Battery Connector gauge help

rdm3

I am connecting 4 12v 225ah flooded batteries in parallel and need to know the safest gauge connectors to use.  You advice is appreciated.

Estragon

It depends on expected charge & discharge rate (ie bigger inverter = bigger wire)

In parallel, you should fuse each individually for the wire size chosen.

MichaelK

Four 12V batteries in parallel is a poor way to design a medium capacity system.  The parallel connections will be very hard to equalize in terms of battery resistance, and quick battery death will be the result.  I've seen others here quote as quickly as 6 months.  You will be much better off designing a system using either 24 or 48 volts and there are many manufacturer's offering high quality equipment running at those voltages.  Because you're only working with 12V, every load you have will have very high amperage, so you're likely to need the heaviest wire gauge normally available, 0000 (4/0).  Use this chart...https://xtronics.com/wiki/Wire-Gauge_Ampacity.html.

Estragon

12v loads aren't necessarily high current.  My 12v bank mainly runs a very efficient DC reefer and LED lights.  They're low current but run for a long time, and bank is sized accordingly.

My wire is heavy anyway though, as charging current could potentially be ~90a.

Don't know OPs application.  For all I know he has one of those crazy 5kw+ 12v inverters hooked up to the bank.

BB.

In summary, need to handle the Maximum expected/needed Inverter loads and Charging currents you expect (i.e., a 5000 Watt inverter and 10 amps continuous load--Does not "have to be" the same as maximum inverter draw).

If you have heavy interconnect cables, you "can" avoid per battery series fuse--The cables take the batteries "flat" before they overheat (sort of like how car battery systems are designed to starter relay).

If you have smaller AWG cables, each should be fused/breakered to the maximum "safe current" you want to allow (different specs, different allowed currents for NEC, Marine, what loads, etc.).

In theory, each battery string should carry 1/N current of loads (i.e, 4 batteries and 100 amp load, then 25 amps "typical"). Personally, I would go with an expected load of 1/2 (i.e., 100 amp load on 4 strings, then 50 amp "worst case sharing loads" per string).

It is purely a personal suggestion, but given that you can have cell(s)/connection(s) fail "silently", it is very possible to see "higher than nominal" shared current (I have seen this multiple times, failing wires/connectors on large computer systems).

The basic problem is not (usually) the "bad connections", it is the "lowest" resistance current path that lets more current through... And Power=Current^2*Resistance... 2x more current, 4x more heating.

-Bill

Marc Kurth

MichaelK said:

Four 12V batteries in parallel is a poor way to design a medium capacity system.  The parallel connections will be very hard to equalize in terms of battery resistance, and quick battery death will be the result.  I've seen others here quote as quickly as 6 months.  ....................................................

I will respectfully disagree with this, as presented. Yes, I prefer series strings for many applications but parallel connections do not always mean quick battery death. I can show you a whole lot of projects and systems running well for many, many years with multiple parallel connections.

The devil is in the details like charge/discharge current, cycle depth, cycle frequency and float time, average battery temperature, etc. - so sweeping generalizations don't always work well.

Marc

rdm3

So much help... still a bit confused.  This is a very low use situation... off grid cabin used about 1week /mo.  Max use is 12v on demand water pump for dishes, a modem/router, 2-3 9w led bulbs, and a laptop charger.  I want the extra amp hours (parallel configuration) for periods of cloudy days... not usual - it's the desert.  For safety I feel that I should go with 4/0  18" battery connectors, a new 60amp mppt controller and a 2500 watt pure sine wave inverter.  The panels are 2 lg mono 360 watt panels.  Again the batteries are 12 v 125ah flooded.  To the point of equalization I am planning to link batteries using 'balanced draw' configuration.  So... is this good thinking and is it safe (fire)?

BB.

My personal suggestion is to use 2x 6 volt batteries in series, then xN parallel strings for a 12 volt bank.

I suggest using a voltmeter across each 6 volt battery is a very quick/easy method to confirm the batteries are working correctly. If you see one (or several) 6 volt battery with high or low voltage compared with rest in the string, then you have an indication that something may not be right.

