Fusing Batteries

animatt
animatt Solar Expert Posts: 295 ✭✭✭
in Off Grid Solar & Battery Systems #1
Have 8 6v crown 235ah 20hr rating. Golf Cart Size.

This is for my motor Home. Will be living in it for atleast a year maybe longer.

Wiring for 24v system.

So there will be 2 strings of 4 batteries.

This will be powering 12v loads and a 24v inverter. 12v loads will come from a dc-dc converter, not center taping the 24v array. INverter is a 2000w inverter, but supposed surge up to 6kw. I do not see surges getting above 3000w.

3000/21v= ~ 142amps at 21v.

Battery wiring is/will be done in 4/0 cabling. Have 50' of 4/0 going to be making my own cables. Already made a few.

Being inverter could possibly surge to 6kw, I am using a 250amp breaker between batteries and inverter.



Now I have read with parallel strings batteries should be fused. attaching a drawing. Not sure what fuse values should be 150amp each string? 125a?

Also if I wire in example B in diagram do fuses help? I would think Diagram B would help balance array better. The addition 4/0 wiring in Blue.


So looking for recommendation on fusing and I guess confirming if wiring Diagram B would be beneficial It does cost a bit more but I already have the material.

Thanks
Matthew

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Comments

  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #2
    Re: Fusing Batteries

    If I understand the idea of fusing correctly (which I may not), it is to protect one sring form another in case of failure.

    The failure may be a short of one of the batteries. In this case, one of the strings becomes 18V bank and the second string, being 24V bank, will start charging 18V. This is charging with 2.67V per cell - a regular equalization voltage. It's extremely unlikely that currents in excess of 125A develop during such charging. Moreover, the current is likely to be comparable with the current you incur during normal operations. Therefore, it is impossible to protect against such failure with a breaker.

    As to the layout B, it will do much worse in case of shorted battery, because if one of the batteries shorts, it'll short the parallel battery too.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: Fusing Batteries

    I understand the appeal of #B cross wiring battery strings. However--It only really helps if one of the batteries/connections is having a problem, and therefore, "hides" the failure(s).

    If all of the batteries are balanced, and the wiring is good--There will never be any current flowing in the cross links.

    The size of the series fuses per string--Just be no larger than the current rating of the string wiring, and no more than you would want to draw from the battery bank.

    With 230 AH per string, using C/2.5 as the maximum (in my humble opinion) surge current capable would be:

    230 AH / 2.5 = 92 AH maximum surge current
    92 AH of surge current * 1.25 NEC derating = 115 Amp breaker/fuse

    So, I would not expect you to "need" a minimum fuse of than a 125 Amp based on battery bank capacity. You could go to a larger fuse that is no larger than the maximum rated current for your interconnect cabling. 4/O with a 250 Amp fuse should be OK.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    #4
    Re: Fusing Batteries

    So it looks like if wiring A is the winner with the fuses sized 110-150amp would be in good shape? Something like
    http://www.solar-electric.com/f-110t.html
    I know it is slightly lower than nec. But just barely.

    I already have fuse holders, but would it be a big no no to mount the fuse directly to the + post of the battery? Seems silly to have a 3" 4/0 cable from battery to fuse holder.

    matthew
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Fusing Batteries

    I am OK with "larger fuses" as the cabling will still support blowing the fuse if there is short somewhere, without melting the cable.

    Having fuses blow because they are too close to rated load--Or even because fuses to "fatigue" when operated at high currents (fuses are heaters and do fatigue with temperature cycling).

    In any case, make sure you have a few spares--You don't want to "go dark" in the middle of a trip.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    #6
    Re: Fusing Batteries

    Just to clarify a bit more. I have http://www.solar-electric.com/mndc250.html

    Since there are 2 strings figuring the http://www.solar-electric.com/f150ampcltfu.html per each string would add extra protection.

    Can I use the mounting hole on the fuse and bolt it directly to a battery stud?

    matthew
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #7
    Re: Fusing Batteries

    I would purchase at least one fuse and see how "hard" the base metal is.

    Also, with any component that has a battery on one end and heavy AWG cable on the other--You would want to make sure that the fuse holder does not support any loads/strains/twisting motions.

    If you can restrain the cables and keep batteries from shifting (moving vehicle), then mounting the fuse holder to the battery post is probably OK.

    If, however, your cables and batteries can shift during driving on an ongoing basis--Then these holders are probably not rugged enough to survive safely.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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