# battery fuses

Posts: 14Registered Users
I have an Off Grid system that has 14 L-16 6 volt batteries. They are for a 12 volt system so there is a string of 7 with two batteries in series. The only fuse is between the batteries and the inverter. From reading this forum I feel that for safety I need to put fuses on them. I would like some advise on the size of fuse to use and the placement of the fuses. Since two 6 volt batteries are connected together to get 12 volts I was thinking of attaching a fuse between the batteries. The connection on the battery is a 3/8 inch bolt, if I get a fuse that has a 3/8" connection at each end could I just connect the fuse between two batteries. Using 7 fuses. Would that work?

I am interested any suggestions and ideas concerning size and placement.

Thanks

Dan

Re: battery fuses

Dan,

Welcome to the forum...

Yes, paralleling lots of big batteries together has the potential for lots of short circuit current (for me, I am probably more scared of your size battery bank than working on a home breaker panel...

Fuses are there to protect the wiring and be the weak link in the system. And in the case of paralleled battery systems (with 3 or more strings), it prevents the other batteries from feeding a dead short on one string...

So, the questions:
• What is the maximum current you expect to pull (i.e., 1,500 watt inverter with 3,000 watt maximum surge).
• What is the gauge / cable type of the individual battery cables.
You will have all the + and - battery connections end at a common +/- bus connection point. Each cable leaving that point should have a fuse or breaker there to protect the wiring leaving the + bus connection point (assuming negative ground).

For example, you have 60 amp charge controller (solar/AC/etc.) on a:
• 60 amp * 1.25 NEC safety factor = 75 amp minimum circuit
Next size fuse/breaker would be 80 amps (note, Outback Solar rates some breakers for ~63 amps for a 60 amp circuit--It sounds like they are really closer to 80 amp breakers--you will need to look at that closer if you choose Outback hardware).

And, you will need to size the cable for 80 amps using NEC or other equivalent cable rating chart (aka boating chart, etc.). NEC is conservative, others not so much.

Note that with solar, and especially 12 volt solar, you have very little room for voltage drop (i.e., 12 volt battery, 11.5 minimum voltage and 10.5 cutout on the typical inverter => 1.0 volt maximum drop). For long cable runs, you will probably need to use a voltage drop calculator and up-size the cables for low voltage drop.

Personally, when I look at paralleled power circuits--I would size the fuse/breaker for your bank to be at least ~1/2 of the maximum current you expect to draw from the bank (and not 1/7tth even though you have 7 parallel strings). This should reduce "nuance" trips.

Current sharing can be difficult on paralleled battery banks... The battery and cable resistance if very low, so small issues (loose/dirty connection, bad cell, etc.)( can dramatically affect current sharing.

Lastly, as you reconfigure your parallel battery connections, you should aim for equal path length through all of those various parallel battery paths... An extra long path will add resistance and reduce current load on that string. An extra short path will increase charging/discharging current on that string.

A nice explanation of how to do the path balancing:

www.smartgauge.co.uk/batt_con.html

And, if you can justify, I would suggest a DC Current Clamp meter (such as this one for ~\$60, same as this model--just private labeled)... It can make the monitoring of shared current flow in your bank much easier and quicker (bad connections, shorted/open cells, etc.)... Just do your measurements both under heavy load and heavy charging currents (note: meter is just a low cost suggestion--pick your own based on your needs).

Fuses, breakers, switches... some starting points:

High Current Fuses and breakers
Battery Bank Switches

Remember one issue about cable termination--Taking terminal tabs and soldering them on cables is usually not a good idea--Cables can work harden and break at solder point and/or in a short circuit, the solder can melt and the cable comes loose--causing more issues.

If you want to terminate your own, you might find a reasonable priced hydraulic crimper or a "hammer crimp tool".

Questions?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 95Solar Expert
Re: battery fuses

Now I have a question. The diagrm shows one row of batteries, what happens if there is two rows of batteries? is each pair of batteries treated as a single battery or is each group wired as showen? What my mind is telling me is each group is wired as diagramed and then they geat seriesed?

Chuck
Re: battery fuses

Chuck,

I am not sure I understand your question... But if it is, for example, you have 2x 6 volt batteries in series for 12 volts--Yes, that pair of batteries is treated like "one battery"--And then each pair of series connected 6 volt batteries is then wired in parallel according to the Smartguage chart.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 17,615Banned ✭✭
Re: battery fuses
Chuck46 wrote: »
Now I have a question. The diagrm shows one row of batteries, what happens if there is two rows of batteries? is each pair of batteries treated as a single battery or is each group wired as showen? What my mind is telling me is each group is wired as diagramed and then they geat seriesed?

