I have to build a huge battery pack with 40 V current. How to do it?

Projects General Guidance
1

Hey guys,

i am working on an innovative motion control project.

The main power drain in my system is a NEMA23 stepper motor. It runs at 40 V on a DM556 stepper driver and can draw 4 A RMS (5.6 A peak).

The 40 V circuit is the main circuit, now it is supplied via a Meanwell-320-48 power supply. To power the controlling Arduino Due, a step-down converter is used which takes the 40V and outputs 7 V to the Arduino voltage regulator.

The Arduino Due runs several wireless interfaces and other components, which together consume approx. 1 A total. As a suited step-down converter, i got recommended this model
which makes sense to me.

So basically it comes down to 1 single 40 V power supply because i step down between this and the Arduino.

NOW, i want to make the whole system mobile by using a battery pack. The battery pack has to stand the total current draw, least for around 3-4 hrs of use and deliver relatively stable voltage.

I know its a big project but i think it is doable. What i dont really know is how to design it, since i have not much experience with battery technology.

Do you have any ideas how to build it?

Thanks,

Julia

2

You need a minimum of about 20 ampere-hours of battery capacity. By far the safest choice is to use lead-acid batteries, preferably deep cycle marine batteries, which tolerate a lot of abuse and are very forgiving about how they are charged.

Three batteries in series (nominally 12 V, peak about 14 V) of 20 ah or higher capacity, will give you roughly 40V when fully charged. They should not be discharged to below about 11 V/cell.

3

jremington:
You need a minimum of about 20 ampere-hours of battery capacity. By far the safest choice is to use lead-acid batteries, preferably deep cycle marine batteries, which tolerate a lot of abuse and are very forgiving about how they are charged.

Three batteries in series (nominally 12 V, peak about 14 V) of 20 ah or higher capacity, will give you roughly 40V when fully charged. They should not be discharged to below about 11 V/cell.

Hi,

thanks for the answer. I need a system that is very portable, so i fear lead-acid batteries have a terrible energy/weight ratio. Maybe i have to use LiPo or NiMH batteries.

You made a good point regarding the safety though. With LiPo i would need technical measures to ensure under and over charging as well as loadbalancing of individual cells i guess?

Also, shouldn't i go for higher initial voltage and use a regulator in the battery system to keep voltage more constant? If you say for example that lead-acid cells should not be discharged below 11 V, it means a significant voltage drop during discharge. Maybe some kind of step-down regulator?

Julia

4

I would be reluctant to use any sort of regulator between the battery and the motors - it will only waste energy. However it might be worth considering a step-up regulator that might bring 12v up to 40v just for the convenience of a less complex battery.

Any battery system will be subject to voltage drop as the batteries discharge. You will need to figure out what is the acceptable minimum voltage and plan around that. Don't assume that the motors need the full 40v that they get at the moment.

Flying model aircraft use large LiPo packs - you should be able to get info on them and their management on Forums that deal with those models. Your application (at 4 or 5 amps draw) will be a lot more gentle on the batteries than an aircraft.

How are you going to recharge the batteries? Would it be better to have 2 or 3 sets of smaller batteries that can be swapped when they run down?

...R

5

Maybe i have to use LiPo or NiMH batteries.

I just looked-up a "random" laptop battery and it was rated at 5 Amp-Hours. So, you'd be looking for something rather specialized if you need 20 Amp-Hours.

You might not need 20 Amp-Hours, since you said the peak current is 5.6 Amps, and you'll obviously need less if the motors don't run continuously.

(When you wire batteries in series, you increase the voltage, but not the Amp-Hour rating.)

I haven't found the Amp-Hour ratings for an automotive battery, but I have a feeling you are getting into car-battery territory... Maybe a batteries for a lawn tractor would do it? They do list "Reserve Capacity" for car batteries, and if you look-up how that's measured you might be able to estimate Amp-Hours.

Or, maybe a gas-powered AC generator?

:smiley: Or, maybe build the whole thing around of a battery-powered golf cart? ...Plenty of batteries and it's self-propelled. :smiley:

6

LiPo battery packs with that sort of capacity are extremely expensive and are very dangerous to charge -- they should be charged in fireproof containers and never unattended. I've had good luck with products from all-battery.com, but see the warning notices Tenergy Power

Lead acid is much cheaper and three of these would work (when in stock) Tenergy Power

I believe that automotive batteries are 30--50 ah or higher.

7

LiPo battery packs with that sort of capacity are extremely expensive and are very dangerous to charge -- they should be charged in fireproof containers and never unattended.

Yep - just ask Boeing!

8

Use LiFePo4 .... perhaps multiples of this 4500mAh 6S2P pack or similar.

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=31941

or these 8400mAh 4S2P

perhaps you could build your own from individual cells like B.M. ( Burgerman ) has.

9

Robin2:
I would be reluctant to use any sort of regulator between the battery and the motors - it will only waste energy. However it might be worth considering a step-up regulator that might bring 12v up to 40v just for the convenience of a less complex battery.

