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Need rules for sizing (matching) generator to battery bank

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  • Need rules for sizing (matching) generator to battery bank

    What are the general rules (formulas) required to properly match (size) a generator to a battery bank for the purpose of keeping the bank properly charged when PV is insufficient due to clouds, winter insolation causing PV insufficiency, etc... ?

  • #2
    First you need a AC charger capable of generating a C/8 hour charge current where C = the battery rated Amp Hour capacity. Example an 800 amp hour battery. 800 ah/ 8 h = 80 amps. Next to minimize fuel burn you want to run the genny around 75% of its rated capacity. So lets say the 800 AH battery is 48 volt nominal. At 80 amps x 52 volts = 4160 watts. So you would need at least a 4160 watt / .75 = 5546 watt or 5.546 Kw. Go shop for a 6 Kva genny.
    MSEE, PE

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    • #3
      IMPORTANT - find out the Power Factor (PF) of your charger, and use the corrected input watts to size your generator with Sunkings method. Many chargers have poor PF, and will force you into a larger genset to "conceal" their problem.

      What is the largest, stand-alone load, you need power for ? Well Pump ? Generator voltage and wattage should be able to support that. Also look up the PF for that load too. Motors have poor PF, and often need starting surge, which genset can deliver easier than inverters can. Inverter Gensets have poor surge capacity, their inverter is already maxed out.

      Another school of generator sizing thinking, is to utilize the inverters Gen Support function & onboard battery charger. A small genset can power loads, and provide some charging via the inverter, and if the microwave is turned on for 5 minutes, the inverter throttles back off charging, and can even assist the generator with large loads. ChrisOlson is the guru on setting that up, he's helped me with it a lot, with non-intuitative inverter manuals and hidden menus.

      Read his stickie here http://www.solarpaneltalk.com/showth...ng-Gen-Support

      If you think making the move to off-grid is "going green" you are dreaming. You only make the move to off-grid because you can't get utility power where you want to live. It has nothing to do with "going green" except for the fact that it takes a lot of green (money) to do it.
      Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
      || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
      || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

      solar: http://tinyurl.com/LMR-Solar
      gen: http://tinyurl.com/LMR-Lister

      Comment


      • #4
        ChrisOlson is the guru on setting that up, he's helped me with it a lot, with non-intuitative inverter manuals and hidden menus.

        Read his stickie here http://www.solarpaneltalk.com/showth...ng-Gen-Support

        If you think making the move to off-grid is "going green" you are dreaming. You only make the move to off-grid because you can't get utility power where you want to live. It has nothing to do with "going green" except for the fact that it takes a lot of green (money) to do it.
        Mike

        Nice way to get someones attention concerning "off grid" not being "green". Maybe using a larger font will help it sink in those that read it.
        Last edited by Mike90250; 09-29-2014, 10:48 AM. Reason: inserted quote

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        • #5
          Thanks for all of the above advice!

          Comment


          • #6
            Originally posted by Sunking View Post
            First you need a AC charger capable of generating a C/8 hour charge current where C = the battery rated Amp Hour capacity. Example an 800 amp hour battery. 800 ah/ 8 h = 80 amps. Next to minimize fuel burn you want to run the genny around 75% of its rated capacity. So lets say the 800 AH battery is 48 volt nominal. At 80 amps x 52 volts = 4160 watts. So you would need at least a 4160 watt / .75 = 5546 watt or 5.546 Kw. Go shop for a 6 Kva genny.
            My 2 cents on Genny selection:

            Portable Gasoline: Honda EU units and Yamaha are proven, but expensive.
            Stationary / RV Gasoline: If you can find a Honda EV6010 (6KV) with low hours, they are very reliable, and water cooled.
            RV Diesel: Any newer Onan but tend to start at 8KV and go up from there.
            I have no recommendation for the home standby units. Most are Generac, and their RV units are not great. I think they start around 12KV anyway.

            The hondas/yamahas and some others can be converted to LPG via one of several kits from propane-generators dot com.

            Pinellas power is a Honda specialist with mods too such as remote starts and extended run tanks.

            Some kits for some generators (Yamaha) are tri-fuel so can run on LPG, Natural Gas, and Gasoline, so fuel options are better if TSHTF.

            LPG conversion reduces wattage by 15% or so, due to less power, the engines run cleaner, but also hotter, and are harder on valves.