Chuck

Other way 'round. When you connect two batteries in series they become one higher Voltage battery, so to speak. As in the common practice of two 6 Volt "golf carts" making "one" 12 V 225 Amp hour string. Parallel those as per the SmartGauge diagram. As Bill said, each string would have a fuse for best safety. And yes it can be between the batteries, as that connection is also part of the circuit.

Attempting to wire them with extra inter-battery connection (paralleling the inter-cell links) isn't a good idea as it creates alternate current paths and eliminates the efficacy of fusing as described above. Someone on the forum recently tried that, with firework-like results.
• Posts: 14Registered Users
Re: battery fuses

Bill:

Thanks for the reply. I have a Trace 2512 inverter that was put in service in 1999. The batteries are also 12 years old. I purchased the cabin a couple of years ago. For most of the life of the batteries they we only charge with a 6kw generator but last summer I added 4 175 watt sharp panels with a FM80 charge controller. I also put up a Whisper 100 wind turbine. Between the solar and wind my diesel consumption was cut in half. The cabin is in the mountains and is use mostly for 2-3 days on weekends. My thoughts were since in the middle of winter use is much less (dark and cold) giving the batteries a little charge may help the out. We get many more -30 degree days than 70 degree days over the year.

The battery bank is wired like method # 1 in the smartgauge web site. There use to be 8 strings on the battery bank but one battery was leaking and another was not putting out much and bulging, it probably had frozen. These two batteries were the furthest from the inverter. There is a large fuse between the inverter and the first battery but I do not know what size it is but will find out the next time I am there.
The wire from the inverter to the batteries is 2/0 and the jumpers between the 6 volt batteries is 1/0.

I understand the the batteries are getting toward the end of there service life but at the present time they seem to be serving our needs very well. The specific gravity is good in all the cells and when charged they are pretty close. We use between 100 and 200 amp/hours per day when there. The loads are mainly lights and some electronics. There are a couple of fuel oil heaters that have small fans. The stove and fridge are on propane. I also added a well last summer. It has a one HP pump but I only use it to fill the tanks in the garage. I always start the generator when pumping out of the well and the tanks are good of 2-3 days. We have a 1/2 hp pressure pump to charge the system that run off the batteries.

I consider this set of batteries to be my learner set. If I can keep them going a little while longer I can probably take good care of a new set when I need them.

So my present plans are to switch the battery cable set up to method # 2. I also need to fuse all the battery strings. Advise on how to do that would be welcomed.

Thanks

Dan
• Posts: 95Solar Expert
Re: battery fuses

Well what I ment is I have 4 6volt batteries in series for 24 volts this is row one row two is the same 4 6volt for another 24 volts, now they are paralleled to up the amps. So is each row of 4 for 24 volts treated like one battery?

Chuck
• Posts: 17,615Banned ✭✭
Re: battery fuses
Chuck46 wrote: »
Well what I ment is I have 4 6volt batteries in series for 24 volts this is row one row two is the same 4 6volt for another 24 volts, now they are paralleled to up the amps. So is each row of 4 for 24 volts treated like one battery?

Chuck

Yes; the series string of four works like one big battery. In essence, and single 6V battery is actually three 2 Volt cells in series, it's just that the connections are internal rather than external.
Re: battery fuses

Dan (and Chuck),

My computer ate my post (actually windows decided that it was the perfect time to run updates):grr.

Anywho... As Caraboo/Marc said.

Shorter reply on fusing/breakers... I am a hacker--and even if I try my wiring still looks messy. Here are some suggestions to start looking:

Solar Panel Array Combiners, Breakers, Fuses
High Amperage Inverter Fuses & Breakers

Marine fuse holders are also good for 12/24 volt systems (typically 32 volt maximum rating):

1 or 2 fuse holder for mounting on a battery directly (very compact)
Various Marine fuse holder systems
Some Marine holders also have a negative bus too

You can look at automotive fuse holder systems... Those stereo guys can use a lot of power:

Various automotive holders

And a very simple/inexpensive fuse block for 6x AGC glass fuses:

Prime Products

If you are making up your own large gauge battery cables, a hammer crimping tool can be useful (if you cannot justify the hydraulic crimper).

Hammer Crimp Tool

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 14Registered Users
Re: battery fuses

Bill thanks for the info. I think I will use these

http://bluesea.com/category/5/21/productline/overview/378

I am planning on putting them on the jumper between the 2 six volt batteries.