Any battery system will be subject to voltage drop as the batteries discharge. You will need to figure out what is the acceptable minimum voltage and plan around that. Don't assume that the motors need the full 40v that they get at the moment.

Flying model aircraft use large LiPo packs - you should be able to get info on them and their management on Forums that deal with those models. Your application (at 4 or 5 amps draw) will be a lot more gentle on the batteries than an aircraft.

How are you going to recharge the batteries? Would it be better to have 2 or 3 sets of smaller batteries that can be swapped when they run down?

...R

I like the idea of using a step up voltage regulator. I could use a 12 V battery pack and transform up to 40 V, but how efficient would it be? If it reaches 90% in real life usage i would consider it.
I still dont know how to recharge the batteries because i havent decided which type of battery i use.

Btw: My Arduino Due is also powered by this battery. So i need a step-down regulator for a 40 V battery system to supply the arduino. The other option would be to use 12 V battery, directly power the Arduino with 12 V and step up to 40 V for the stepper motor. I am not sure whats the better option...

DVDdoug:
I just looked-up a "random" laptop battery and it was rated at 5 Amp-Hours. So, you'd be looking for something rather specialized if you need 20 Amp-Hours.

You might not need 20 Amp-Hours, since you said the peak current is 5.6 Amps, and you'll obviously need less if the motors don't run continuously.

(When you wire batteries in series, you increase the voltage, but not the Amp-Hour rating.)

I haven't found the Amp-Hour ratings for an automotive battery, but I have a feeling you are getting into car-battery territory... Maybe a batteries for a lawn tractor would do it? They do list "Reserve Capacity" for car batteries, and if you look-up how that's measured you might be able to estimate Amp-Hours.

Or, maybe a gas-powered AC generator?

:smiley: Or, maybe build the whole thing around of a battery-powered golf cart? ...Plenty of batteries and it's self-propelled. :smiley:

A generator would be very nice, but my application must not produce any noise. Lawn tractor batteries are a convenient idea, i saw some good ones from samsung:

36 V is still ok and a pre fabricated battery pack will be in a nice shell and maybe have protection systems (depending on the type, will have to look more into this)

woodygb:
Use LiFePo4 .... perhaps multiples of this 4500mAh 6S2P pack or similar.

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=31941

or these 8400mAh 4S2P

http://www.hobbyking.com/hobbyking/store/uh_viewitem.asp?idproduct=14074

perhaps you could build your own from individual cells like B.M. ( Burgerman ) has.

BM3 Lithium Powerchair Battery. A safe LiFePO4 wheelchair!

LiFePo4 looks interesting. I will read into this type of battery today. Anybody else has some opinions and experience about using LiFePo4? Seems to be safer than LiPo at least, maybe Boeing shouldve considered using those :wink:

Finally, my first decision will be:

Use high voltage Battery directly with motors and step down voltage for the Arduino OR use something like a 12 V battery and step up the voltage for the motor and use Arduino directly with 12 V directly??

10

Hi, avoid automotive batteries, they are designed for quick short high current loads and long float times.

Deep cycle if you use lead acid, and most manufacturers do provide performance specs.

Tom...... :slight_smile:

11

LiFePo4 chemistry is considered VERY safe ...unlike Lipo's

Lipo

LiFe(Po4)

I use 4 of the Zippy 8400mAh 14.8v 4S2P packs in my Heavyweight Robot ...configured as a 8 serial 4 parallel.... they are not treated gently.

Note that those machines mass 100kg's.

12

woodygb:
LiFePo4 chemistry is considered VERY safe ...unlike Lipo's

Lipo
http://www.youtube.com/watch?v=YCWdnjLqVWw

LiFe(Po4)
http://www.youtube.com/watch?v=XymqQ-YlfJ0

I use 4 of the Zippy 8400mAh 14.8v 4S2P packs in my Heavyweight Robot ...configured as a 8 serial 4 parallel.... they are not treated gently.

http://www.youtube.com/watch?v=al9ULLuAPFM&list=PL9xznNA0LgWaJwuftkT_3tDbXBcPUulV3&index=1

Note that those machines mass 100kg's.

Bump..

Hi,
i think LiFePO4 batteries may be a good ( and safer) option than LiPo, but fr using any of those packs with 12 or 14.8 V i need to figure out if its feasible to use a step-up converter to get my 36-40 V for the motor.

What do you guys think about using a stepup converter? Will i have to much energy loss?

13

Step up converters can have a very high efficiency ... somewhere around 90% as I recall.

Thus your energy requirements of 3 or 4 std 12V SLA 20Ah batteries in series...could then become one 12v SLA 60 - 80 Ah.

NOTE:- Using ALL the energy in a SLA battery is NOT a good thing ... 80% D.O.D is really the MAXIMUM that you should use... 60% D.O.D is much better.

DOD = Depth Of Discharge

So the original SLA battery Ah requirement needs revising.

