Battery Sizing and Charging Considerations for a Hydro System

turgo
turgo Registered Users Posts: 7
I have a 135 amp 12 volt hydro system. For years I have used a motley used and free collection of batteries. They are mostly dead, and its time to buy new batteries. I want to conserve the new batteries, and I am concerned that the dump load controllers that I use to regulate battery voltage may not charge the batteries reliably. I use a dump load controller on my Outback VFX2812 to switch a 900 watt water heater element, and an Watlow temperature controller to switch a 600 watt water heater element. Depending on loads in the house, this keeps battery voltages mostly in the 13 to 14.5 volt range. Below is a snapshot of graph of battery voltages captured by an Arduino microcontroller and stored on ThingSpeak:
Attachment not found.
See the sawtooth pattern as the heater elements switch on and off. This is pretty typical of the system.
Is this a good charge pattern to care for batteries?
Should I consider adding a charge controller? If so, how would it work with my dump load controller?

What size battery bank should I buy? What model batteries? (Price is a primary consideration.)

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Battery Sizing and Charging Considerations for a Hydro System

    Welcome to the forum.

    That is not a very good pattern for battery charging. I'm not sure how much of the swing is due to bad batteries 'collapsing' under the sudden addition of the dump load and how much is due to uneven regulation by the controller. What is the controller? Some home brew unit? You would probably be better off with a commercial charge controller that can vary the load rapidly (PWM) to maintain battery Voltage at proper set points and achieve the Bulk/Absorb/Float levels normally associated with RE systems. Simply switching hundreds of Watts on/off of the inverter is not the smoothest way of attaining this.

    135 Amps is quite a lot of charging current. Is it steady? In theory that could support a whopping 1350 Amp hours +/-. When not used for charging you would have roughly 1600+ Watts of "extra" power available so your 900 Watt & 600 Watt loads may even be a bit undersized.

    Considering the amount of current available one charge controller would not work here as a straightforward diversion control (see this page for reference: http://lib.store.yahoo.net/lib/wind-sun/TSdatasheet.pdf) because the biggest controllers can only handle 60-80 Amps. Even two 60 Amp controllers couldn't handle 135 Amps. You are in expensive territory here.

    I think before you get any more input from us we're going to need more details from you. Particularly about the turbine itself, but also about your load requirements as far as sizing the battery bank goes. You would appear to have significant surplus from that hydro power. :D
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Battery Sizing and Charging Considerations for a Hydro System

    You are seeing the "problem" with dump controllers. They are not "great" at properly recharging batteries. And I assume that voltage profile is when the batteries are "full" and your system is "floating"?

    Anyway--What are your loads (kWH per day)? What is your "peak average load".

    Is your hydro available all year long, or do you have times when it is not available (winter freezes, summer low water levels?).

    What is your average peak current from the hydro system? At least during your above sample, it is less than 900 watts (or ~67 amps).

    Does your hydro supply all the power you ever need?

    Do you cycle your batteries down to 75% or so at least a few times a month?

    How much water do you add to your batteries... Ideally, you want to add water about every 2 months... If you are adding water every month or less, they probably are being over charged. If you add water hardly ever, then they might be under charging.

    Are you happy with the performance of your hydro system? Will you or do you have alternative power (solar, backup generator, utility power, etc.)?

    Have you thought about 24 or 48 volt battery bank, larger inverter, etc.?

    -Bill "more questions than answers" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • turgo
    turgo Registered Users Posts: 7
    Re: Battery Sizing and Charging Considerations for a Hydro System

    Thanks for the kind replies.

    More details about the battery regulation: When these used core batteries were younger, the voltage swings were slower. Now the rate of the voltage swing is mostly limited by a 350 Farad bank of ultra capacitors. There are two different 120 volt water heater elements powered by the inverter that switch on and off independently at different voltage set points. I tried a duty cycle Watlow Dinomite power controller from the Watlow temperature controller, but it resulted in unacceptable flickering of the light. Simple on and off is better.

