Simple series/parallel question?

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chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
I don't have time for this now and can't seem to wrap my feeble mind around this. Any players?

I have five 12V batteries and need to get 36V from them. Easy with 3, but the cranking power of the other 2 is required. Show me how to wire this.

Thank you.
 

Caesium

Member
I can't seem to think of a good way of doing it.
Any way you connect it you will be limited by the internal resistance of one of the batteries.
Are they all the same?

Nathan.
 

cadpoint

Senior Member
Location
Durham, NC
How many Amp's of cranking power that in each battery is what you need to zero in on.

What's the spec's, Etc, Trickler change and Maintaince, all in play ... and then theres the equipment.
 

480sparky

Senior Member
Location
Iowegia
I don't have time for this now and can't seem to wrap my feeble mind around this. Any players?

I have five 12V batteries and need to get 36V from them. Easy with 3, but the cranking power of the other 2 is required. Show me how to wire this.

Thank you.


One: It don't get that cold in Florida, Chris. I know, you think 60 is cold. But you will never need that many CCAs.



Two: 36 volts? Who you kidding? What do you drive that runs on 36 volts? I know tractors do, but c'mon...... they don't drive tractors in Florida!



Three : OK, I'll show you:

5batteries3.jpg
 

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
Thanks all, this is a real world application. Some kind of scooter that came with 3 batteries in series. There is only room to rack 2 more.

Is the installation in Ken's picture safe?
 

gar

Senior Member
100205-0806 EST

The available energy is primarily limited by the single 12 V battery in the series combination. Once you exhaust its ampere-hours (really watt-hours), then its voltage drops drastically, and even if this one battery looks like a short circuit the output of the total remaining 4 batteries would be 24 V.

The two paralleled 12 V batteries need to be separated from the others, used as a series pair to drive an electronic boost circuit, and this connected in parallel with the three series batteries.

This will provide the total energy of the five batteries minus maybe 10% of the energy of the two series batteries from losses in the boost circuit.

.
 

480sparky

Senior Member
Location
Iowegia
Thanks all, this is a real world application. Some kind of scooter that came with 3 batteries in series. There is only room to rack 2 more.
I didn't think you were that old, Chris. Or are you just trying to get a senior discount at restaurants now?
groucho.gif


Is the installation in Ken's picture safe?

As safe as I can make it. The only problem is, the far right battery will only last half as long as the other four. Once it gets low, you'll see a reduction in output. So adding an additional two batteries won't gain you much.
scooter.gif
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
The only problem is, the far right battery will only last half as long as the other four.
scooter.gif

Depending on the discharge rate, it may not last even that long. It might die almost immediately if you are well above its rated discharge rate.

The faster you discharge a battery, the less watt hours you get out of it.

Steve
 

broadgage

Senior Member
Location
London, England
Thanks all, this is a real world application. Some kind of scooter that came with 3 batteries in series. There is only room to rack 2 more.

Is the installation in Ken's picture safe?

It would be safe for temporary or short term use at limited power levels, but potentialy unsafe for regular use.
On discharge, the single 12 volt battery would run down first, and might well become reverse charged which is most undesireable.
On charge, the single 12 volt battery would be liable to overcharge and damage.

36 volts suggests not an engine starter battery, but a small electric vehicle.

6 volt batteries might be an option since these normally have a greater capacity than 12 volt ones. Use 6 in series for 36 volts, if space permits.

8 volt "golf cart" batteries are available, and you might be able to use 5 of these in series for 40 volts.
That might overstress the electrical system designed for 36 volts, but it is a small difference and would probably be fine in practice.
Remember that the charger will need replacing with one for 40 volts, which is not a common voltage.

Batteries in series should be the same type, age, capacity, make, and preferably voltage.

Any odd batteries that are to hand may be used for brief tests or expeiments, but for regular reliable service all batteries in a bank should be identical.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
For this sort of application, I'd be inclined to replace the controller with one that can handle higher voltage, and then simply connecting all the batteries in series.

Intentionally unbalanced series strings are a recipe for disaster. Broadgage's comment about reverse charging is a key issue; the 'non-paralleled' battery is ripe for being reverse charged and severely damaged.

-Jon
 
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