Charging duration?

solarabcd
solarabcd Solar Expert Posts: 38
Calculating Generator run time questions.
Example:
Continuous running loads, 48Vdc, 2035 Amp Battery bank, 40% DOD, DC Gen output = 142amp, Load draw during charge 25amp. Solar output: 3.72kw (available for recharging i.e. after derating).

Use 85% eff for battery, 75% for Genset charger eff.

Battery can accept the full 142 amps up to 80% (bulk) level.

With the above details: From DOD up to 80% = 407 amps, and therefore 407 amps for the last 20% of battery capacity. (814amps total at DOD)

Using the Gen to charge up to 80%: (407/85%) / (142x75%-25)= 478/82 = 5.8 hrs


So how long would you estimate that the full charge with absorption would take considering the current rate will reduce?

Also if the Genset start is triggered by DOD then it may well start in the night time and get the battery back to 80% and start draining again.
I understand the concept of having the Genset start each morning rather than waiting to reach DOD so it can work in tandem with the solar to achieve the absorption phase, but this means noise every day rather than just a longer run time less often. Any ideas?

I have seen basic calculations using the above maths but substituting the 407amps with the full DOD (814amps) and then adding 4 hrs. This doesn't seem accurate considering I have different charge rates (genset and array) available.

Thanks.
B.

Comments

  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Charging duration?

    The less you stress your batteries - the longer they last. Daily AM charging with genset is a lot less expensive then a early failure of the batteries. And genset can be on far side of garage, where you don't hear it.
    Powerfab top of pole PV mount | 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 ,

  • solarabcd
    solarabcd Solar Expert Posts: 38
    Re: Charging duration?

    I understand that is the case. In this case with a large battery capacity and depending on how much power is consumed overnight the battery capacity may still be well above the bulk charge level by the next morning.

    I just want a more accurate way of calculating how often the Genset will need to run to predict the running costs etc.
  • blackswan555
    blackswan555 Solar Expert Posts: 246 ✭✭
    Re: Charging duration?

    Trying to use time as an end of charge trigger is not accurate at all, To many variables involved to get it consistently accurate, Bat temp, Equipment temp, DOD, Speed/load of discharge, if the sun is out etc ( + your gen gives 142 amp, I presume @ 48 v ? whats it giving @ 56 v ?)
    But from a bat manufactures manual, The first 80% of the charge takes 20% of the time, the last 20% takes 80% of the time, so you can see how distorted that could get (and very expensive on wasted gen run time)

    Please give more info on the system, it`s components and it`s requirements, There may be several different ways to achieve your goal,
    Use of an AH "counter" eg FNDC if you use Outback http://www.outbackpower.com/products/communications/flexnet_dc/ will give you some good options,
    eg let the solar do most of the work, then only charge to full (using gen) say once a month or if SOC go`s below a certain level ( quiet times can also be "dialed in ") They terminate charge when the end of charge amps drop below a programed level, above a voltage, for longer than a time, ( the second best method of charge termination after dipping SG`s)
    Have a good one
    Tim
  • solarabcd
    solarabcd Solar Expert Posts: 38
    Re: Charging duration?

    Thanks Tim
    The 142 amps from the Gen is at 56v. i.e. 8kw DC genset. The Flex Net sounds neat and will pretty well achieve what I am looking at. Does morningstar products work as well as the outback? I like the Web based warnings and alert concepts they have which appear to be all in-house design, as opposed to the outback Web partners.

    At the moment I am mainly concerned with a way to plot generator run time in a spread sheet I have made up and more accurately than all the ball park figures out there. I know in reality it is hard to be accurate for every system with just one calculation.

    Doing a full charge only once a week/month with the genset is something I think all charge controllers should accommodate for the obvious reason you stated. Is the connection of the flex net AUX relay control reasonably simple and compatible with any genset?
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Re: Charging duration?

    AND Solarabcd, you may be missing something: Bulk is the highest charging voltage your batteries see in a normal daily charging cycle. For a 48 Volt bank, of FLA batteries, it is around 58 Volts. Float is usually 53-54 Volts. When charging ceases, the battery voltage drops. Fully charged 48 volt bank rests around 50.8 to 51 volts at 25 degrees C, with NO LOAD, and a few hours after charging ceases.

    The next morning, or when charging begins, Bulk is the mode the CC will start in. It will stay there until the Asorb target voltage is reached -- around 58 Volts for common FLA batteries. If the batteries are fully charged, the CC will still go into Bulk, and stary there for some time, depending upon the way the CC is set. Bulk will take 30-90 minutes, perhaps more, and, if Solar input is the only charge source, usually in the moring, the Solar current available is low -- the sun is at a very oblique angle. If the solar panels are heavily shaded in the morning, the available solar charge current will ramp up more quickly.

    There are a few variables that will change the above a bit, but generally, this is my experience.

    YMMV Good Luck Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • solarabcd
    solarabcd Solar Expert Posts: 38
    Re: Charging duration?

    Thanks Vic.
    Yes I understand those parameters. What I should point out is that with the DC Genset it has it's own charge controller/monitoring but potentially not very programmable. It sounds like it will be best to have the standard solar charge controllers do all the monitoring and use their remote aux relay starting/trigger options. If that is the case I assume just one of multiple charge controllers do the monitoring and starting of the genset with no need to have the other controllers connected to something like the Flex net?

    Lets say the genset and components are programed to do a full charge say once per week for example then I can determine run time up to the 80% level by adding the genset ouput + the solar output divided into the capacity at DOD up to 80%. The batteries can handle up to 275 amps to 54V.

    What I need to figure out is the time the Generator needs to stay on to complete the absorb phase at a reduce current rate and add that to make a total run time figure....Using the previous 3.62kw solar input (after full derating including battery efficiency etc)