I was thinking a 100 amp fuse

Does that sound like a reasonable approach??

Dan
Re: battery fuses

Assuming these are 420 AH flooded cell batteries, the maximum useful surge current of C/2.5 would be:
• 420 AH * 1/2.5 = 168 Amps
You have 7 parallel strings of batteries driving a 2,500 watt 12 volt inverter (assuming 85% efficient inverter):
• 2,500 watts * 1/10.5 volts * 1/0.85 eff * 1.25 NEC factor = 350 Amp Circuit
For a battery bank with lots of paralleled strings--I would, personally, try to design for 50% of the load to be carried by one battery (lots of paralleled batteries do not always share current well--designing fusing to 50% will prevent blowing fuses unnecessarily vs 1/7th of load or 50 amp fusing):
• 350 Amps * 0.50 = 175 Amp circuit per battery.
Depending on lots of things, a 1/0 cable is good for ~150-170 amps or so (even upwards of 245 amps in free air--NEC is pretty conservative).

So--I would probably pick the 175 amp fuse myself... This will limit issues with false trips if you have current sharing issues (bad cables, cells, etc.) but still protect wiring/batteries from excessive current (you still need to protect wires leaving the + bus bar/battery common point).

Since this is an old battery bank and system--Before you put a lot of money into the system wiring up the fuses/parallel battery bank... Does this make economic sense to you? New battery bank, new inverter, changing to 24 or 48 volt bank?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 14Registered Users
Re: battery fuses

Bill:

Thanks for the help. I know that some day they will have to be replaced. It just comes down to a few hundred dollars for fuses and battery cable or a few thousand for a new battery bank and inverter.

I will probably go with the fuses at this time but I have been thinking about what I will replace them with when the time comes. Since we use 100-200 amp/hrs per day and are there 2-3 days at a time we could use a battery bank that would supply 3-400 amp/hr over two days at 20-25% DOD. I was thinking of 8 L-16 batteries in two strings at 24 volts. Is there any advantage in using one string at 48 volts? Also I do not know how to figure the size inverter I would need.

Thanks

Dan
Re: battery fuses

One simple way to assess your AC loads is with a Kill-a-Watt meter... They are cheap and easy to use. You want to:
1. Measure running Watts
2. Measure running VA (Volt*Amps)
3. Estimate Peak Watts and VA (k-a-w is not good at peak power)
Running Watts are used to size your inverter / battery / array...

VA and Peak VA (Watts) is used to size your wiring, inverter rating, and wiring--and one of the checks on Battery AH sizing.

kWH per day will be used to estimate the AH rating of the Battery Bank and the Size of the solar array.

In general, for off grid power--conservation is king. For a small system, a 12 volt 300 watt TSW inverter is something good to aim at... A small system can drive a well pump or other short usage items, but everything has to be much larger to handle the short term loads/surge current.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 10Registered Users
BB. wrote: »
Re: battery fuses

Dan,

Welcome to the forum...

Yes, paralleling lots of big batteries together has the potential for lots of short circuit current (for me, I am probably more scared of your size battery bank than working on a home breaker panel...

Fuses are there to protect the wiring and be the weak link in the system. And in the case of paralleled battery systems (with 3 or more strings), it prevents the other batteries from feeding a dead short on one string...

So, the questions:
• What is the maximum current you expect to pull (i.e., 1,500 watt inverter with 3,000 watt maximum surge).
• What is the gauge / cable type of the individual battery cables.

You will have all the + and - battery connections end at a common +/- bus connection point. Each cable leaving that point should have a fuse or breaker there to protect the wiring leaving the + bus connection point (assuming negative ground).

For example, you have 60 amp charge controller (solar/AC/etc.) on a:
• 60 amp * 1.25 NEC safety factor = 75 amp minimum circuit

Next size fuse/breaker would be 80 amps (note, Outback Solar rates some breakers for ~63 amps for a 60 amp circuit--It sounds like they are really closer to 80 amp breakers--you will need to look at that closer if you choose Outback hardware).

And, you will need to size the cable for 80 amps using NEC or other equivalent cable rating chart (aka boating chart, etc.). NEC is conservative, others not so much.

Note that with solar, and especially 12 volt solar, you have very little room for voltage drop (i.e., 12 volt battery, 11.5 minimum voltage and 10.5 cutout on the typical inverter => 1.0 volt maximum drop). For long cable runs, you will probably need to use a voltage drop calculator and up-size the cables for low voltage drop.

