# wiring in parallel



## Coley (Jul 26, 2007)

Nothing wrong with putting the two packs in parallel. I am going to do the same thing.

Charging them is not a problem, just takes a little longer.

I want to split them with solenoids, so that I can run either pack individually.

That way you won't need the whole 2 packs for short trips.

The charger can be split also just to charge one pack at a time if desired.


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## Qer (May 7, 2008)

Coley said:


> I want to split them with solenoids, so that I can run either pack individually.
> 
> That way you won't need the whole 2 packs for short trips.
> 
> The charger can be split also just to charge one pack at a time if desired.


I think splitting the pack wouldn't be very wise, or at least not beneficial in any way. The packs survive better if the current and the DoD is kept as low as possible, so splitting the pack will probably wear the used pack more than if you keep them mated all the time.

Charging them separately probably won't do any difference except if you can run two separate chargers at the same time to cut the charging time in half. Connecting two chargers in parallel to the same pack is probably a very bad idea so there the possibility to split the pack might come in useful.


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## TX_Dj (Jul 25, 2008)

Here, have a look at my scooter.










This setup has two 36v packs in parallel.

1) If this was set up as a 72v series pack, the amount of power available and the Peukert Effect under load would be the same. (36v * 6 Ah = 216 Wh * 2 = 432 Wh, or 72v * 6 Ah = 432 Wh)
2) The lower voltage of parallel packs will not give you the same speed and acceleration capabilities as one series pack.
3) The lower voltage may be necessary due to limitations of the controller. (In my case it would have been darn near impossible to fit a 72v controller and motor in this scooter, and the event I built it for did not have a 72v class for entry and I could only fit 6 of these batteries, thus I didn't enter as a 48v)
4) If one pack is mismatched or unbalanced from the other pack, the stronger one WILL discharge into the weaker one, trying to charge it without any form of charge regulation. (Read: Max amps, if the weaker battery demands it, which is sure to fry it or melt something down).
5) My setup uses a large-ish (200V 30A) fast-switching dual diode package to act as the parallel union. This prevents the two batteries from discharging into each other.

A note about the diode- if you don't have one, and one pack is weaker than the other, you will waste power trying to charge the weaker one and the weaker one will not supply any (or much) power to the drivetrain, and the stronger one won't either, as it's also trying to charge the weaker one.

With my setup, when the batteries become unbalanced (and they do) the diode acts somewhat like a "valve" letting more "pressure" out of one battery than the other. I.E. if the strong battery can provide 10 amps but the weak one can only provide 15 amps, they will do exactly that.

I wouldn't recommend connecting batteries in parallel, but many many people have done it without bad things happening... but the risk is very real, and it's always there so long as those batteries are connected to each other.


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## Coley (Jul 26, 2007)

One reason for having the packs hooked to relays, is that a range check can be made with one pack, then get back with the other one!!

It wouldn't be used much, as it is best to run them together, but what if a battery fails?...run on the other pack to get home to do repairs.


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## Chris Sandberg (Sep 13, 2008)

DJ, can you expand on the diode idea a little more as I am looking at a series / parallel system also and this sounds like a good BMS tool to have. Did you build this or is it something that is available to buy and if so from where.
TX Chris 
a newbie/ beginner


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## Qer (May 7, 2008)

Coley said:


> It wouldn't be used much, as it is best to run them together, but what if a battery fails?...run on the other pack to get home to do repairs.


Yep. That is a very good point and a reason for me to consider two chains since I don't expect my gal to even consider rewiring the pack if one cell breaks on the road. She absolutely hates the idea of electrical shocks and even static electricity makes her whimper so to tell her to rewire a 144 Volt pack into a 138 or 132 Volt pack to get home is just not going to go down well. 

Switching from A+B to A or B on the Big Safe-Looking Breaker that's Well Insulated is an entirely different thing...


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