            Older Onans are heavy duty, but many have flatheads and they need periodic tweaking/decarboning due to the old fashioned points ignition. They are easy to work on, and the electronics can be bypassed to keep them running if it dies. There are newer electronics for them from Dinosaur boards, and pertronix electronic ignitions are also available. I'd Avoid the newer and discontinued Inverter units from Generac. They don't last, and the inverters don't deal well with high current startup loads like AC units.

            I am partial to RV generators as the Diesel units and most older gasoline models are 4 pole and run at 1,800 RPM (Kohler/Onan). That is contradictory to my recommendation for the honda (It's a 2 pole @ 3,600 RPM) but the water cooled OHC engine is probably the best out there and parts are still in production. The GX360 engine is used on tractors and such as well.

            I have a Honda 4010 (4KV) and it's just about right for my 216AH bank, although calcs dictate 4320 for me, 4KV is fine.

            ... More than you ever wanted to know about small generators...
            House-Sun Earth Hot Water.
            RV-390W Kyocera, Kid.

            Comment


            • #7
              Originally posted by ZoNiE View Post
              My 2 cents on Genny selection:

              Portable Gasoline: Honda EU units and Yamaha are proven, but expensive.
              Stationary / RV Gasoline: If you can find a Honda EV6010 (6KV) with low hours, they are very reliable, and water cooled.
              RV Diesel: Any newer Onan but tend to start at 8KV and go up from there.
              I have no recommendation for the home standby units. Most are Generac, and their RV units are not great. I think they start around 12KV anyway.

              The hondas/yamahas and some others can be converted to LPG via one of several kits from propane-generators dot com.

              Pinellas power is a Honda specialist with mods too such as remote starts and extended run tanks.

              Some kits for some generators (Yamaha) are tri-fuel so can run on LPG, Natural Gas, and Gasoline, so fuel options are better if TSHTF.

              LPG conversion reduces wattage by 15% or so, due to less power, the engines run cleaner, but also hotter, and are harder on valves.

              Older Onans are heavy duty, but many have flatheads and they need periodic tweaking/decarboning due to the old fashioned points ignition. They are easy to work on, and the electronics can be bypassed to keep them running if it dies. There are newer electronics for them from Dinosaur boards, and pertronix electronic ignitions are also available. I'd Avoid the newer and discontinued Inverter units from Generac. They don't last, and the inverters don't deal well with high current startup loads like AC units.

              I am partial to RV generators as the Diesel units and most older gasoline models are 4 pole and run at 1,800 RPM (Kohler/Onan). That is contradictory to my recommendation for the honda (It's a 2 pole @ 3,600 RPM) but the water cooled OHC engine is probably the best out there and parts are still in production. The GX360 engine is used on tractors and such as well.

              I have a Honda 4010 (4KV) and it's just about right for my 216AH bank, although calcs dictate 4320 for me, 4KV is fine.

              ... More than you ever wanted to know about small generators...
              Actually it is pretty good info.

              I have 3 small generators that I have purchased over the last 10 years for numerous reasons. Two are gasoline type; Coleman 1800w & B&S 5500w. The third is a Generac LP 3250watt. I also built a small solar 12v 200Ah battery system with a 600watt Pure Sine wave inverter for about $2500. I should have spent that $2500 on either a Kohler or Generac whole house system that could have gotten me an 8 to 10kw system plus the cost of a 100 gallon LP tank. Hind-site is great.

              Comment


              • #8
                Originally posted by Sunking View Post
                First you need a AC charger capable of generating a C/8 hour charge current where C = the battery rated Amp Hour capacity. Example an 800 amp hour battery. 800 ah/ 8 h = 80 amps. Next to minimize fuel burn you want to run the genny around 75% of its rated capacity. So lets say the 800 AH battery is 48 volt nominal. At 80 amps x 52 volts = 4160 watts. So you would need at least a 4160 watt / .75 = 5546 watt or 5.546 Kw. Go shop for a 6 Kva genny.
                What if you need to run loads while charging the batteries, like at a microwave site, do you add that load to the generator capacity? A lot of our sites have big old Ratelco 48 volt chargers that run on AC from the generator to charge the batteries, but the radios are always running on the batteries while they're being charged.

