# paralleling 3 lead acid batteries

#### tracecom

Joined Apr 16, 2010
3,944
I have three 12V deep cycle/marine 24 size batteries; they have never been used in parallel. All have been recently charged and allowed 48 hours (or more) for the surface charge to dissipate.

With no load, A is at 12.87V, B is at 12.95V, and C is at 12.53V. I ordered and received the necessary 2/0 cables to place them in parallel, but I don't know if I should just connect them together, or is there something more complicated required?

Thanks.

#### wayneh

Joined Sep 9, 2010
16,124
I'd probably just do it. If I was really worried about it, I'd connect them first through a headlight. This would allow some current to flow to "equalize" them. But again, I don't think I'd bother. The small voltage difference combined with at least some internal resistance will limit any current to a non-damaging level.

#### AlbertHall

Joined Jun 4, 2014
8,756
You should be able to parallel them for both charging or discharging.

#### #12

Joined Nov 30, 2010
18,167
I've seen some serious raving on here about connecting 2 batteries in parallel, but I never bought it. Considering you have to be at least a volt above battery voltage to do any significant charging, what's half a volt difference going to melt? This is your chance to do the measurements!

#### GopherT

Joined Nov 23, 2012
8,012
Since they are all the same chemistry (lead acid), and all are rechargeable, and within a few tents of a volt, I'd do it.

Also, yesterday, connected a perfectly good battery in a running vehicle that is getting a change at 14.5 V to a battery in sad shape at about 9 to 10 volts and nothing exploded when car that sad battery was connected to was jump started.

#### Dwarvensilver

Joined Sep 9, 2010
3
Not enough voltage difference to make a difference, so to speak, I'd just hook them up. If batteries needed to be exact voltage to hook them up in parallel we would'nt have any electric cars at the minimum. Even dry cells vary in voltage and they are connected in multiple parallel/series configurations. Think about solar energy battery banks.
Cheers
Dwarvensilver

#### crutschow

Joined Mar 14, 2008
23,741
One possible problem with paralleling batteries is if one of them develops a shorted cell.
Then the other two could rapidly discharge through the one with the shorted cell.
Not sure if that would create enough heat to be of concern.

#### nsaspook

Joined Aug 27, 2009
6,580
Run the power cables from the batteries on opposite ends of the stack to equalize wire resistance. You might be surprised how much even a tiny imbalance can effect low resistance charge/discharge paths.

Over time the pack will quickly get unbalanced if wired incorrectly.
http://www.smartgauge.co.uk/batt_con.html

#### tracecom

Joined Apr 16, 2010
3,944
Okay, all hooked up. No smoke, no sparks, no smell, no spewing, no splosion. Bank measures 12.75V.

How long do we expect it to take for the batteries to equalize? Do I need to wait before I hook up charging system?

#### nerdegutta

Joined Dec 15, 2009
2,675
At work, we use a lot of lead batteries. 12v 33Ah and 12v 5Ah. When they start to get old, I measure a few and find the ones with the most equal voltage. I connect them in parallel and put them on an old charger. I do not mix 33Ah and 5Ah, and I don't wait for anything... A few days later, the batteries hold the charge at bit longer.

#### nsaspook

Joined Aug 27, 2009
6,580
Okay, all hooked up. No smoke, no sparks, no smell, no spewing, no splosion. Bank measures 12.75V.

How long do we expect it to take for the batteries to equalize? Do I need to wait before I hook up charging system?
Just run a quick full power load test to be sure all the connections are good and nothing gets hot. With FLA parallel banks it's important to run an equalization charge after many full discharge cycles to get a level recharge on all the cells because with so many it's a pretty sure thing some are weaker than others. I normally run four 6 Volt volt 204AH batteries in two 12 Volt strings using Costco CG2 (traction) flooded types for my emergency power backup. The old set lasted about 5 years with pretty heavy summer abuse running a small AC unit.

http://batteryuniversity.com/learn/article/equalizing_charge

#### tcmtech

Joined Nov 4, 2013
2,868
One possible problem with paralleling batteries is if one of them develops a shorted cell.
Then the other two could rapidly discharge through the one with the shorted cell.
Not sure if that would create enough heat to be of concern.
Hasn't been a major problem in the billions of other applications in the world that run parallel battery systems so far. It just cuts the life of the good batteries down if not caught soon enough.

#### crutschow

Joined Mar 14, 2008
23,741
Hasn't been a major problem in the billions of other applications in the world
Are you sure it isn't trillians?

#### tcmtech

Joined Nov 4, 2013
2,868
Are you sure it isn't trillians?
Well since you have doubts what is your estimate on how many machines and backup power systems plus whatever other sort of device there may be in that has ever been made that runs with 2 or more parallel batteries?

The lead acid battery alone has been around over 150 years so by that I say Billions of them have been used in parallel battery applications by now.

#### crutschow

Joined Mar 14, 2008
23,741
The lead acid battery alone has been around over 150 years so by that I say Billions of them have been used in parallel battery applications by now
Well, most of the lead acid battery applications (in vehicles) I'm aware of use series cells, not parallel.
I didn't think there were that many applications that used parallel cells.

#### nsaspook

Joined Aug 27, 2009
6,580
The disadvantages of premature battery aging and load sharing with many parallel increases with the type of application. For systems where the battery packs stay at a near fully charged state like a backup UPS that's rarely used or steady low rate applications on a massive redundant bank like in telecom battery systems the large parallel string operation usually works fine. In applications where there are frequent, high rate discharges and likely operation at less than 80% SOC like in an EV or a daily Solar recharged energy bank the slight differences in cell SOC, internal and connection resistance quickly cause the strings to unbalance and to stress cells unevenly.

#### tcmtech

Joined Nov 4, 2013
2,868
Well, most of the lead acid battery applications (in vehicles) I'm aware of use series cells, not parallel.
I didn't think there were that many applications that used parallel cells.
So there's no large diesel vehicles or heavy commercial/industrial vehicles and larger equipment in your world?

What about large battery based backup power supplies or electrically powered vehicles or other mobile electrically powered machines? None of those either?

#### shortbus

Joined Sep 30, 2009
7,230
@crutschow and tcmtech, may be wrong but ,think the two of you are talking different animals here. Cruts - - cells, all lead acid batteries I'm aware of are made up of series CELL's in one case making up a battery. tcm - - aren't you talking parallel BATTERIES of series cells?

#### nsaspook

Joined Aug 27, 2009
6,580
@crutschow and tcmtech, may be wrong but ,think the two of you are talking different animals here. Cruts - - cells, all lead acid batteries I'm aware of are made up of series CELL's in one case making up a battery. tcm - - aren't you talking parallel BATTERIES of series cells?