Grid powered dc system

Alterna
Alterna Registered Users Posts: 6
I have a bunch of 12v dc equipment that I want to power super reliably.

I was thinking of just some 12v deep cycle batteries tied to a smart charger and put my dc loads on that. My loads draw 5-20A continuous so I would use a large sized charger.

Will the loads confuse the smart charger causing overcharging? What if the batteries are low, will they draw so much current that the equipment gets starved of current?

Is there a better way to do this?

Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Grid powered dc system

    not all 12v equipment can reliably be used in conjunction with 12v batteries and their charging systems. the voltage varies widely enough to cause problems with some stuff and some just need a steady voltage which will not happen with 12v batteries unless you can obtain a high current dc to dc converter.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Grid powered dc system

    To add to what Neil said, if it's 12VDC battery equipment it may not have sufficient filtering to keep out line noise supplied by the charger.

    So it comes down to your specific equipment's specific power needs, which may require regulating/filter after the battery bank.

    But that doesn't mean it can't be done. :D
  • Alterna
    Alterna Registered Users Posts: 6
    Re: Grid powered dc system

    The gear will accept 9-28V.

    It is a bunch of Wifi routers with integrated DC-DC regulators.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Grid powered dc system

    ok, let's revisit your original post,
    Alterna wrote: »
    I have a bunch of 12v dc equipment that I want to power super reliably.

    I was thinking of just some 12v deep cycle batteries tied to a smart charger and put my dc loads on that. My loads draw 5-20A continuous so I would use a large sized charger.

    Will the loads confuse the smart charger causing overcharging? What if the batteries are low, will they draw so much current that the equipment gets starved of current?

    Is there a better way to do this?

    now you have to define reliably for how long? 5-20ah rates of discharge means a large battery bank and you can't take it to dead as you want at least half of the power left in the batteries for preserving some lifespan. if it's a 20ah rate going 24hrs a day this is 480ah and the battery bank needs to be at least double that for 960ah. if at 5ah it will all be 1/4 of that stated. how many days are you going to try this for as each day will require between 240ah and 960ah depending on exactly the draw you average?
    charging this is another problem as you will need between 5 and 13% of the ah rating of the batteries to charge it with and trojan i believe wants 10-13%. you also have a need to totally replace any backup ah used as well as keep up with the normal drain everyday. in other words it can't just replace what is used and should be a 3 stage charger or chargers.
    now when you are drawing a load and charging at the same time the charger must deliver more than the load or there will not be any charge to the batteries. this is not an overcharge either as it could be the opposite if it's not planned for as it would take more time to reach full charge if it could reach full charge. i recommend to try and have the 5% rate beyond the loads that are being drawn. if you as an example would draw 20a with a 960ah battery bank you should have the 20a + 5% of the total ah which is .05x960ah=48a for a total of 68a. this will insure at least a 5% rate to the batteries to keep charging with and would represent a total charge percentage of 68a/960ah=7.083%. this would be the minimum you'd want in that example.
    as to your last question if the batteries are that low you could be in for some troubles, but the charger(s) need to run 24/7. if you can narrow down the actual daily ah draw and how many days you need to insure this to run things you may get a better handle on the battery requirements. this won't be a cheap undertaking given the costs or batteries and chargers.
    hopefully i got this answered in the ballpark of what you were looking for, but if not i hope you have a better perspective of what you may need.
  • Alterna
    Alterna Registered Users Posts: 6
    Re: Grid powered dc system

    There is no daily drain (aside from maybe internal resistance of the packs?)

    This is not an off grid installation, the batteries only drain when the power grid fails which is only a few times a year.

    I will probably be able to count the annual discharge cycles on one hand. I am planning to have 250-500AH of capacity, and if it takes 3 weeks to charge I don't care.

    Most of our handful of power outages are under 3 hours, we did recently have a record 18 hour outage at one location recently, up until recently, the record was 7 hours.

    In the old days my father worked for the phone company, they just had a huge bank of 48v batteries, ran them at 52v 24/7/365 on a big power supply and ran the telephone exchange at 52v. If the grid failed, power dropped to 48v and everything ran fine.

    I am trying to do something similar but I am concerned about shortening the life span of the batteries and what the charge cycle would do for the few times a year it happens.

