What limits a 12V parallel bank in size?

Liam M

Solar Enthusiast
I've got a 4P 12V flooded lead acid bank 460AH.
In researching how many can be paralleled, I've seen "no more than 3", but "4 is ok".

My system is 12V now, but eventually plan to work up the voltage ladder.
I'd like to accumulate, and increase the capacity of my 12V parallel bank now, and later step up to 24V in series/parallel.

In doing this, what are the numeric limits to a 12v parallel bank, and why?

I understand the length of cabling issues, but what else?
 

gnubie

Photon Sorcerer
Practicality in terms of space, charging current, money etc. If done properly you have fuses between each battery and the common wiring, lots of issues but none unsurmountable. I've seen some rather large lead acid batteries* in the industrial world that make limits of '3' or '4' look silly.

*a battery can be a battery of batteries, each of which may be individual cells in series
 

gnubie

Photon Sorcerer
Google image search telephone exchange battery, datacentre battery that sort of thing for a start but there's quite a bit out there once you go down the rabbit hole.

There are issues to consider but that you have flooded batteries to start with a lot of them go away but others appear. You don't want to put 10 flooded cell batteries in an enclosed unventilated space because of the real risk of explosion for example. You also need to pay attention to how they are wired up to ensure each battery gets the same load placed on it for longevity etc.

Amusingly a lot of the people that made all sorts of dire predictions about lead acid in parallel in the past have stopped making them about lead acid and are now waving the red flag for lithium batteries.

Re-reading your first post you are going to add more batteries to the existing set. This may cause technical issues for you but maybe not practical ones. Effectively the new batteries will have a higher amp hour rating than the existing ones. If it were possible to parallel them then place a good load on the batteries, quickly remove the load, break the parallel wiring and monitor the voltage on all the batteries you'd see them rising up to resting voltage at different rates. If instead you put a amp meter on each batteries + wire you'd see some current flowing from the high capacity battery to the lower ones until they come back into charge equilibrium. Effectively the newer batteries will cop a bit more cycling until they fall in line with the cycle aging of the existing batteries. Since they aren't diabolically out of step there won't be high current flows so its not a problem but something to be aware of. This assumes your existing batteries have been correctly equalise charged periodically.
 
Last edited:

RCinFLA

Photon Sorcerer
Primary issue is balancing of current for charging and discharging. Batteries, connectors, and cabling all contribute to mismatching.

It is very likely you will have different currents though each of the parallel batteries, 20% to 50% difference.

If you are doing it for increased capacity you can live with some mismatch. If doing it to run a higher power inverter the risk is overdrawing current on a single battery to point of damaging it. Also have to be careful of charging current as one battery may hog more current charging at greater then 15-20% of its C rating in amps.

Then there is the ramifications of a single battery developing a defect like a severe shunt leakage cell. That battery, in the parallel group, will get hot. At the least it can spit out electrolyte. At the worse it can explode the case.

Safest way to put more in parallel is to put current shunt monitors on each parallel leg and monitor current distribution between batteries.
 

Zil

Solar Addict
I built large paralleled LA batteries. Must use exactly the same length positive cables to a buss bar. Then the same with negative to neg buss bar. The negative do not have to be same length as positive but each negative must be same as other negatives. Same as with positives. Using six volt battery, each interconnecting cable must be the same as the other interconnecting cables. This gives the best balance between each battery.
 

Liam M

Solar Enthusiast
I built large paralleled LA batteries. Must use exactly the same length positive cables to a buss bar. Then the same with negative to neg buss bar. The negative do not have to be same length as positive but each negative must be same as other negatives. Same as with positives. Using six volt battery, each interconnecting cable must be the same as the other interconnecting cables. This gives the best balance between each battery.
That is what I was thinking was the only way to do it right.
I'm thinking too, the bus bar ratings required would be solely dependent on charger/inverter, and not the size of battery bank, correct?
 

