# Is 4 12v 135ah fla deep cycle in parallel ok?

Registered Users Posts: 73 ✭✭
Is having my battery bank in parallel ok? I have 4 135 ah 12v FLA deepcycle batteries connected in parallel. Is there any risks? Should the be connected differently?

It is not "wrong" to parallel a bunch of batteries together into a larger AH rated battery bank... It, many times, is less than optimum.

First, you want to have "equal resistance" in the wiring to each battery so that they charge and discharge evenly (share the current). This website shows how to wire batteries to get even charging and discharging:

http://www.smartgauge.co.uk/batt_con.html

Next--Having a lot of batteries in parallel--you can have corrosion, loose terminals, bad wire connections, etc. that "drop" a battery out managing "its share" of current. When you get >3 parallel strings, it is can be difficult to find these issues (more things to check, more things that can go wrong).

Similar checking lots of caps for water/specific gravity/logging.

For example 4* 135 AH @ 12 volts (1s * 4p) vs 4 * 270 AH @ 6 volt batteries (2s * 2p)
• 4 x 6 cells per battery = 24 cells to check (1s x 4p) = 540 AH @ 12 volts
• 4 x 3 cells per battery = 12 cells to check (2s x 2p) = 540 AH @ 12 volts
Also--When you have a bunch of batteries in parallel--In theory, each battery needs its own fuse/circuit breaker to prevent a battery short circuit from being fed energy from the other batteries in the string (cooking battery, frying wiring). Not many people install fuse per battery string--And if you have a bunch of parallel batteries lots more money on fuses and such--Have seen 12 batteries in parallel before).

Lots of paralleling batteries--Each new battery added, adds little to the total battery energy storage (i.e., 10 to 11 batteries, only 10% more added storage--Even difficult to measure).

Also you have less copper interconnect cables (for every parallel added battery string, need two cables, + and -).

Debugging and monitoring series batteries is easier than parallel.

For example 1s x 4p battery bank (12 volt batteries in parallel for larger AH capacity). All batteries have the "same voltage" (you have to disconnect the batteries to measure the resting voltage of each battery). And it is not as easy to measure shared current per battery (have to measure current before and after battery and subtract to measure current per battery).

With 4x 6 volt batteries (2s *2p) for 12 volt bank... You can measure the voltage of each battery resting/under load/under charge--A quick of each battery under various conditions--The weak or "problem" battery sticks out from the rest.

And with series connections--There is a single wire between series batteries--You can use a DC Current Clamp Meter to measure the current directly in that string. Just easier and faster.

There is an interesting issue with failure mode with lots of parallel connections. One would think that a "failing" or going high resistance connection can overheat and fail worse. I saw this with parallel wiring for computers--A bunch of + and - wires carrying current from power supply to a circuit card... The connections with "unzip". With one connection at a time failing (overheating).

It turns out in parallel power connections, it is the "lower resistance connection" (the "good connection" the can overheat and fail. Lower resistance means that battery passes more current, and since resistance heating Power=I^2*R ... Double the current, get 4x more heating.

More connections, more chances of things going wrong (typically high resistance connections simply reduce battery bank overall capacity--Those batteries simply supply/absorb less current than the ones with the "good connections").

I have a personal rule where if there are more than 3 parallel strings required--I suggest looking for another solution. A larger AH battery. Or lower voltage battery (i.e., 12 volt @ 100 AH stores the same amount of energy as a 6 volt @ 200 AH battery--same size and weight). But now I can use 2x 6 volt batteries in series for a 12 volt 200 AH battery bank with no parallel connections, just one series connection.

Anyway--My personal thoughts on the issue... If you have good wiring and good batteries in parallel (keep up on maintenance, monitoring electrolyte levels and SG readings)--Either bank setup will give similar life.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 5,995 ✭✭✭✭✭
I would generally disagree with Bill on this subject. While you can often get similar life, you will equally have poorer life expected.

Very rare to get perfectly even resistance across several strings of batteries. Even when you've measured and sized the wiring perfectly, a loose connection and the resistance isn't balance correctly.

...add that a single cell failing or having any issue will have the other batteries trying to keep it equally charged. So often a system with multiple strings will work fine for a couple years as minimal out of balance issues go undiscovered. These little out of balance will grow over time. The failure happens later in the life of the battery bank as one string brings down the others. I would expect a shorter battery life in any system with multiple strings.

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.
• Solar Expert Posts: 1,095 ✭✭✭✭
edited March 2022 #4

I am no expert on this subject, but my own observations over the years have shown me that:

Battery banks with multiple parallel batteries can live a long life if they spend a lot of time in float in order to balance with each other.

Battery banks with multiple parallel batteries that have a deep daily DOD and spend very little time in float will die young with big imbalances between strings.

