Multi Voltage from Series Batteries 2

[twinsavage]

hello, i would like to know hypothetically if there would be any adverse (mainly the cells being damaged) effects of wiring a battery system like this:


there are three 7.4 volt lipo cells and they are wired in series to provide 22.2 volts, would it be wise to also wire them in parallel to provide 7.4 volts to another system, so that 7.4 and 22.2 volts are coming off the same battery system? or would it be better to have two distinct battery systems?

 

[bpelec]

You could not wire all three batteries in series and parallel at the same time, even with different loads. The necessary interconnections between the batteries would short them out I think...

However, you could wire the three batteries in series and take 7.4 volts off one of them to a different load. If you did this then the battery that is supplying the 7.4 volts is going to discharge faster than the other two, but this need not be a problem. I would suggest using a common ground reference with the two circuits to avoid any undesirable circulating currents between your two systems.

I hope I haven't missed the point, and this information is useful.

Kind regards,

BPELEC.

 

[bpelec]

Here's a sketch of what I was suggesting...

http://files.engineering.com/downlo...-7d25-4ad4-8dc2-955146d653a0&file=circuit.jpg

 

[twinsavage]

yes this is very helpful, thank you for your insight.

i could even do something like this:

http://files.engineering.com/getfil...-45a0-a9dd-5e1f7be45852&file=circuit_mod1.JPG

to offset the discharge rate of the 7.4 volts

 

[bpelec]

Absolutely, although if you are adding an extra battery then you may also consider running your 7.4 volt system separately using that 4th battery, and having two distinct battery systems (as you suggested originally).

 

[itsmoked]

Sorry gents but any mixed series-parallel scheme will result in damaged batteries in a few charge cycles. Battery sets require uniform discharge and charge profiles to maintain capacity balance. Once you pull extra load from any one battery, you are working it harder. The "pack members" will then diverge rapidly.

If you want to run a lower voltage load while running a higher voltage load use a DC to DC to drop the higher voltage, efficiently, to your lower required one while uniformly continuing to discharge your pack.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[slavag]

Keith!!!! Star!!!!
Regards.
Slava.
Or use two separate system,as wrote in your OP

 

[itsmoked]

Thanks slavag. Happy new year man!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[twinsavage]

i will probably opt for two distinct battery systems in the end, due to lipos under voltage cut off requirement

thank you for you help


and just for kicks this is what this is being used for:

http://files.engineering.com/getfile.aspx?folder=f3407d9a-9e24-47a9-8db1-94ffdcc2b88a&file=1.jpg

 

[Nealsn]

Just a suggestion - but you could add a single chip to your circuit to get your lower voltage from the combination of the batteries - called a voltage regulator

We used these a lot when I was still in School (May)

Here is a fixed voltage of 5v out - these take any DC voltage in 6-35V and turn it into a fixed 5v output

http://www.radioshack.com/product/i...&cp=2032058.2032230.2032279&parentPage=family

They make the chips with several different preset outputs. Also they make adjustable ones where you can change the output

Stephen

 

[itsmoked]

Wow! I hope I never meet that thing on a 3AM trip to the bathroom..

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[stevenal]

That regulator only goes to 1A, is that enough? And the extra power needs to be dissipated by a heat sink. Since this thing is run by batteries, battery life is likely to be an issue. Wouldn't want to run the battery down before that 3AM bathroom trip. If efficiency is needed, use an efficient DC-DC converter like itsmoked recommended.

 

[twinsavage]

The 7.4 volt circuit will probably pull around 30-40 amps during activity
(at least its not as bad as the 22.2 volt circuit, which can pull 90 amps peak but should go down to less than half of that while in normal use)


i'm incorporating some DC-DC converters into the design, but for the lower amperage subsystems (solenoid valves, micro controllers, sensors)

i guess the best path would be to use 2 separate battery circuits

 

[itsmoked]

Why do your two different systems each draw that much power? Don't you have a high power motive system and then all-the-rest?

Are you confusing an inrush on something for draw?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[twinsavage]

there is a 1.8kw 22.2v brushless motor in charge of air compression, and there is a large counter weight on the crankshaft of the compressor which will cause the motor a great deal of load during spin up, so i am expecting full load for a couple of seconds on the batteries just to be safe.

the 7.4v systems is comprised of twelve 98w motors, but they will never be running all at the same time, they swing the legs and may have force against them from the leg trying to move something out of the way, so i expect to see full current draws from some of them under certain circumstances.

 

[slavag]

Hi.
May be check some good AC-DC convertor instead batteries.
Something like to this catalog:

http://www.lambdapower.com/products/ac_dc.htm

Regards.
Slava

 

[itsmoked]

Well... Sounds about right then.. How big is this monster? Starting to sound like its top plate is what, 3 feet across?

If that's the case might I suggest an inverter generator instead? Very quiet, light, and an insane energy density. For debugging indoors etc., you then just bypass the generator and extension-cord it. This way you can use off-the-shelf power supplies to provide just what you want. You won't be limited to short runs with batteries followed by down times for recharging.

Here's an example. And they are using a LOUD generator, they can be MUCH quieter.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[twinsavage]

the top plate is about 26 inches across, good guess.

for the 22.2v system, just to test it (very briefly) i was going to use 18 "D" batteries in series, but i think i like the idea of the inverter generator better. there will be much debugging so there is no way i will be able to do it all on batteries.

i did however snag a U1 variable power supply, 1-20 volts, 1-50 amps off ebay for about $100. so this could leave the inverter generator to power the compressor motor, and thus letting me get a smaller inverter generator (quieter).

 

[itsmoked]

Yeah! 18Ds would have crippling internal resistance to boot.

Nice find for $100!!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com