    The hydro makes 135 amps @ 12 volt nominal 24/7/365. The DC load to battery and inverter must dissipate this power (about 1.2 kw @120 VAC), or DC voltage will swing to unacceptably high levels.

    The first graph was for a time when there were light constant house loads, and the size of the dump load was mismatched to the available power. In this example, the 600w loads switches on @ 14 volts, and the voltage rises to 14.1. Then the 900w heater switches on. The voltage falls to 13.0 v where the 600W load switches off. voltage continues to fall to 12.75 where the 900w heater switches off. Usually battery voltage is above 13.0 volts, so I assume that batteries are charged. There are brief periods a few times a day for 10s of minutes when voltage goes below 12.7 volts, and power is then drawn from the batteries. The batteries are so dead that they can't provide much duration - they might total 100 AH. Below is another battery voltage graph for a time when the loads better match the supply:
    Attachment not found.
    While my current system does meet my needs, I'd like battery capacity to supply an extra 2 KW for a few hours several times a week. At a C/10 rate, I could have a 1320 AH bank of 24 T105s, for example. If I derate the system to 1000 ah to account for aging, and use a 50%DOD, that gives me 6KWH to use, which is 2KW for three hours. How do these numbers sound?
    Bill: Some answers:
    I should have built a 24 volt system, but the equipment is already bought.
    I add about 12 gallons of water three times a year to my 20 T105s and 20 car batteries. Sometimes the plates are exposed.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Battery Sizing and Charging Considerations for a Hydro System

    Your number sound about right.

    Twenty T105's? That would be two in series for 12 Volts times five in parallel for about 1100 Amp hours just from them. The car batteries are probably more of a hindrance than a help. Five parallel battery strings is a problem too, especially on a 12 Volt system.

    You should be looking at much larger capacity 6 Volt or forklift-type 12 Volt batteries here; getting rid of parallel battery connections and having good batteries should smooth out some of the Voltage spiking.

    It would be better to regulate the current on the DC side, but that may not be practical here.

    One concern I would have is if the diversion loads coming on can pull the charging Voltage below the Absorb level at that stage. Fluctuations during Bulk or Float are not that critical, but Absorb depends on maintaining a steady Voltage for long enough to complete charging.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Battery Sizing and Charging Considerations for a Hydro System

    Never expose plates... That is a killer for batteries.

    I am not sure what is best for your system... The batteries are there for "ballast"/Voltage control. They are not (usually) need to supply any current (possibly when starting a water pump, etc.) is about it.

    I read once about a phone company "battery" device that was really a electrolysis cell (did not use lead/sulfur reaction/components)... It did not store any energy--Any excess voltage/power was simply converted to hydrogen/oxygen gases (as I recall, was a long time ago and did not have a lot of details what I read) to dump excess DC power (more or less automatically).

    Larger battery bank and "slow" action dump controller (seconds to minutes between charge/dump cycles) or finding a controller that is "really" fast (100's to 1,000' of cycles per second) and use a smaller battery bank/"super cap" instead (assuming you don't have any significant loads to support from the battery).

    I am not a big fan of super caps (they have very tight voltage margins, don't store much power, or last very long, and are very expensive).

    Batteries that are >90% full don't do a 'good job' of storing excess energy/limiting DC bus voltage (other than converting to Hydrogen/Oxygen gasses--uses water and is hard on batteries).

    You could try other cycling (i.e., 100% to 75%, or even 80% to 50% for week at a time with recharging >90% once every week), etc... But that is just putting the batteries "to use" which changes the "wear out" mechanism from float/gassing to cycling.

    Hard pressed to figure out an "optimum" solution that is much different than what you have today. Used batteries (maybe some "big" fork lift batteries) may be your best bang for the buck (with, possibly an "improved" shunt controller).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • turgo
    turgo Registered Users Posts: 7
    Re: Battery Sizing and Charging Considerations for a Hydro System

    What are the problems with parallel batteries? Do they make a 1200 amp hour battery? Could it be cost competitive with a T105?