With 12 volt batteries, you cannot measure their voltage (resting/under load/under charge/etc.) with just 4x 12 volt batteries in parallel. You are left with measuring Specific Gravity (if not sealed/agm) or current during charge/discharge cycles with a DC Current Clamp Meter (if you have one--Don't get me wrong, I think it is a great tool with DMM to check your system's health, but this is your money/time/etc., not mine).

The other thing is a 12 volt @ 225 AH battery weighs the same as a 6 volt @ ~450 AH battery... And you only need 2x parallel strings of the larger 6 volt cells, vs 4x parallel strings for the 12 volt batteries... And 1/2 the number of cells to check/water (12 cells for 6 volt x 2 parallel strings, vs 24 cells for 4x 12 volt parallel strings of the "same AH capacity").

Also, I see that you have a difference between your first and last posts... Are these 12 volt @ 225 AH or 12 volt @ 125 AH batteries?

If the 12 volt @ 125 AH batteries, I would really look at 6 volt @ ~250 AH batteries (~200-250 AH typical for "golf cart" batteries). A 2x 6 volt @ 250 AH string by 2x parallel strings for 12 volts @ ~500 AH bank... Still 4 batteries at roughly the same weight/stored energy... But 2sx2p with the 6 volt vs 4p with the 12 volt.

Manytimes, the 6 volt golf cart batteries can be found easier/cheaper. The below are guesses, but "good enough" to get a decent estimate of your system usage.

To estimate your current under load:

• 9 amps x 12 volts (RV water pump) = 108 Watts
• 3 LED * 9 Watts (LEDs) = 27 Watts
• Wired standard Modem/Router = 24 Watts (if you use a cell phone and WIFI, you can get down to 5-10 Watts pretty easily)
• 60 Watt laptop computer charger = 60 Watts
• 195 Watts estimated max continuous load total
• 219 Watts / 12 volts nominal = 18.25 Amps

The estimated average "best case" charging current:

• 2x 360 Watt panels * 0.77 panel+controller losses * 1/13 volts nominal low battery bank charging voltage = 43 amps
• Using 60 Amp MPPT solar charge controller = 60 amps physical system maximum (MPPT limited, safely, normal operation)

Assuming nothing else like a large AC inverter, generator charger, etc, then your "worst case" nominal loading it around 43 Amps during charging. I like to derate wiring and fusing for battery system from NEC by 0.80 or (1/0.80) 1.25 times:

• 43 amps * 1.25 = 54 Amp branch circuit wiring/fusing
• 60 amp * 1.25 = 75 Amps for fully paneled MPPT charge controller

If you choose 54 or 75 amp circuit max current--Your choice. Either is a pretty conservative number for your system.

Sort of depends if you pick 4x 12 volt batteries in parallel or 2p*2s  for 6 volt batteries (2 parallel strings).

For 4x parallel strings, I would size the battery wiring to carry 1/2 of your typical maximum current (in theory, each string should carry 1/4 the current, but I would use 1/2 to assume that one or two strings fail)

For 2x parallel strings, assuming one fails, then each string should carry 100% of the rated system current.

4/0 cable is good for roughly:
https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

195-260 Amps NEC "Nominal" (lots of other factors in full NEC tables), but obviously way larger cable than you truly need...

If you assume 75 amps (round up to 80 amps), then 4 or 2 AWG cable is more than good enough.

If you choose to put breakers/fuses on each battery string--Your choice. Blue Sea makes some nice fuse holders that work well with battery string fusing:
https://www.bluesea.com/products/2151/Dual_MRBF_Terminal_Fuse_Block_-_30_to_300A
https://www.bluesea.com/products/5191/MRBF_Terminal_Fuse_Block_-_30_to_300A
https://www.bluesea.com/search/results/mrbf (fuses)

Note that the fuses are not included, and are not cheap... Using Circuit breaker(s) both for fusing and as an on/off switch is not a bad way to go.

I will stop here... Questions, corrections, confusion?

-Bill

BB.

One thing I forgot to add to the overly long post above... Voltage Drop. 12 volt battery systems frequently need "heavier cables" to ensure that you do not have too much voltage drop.