14

Also the Amp Hr's that you can get out of any given SLA battery varies with the continuous Amps being drawn over a given period .
The 20 hr rate ( which is that quoted by the battery manufacturer to 100% D.O.D. over a 20hr period ) will be much lower Ah's than you expect if using a higher discharge rate due to the Peukert effect.

http://www.smartgauge.co.uk/technical1.html#peukert

http://www.sunwize.com/documents/MK-Battry-8115_AGM_LG_v2_r5.pdf

15

Hi,

i had some talks today with colleagues and revised my specifications for the battery system. We found that:

We will use 2 swappable battery packs with 8000 - 10000 mA each.
Charging of one pack should be possible in 2 hours.
Lead seems to be a bad option, because it has a energy density of 30 Wh/ kg. LiFePO4 has 80-140 Wh/kg and LiPo 140-260.

Please, if anybody can give me a good reason to use a lead battery instead of LiFePO4 (they are explode/fire safe), tell me. I dont want to miss something. I just think from all the facts i read so far, that LiFePO4 is as safe as lead acid but gives me way higher energy density.

BIG question is still: Should i put 3 12 V packs in series to get 36V or should i get a bigger capacity pack and use a step-up regulator to get around 38 V?

Quite a battery jungle, jeez!

Julia

16

woodygb:
NOTE:- Using ALL the energy in a SLA battery is NOT a good thing ... 80% D.O.D is really the MAXIMUM that you should use... 60% D.O.D is much better.

In my view this is because the manufacturers grossly overstate the Ah capacity of lead-acid batteries. I always assume the true practical capacity is 50% of what the label says.

...R

17

The 20hr rate is industry std .... the amps draw at that 20hr rate is a trickle and until the battery is completely drained.

Use a higher Amps draw than the 20 hr rate and you get less Amp Hr's out.
The MK pdf has a chart showing this.

Why not take the Peukert link?

18

woodygb:
The 20hr rate is industry std .... the amps draw at that 20hr rate is a trickle and until the battery is completely drained.

Why not take the Peukert link?

I'm fed up to the back teeth with Peukert.

Yes, you can draw X amp-hrs at the 20hr rate. But what state is the battery in when you have done that? And how long does it take to recharge so you can do the same thing again? and how many times can you do it and get the full capacity?

This is not a practical measure of capacity for a battery system that has to provide N amp-hrs every day.

...R

19

Theres no such thing as a battery system that can provide N amp hours every day.
Every type of battery chemistry loses capacity per discharge cycle and over time regardless of discharge cycles or rate.
Higher rates make it worse.
Theres no absolute method of measuring battery capacity over time.
Even the computerised systems in EV cars struggle to accurately predict how much energy is left in the battery at any given time.

20

Lipo batteries do require caution, but the people I have known who use them are well aware of that and
take the required precautions. They transport them in WWII surplus 50 CAL amo boxes. They NEVER
charge them INDOORS. They have a fire extinguisher handy (auto parts store mini extinguisher) and they
always use the proper charger. They check them to make sure they are not heating up or puffing while
charging and they avoid over discharging them. I used them for 7 years and only had one mishap when
I accidentally punctured one with my pocket knife while trying to separate the cells which were stuck
together. The battery pack immediately burst into flames in my hands (and I of course dropped it like a
hot potato !). It scorched the parking lot asphalt burning up. That being said,
3S (3 cells in SERIES) charges to a voltage of 12.54V . Three of these in series (3S X 3= 9S) would give
your 37.62V.

9S , 5.0 Ahr would give your 37.6V at 5A.
9S/4P (four in parallel) would give your 37.6V at (4 X 5=20 Ahrs) or 5A for 4 hours.

If you take a 3S , 5.4Ahr battery, ($70)

and a 6S, 5.4 Ahr battery ($140)

Put them together in SERIES, and you have 9S, 5.4 Ah. ($70 +$140=$210)

If you then made 4 of these and put them in PARALLEL, that would give you a 9S,4P, 5.4 Ah,
Which ,if you do the math, is 4 x 5.4Ah = 21.6 Ah @ 37V, which if your load drew 4 A, would last 5.4 hours. By now you realize that 4 x $210 = $840 . That's what they meant when they said Lipo batteries
are expensive. If you had the money, and if you observed all the precautions, it should do what you want
and more. If you have problems, they could be problems that the word "problems" does even begin to
describe. Point being, you need to think it through and cover ALL the bases. So what happens if your
load suddenly develops a SHORT across it ? What would happen to all those batteries if you dead shorted them ? Well of course they would burst into flames and possibly explode but my experience is that they
never actually explode, they flare up like a box of red-tipped wooden matches. There is an initial flareup
which lasts 30 to 60 seconds and then it just burns at a subdued or constant rate. Can this be prevented ? Well by now you should be thinking "FUSE" because that is the make or break safety device you can't
live without in a world of Lipo batteries. You should be all about fuses and never use your batteries without them. Would any sane person use the above battery configuration ? Absolutely. They do it every
day, or at least every weekend. Most of them go 5 years without a single incident because they are care-
ful. When something does go terribly wrong they are prepared for it because they have been waiting 5
years for just such a thing to occur and therefore are not caught by surprise. Safety first. If you're going
to play with matches, where asbestos gloves.

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