# Batteries in parallel



## GizmoEV (Nov 28, 2009)

If they are LiFePO4 batteries you will not have any problem putting them in parallel, even if they are of different capacity. Look at the charge/discharge curves and you will see why. For all practical purposes, each cell will be pulled to the same voltage under load/charge. The lower resistance ones will put more power out initially but will then drop in SOC to the point that the higher resistance ones will start sharing the load. By the time you get to the full/empty point they will all be at the same capacity.

FWIW, I parallel 100Ah cells in my car and have had no problems in over 10kmiles and two years.


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## Sunking (Aug 10, 2009)

Mostly a problem with lead acid chemistry.


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## PZigouras (Jun 5, 2010)

Sunking said:


> Mostly a problem with lead acid chemistry.


When doing this with lead-acid packs, will the batteries eventually balance themselves (when you are off the throttle)?

The reason I ask is because there are a lot of people building EVs with used batteries around here... and some are from the salvage yards, so you know the capacity will always be different.

I try to discourage this because you have no idea what the capacity of a used battery is, but I just saw someone locally running 30 used batteries without any balancing and he is obviously having range issues.


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## lithiumlogic (Aug 24, 2011)

With Lithium it's fine. Recommended to wire in parallel first, then connect these groups of paralleled cells up in series, rather than paralleling series strings.

NiMH batteries hate being paralleled.. i found this out myself back in the 1990s. The problem is the cell voltage peaks at full charge, then it actually drops a bit as it gets into an overcharged state.

My project was a homemade bicycle light, with 4 x 1800 mah AA NimH cells stuffed down the seat tube, a step-up switch mode power supply, and a load of LEDs.

Desiring a longer run time, i paralleled another string of 4 x 3000 mah AF for extra capacity. But my run time actually got worse. 

I didnt understand what was causing this, but i ditched the AA string and got an extra AF, to make a 5 x 3000 mah series pack. The switcher could handle the extra input voltage and this setup performed well.

It's only just dawned on me what was happenning. On a charge, the smaller string was getting full first, and it's terminal voltage would drop below the large string. This caused the lion's share of the charge current to go into the already full string, in fact the 3000mah cells probably started dumping their own current into it as well.

My charger was just a simple constant current, 16 hour charger. After a 4 hour night ride, my own recharge time was longer than that of the batteries. So, there were no explosions, loud venting events etc. But the severe overcharging undoubtedly reduced the capacity of the smaller string further each time i charged up, exacerbating the problem.


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## GizmoEV (Nov 28, 2009)

PZigouras said:


> When doing this with lead-acid packs, will the batteries eventually balance themselves (when you are off the throttle)?
> 
> The reason I ask is because there are a lot of people building EVs with used batteries around here... and some are from the salvage yards, so you know the capacity will always be different.
> 
> I try to discourage this because you have no idea what the capacity of a used battery is, but I just saw someone locally running 30 used batteries without any balancing and he is obviously having range issues.


If they are flooded batteries they can be overcharged to balance them. While overcharging does diminish the life of the battery it is less of an issue than undercharging them or reversing them. They may have some cells in their batteries which have been reversed which will cause a major range issue. If you have two batteries paralleled and one has a reversed cell then there could be a 2V difference between the two batteries and the "good" battery will constantly dump into the bad battery, further reducing the range.


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## IamIan (Mar 29, 2009)

marc02228 said:


> I recently asked myself a question. What happens if I build a batteriepack with, let's say 10 Headway batteries, in parallel. All the batteries have different internal resistances. When I draw a current out of the batteries, there will be a different voltage drop at the different batteries. Because of that there will be a current between the batteries in parallel.
> 
> Now my question..Can I put the batteries without any other parts in parallel? Is there a mistake in my brainstorming i wrote above?


Only mistake I see is :

All cells connected in Parallel have the same final terminal voltage... not amps of current ... Ignoring for a moment the resistance of the connectors which are in series... connector resistance is usually very small anyway.

So when you drawl power out of the battery the voltage drop is seen equally by all cells on parallel ( Ignoring connector series resistance ).

Different cell level resistances will result in different Amp rates of discharge from different cells in order to preserve the same final terminal voltage ... those with lower resistance will be pumping out more amps than those with higher resistance as they all go to the same dV.

This can result in some interesting current flows ... but as long as the cells remain connected in parallel they all have the same terminal voltage ( ignoring the series resistance of the connectors ).


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