batteries for. high charges will cause excessive boiling of the electrolyte in any standard lead acid battery thusly increasing maintenance.

BB. · May 2008

Re: battery sizing and maintenance with grid tie

It is hard to find a nice concise link/website with everything you want to know about a particular battery in one place. Here is a Crown Deep Cycle battery PDF with some useful information (real numbers).

-Bill

battery sizing and maintenance with grid tie

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