For example, say you run the battery bank down to 11.5 volts (under load). And the AC inverter cutoff voltage is 10.5 Volts. That gives you ~1.0 volt maximum wiring (fuse/breaker/connection/etc.) drop too. For inverters, they can surge 2x rated power, so I use 0.5 volt drop maximum for those circuits to support starting loads (like refrigerator compressors).

So, for a 12 volt bank, 4 AWG wiring, running 43 amps, 8 feet one way run (from source to load), using a simple voltage drop calculator we get:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.8152&voltage=11.5&phase=dc&noofconductor=1&distance=8&distanceunit=feet&amperes=43&x=55&y=11
Voltage drop: 0.17
Voltage drop percentage: 1.49%
Voltage at the end: 11.33

We get ~0.2 volts drop... Which is fine.

Now, for the solar charge controller, generally at 12 volts, we want to see 0.05 to 0.10 volts maximum drop (controller measures accurate voltage under charge). Say 60 amps, 4 AWG, 3 feet from controller to battery bank (including battery bank series wiring):
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.8152&voltage=12&phase=dc&noofconductor=1&distance=3&distanceunit=feet&amperes=60&x=65&y=19
Voltage drop: 0.089
Voltage drop percentage: 0.75%
Voltage at the end: 11.911

We get ~0.09 volt drop at maximum charging current (if you add another panel).

You can see that at these low battery bank voltages, and under heavy current, it does take heavy copper wiring, kept as short as practical to keep the voltage drop under control (and not spend too much money on copper wiring).

-Bill

rdm3

Thank you Bill... this is very helpful.  I've already got the 4 12v 225 ah flooded batteries so the investment has already been made.  Based on your calculations... it seems worth it to me to go with the heavy 4/0 connector wire for both safety and efficiency - correct?  What do you know about the 'balanced draw' parallel connection method?  Is it helpful or necessary?

Estragon

Info on wiring for balance:
http://www.smartgauge.co.uk/batt_con.html

IMHO, method 3 would work well and be easy to fuse.

Unless you have AC load(s) requiring a 2500w inverter, you may want to consider something smaller.  Large inverters tend to use a lot of power just being on powering very small loads.  Running a couple of AC led lights on a big inverter might actually use nearly as much a full size fridge, for example.  A nice smaller inverter that should run the loads you listed fairly efficiently:
https://www.solar-electric.com/morningstar-si-300-115v-ul-inverter.html

Many "wall wart" type loads are actually DC loads that can be run with 12v adapters/converters much more efficiently than with AC wall warts or transformer bricks on a big inverter.  It does depend on the load though.

BB.

4/0 is not going to hurt anything... So go ahead and use it.

Having fuses/breakers on each circuit that leaves the battery bus is important too. One for the smaller DC loads, another for the AC inverter, and another yet for the solar charge controller.

Regarding balanced current for the bank, Estragon has that with the Smartgauge link.

And the AC inverter--For a 12 volt bank, a 1,200 to 1,800 Watt AC inverter is about the maximum practical to support (high current, voltage drop issues etc.):

• 2,500 Watt inverter * 1/0.85 AC inverter eff * 1/10.5 volts battery cut off = 280 Amps @ 12 volt input
• 280 amps * 1.25 NEC current derating = 350 Amp rated branch circuit wiring/breaker/fuse

For a small cabin, generally suggest something like 300 Watt AC inverter (just enough to run the smaller loads). And use an AC generator for power tools/etc. when needed. Use propane/wood/etc. for heating/cooking/hot water/etc.

I had missed the 2,500 Watt inverter--As Estragon says, the larger AC inverter uses more energy (Tare losses) just being "turned on". A large inverter can use 20-40+ Watts, a 300 Watt inverter can use just 6 Watts.

If you really need the larger inverter at times... Wiring two inverters up, one for smaller loads (lighting, cell phone charging, laptop charging, etc.) and the second large one only turned on when needed.

And for a 2,500 Watt inverter, 4/0 cable is even on the small side. Sorry I missed it. That inverter is a game changer. And if the inverter is used, your present solar panels / battery bank would need to be much larger / and a higher voltage battery bank (24 or 48 volt) to supply those larger AC loads.

-Bill

rdm3

Thanks Estragon!  I've got a 1500 watt pure sine wave now but was going to upgrade... now I will save the $ and continue to use it.