    If the gear say draws 10A, I would be looking at a 20-40A power supply. I have some Xantrex 20A and 40A chargers here I can try.

    This is not so much an alternative energy question as a battery question.

    The alternative would be some huge APC AC UPSes. To give 12-48 hours of backup power it is about $10,000 just taking into account the power needed to run the built in inverters.

    I am trying to use a pure DC system since the equipment is all DC anyway. Xantrex 20A chargers are on sale for $200 at the local store and 120AH wet cell deep cycle batteries are about $130 each. 4 Batteries, a Charger, a low volt shutoff, fuses and etc, are way under $1000. A great DIY solution, if I can be sure I am not setting it up for bigger problems down the road. We currently use APC AC UPSes and get a few 1-3 hours of power, best case. Inverters, power bricks, etc suck up a huge percentage of battery power, and a 1000VA UPS has tiny batteries.

    It is part of a community Internet access plan.

    Each location has different current draws, so we want a solution that will work at each varied location and we can add on battery packs to vary the run time based on power stability and load. If we build a custom solution at each site, volunteers etc, will have a harder time understanding each situation. A single solution with modular capacity is the best choice.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Grid powered dc system

    ok i guess i now have a better perspective on where you are going with it. obviously i did not think this to be off grid or i would not have mentioned any chargers as they require utility power. i suppose design it with 10hrs of backup and 10a and that represents 100ah. unless you plan on buying more batteries you better not drain them dead as they aren't meant to be drained down that far. i don't think with the 100ah requirement and you going for 250-500ah that there will be any problems. minimum charge rate for the charger(s) will be 5%. for 250-500ah that is a range of 12.5-25a well within the range of your chargers. the 50% soc design point would be 200ah so you will be doing better than that with your specified range.
    i figured 10hrs to be a good figure as another record 18hrs will still show 20% soc in the batteries if you designed for the 50% soc point and it will cover all of your normal outages. just be sure the chargers will deliver about in the 5-13% range with as low as 3% as a possibility for longterm charging depending on the battery used and if it gets abused.
    make sure you have a fuse at the battery in case the battery leads short.
    does this sound better to you?
  • Alterna
    Alterna Registered Users Posts: 6
    Re: Grid powered dc system

    I think we are on the same channel now :)

    But I am still unsure of how the smart charger will feel about having a load other than the batteries on it.

    I am under the impression that the charger varies output voltage based on current draw. So if there is other equipment on the charger as a load, how does the charger know the difference between equipment or a low battery.

    As the equipment draws current, will the charger think the batteries are low and run at a higher voltage? If the batteries are already full, will this overcharge them?

    I may be misunderstanding this, I am not sure.

    I did this on my bench and all works great, but I do not want the batteries to fail 6 months down the road. I would like to have a 3+ year lifespan. 5+ years would be awesome, but may be a bit to ask, given that some of these locations do not have heat or aircon.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Grid powered dc system

    should not be a problem as the same thing occurs with controllers and they start a new charge cycle, but the things aren't going to overcharge anything as long as they are of the 3 or 4 stage variety. technically the power is going partially to the loads and partially to the batteries and it will deliver up to what it all needs and should be fine. if you feel the charger cycles too much then shut the charger off for awhile or put one on that has a bit lower amps, but this isn't necessary imo.
  • Alterna
    Alterna Registered Users Posts: 6
    Re: Grid powered dc system

    What you say makes sense.

    Since these chargers will be installed in a place without humans around, I am not sure if we would know if they are cycling too much.

    This all seems to work, I am just trying to find someone who has done this for a period of time and can report result. :)
  • Alterna
    Alterna Registered Users Posts: 6
    Re: Grid powered dc system

    Google!

    After many hours of searching, I think this product is exactly what I need:

    http://www.samlexamerica.com/products/productdescription.asp?ProductsID=8001

    And right in my price range for both 15A and 30A

    It explicitly supports batteries with a load!

    I can use the 15A for smaller locations and the 30A for big locations.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Grid powered dc system

    technically a battery is a load when it's being charged by the charger, but that charger should work ok too. out of curiosity, what kind of batteries do you plan on using with this stuff? make? model?

    come to think of it if this is to be out in the elements you may want a charger that has a temperature sensor if you can find one. not to sure if the xantrex chargers you have have that feature or not, but i think they might.