    BB, Indeed, my batteries currently function as ballast. I'm looking forward to batteries as storage again. My ultra caps cost $175 for the bank of (14) 2200 Farad Maxwell caps on ebay. Seven groups of 2 caps in parallel provide about 19 volts of peak voltage. They provide amazing supply stiffness for motor starts. I've observed 250 amps drawn from them when starting my 15 amp AC chop saw.

    I could raise the set points on my dump controller to charge at a higher voltage. New batteries will absorb more power and reduce the frequency of voltage swings. I'm not sure what I could do about the absorb cycle. Hydro doesn't have a diurnal cycle of charge and discharge, so I don't see how to create the classic bulk/absorb/float regimen.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Battery Sizing and Charging Considerations for a Hydro System

    The problem with paralleling batteries is that the resistance in the wiring inevitably becomes different per string, meaning some of them will get/supply more current than others. This changes the rate of "aging". The further apart they get, the worse it works.

    Page on battery wiring that should be helpful: http://www.smartgauge.co.uk/batt_con.html

    They do make batteries of higher capacities. Some examples from Crown:
    6 Volt: http://www.solar-electric.com/6voltbatteries.html
    2 Volt: http://www.solar-electric.com/2voltbatteries.html
    Forklift: http://www.solar-electric.com/12voinba.html

    Yes, they are expensive. More so per Watt hour than T105/GC2's regrettably. But there would be a performance improvement.

    Your system as it is now runs basically steady with the batteries/capacitors acting only as filters. To actually cycle the batteries would require some really big loads as you'd have to use all the power from the turbine plus capacity from the batteries.

    Thinking out loud here, I wonder if under the circumstances you might be able to use less battery rather than more? Something along the lines of minimum filtering capacity as usually recommended for battery-based GT systems: 100 Amp hours per kW of inverter or about 300 Amp hours. You would have to limit the charge current some way to prevent dumping 30%+ to the batteries. And you'd still need someplace for that 1600 Watts to go when not in use. Another reason why regulating on the DC side is better. Hmm. Tricky.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Battery Sizing and Charging Considerations for a Hydro System
    Thinking out loud here, I wonder if under the circumstances you might be able to use less battery rather than more? Something along the lines of minimum filtering capacity as usually recommended for battery-based GT systems: 100 Amp hours per kW of inverter or about 300 Amp hours. You would have to limit the charge current some way to prevent dumping 30%+ to the batteries. And you'd still need someplace for that 1600 Watts to go when not in use. Another reason why regulating on the DC side is better. Hmm. Tricky.


    By the way, I think Marc "Cariboocoot" is typing about 100 AH @ 48 volts per 1kW of inverter rating... Or ~400 AH per 1kW at 12 volts.

    You could look at a feedback circuit that regulates water flow, etc... But all of that adds complexity and cost--Most off grid systems have too much of "both" already.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Battery Sizing and Charging Considerations for a Hydro System
    BB. wrote: »
    By the way, I think Marc "Cariboocoot" is typing about 100 AH @ 48 volts per 1kW of inverter rating... Or ~400 AH per 1kW at 12 volts.

    Yes, along those lines - trying to adapt that recommendation to what is a steady supply of 1.6 kW of power.
  • turgo
    turgo Registered Users Posts: 7
    Re: Battery Sizing and Charging Considerations for a Hydro System

    Thanks for the ideas!

    Lets assume for now that I will use T105s as they are a third as expensive as Crowns for the same AH. This means that I could replace the pack three times as often for the same money. A bank of (24) six volt batteries would require a star wiring pattern to balance lead resistance (thanks Cariboocoot). Should it be configure as 12 parallel groups of two batteries in series? During operation, I could use my DC clamp meter to see if any parallel groups are charging at widely higher/lower rates. Then I could mix the highest charging pair with the lowest to rebalance the pack. How does this sound?

    You write, "To actually cycle the batteries would require some really big loads..."