                Comment


                • #9
                  Originally posted by sdold View Post
                  What if you need to run loads while charging the batteries, like at a microwave site, do you add that load to the generator capacity?
                  Yes is the short answer. I can help you there as that is what I do for a living of 35 years. A typical cell site tower has a 600 to 1200 amp 24 volt DC plant. They also have dual 3 to 5 ton Air Conditioners and NO HEAT because they have no use for heat. So the generator is sized based on worse case assuming the site is down and batteries dead at 120 degree inside temp. What I am driving at is when the genny comes on, both Air Conditioners will run and the DC plant will go into Current limit until the batteries reach about 80% and then current tapers. Makes life real easy a cell tower site uses 30 to 50 Kw in either diesel or LPG. Verizon makes it real easy, they use 50 Kw for everything as it eliminates an engineer on the payroll.

                  As for your requirements total up worse case demand load and multiply by 1.5 and you got your answer. I assume since you said microwave I am guessing a mw relay for long haul. MW radios don't use much power as they are less than 1 watt radios. Its the modulators and controls that use the power. Some of the new packet Alcatel and Aviat radios burn around 100 watts. So when you get into commercial grade gennies about the smallest you are going to find for micro sites is 7 Kw running LPG.

                  I have designed about just over 100 solar micro cells and they use a 6 Kw solar array and 10 Kw lpg genny. A micro cell site pulls 250 watts continuous and are located from TX to CA along I-40, I-20, and I-10 corridor and Indian reservations.

                  FWIW a cell site uses a 200 amp service. If the site goes down and the genny fails to start, or no generator on site. A tech has to go to the site and unplug all but a few rectifiers, open the door to cool it and wait for for commercial AC power to restore. A 200 amp service is supplied with a 15 to 17 KVA transformer and they will blows it fuse if you hit it with the DC plant and dual Air Conditioners as the load exceeds the transformer capacity. All a 200 amp service means is your Main Breaker Panel has a 200 amp buss with 42 positions. No POCO is going to use a 50 KVA transformer for a 200 amp service unless you specify and pay extra for it. POCO plays by a different set of rules.
                  MSEE, PE

                  Comment


                  • #10
                    Thanks for the response. All of our solar sites that I can think of are on mountain tops and have no A/C. I've wondered if we might be going through some of our batteries faster than we should because of that, especially at the VHF-only sites where the vaults get below freezing in the winter. The bigger solar sites are concrete block with a lot of heat-generating microwave radios, so the temps seem to stay somewhat constant, around 80F in the summer and maybe 40 in the winter.

                    The microwave radios put out one watt of RF, but use a surprising amount of power. The digital radios running hot standby (two of everything) plus at every site we bring the "baseband" down to digital switches, channel banks. and then re-convert it all back to go into the next radio. We can't just repeat the baseband because some circuits have to be split out to drop at the site or maybe go on another path. So if we have a site with three microwave paths for example, the 48V power used ends up being about 1kW per MW path, unless it's a small stub path with a low density radio and less equipment. Everything runs from straight -48V except for the 12V for the VHF radios, which we usually have a separate set of panels and batteries. At least we don't need inverters.

                    "No POCO is going to use a 50 KVA transformer for a 200 amp service unless you specify and pay extra for it. POCO plays by a different set of rules." Thanks for that bit of info, I didn't realize that.

                    Comment


                    • #11
                      Originally posted by sdold View Post
                      Thanks for the response. All of our solar sites that I can think of are on mountain tops and have no A/C. I've wondered if we might be going through some of our batteries faster than we should because of that, especially at the VHF-only sites where the vaults get below freezing in the winter. The bigger solar sites are concrete block with a lot of heat-generating microwave radios, so the temps seem to stay somewhat constant, around 80F in the summer and maybe 40 in the winter.

                      The microwave radios put out one watt of RF, but use a surprising amount of power. The digital radios running hot standby (two of everything) plus at every site we bring the "baseband" down to digital switches, channel banks. and then re-convert it all back to go into the next radio. We can't just repeat the baseband because some circuits have to be split out to drop at the site or maybe go on another path. So if we have a site with three microwave paths for example, the 48V power used ends up being about 1kW per MW path, unless it's a small stub path with a low density radio and less equipment. Everything runs from straight -48V except for the 12V for the VHF radios, which we usually have a separate set of panels and batteries. At least we don't need inverters.
                      Steve I am real familiar with what you are doing. You need to look at two things.