Personally, when I look at paralleled power circuits--I would size the fuse/breaker for your bank to be at least ~1/2 of the maximum current you expect to draw from the bank (and not 1/7tth even though you have 7 parallel strings). This should reduce "nuance" trips.

Current sharing can be difficult on paralleled battery banks... The battery and cable resistance if very low, so small issues (loose/dirty connection, bad cell, etc.)( can dramatically affect current sharing.

Lastly, as you reconfigure your parallel battery connections, you should aim for equal path length through all of those various parallel battery paths... An extra long path will add resistance and reduce current load on that string. An extra short path will increase charging/discharging current on that string.

A nice explanation of how to do the path balancing:

www.smartgauge.co.uk/batt_con.html

And, if you can justify, I would suggest a DC Current Clamp meter (such as this one for ~\$60, same as this model--just private labeled)... It can make the monitoring of shared current flow in your bank much easier and quicker (bad connections, shorted/open cells, etc.)... Just do your measurements both under heavy load and heavy charging currents (note: meter is just a low cost suggestion--pick your own based on your needs).

Fuses, breakers, switches... some starting points:

High Current Fuses and breakers
Battery Bank Switches

Remember one issue about cable termination--Taking terminal tabs and soldering them on cables is usually not a good idea--Cables can work harden and break at solder point and/or in a short circuit, the solder can melt and the cable comes loose--causing more issues.

If you want to terminate your own, you might find a reasonable priced hydraulic crimper or a "hammer crimp tool".

Questions?

-Bill

Hi,
kindly advise which breaker ratings to use in this case (system not yet installed):

1. two charging sources: 25Adc (inbuilt inverter charger) & 50Adc (external charger)
2. inverter/battery operating voltage :96Vdc
4. 4nos parallel strings of 200AH/string, each requiring a breaker.( 4 parallel strings is not ideal, but i am trying to work with what i've got)
5. cooling system in place where batteries are going to be kept on racks, not sealed enclosure.
6. maximum wire length 5m (+ve & -ve wires combined)

i understand there are to be breakers in the following places:
-DC breaker on each of 4 parallel string
-DC main battery disconnect/breaker before connection to DC + bus
-DC breaker between charger 1 and battery at DC + bus
-DC breaker between charger 2 and battery at DC + bus
-DC breaker between charger 1 and battery at DC + bus

i have read breakers are to be rated in event of string failure and excessive charge / discharge currents. please advise accordingly.
i would also appreciate if appropriate wires gauges can be suggested.
many thanks.
I will take a stab at this... The numbers I will give reference US Code and practices (again, not a code person, always consult the codes for correct interpretations)... You will have to translate you your local materials and requirements.
lexington wrote: »
1. two charging sources: 25Adc (inbuilt inverter charger) & 50Adc (external charger)
I suggest that all of your major devices should have their own wiring that comes from a battery "bus bar/common connection". The usual technique is to have +/- wiring leave the bus bar common (with fuse/breaker in + lead from bus bar, close to bus bar). So, you take the above maximum continous rated current, multiply by 1.25 National Electric Code safety factor, then pick wiring/fuses/breaker that support that current.

If you have a PWM (less expensive) type solar charge controller, NEC requires a second 1.25 safety factor for variations in solar energy:

1.25 * 1.25 = 1.56 derating for PWM solar charge controllers/solar panel wiring
2. inverter/battery operating voltage :96Vdc

You have to look up the minimum battery voltage cutoff for your inverter--But typical value would be:
• 10.5 volts * 8 (12 volt to 96 volt battery bank conversion) = 83 volt estimated battery cutoff voltage
• 3,710 Watts * 1/0.85 inverter eff * 1/84 volts cutoff = 52 amps max continuous current
• 52 amps * 1.25 NEC derating = 65 Amp minimum Branch Circuit Rating for wiring/breakers/fuses
So, take the 65 amps, round up to next standard fuse/wiring size.

Note, with "low voltage" DC power systems, you also need to keep cable runs short, and check that you don't have too much voltage drop for the length of wire run. For a 96 volt system, I would suggest a maximum of ~4 VDC voltage drop for your wiring system.

