Stacking battery chargers

slyttle
slyttle Registered Users Posts: 9 ✭
in Solar Beginners Corner #1
Got what I hope is an easy question.

I am creating a battery backup system for some critical circuits. I am starting very small, with just 200 AH of deep cycle battery capacity. I am also running some continuous loads off this battery bank, amounting to about 200 watts. Eventually I want to build up the system to about 800 AH.

right now, I have just a small 20A plug-in AC charger that keeps the batteries topped up. I realize this charger is already too small, and am looking to replace it with a larger one.

Is it better to get a BIG charger now (100 Amp), anticipating that I will eventually build up my system to 800Ah, or can I get a smaller one, and stack another charger on top of it later on?

My concern is that if I try charging a 200 Ah bank with a 100Amp charger there might be 'unfortunate' results (the explosive kind), but I don't want to buy another small charger and end up selling it a year from now when I build up the capacity of my battery bank.

A little advice would be appreciated.

thx

-Sheldon
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Stacking battery chargers

    For the most part there is no problem with 'stacking' charge sources, whether solar or AC sourced. Their outputs are Voltage regulated and since the internal components are semi-conductor based there's not much chance of one 'back feeding' another and overwhelming it with current. This is especially true if they're trying to feed a battery with a load on it as that will be the low resistance in the circuit and take the current.

    It is more efficient to run one larger charger rather than two smaller ones that add up to the same amount because less power is consumed by the single charger as opposed to two.

    Unregulated charge sources are another matter, as they just push current as a function of higher Voltage available with nothing to scale them back. They expect someone to be watching.

    As it is, your 20A charger is fine for 200 Amp hours. The load will draw this down of course. 200 Watts at what Voltage? That's what changes the current. That would be about 16-17 Amps on 12 VDC which would bring the actual charging down to 4 Amps.

    100 Amps is a lot of charge current potential. On 800 Amp hours it would be fine, though.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: Stacking battery chargers

    cutting straight to the application of a 100a charger to a 200ah battery bank will have detrimental results. it will boil the electrolyte out and the high heat of the fast charge could possibly warp the plates and may short them out. i don't believe there will be an explosion if there aren't any sparks to ignite the hydrogen gas being given off by the batteries and there would be a higher amount of hydrogen gas due to the higher charge rate. ventilation would be a good thing to do if high charge rates are employed, but for 2 batteries i don't normally feel it mandatory. you will be expanding on the battery bank so plan on venting it.
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  • slyttle
    slyttle Registered Users Posts: 9 ✭
    #4
    Re: Stacking battery chargers

    Many thanks. That helps a lot!

    Yes, the loads are run at 120 volts so we are talking about 200/12 = 16.667 amps of load on top of the battery charging. Soo even though it isn't quite as efficient, I think what I might do is sell my 20amp charger and get a 50 amp chargers (3 stage) ... and possibly one additional 100 AH battery. Then, when I eventually increase the number of batteries, I will also buy an additional charger.... or perhaps make up the difference with solar or wind ... we'll just see how it goes.

    A reasonable compromise I guess. (beats boiling my batteries :) )

    Thanks!
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #5
    Re: Stacking battery chargers
    niel wrote: »
    cutting straight to the application of a 100a charger to a 200ah battery bank will have detrimental results. it will boil the electrolyte out and the high heat of the fast charge could possibly warp the plates and may short them out. i don't believe there will be an explosion if there aren't any sparks to ignite the hydrogen gas being given off by the batteries and there would be a higher amount of hydrogen gas due to the higher charge rate. ventilation would be a good thing to do if high charge rates are employed, but for 2 batteries i don't normally feel it mandatory. you will be expanding on the battery bank so plan on venting it.

    I once, unintentionally, charged my batteries with 23% of C20, for an hour. Temperature rose 7C. With 100A charger on 200AH bank, that would be 50% of C20, which should produce approximately 4 times as much heat. That would be rise 28C per hour. If the bank was at 50% DOD, this cannot last for more than an hour. If ambient was 25C, it'll heat up battery to 53C.

    This definitely is very bad for batteries and possibly may warp plates, but I don't think it'll boil out the electrolyte.

    However, if the plates short due to warping, it'll produce much more of additional heat, which has a potential to boil electrolyte. If plates get exposed to the air, and at the same time are warped, there could be an arc between them, which is very likely to cause an explosion.


    Fortunately, charger probably can be configured for lower current.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #6
    Re: Stacking battery chargers
    NorthGuy wrote: »
    ...charged my batteries with 23% of C20, for an hour. Temperature rose 7C. With 100A charger on 200AH bank, that would be 50% of C20,...

    Hi North Guy, C20 would normally discribe a charge or discharge rate, like saying 1/20th of the battery banks capacity, so your reply is a bit confusing.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,590 admin
    #7
    Re: Stacking battery chargers

    I think he is trying to be clear--20% of the battery banks capacity as defined by the 20 hour rate, as opposed to some other C/10, C/8, etc. battery capacity number.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #8
    Re: Stacking battery chargers
    Photowhit wrote: »
    Hi North Guy, C20 would normally discribe a charge or discharge rate, like saying 1/20th of the battery banks capacity, so your reply is a bit confusing.

    Sorry for the confusion. I meant C20 to be 20-hour rate. I've seen that notation in many places. 1/20th of the rate would be C/20, or C20/20.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #9
    Re: Stacking battery chargers

    Probably the best way for the long haul if you know the Inverter / Charger your going to end up with, if that where your heading is to buy it and forget the charger. With them you can control the AMP output, but you have to have a remote on most.. There a few chargers that have a variable output, Black and Decker Smart Charger is around $150 or so for a 40 AMP. Xantrex has the True Charge 2, but it's real pricy, $500 for a 60 AMP.

    I am sure there others.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #10
    Re: Stacking battery chargers
    NorthGuy wrote: »
    Sorry for the confusion. I meant C20 to be 20-hour rate. I've seen that notation in many places. 1/20th of the rate would be C/20, or C20/20.

    You will generally see that here too. For AGM batteries, you may also see things like 4C to denote 4 times the C value(rate unspecified). If you want to be unambiguous you would print (or type) something like. 4C20 or C20/5 (use Go Advanced to get subscripts).
    SMA SB 3000, old BP panels.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #11
    Re: Stacking battery chargers
    inetdog wrote: »
    You will generally see that here too. For AGM batteries, you may also see things like 4C to denote 4 times the C value(rate unspecified). If you want to be unambiguous you would print (or type) something like. 4C20 or C20/5 (use Go Advanced to get subscripts).

    C20 is always less than C100. I can do that! Thank you, inetdog.
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