Battery banks with multiple parallel batteries that get cycled deeply, but also spend a lot of time in float – will be fine.

Variations in the internal impedance of individual "identical" batteries from the same production run can be 10% to 30%. This is normal and within manufacturers' tolerances for QC.

Properly designed off-grid systems present a low load average on the battery bank compared to its size. By definition, a low daily DOD equals a low average load.

Cables and connectors are designed to have low resistance at peak loads. At the low average running load of a typical off-grid system, the variations in resistance between cables and tight connections are often far less than the variations in the batteries themselves.

I always have more questions than answers. That's the nature of life.
• Solar Expert Posts: 1,966 ✭✭✭✭✭

I am no expert on this subject, but my own observations over the years have shown me that:

Marc, You're so modest. I wish I knew half of what you may forget in all your years of battery experience.
You are one of the handful of posters here who's comments I try and absorb because of your credibility..

2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

• Solar Expert Posts: 1,095 ✭✭✭✭

I am no expert on this subject, but my own observations over the years have shown me that:

Marc, You're so modest. I wish I knew half of what you may forget in all your years of battery experience.
You are one of the handful of posters here who's comments I try and absorb because of your credibility..

Always remember that my advice is worth exactly what you paid for it!
I always have more questions than answers. That's the nature of life.
Send Marc money. Then the same advice will be worth more.

-Bill

Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 233 ✭✭✭
Perhaps a better way to answer this question is to ask why you need this level of power at just 12V.  Once you start having to make as many a four parallel batteries to get the power you need, wouldn't it be better to upgrade to a higher voltage.  Four batteries in series to make a 48V system would provide exactly the same power level without the problem of balancing batteries.

You appear to be what other solar mavens call "stuck in a 12V box".  Once you start thinking outside the box, your options are greatly increased.  I started out at my own cabin with a 12V system, and I very quickly saw it's shortcomings.  I've since upgraded to 48V and I couldn't be happier!
System 1) 15 Renogy 300w + 4 250W Astronergy panels,  Midnight 200 CC, 8 Trojan L16 bat., Schneider XW6848 NA inverter, AC-Delco 6000w gen.
System 2) 8 YingLi 250W panels, Midnight 200CC, three 8V Rolls batteries, Schneider Conext 4024 inverter (workshop)
• Registered Users Posts: 73 ✭✭
edited July 2022 #9
But what about all the devices and light and everything that's 12 v. Would I have to get all new equipment if I upgraded to a 24 or 48 volt system?

The other thing is was wondering is if you have multiple batteries connected in parallel does it become possibly dangerous because of the high amps? I don't want to get electrocuted.
• Registered Users Posts: 73 ✭✭
I changed to three durecell 31 series marine deep cycle. Going to add a fourth once I get more solar panels. Connections look good?
You have the +/- bus connections on the battery "Kitty Corner" to each other--So that is fine.

You have some smaller AWG cables leaving the battery bus... You might wish to put a fuse/circuit breaker on the positive lead leaving the battery to prevent a short circuit somewhere turning the wiring red hot and possibly starting a fire. Typically one fuse/breaker per cable leaving the battery (assuming for example, 2x 14 AWG cables and 15 amp circuit protection per cable).

Another suggestion--Wrapping (or heat shrink) black for the negative wiring and red for the positive wiring (don't want to connect your DC wiring backwards).

And lastly, route the cables so you can get to all of the cell caps so you can check electrolyte levels and specific gravity for all cells.

Longer term... I like to use 2x 6 volt batteries in series then parallel into a 12 volt bank. I suggest this because I can now measure the voltage of each 6 volt battery (checking for weak cells/batteries). When you have 12 volt batteries in parallel--You have to disconnect the batteries so you can check the voltage of each battery.

Also, if you have 4x 100 AH 12 volt batteries in parallel, you have 24 cells to check. You can use 4x 6 volt @ 200 AH batteries (same physical size battery bank) for 2 parallel strings and 12 cells to check--A bit less work.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 73 ✭✭
Still one more question. Is there an electrocution risk with this many batteries in parallel. And I do have a fuse on the wire going into the camper. Can't see it in the picture.
• Registered Users Posts: 73 ✭✭
Another guy told me these were maintenance free batteries. True?
• Solar Expert Posts: 5,995 ✭✭✭✭✭
bgu1982 said:
Another guy told me these were maintenance free batteries. True?
I'm 99.9% sure they are lead calcium batteries, because of their designation of 'marine' and the recessed battery caps. They are liquid electrolyte, worth checking from time to time. Of course only add distilled water.

'Real' deep cycle batteries are lead antimony and will require regular maintenance in exchange for long lives. They will have raise battery caps.

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.