    I have these loads now, I just can't use them. Combinations like microwave and washer, freezers and lot of lights, air compressor or table saw and anything else, etc. Currently I have a low voltage alarm fitted to my Watlow temperature controller, which tells me to turn stuff off.

    Here's a look at the last 36 hours battery voltage:
    Attachment not found.
    Wow, it averaged out the detail, here's the last five hours:
    Attachment not found.
    The average voltage is about 13.5 volts. Should it be 14.0 instead?
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Battery Sizing and Charging Considerations for a Hydro System
    turgo wrote: »
    Lets assume for now that I will use T105s as they are a third as expensive as Crowns for the same AH. This means that I could replace the pack three times as often for the same money. A bank of (24) six volt batteries would require a star wiring pattern to balance lead resistance (thanks Cariboocoot). Should it be configure as 12 parallel groups of two batteries in series? During operation, I could use my DC clamp meter to see if any parallel groups are charging at widely higher/lower rates. Then I could mix the highest charging pair with the lowest to rebalance the pack. How does this sound?

    This sounds like a 2640AH battery bank. You will routinely need 300A currents to be able to charge it. To power a 3kW load you'll also need 300A of current. This will require huge wires, breakers and will be very inefficient.

    It would look much better in 48V system - 3 strings, 660AH bank, 80A charging current.

    Whatever you loose on selling your 12V inverter(s), you'll make back up by going to thinner wire and smaller breakers.
  • turgo
    turgo Registered Users Posts: 7
    Re: Battery Sizing and Charging Considerations for a Hydro System

    Thanks for catching my math error. Sounds like I can get away with 12 batteries. It also means that the Crowns are 5 times as expensive.

    I have the wires and breakers. The only penalty in using 12 volts is efficiency.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Battery Sizing and Charging Considerations for a Hydro System

    Using T105's @ 220 Amp hours 6 Volts, you make up strings of two (to get 12 Volts) then parallel the strings to increase Amp hour capacity.

    The closest to the 1350 Amp hour "ideal" would be six parallel sets (twelve total) for 1320 Amp hours. Even using the bus bar/common connection point ("star" as you say) you can have trouble over time with that many parallel connections. You could use as few as four strings (880 Amp hours) and not be too heavy on charging current. Since your power supply from the turbine is just about constant the battery bank is "secondary" in terms of supply power.

    At full power your inverter will pull approximately 235 Amps, which would be full turbine output plus 100 Amps from the batteries. An 880 Amp hour bank will not be adversely affected by a 100 Amp draw from time to time.

    Be advised you should have a battery post terminal fuse on each string for maximum safety.

    It is unfortunate that the turbine is fixed at 12 VDC (it is, isn't it?) because it would be more efficient and easier to handle the power at a higher system Voltage. But bucking 12 Volts up to charge batteries at 24 or 48 is probably more trouble than it's worth (especially at 1600 Watts).
  • turgo
    turgo Registered Users Posts: 7
    Re: Battery Sizing and Charging Considerations for a Hydro System

    When I replace the battery bank, I will be adding a second Outback VFX2812 and a balancing autoformer to make 220VAC. This means that I could supply 5.6 KW. How big a battery bank would I need, and could I support? If a larger equalizing current is needed, My 10 KW diesel can supply 125 amps thru each Outback, plus 150 amps from another power supply I have, for a total of 400 amps; 535 amps with hydro output.

    Regarding the idea of moving to a different system voltage: now is not the time - I need only to buy batteries in multiples of four to preserve the ability to change to 24v later- and at great expense. The alternator is an EcoTech 305 amp alternator with a hybrid permanent magnet/wound field. Its designed for 12 volts. They also make a 24 volt version.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Battery Sizing and Charging Considerations for a Hydro System

    You are really getting into the realm of the absurd with 5kW from 12 VDC. That would be roughly 467 Amps to supply that, and the wiring becomes a nightmare. Each inverter would have its own 240 Amp wiring: 4/0 stretch to its limit to keep Voltage up at those current levels. Plus the battery bank wiring would be a lot of very large wires to really big bus bars and fuses all over the place.

