Connecting 2 12v batteries in parallel

[luvini]

I have a project which requires a lot of battery capacity and must operate at 12v

we have decided to use Aliant YLP24 batteries, which are lithium (LiFePO4) batteries for motorcicles and have 24Ah capacity

however, we need to connect them in parallel to get to 48Ah capacity

is it safe? do i need balancing/equalizing? if so, how?

[Psi]

Yes, this is fine.
Cell balancing is only an issue if you connect them in series.

When in parallel the load current will flow out of the cell with slightly higher voltage first.
This discharges that cell a little and now the other cell has a slightly higher voltage so current flows out of that one.
This system makes it find a nice equilibrium point all by itself where equal current flows out of each.
It even works if they are different capacity cells.

Just make sure they are both fully charged before you connect them in parallel.
Ya don't want one trying to force-charge the other because they were at different states when you connected them together.
Also obviously make sure you connect + to + 

[luvini]

...
Ya don't want one trying to force charge the other because they were at different states when you connected them together.
...

that's what i'm worried about might happen over time? what if one cell starts losing capacity over time faster than the other? will they SAFELY self balance?

[Psi]

Ya don't want one trying to force charge the other because they were at different states when you connected them together.

that's what i'm worried about might happen over time? what if one cell starts losing capacity over time faster than the other? will they SAFELY self balance?

That wont happen once they are connect in parallel.
If one cell loses 50% capacity they will still both be at the same % state of charge.
They will self balance and it will simply affect how they share the discharge and charge current, more current will flow out/in from the cell with more capacity.

It's only a risk when you first connect them together.
If one was charged and one was flat the current between them would be huge and likely too much for each to handle. They would still start to equalize and eventually be at the same state of % charged but the initial huge current would be bad for them.

[Psi]

Note that i'm talking about 'bare' cells in parallel.
i'm unsure exactly whats inside those batteries you posted.

[digsys]

I would DEFINITELY add some sort of fuse from EACH parallel battery to the +ve "bus bar". Common practice.
12V Lead acids or Li packs have 3-5 single cells in series, and should 1 ever go short cct, the others in the bus will try to destroy it !
If your max current draw is 100A, and each "battery" supplies 5A, then use a ~60A fuse. It can be a re-settable type, and if it is, maybe add an indicator or
warning. It's a safety issue.

[luvini]

...
add some sort of fuse from EACH parallel battery to the +ve "bus bar"
...

Attached image with a fuse for each battery added

...
If your max current draw is 100A, and each "battery" supplies 5A, then use a ~60A fuse
...

the max current is >20A

Note that i'm talking about 'bare' cells in parallel.
i'm unsure exactly whats inside those batteries you posted.

The battery datasheet: http://www.go-aliant.com/lithium/images/datasheet/Datasheet_Aliant_YLP_Series_EN_201612_F.pdf
says it has "Active Balancing embedded control electronics with Microprocessor control"

[digsys]

.... Attached image with a fuse for each battery added ... the max current is >20A ....

Yup, your choice on type. I'd say use a ~30A fuse. If 1 battery in the chain is supplying more than its share, somethings up. Like Psi says,
they should self equalize, and have no reason to drift too far apart.

  .... "Active Balancing embedded control electronics with Microprocessor control"

Yeah, active balancing WITHIN each battery pack, stand alone. Each pack has NO idea what the other packs are doing. PLUS, I have rarely found the
embedded BMSs to be true active. Usually they only SHUNT excess voltage to try to match up all cells. So, if only 1 cell goes low, they shunt ALL the others
to match it. Adding voltage is a much more complicated task - using bi-directional DC/DC converters. Done a few, it requires a LOT more bulky electronics.

[floobydust]

Careful- with a 20A fuse on each battery, you can supply 40A to the load. You need to wire things to handle 2x the fuses' rating, or a third fuse for the load only.

[luvini]

Attached image of how i connected the batteries up

i first tried to make their voltage be closer to each other by discharging the one with higher voltage

after that i let the battery rest for around 14h and checked their voltage are close enough to continue

i connected both GND together, added a multimeter to VCC (measuging DC current) in series with 4A fuse and the other battery

the current measured was 15mA
after 2h30 it lowered to 4,04mA

the multimetter is opto-connected to the computer and is logging the current

i will let then connected like that while i work with other stuff

[digsys]

Great news. You don't have to be that nit-picky with the balancing currents. Initial leveling can go as high as 0.5C for short periods. That is equivalent to say
boost charge at 14.4V. Then you can maintain 0.1C for quite a while -equivalent to 13.8V trickle. The important thing to watch is the current tapering off.
Usually quite rapidly. You may even see residual current imbalances of 10-20mA for ever (under load only!!). That's not ideal but fine, just small variations in
chemistry between the batteries.
When I am in a hurry to match sets, I let them go as high as 2C, with no ill effects :-)  It is all in spec.

[David Hess]

A parallel connection will not be a problem with LiFePO4 batteries.

The one thing which should be observed in a high current drain application is maintaining the same lead resistance to each battery which can be done as shown below.

[Psi]

Cells in parallel is totally fine, (agreed, a fuse is always a good idea)

The potential issue is that connecting two batteries isn't connecting each of their cells in parallel. It's connecting two 'strings' of cells in parallel.
With batteries that contain electronics inside, depending on what exactly that is and how it's connected, it's very hard to conclusively say if putting two in parallel will work ok.  But i would say it's more likely to work than not.

Unless the battery datasheet says its ok to put them in parallel, or you get an 'ok' from the manufacture, then the only way to know is to test it yourself.

[jonroger]

As drawn in the original question and assuming common wire sizes, the batteries will provide substantially different currents when under heavy load or charging.   They will wear unequally.   Response #11 mostly solves the issue - measuring current balance is even better.