                      1. You hit on something, temperature. A FLA battery looses capacity as the temp drops from 77 degrees F, so that has to be factored in.
                      2. Peuket effect. Batteries are specified at some discharge rate. Consumer grade batteries are spec at the 20 hour discharge, and commercial applications are usually spec at the 8 hour discharge rate. You need to know what discharge rate your equipment is pulling. For example let's say your batteries are rated at 220 AH at C/20, but in service are discharged at C/3 or 43 amps turns that same battery into 130 AH. Click this link and you will see what I am talking about.

                      So you need to first check your battery spec to see what the derate factor is for temp and discharge rate. I think that is where you might find the problem. Good Luck.
                      MSEE, PE

                      Comment


                      • #12
                        I'll look at the battery specs. All of our batteries are AGM, do they show the same temp derating effect?

                        I normally work with the radios, towers and antennas, but reading this site has gotten me interested in the solar and battery part of our work. The guys that have been doing that are slowly retiring.

                        Comment


                        • #13
                          Originally posted by sdold View Post
                          I'll look at the battery specs. All of our batteries are AGM, do they show the same temp derating effect?
                          Same capacity derating in cold, but since they are lower internal resistance Peukert is not quite as bad but still has significant effect. If these are Telecom AGM's the manufacture should have published discharge curves for many discharge rates. C&D Exide, Enersys, Johnson Controls etc all have very detailed discharge curves. If you by chance have GNB Absolyte-II batteries take them to the nearest Marina and give them away as boat anchors.

                          Good news is Lead Acid batteries have better cold performance than lithium batteries. Both suck, but Lead Acid sucks less. Ask a Leaf owner. Very early in my career I did a bit of commercial radio, and even did my Internship in College working for Motorola two-way radio engineering. Also a ham operator since age 14 and have built quite a bit of my gear for 80/160 meter stuff including all my antennas. In 1989 I switched from Power Generation and Transmission to telecom as a transport engineer in MW radio and optic transmission with a little switching thrown in. Then about 1994 the telocom market caught fire and the company I worked for at the time (Willtel in Tulsa) and was in desperate need of a Facility Engineering which is all aspects or electrical power. They finally figured out what my background was and had me start up a whole new Facility Engineering department. By the end of 2003 when it went bust, MCI-Worldcom Facilities Engineering had 5 Regional office with over 100 employees. They sent us all packing and farmed out all our work. Good news is I started my own company and got a lot of their work and got paid a lot more money doing it that way. Kind of funny how things work out.
                          MSEE, PE

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                          • #14
                            I just found these on my computer. It was the first solar site I helped design, my part was mainly making the drawings and figuring out how to wire everything (Generator, AC chargers, charge controllers, panels) together with a lot of load-shedding features on the generator AC circuits since we only had 15 KW of AC from the generator to work with. The other guy I worked with sized everything based on our rule of thumb, which was: Figure out the maximum anticipated site loads, and size the panels seven times that amount, and size the batteries for something close to the 5-day autonomy/2.5 day to 50% DOD that Sunking recommends. Everything runs on two separate systems, -48V for the microwave and +12V for the VHF radios. When needed the 15 KW generator starts and powers four 100A AC chargers to charge the batteries. It's funny, it looks really impressive but we had no idea what we were doing. Somehow it has worked for about 12 years now with only a generator replacement, probably because there are only about half of the microwave terminals we planned for were actually installed. One of these days I'm going to use the knowledge I've recently gained on this forum and figure out how close to correct we were with everything. All of the panels are Siemens 110 watt 12V panels, I'm not certain but I think we paid something like $800 apiece!* The charge controllers are several small units in parallel made by a company (or person) called "Bobier" I think, that seemed to disappear right after we put these in. Luckily none have failed. Some charge the 48V battery and some charge the 12V battery. I think it would be fun to hypothetically re-design this using modern panels and MPPT charge controllers. This site gets 10-15 feet of snow almost every year.

                            *EDIT: Actually. a cell company paid for the whole thing, since they needed one of our 1.8 GHz channels and had to pay to move us to 6 GHz, and this new site was needed as an intermediate site.

                            leek panels.jpgleek batteries.jpgwall of charge controllers.jpgoriginal generator.jpgpanels snow.jpgdoor snow.jpgcontroller.jpg

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