# regarding current

hi ,i am using a 29 volts lithuim ion battery but with less current rating ,i want to run a motor of 24 volts 16 amps,but i am not getting enough current going to motor.
i am using a dc to dc bucky convertor to convert voltage from 29 volts to 24 volts,but i am unable to increase the enough current to the motor,what should i do now?

what should i do now?

You could use two batteries in series if your DC to DC converter can stand it. That will let it double the current again if the DC to DC converter can give that current.

Might this be the battery protection circuit kicking in? Could you provide details of the battery and motor?

What is the amperage rating of your DC convertor. If it is not capable of outputing say 100 amps you will find it difficult to get the motor started
A motor rated 24V at 16A consumes some 384watts once the motor is running at rated speed but maybe 5 or 6 times that to start.
What do you mean "I m unable to increase the current"

Depends on how you ramp up the PWM though - stall current is best avoided in larger motors. You will need
to use PWM at high powers for this reason.

Well,

First to increase the current you would put batteries in parallel not series...

For the buck converter if you put a relatively large capacitor at it's output then the capacitor can handle the large startup currents without requiring the converter to handle start up and running currents.

Cheers,
Terry

TerryVA:
First to increase the current you would put batteries in parallel not series...

For the buck converter if you put a relatively large capacitor at it's output then the capacitor can handle the large startup currents without requiring the converter to handle start up and running currents.

Grumpy_Mike said to put them in series at the input of the DC-DC, so it is a valid way to increase the output current of the DC-DC. That said, I don't see a reason to do that instead of putting them in parallel.

I think the OP needs to post links to the battery, DC-DC (if he needs it), and motor if he really wants answers to his questions.

Putting batteries in parallel is problematic because they cross charge. It is a very bad idea.

Grumpy_Mike:
Putting batteries in parallel is problematic because they cross charge. It is a very bad idea.

I'm sorry, but that's not true. I worked for a Li-Ion battery company for 2 years, and we made many batteries that had cells in parallel. Nowadays I support many battery companies that buy my company's balancers and monitor ICs. Battery packs use cells in parallel all the time.

Yes, you do need for them to be close in voltage when you first bus them together, but after that parallel cells are self balancing.

BigBobby:
Battery packs use cells in parallel all the time.

I don't think all battery technologies will work that way. Li-Ion are probably a special case.

I know all of the common chemistries can be parallelled. If you know of an odd one that can't plz post a link, but I can't imagine why they couldn't.

A single cell is often modelled as several cells in parallel. Due to the geometry of its construction, localized parts of the electrodes lose potential faster than other parts, but they all self balance to provide the average capacity of the cell. Two physical cells in parallel aren't much different from taking the chemicals in each can and combining them in one larger can.

A single cell is at its heart a pair of plates, each hypothetical portion of the plates is electrically in
parallel with all the other portions, so if parallel did not self-balance, electrochemical cells would
not be possible.

The thing to avoid is connecting two cells that are different in parallel (different in type, capacity,
manufacturer, or history). They must have the same chemistry and history to be matched. For
chemistries with a flat discharge voltage curve the match has to be really good (NiCd and NiFe cells
I think are fussy).

Series connection is more problematic as the first cell to drain then gets back-charged by the
others, so you have to be much more careful to avoid over-discharge. We have to live with series
in practical batteries so we view it as normal, but its the worst case and its so easy to damage
rechargable series packs by over-discharge - most people never see the full battery cycle life out of their
rechargables as a result (except for Lithium-ion laptop batteries with built-in management and
protection circuits)

One gotcha with parallel connection is when a cell fails by shorting (whiskers of metal bridging
the cell are possible in certain chemistries). Then all the parallel cells will discharge through
the shorted cell, possibly causing it to overheat / leak / burst. This isn't a big issue with series,
you just lose one cell's voltage. High power battery banks use fuses/protection circuits to
protect against such failure modes.

So if connecting in parallel its probably a wise precaution to fuse each battery separately.

i ,i am using a 29 volts lithuim ion battery but with less current rating ,i want to run a motor of 24 volts 16 amps,but i am not getting enough current going to motor.
i am using a dc to dc bucky convertor to convert voltage from 29 volts to 24 volts,but i am unable to increase the enough current to the motor,what should i do now?

There's no way that lithium ion battery can handle motor current . That's just plain ridiculous. Find another battery type. Lithium ion are not designed for high current.
Use Lithium Polymer. (LIPO) . Lipo batteries can deliver ridicuous high current.