    As to sizing a bank for that capacity, you have 135 Amps coming from hydro so you deduct that from the total (this load demand will not be constant so there will still be charging capacity), leaving 332 Amps for the batteries to supply. That is an awful amount of current in itself, and the battery capacity would need to be at least four times that (a 25% discharge rate, so to speak) to keep from "going flat" under load. Now you are back up at that 1328 Amp hour range as before. Fortunately the turbine should still be able to charge that much (as previously calculated).

    Personally I wouldn't do it. But you're stuck with a high current, low Voltage turbine. If it were possible to modify it to put out 24 Volts (depends on how they set up the thing) you would be much better off.
  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Battery Sizing and Charging Considerations for a Hydro System

    Can more gnats dance on the head of a pin if half are standing upright and other half are upside down ??
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Battery Sizing and Charging Considerations for a Hydro System
    Can more gnats dance on the head of a pin if half are standing upright and other half are upside down ??

    :confused:
    Normally I'm needle sharp, but this comment has me thinking I'm more like the surface the gnats are dancing on: obtuse.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Battery Sizing and Charging Considerations for a Hydro System
    If it were possible to modify it to put out 24 Volts (depends on how they set up the thing) you would be much better off.

    I've heard people rewind their turbines to meet their needs.

    If you look at the prices of 600A DC breakers, may be buying a new 48V alternator will not look as such a big expense.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Battery Sizing and Charging Considerations for a Hydro System

    since you want to go to 5Kwh from the bank and have a constant hydro power source, have you considered taking the 'hydro' and inverting it to 110AC and then charging a 24 v bank?
     
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: Battery Sizing and Charging Considerations for a Hydro System

    Just an FYI. There is a pretty active microhydro Yahoo group that has some pretty knowledgable folks. Some of which may have experience sizing battery banks in your situation. Link HERE.

    If/when you get to a point of replacing your current turbine, you might consider a newer, higher voltage type turbine designed to be used with an MTTP charge controller. Makes wiring, battery choices and charging efficiency easier.
  • turgo
    turgo Registered Users Posts: 7
    Re: Battery Sizing and Charging Considerations for a Hydro System

    Thanks for all your thoughtful posts. They have helped me direct my research. Here's my current direction:

    A charge controller would help my battery life, but at 12 volts and 135 amps, two charge controllers would be very expensive. However, I could use a diversion load charge controller along with my existing dump loads. The hydro output would be connected directly to batteries as it is now. The diversion load charge controller would regulate battery voltage for optimum charging voltage by diverting DC PWM switched power into a DC load. When the amount of diverted DC power exceeded the power level of my dump load, it would switch on; when the DC diversion load falls to zero, the dump load switches off. My existing temperature controller could switch on and off the dump load, regulating based on charge controller output current instead of battery voltage. Will this work? Is there a diversion load charge controller that is suitable for this? Since I don't need MMPPT capability, I could use a diversion load charge controller which only had a pwm output signal, and drive a large solid state 12volt relay with that signal.
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    Re: Battery Sizing and Charging Considerations for a Hydro System

    If I've understood correctly, since you don't need all the available power to complete an absorb stage, what you're suggesting is to split the dumped power between:
    a) a PWM diversion dump and
    b) a fixed power dump load turned on with a simple relay.

    Sounds feasible to me. If the total charging power available is 135A, then since 60A diversion controllers are easily available like the Morningstar Tristar 60A, your fixed power dump load should draw 75A only. You don't need to go through the complication of turning on the fixed dumped load using current, you could still use the battery voltage as you're currently doing.

    E.g. configure your current dumping system to start dumping at 14.3V. So this should only dump 75A. Since you're pumping 135A into the battery the battery voltage should continue to climb even with the dump load on, then you configure the Tristar to perform an absorb at 14.4V, and it will then be able to dump the additional 60A giving you a nice clean absorb phase (although it will have a jerky start because of the fixed dump load).