Dzl

Perpetual Newbie & Unofficial Forum Librarian
Staff member
Moderator
Edit: oops, didn't read the post clearly enough or realize this was the lead acid subforum. My B

Here is 10P8S using pouch cells:
https://01319721522019860390.googlegroups.com/attach/a948f37c1273/P1220395.JPG?part=0.4&view=1&vt=ANaJVrG4bEljpwCyWWkJvVsrASQSRFlIePJunZCkHj2rBcZj-Xx0EeCNnG4n6geJPTp8jw9kMcwKGFZlNGz_Ory4pe3iJ8-p1r-D9KsYeyH68qUdZfjmy88
 
Last edited:

chrisski

Photon Sorcerer
This page has some math and diagrams for four batteries, but mentions at the end the wiring method 3 of 4 should be good to 8 in parallel.


I'm finding it very hard to not parallel batteries. I looked at two volt and four volt high Amp Hour Batteries in series, but them it became either a space issue in my RV where the batteries were going, or devising a way to vent the batteries to the outside in the enclosed storage based off the dimensions of the batteries.

I decided on two 1" taller than normal golf cart 6V batteries for 2S2P, but with my first build, getting a vented battery box for that is extremely difficult, and I should be finishing that up tomorrow.

This build would have been much easier and complete weeks earlier had I only put 4 standard 12 Volt 100 AH batteries in series.
 

Zil

Solar Addict
Wires and buss bars should be sized to the amount of amperes expected. I use a large safety margin.
As deep cycling banks, four 6 volt golf cart battery is much better than two 12 v battery.
 

chrisski

Photon Sorcerer
Wires and buss bars should be sized to the amount of amperes expected. I use a large safety margin.

I installed my busbars today and for future for expansion and upgrades went with an 8 post busbar for both positive and negative, and I have plans to use up to 7 of the posts used by the end of next year. I got these busbars to twice the amperage I thought I'd use. Between the buss bar and battery, battery cable is fused to 1/3 the busbar rating. Plenty of safety margin there.

Actually, plenty of overkill. After installing, each of the two busbars was almost 12" thick and 2" wide, I realized I probably had overkill and I did not need so many 3/8 posts and could have went with a mixed busbar. Also read in the tech data that I could have connected up to four lugs to each terminal, but I think so mony lugs on one post would have been quite difficult and seems to me elsewhere I read not to connect more than one terminal to a lug. Also, one of the upgrades I plan on doing is going from 24 VDC to 12 VDC, so the amps will be cut in half for the same load at twice the volts.
 

eXodus

Solar Addict
usually when you need that much capacity in lead acid, people just get bigger single batteries.

You have 4x batteries for 460Ah, 115ah each
I got two 12V for about the same capacity. (450ah 2x 225ah @ 12v are actually 4x 225 6V Golfcarts)
 

Liam M

Solar Enthusiast
I installed my busbars today and for future for expansion and upgrades went with an 8 post busbar for both positive and negative, and I have plans to use up to 7 of the posts used by the end of next year. I got these busbars to twice the amperage I thought I'd use. Between the buss bar and battery, battery cable is fused to 1/3 the busbar rating. Plenty of safety margin there.

Actually, plenty of overkill. After installing, each of the two busbars was almost 12" thick and 2" wide, I realized I probably had overkill and I did not need so many 3/8 posts and could have went with a mixed busbar. Also read in the tech data that I could have connected up to four lugs to each terminal, but I think so mony lugs on one post would have been quite difficult and seems to me elsewhere I read not to connect more than one terminal to a lug. Also, one of the upgrades I plan on doing is going from 24 VDC to 12 VDC, so the amps will be cut in half for the same load at twice the volts.
Have a picture of those busbars by chance?
 

chrisski

Photon Sorcerer
It's a Blue Sea Bussbar. I put two of them on a 2' wide board and they take up almost the entire board.
 

Attachments

  • Buss Bar.jpg
    Buss Bar.jpg
    29.8 KB · Views: 21

Zil

Solar Addict
Depending on the lugs used. I use FTZ heavy duty battery lugs on 2/0 awg. I am comfortable with two lugs on a 3/8 buss bar terminal. I guess starter lugs could fit three.
 