I don't want to over simplify the issue but in my opinion, your problem is that you are trying to do something you shouldn't, namely use batteries that are rated for less that your load. As long as you continue to do that , you will continue to have problems. As already stated, basically any voltage or current rating can be obtained by combining series and parallel batteries (of whatever type). series parallel, parallel series, etc. You start by putting them in series if the battery voltage is less than the load voltage. If the battery voltage is greater than the load voltage then put as many of them in parallel as necessary to obtain the needed load current + 20%.

Your barking up the wrong tree. Give up and change battery type.

Two physical cells in parallel aren't much different from taking the chemicals in each can and combining them in one larger can.

Sorry but they are.
The thing about physically having one larger can is that any local chemical changes will balance out through the natural process of diffusion. Where as if it is in two cans then the only "balancing" that can occur is by cross charging.

BigBobby:
I worked for a Li-Ion battery company for 2 years, and we made many batteries that had cells in parallel.

Yes but you had a vested interest in selling more batteries when their life was prematurely cut short. Something that would not be noticed by the majority of users. The point is that all things age slightly differently so what you might get away with at the start by being closely matched, gradually gets worse.

However, to the problem in hand that the OP has then raschemmel is quite correct, wrong solution to the problem.

Look at all the applications that use Li-ion batteries. They are electronic equipment like laptop computers and anything with circuit boards. You never find them used to drive motors. Your lack of experience with electronics applications is misleading you into thinking that you can "manipulate" components to obtain your solution. You would not put ordinary tires on a high speed race car. ( The Bugatti Veyron tires are rated for 12 minutes life @ 255 mph . Cost: \$12,000 each. Different applications require different technology. Motors can be powered with either lead acid or Lithium Polymer. (LIPO) A 45C , 5000 mAh 6S Lipo
can deliver 45 *5A =225A for 1.3 minutes, or 20A for 15 minutes.. If you put 2 in series you have 2 12.54V=25.08V , If you parallel two of these 12S strings (26S) you could get 20A for 30 minutes. It won't be cheap but it will definitely work I see people drawing 100+ A from lipos all the time. I personally was getting 60A for about 5 minutes every weekend for more than a year before I fried the motor in my airplane. Lipos are lighter than lead acid but more expensive. (and of course there is the fire hazard issue, which I will let you research on your own)

LiPo actually is Lithium-Ion (as opposed to Lithium Metal), although people rarely say "Ion" when talking about them. The Tesla Model S battery uses 69 Li-Ion NCR18650A cells in parallel to drive its motors. The EV batteries that I used to design were LiFePO4, which is actually pretty rare compared to other Li-Ion chemistries which are the most common cells to find in EVs today.

Everything MarkT said is true. Heh...I'd never imagine paralleling cells of different chemistries but he's right not to do it. Individually fusing each cell is common too, as he mentioned. I have a 27s25p pack in my lab right now using A123 26650 cells. Every single one of them has a fuse stamped into its weld straps, so that the 25p isn't just to add ampacity but redundancy as well. He's also correct about the dangers with imbalanced series cells, which is why I'm developing an active balancing algorithm for that pack using the LTC3300.

As far as companies purposely using parallel cells to reduce the life of their packs so that they can sell more...it's an interesting theory, but they do still need their packs to last for the warranty period. Tesla is paralleling cells because they need the current, not because they want the packs to die quicker so they can sell more.

LiPo actually is Lithium-Ion (as opposed to Lithium Metal

Perhaps true, but I only know that a battery labeled Lithium Ion is not going to deliver near the current as a battery labeled Lithium Polymer , regardless of the above. .Lipo batteries are not labeled Lithium Ion. If you are recommending Lithium Polymer for a motor application I don't see how telling the OP that Lithium Ion (what he has ) is actually Lithium Polymer which is what it sounds like you are saying because what he has is clearly not what he needs. Can we confuse the OP any more than he already is ?

raschemmel:
Perhaps true, but I only know that a battery labeled Lithium Ion is not going to deliver near the current as a battery labeled Lithium Polymer , regardless of the above. .Lipo batteries are not labeled Lithium Ion. If you are recommending Lithium Polymer for a motor application I don't see how telling the OP that Lithium Ion (what he has ) is actually Lithium Polymer which is what it sounds like you are saying because what he has is clearly not what he needs. Can we confuse the OP any more than he already is ?

Yes, you are correct. Most of what I said in that post wouldn't be useful to the OP. I'll try to get better at keeping my responses geared toward helping OPs and less toward theoretical discussion.

I don't know if the OP is coming back though. It's possible that he already did get his motor running by doubling the voltage on the input to his DC-DC and adding a big cap. If he didn't, he'd need to post some more info in order to select a battery to power his motor.

It's sort of unsatisfying on these forums when you spend time helping but never find out how the project ended, don't you think?

That's what I refer to as "OABAs" (OPs abducted by aliens.) or "PAR" (Post and run..)

They get what they want and split...