For example, if you have 4 meters (one way wire run) or ~14 feet of 4 awg cable with 52 amps of current draw:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm (simple version of US wiring AWG vs Current vs Temperature ratings)
http://www.calculator.net/voltage-drop-calculator.html (simple US based NEC voltage drop calculator)
http://www.acbsphl.org/Tips_and_hints/ABYC_Wiring.htm (typical Marine wiring allowances--Much less conservative than NEC)

14 feet (one way run), 52 amps, 4 AWG cable:
Voltage drop: 0.36
Voltage drop percentage: 0.38%
Voltage at the end: 95.64

So, 4 AWG cable is less than the 4 volts maximum drop I would suggest.
4. 4nos parallel strings of 200AH/string, each requiring a breaker.( 4 parallel strings is not ideal, but i am trying to work with what i've got)

Since you are running at 96 volts, your 52 amps maximum current draw for the AC inverter is not too bad... I would suggest the minimum current rating for a single string of batteries be rated to >= 65 amps (your worst case load).

The typical maximum current draw (I would suggest) for a 200 AH Flooded Cell battery would be:
• 200 AH * 1/5 max discharge rate = 40 amps max continuous
So that is less than the 52 amps of your inverter--So the above 65 amp design limit would give you maximum current from your battery string safely (note, you should also calculate voltage drop for battery wiring too--It would add to the inverter voltage drop too).

It has been recommended by others that you should use 2/0 AWG minimum wire gauge for batteries (low voltage drop, ability to manage high surge current from battery bank)--But that is your choice. You can go with 2/0 or heavier cabling, and still stay with an >=65 amp breaker.

Batteries are very poor at sharing current--So while the average current is 52 amps / 4 = ~13 amps per battery string maximum, I would suggest that you rate one string capable of supplying your maximum load current--Just in case. No false tripping of fuses/breakers, no over heating of wiring if something "goes wrong" with a battery cable/connection.
5. cooling system in place where batteries are going to be kept on racks, not sealed enclosure.

A good engineering rule of thumb... For every 10C you keep the battery cooler, it will last about 10x as long. ~25 C is "standard temperature".

If you are in a hot climate, and the batteries run at 35C, they will age about 2x faster (1/2 battery aging life) vs 25 C.

That means the building should be well ventilated/shaded from hot sun. And if it cools at night, getting cool air ventilating the bank should not hurt (assuming humidity/bugs/salt fog on coast/etc. are not issues).

In hot climates in the US, placing batteries in cellar/underground/earth berm type building can be a big help. If you are on rock/have flooding/high water table/etc... You may be very hard pressed to keep the battery bank "cool".
6. maximum wire length 5m (+ve & -ve wires combined)

You have to find the voltage drop for each set of cable runs (inverter, chargers, etc.). Fuses/Breakers are there to protect the wiring from short circuits. The fuses are not there to protect the equipment (other than from over current). Each wire run should have its own fuse/breaker (or if small loads, one fuse for that group of wiring, but the fuse should be sized to a single wire run in case there is a short circuit).

Note that 96 VDC power systems are not very common. You may have difficulties finding circuit breakers/fuses that are rated for those voltages without having to special order from the US/Europe. Most DC protective devices are rated at 24 volts or so (automotive/truck/marine systems).
i understand there are to be breakers in the following places:
-DC breaker on each of 4 parallel string

Yes--A very good idea. But many people do not do that (large breakers/fuses are expensive, extra wiring/components a pain).

It does come down to a risk/reward comparisons. Is the risk/loss by fire/short circuit (rare) vs the relatively high cost of protection worth it to you and your family/work/etc.
-DC main battery disconnect/breaker before connection to DC + bus

If you have 4x circuit breakers (one on each string)--You really do not need a separate battery disconnect... But OK if you want.
-DC breaker between charger 1 and battery at DC + bus
-DC breaker between charger 2 and battery at DC + bus
-DC breaker between charger 1 and battery at DC + bus

-DC Breaker between AC inverter and DC + Bus
i have read breakers are to be rated in event of string failure and excessive charge / discharge currents. please advise accordingly.
i would also appreciate if appropriate wires gauges can be suggested.
many thanks.

Yes both Votlage (AC and DC voltage ratings are different, DC rating voltage is usually much lower than AC) and current (trip current). And AIC Ampere Interruption Capability.

Breaking capacity

A DC Lead Acid Battery bank can easily put hundreds to thousands of Amperes into a dead short. Looking for breaker/fuses with AIC ratings > 1,000 Amperes (or even 10,000 Amperes) is recommended (by me).

Our host has 150 VDC rated breakers--So that is a place to start your research.

http://www.solar-electric.com/installation-parts-and-equipment/midnite/cipr1/stfubr1.html

And type T fuses:

http://www.solar-electric.com/f-200t.html

Hope this helps.

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