Liam M

Solar Enthusiast
It's a Blue Sea Bussbar. I put two of them on a 2' wide board and they take up almost the entire board.
Theoretically, should the charging/loads connection point on the busbars be on a center terminal, or would that even matter?
Thanks for the link, I've seen 4 post busbars but not these 8 posts at high amp rating.
 

chrisski

Photon Sorcerer
Theoretically, should the charging/loads connection point on the busbars be on a center terminal, or would that even matter?
Thanks for the link, I've seen 4 post busbars but not these 8 posts at high amp rating.

My first system. Way I'm wiring because of space, the higher amp loads will get routed so they are closest to the device, and not the center of the bus bar.

This may save 6" and the highest that should be flowing through the inverter or battery is watever it takes to blow a 200 amp fuse, which I expect is 1300 watts at 12 Volts.
 

GXMnow

Photon Sorcerer
There is a big difference between paralleling batteries for "more current" or "more run time".

200 amps at 12 volt will support about 2,000 watt. If 2,000 watts is all you need for peak power, then a 12 volt bank will work. Each 100 amp hour battery can give you that 2,000 watts for just 30 minutes. If you need 2,000 watts all day, 24 hours, then you need 48 Kilowatt Hours of battery. That would be 4,000 amp hours. There is no problem with paralleling batteries to get a long run time, but if you need more current than a single battery can support, then you need to be very careful to ensure the batteries are sharing the current well. Equal length of identical cable from each battery to the buss bar is a good start. Each battery should be fused within it's safe limits. And the wire needs to be sized to handle more than the fuse. The main fuse after all the batteries are combined needs to be less than the buss bar rating. And the wire after the fuse must be able to handle that fuse current.

I am very conservative. My rule of thumb is to only expect 50% more current each time you double the batteries. That way you should be very safe. For example, if each battery can take 50 amps, and you parallel 2, you can expect to safely get 75 amps. Parallel 4 and you can pull 112.5 amps. Double it again to 8 and you have 168.75 amps. As I said, this is VERY conservative and you can certainly pull more current, but if you design around this, you should never have a problem with current sharing. You should still do you best to use equal lengths and feed the groups from opposite ends. Balance the current the best you can, and everything runs cool and you never have a problem. This should also be a low enough current that the separate parallel packs should be able to easily self balance between them through the charge and discharge cycles. If you are running more current, any mismatch between the banks will cause the state of charge between the banks to differ more and more. The less current you have on each bank, the better the BMS will be at balancing the cells as well.

If you do have 2 battery feeds, it does make sense to put them on each end of the buss bar and put the loads in the middle. This way the buss bar material is only taking half of the current. It is like putting 2 buss bars in parallel. I did this on my negative bar between my batteries and the BMS input. It is a 4 position bar, so the battery strings go to posts 1 and 4, and the two cables to the BMS go on posts 2 and 3.
 

chrisski

Photon Sorcerer
200 amps at 12 volt will support about 2,000 watt. If 2,000 watts is all you need for peak power, then a 12 volt bank will work. Each 100 amp hour battery can give you that 2,000 watts for just 30 minutes.

The comment I have on that, is for lead acid batteries on a 12 volt system, my inverter low voltage cutoff is 10.5, at an inverter efficiency of 85 to 95%, the amps will be higher than 200 amps. If the voltages stay around 12, it’ll be less than 200.

I am wondering about this type of stuff as I wire my 12 volt 2000 watt inverter with 2/0 wire on a 440 ah lead acid battery bank. I’m wondering if the microwave gets used, how low my voltage will dip I will see.

I have it fused to 200 amps, and plan on not taking it past 1300 watts.
 

GXMnow

Photon Sorcerer
How long is your run of 2/0 wire between the battery bank and the inverter?

2/0 is pretty safe for the current, as in, it won't burn up, but if you run 200 amps a lot, a little heavier wire might be in order, especially if the length is over 10 feet. 12 volts is not forgiving on voltage drop at all when it comes to length though.
 
Top