Most efficient method to limit voltage to a motor controller. 1

>>>two 12v batteries wired in a series parallel arrangment to18volts<<<

Huh? From that combo, you can get 12V in parallel, or 24V in series.
To get 18V, you need three 6V batteries in series.

Or, as Gunnar said, a switching DC/DC converter. But then, what are you going to do with 18V? Just turn it loose on the wheel motors? And do wheelies?

If the bridges can switch 24V, that's what I would do, turn them on and off rapidly, using the duty cycle to control the speed of the wheel motors.



Mike Halloran
Pembroke Pines, FL, USA

Sounds like he wants to slow it down; says it "was 24 v" and now he wants to limit it to 18 volts. Agree, the easiest way with DC is to start with 18 volts or PWM the switches.

I purchased some "high power" motor controllers from the Pololu site for robotics. I meant to purchase the ones that were rated 24v. The ones I have are rated at 18v. I am going to control speed and torque of the motors via a PWM signal from the motor controllers I just need to step the voltage down to 18v supply to the controllers. I would have trouble going to a 3 x 6v battery arrangment due to the mounting system for the batteries. It may be easier to return the 18v units and pick up the 24v ones. I just thought if I could efficiently reduce the votlage then maybe I could extend the runtime by thoretically having some voltage in reserve.

Good decision to buy the right controllers. You gain nothing but problems if you don't.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.

Okay, you can put a switching regulator in there, but at that power level it won't be cheap, and your first design probably won't work right, at which point you'll get to buy new motor controllers anyway.



Mike Halloran
Pembroke Pines, FL, USA

Guys I appreciate all of the feedback. Question, If I connect a 12v and 6v in a series parallel arrangment will I get 18v? I have never tried to mix the voltages.

What do you mean by series parallel arrangement?

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.

You won't get any "voltage in reserve" by running 24V batteries with something reducing the controller voltage to 18V. The voltage only really starts to collapse when the batteries are dead, and at that point they are dead and are not capable of sourcing more power until they are "more dead".

2 batteries can't be hooked in both series and parallel at the same time. You have to pick one, either series or parallel.

You are better off just returning the controllers and getting the correct voltage controllers. You'll will not find it cost effective to build or buy a device to reduce the battery voltage to 18V.

There is no reason you can't put a 12V and 6V battery in series. Make sure the cells have the same ratings if you're planning on using a 18V charger.

I was taught that if you hook ground to the chassis on 1 battery then the positive lead to the negative lead on the other battery you will get the battery voltage x 2 between the positive of one battery and the chassis or ground of the other. Thus is a series parallel arrangment. Every piece of heavy equipment that uses 24 volts is wired this way.

Thus is a series parallel arrangment.

Click to expand...

Not really.

Some equipment has a 12 volt system, 12 Volt alternator, 12 Volt lights, 12 Volt accessories, etc. But the starter is 24 volts.
A special solenoid type switch disconnects one battery and reconnects it in series to provide 24 Volts to the starter only. This is the series parallel switch. The other battery remains connected to and supplies 12 Volts to the rest of the system. When you turn off the starter, the series parallel switch returns the second battery connections to parallel so that it may be recharged by the 12 Volt alternator. Now if you have an old Kenworth with four six volt batteries (was looking at one the other day) then you have a true series parallel connection when on 12 Volts.
Each set of two six volt batteries is in series to provide 12 Volts and the series parallel switch reconnects all four batteries in series for starting.

Every piece of heavy equipment that uses 24 volts is wired this way.

Click to expand...

Not really.
Alternately, there are equipment systems that are completely 24 Volts. 24 Volt alternators, 24 Volt starters, 24 Volt lights and, most importantly for the operator who was told that cell phones were banned on site, a 24 Volt cigarette lighter. 24 Volts to the charger will fry a cell phone right now.

Bill
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"Why not the best?"
Jimmy Carter

You are right there are some systems that use 24v to start and 12v to run. I meant when a piece of equipment is 24v start and 24v run then they are wired in series parallel. Are you saying that two batteries wired positive to negative with the vehicle connections on the ends--the other positive and negative-- then they are only parallel? And when you have say 4 batteries that are six volts each pair in series and the the two pair wired in parallel then that is series parallel?

Hey heavy, are you here to start a flame war?

Or are you, as a structural guy, trying to teach us seasoned and knowledgeble engineers about electricity and terminology?

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.

no i was actually just trying to make sure i had my terminology correct. I started off as a technician in the Marine Corp and then worked in the crane industry as a service technician before I started into rigging engineering which is basically structural applications of statics. I have a pretty good handle on practical applications of troubleshooting electrical systems mainly PLC systems.

I am NOT and electrical engineer which is why came to this site to ask questions from the experts. I just wanted to make sure I understood the difference.

I attached a sketch with my undrstanding of the two different systems. Maybe it will help with my explanation.

Back on subject, in this case two wrongs probably equal more expensive. Good choice to return the 18 volt controllers.

Two 12V batteries wired to produce 24V is a SERIES connection. It is NOT parallel or series/parallel.

A system with 2 batteries that switches between 12V and 24V is not connected series/parallel either. It switches between a series connection and a parallel connection. It is never connected in both series and parallel at the same time.

You need to look it up series and parallel connections if you don't understand. We're not explaining such basic entry level topics here.

I would like to thank you guys for your rapid responses and for all of the help. I believe I will return them and get the ones rated for the proper voltage. I just didnt want to go through the hassle of mailing them back etc. only to find out that with a couple of components I could have used them.

Thanks again, this forum is a great resource.

I am still interested in outcome of the series-parallel debate.....I actually taught electrical troublshooting for two years for a major crane company, Demag Mobile Cranes. Lucky for my students it was focused on practial troublshooting and not theoretical electrical fundamentals or terminology. I was basically how to read european E-Lan schematics and how to use a multi meter and milliamp driver to test and elimiate sensors etc.

The top drawing you posted shows two 12v batteries in "series". The bottom drawing you posted shows two pair of batteries in parallel, but the wire connecting the two pairs should run from positive to negative (instead of positive to positive) to put the pairs in series. I have attached a drawing...

OK heavy, I was a bit harsh on you. But when complete ignorants try to tell the experts about basic things, I get mean. No bite, though. Just bark.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.

Many 12:24 volt systems use two 12 volt batteries. Parallel for 12 Volt operation and series for 24 Volt starting. A switch called a series parallel switch changes the connection from parallel to series. The only true series parallel connection of the batteries is when four 6 Volt batteries are used and then only when configured for 12 Volt operation and charging,
Regarding your sketch;
1/ The top sketch is a series connection.
2/ The 12 Volt labels on the second sketch should be 6 Volts.
3/ The 24 Volt label should be 12 Volts.
4/ The series label should be parallel.
5/ The parallel label should be series.
6/ The connection as you have shown it is not the connection used in trucks and will not develop 24 Volts.
Hence the frustration of some of the gurus here.
Please do a little Googling on series parallel systems.

Bill
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"Why not the best?"
Jimmy Carter