DIY Capacity test battery packs

[pilotg2]

Hello all

I have built a DIY battery capacity tester and as i’m not electrical engineer I wanted to run a few things by you before I test my good battery packs as there are a few things I don’t understand. I’m using a watt meter to measure the current flow.

The load is from oven, stove and oil heating elements. I am useless and describing things so I have attached pics - have I connected them optimally to maximise the load?

Currently the load varies depending on the battery. So at 40V it’s 4.7A and 188W. I know that’s V x A. With a higher voltage battery the amps are higher. Why is that? Is it that the higher voltage provides more power so I presume the elements will get hotter (motor would spin faster).

I tested a 36v 10ah battery that I bought 2nd hand from full 42v to 30v and the watt meter read 9.1ah. I'm happy with that but is there a calculation I could use to give me the ah of the pack but not have to strain it all the way down to 30v. I like to use my packs between 80% and 30%.

I know i’m going to sound like a right plonker with these questions but we all have to start somewhere.

 

[john61ct]

No you are very much on the right track.

You want if possible to have an adjustable Active load that automatically keeps the current flow constant as voltage drops from your 100% down to 0%.

That then allows you to just use Time elapsed between those two points as your main measurement, the coulometer being the more approximate sanity check.

You should be in control of adjusting the current rate discharged, and set it proportionally to rated (or maybe actual) capacity so your C-rate is a constant. With LI chemistries, Peukert being a minor factor my preference is 0.5C, so the test takes under 2 hours.

With lead batteries you should use 0.05C aka the 20-hour rate.

If your purpose is a (maybe one-time) check against the mfg rating of capacity yhen you should use their definition of 100% and 0%, don't worry about the extra stress you'll be gentler rest of the time.

If you want to just establish a benchmark now, to use for comparison in future as the batteries wear, then you can use whatever gentler, you-do-you begin & end points you like.

Just must match them exactly in subsequent tests to get apples to apples and accurate relative SoH%.

Usually such tests are conducted on each cell separately BTW.

Useful links

https://youtu.be/8xX2SVcItOA

https://hackaday.com/2020/02/11/build-your-own-active-load

https://hackaday.com/2018/09/06/zpb30a1-electronic-load-gets-an-open-firmware

https://hackaday.com/2017/02/28/beefy-100-amp-electronic-load-uses-two-mosfets/
https://hackaday.com/2013/10/28/building-a-dc-constant-currentpower-electric-load

https://www.rcgroups.com/forums/showthread.php?2406395-An-effective-variable-load-automatic-LiPo-discharger

https://www.rcgroups.com/forums/showthread.php?3490399-High-Power-LiPo-Discharger-Project

https://endless-sphere.com/forums/viewtopic.php?t=95930

https://www.rcgroups.com/forums/showthread.php?1107072-How-to-build-a-constant-current-discharger

http://www.myrcmart.com/rcx-3in1-battery-balancer-discharger-voltage-indicator-150w-discharging-p-4767.html

https://youtu.be/htY2mHTx3kQ

https://usa-m.banggood.com/AOKoda-CellMeter-8-150W-Discharge-Module-Set-with-Lipo-Battery-Balancer-p-1196376.html

https://usa-m.banggood.com/150W-Constant-Current-Electronic-Load-60V-10A-Battery-Discharge-Capacity-Tester-1602-LCD-Display-p-1255707.html

https://endless-sphere.com/forums/viewtopic.php?t=98407

https://www.diyelectriccar.com/forums/showthread.php?t=202011#/topics/202011

https://www.rcgroups.com/forums/showthread.php?3189173-New-LiPo-discharger-Maylibet

https://endless-sphere.com/forums/viewtopic.php?t=103092

https://www.rcgroups.com/forums/showthread.php?p=42697527

https://www.eevblog.com/forum/projects/massive-igbt_s

 

[John in CR]

Currently the load varies depending on the battery. So at 40V it’s 4.7A and 188W. I know that’s V x A. With a higher voltage battery the amps are higher. Why is that? Is it that the higher voltage provides more power so I presume the elements will get hotter (motor would spin faster).

Think of voltage as pressure and current as flow, both like water going through a hose. If the voltage (pressure) is higher then you get more amps of flow.

I'd use longer wire to get the heating elements further from my batteries and secure that cable in a manner so no one or no animal can trip on it and cause a catastrophe. I like to also use a fan to blow air across the resistors and the heat away from my batteries as well as keep the resistors (heating elements in your case) cooler.

 

[amberwolf]

The load is from oven, stove and oil heating elements. I am useless and describing things so I have attached pics - have I connected them optimally to maximise the load?

i can't clearly see how they're connected. are they all in parallel, or are some in series?

Currently the load varies depending on the battery. So at 40V it’s 4.7A and 188W. I know that’s V x A. With a higher voltage battery the amps are higher. Why is that?

ohm's law explains this.

see https://endless-sphere.com/forums/viewtopic.php?f=14&t=99030&p=1450002 for one of my explanations of that, and how to remember how it works.

if you want a constant load wih different voltages, bu the load is a constant resisstance, you must use fsomething betwen the batery and load tahat keeps he voltae on the laod itself constant. a brushed motor contrller can do this, if you use somethin like hte cycle analys in power-limitin g modde to monitor the battery curent an voltae,m and control throtle of the controller. th e heatin elements go wheeere the motor woul. insead.

I'm happy with that but is there a calculation I could use to give me the ah of the pack but not have to strain it all the way down to 30v. I like to use my packs between 80% and 30%.

there's no way to know for sure the capacity of a pack without actually charging it at the rate you'd normally charge it at, to the voltage you'd normally use i at, and hten discharge it at the rate you'd normally discharge it at, to th e ovltage you'd normally stop using it at.

you can guesstimate it's capacity for your purposes by taking the stated capacity of the pack and multiplyin g by the range you use it in. in your case, you ae only using 50% of it's capacity,. so if it was a 10ah pack, you only get 5ah from it. if it's a couple years old, you can take 20% off of that and call it 4ah. if you get more, great,

 

[Hillhater]

The load is from oven, stove and oil heating elements. I am useless and describing things so I have attached pics -

Ognoring your questions etc,....
... but that is one hell of an impressive piece of modern art !
Maybe a dash of bright red/orange paint on the elements....and you have a real showpiece !

 

[tigcross]

I fully support the DIY approach, and I was going to do a similar thing. Then I found these discharge cells on ebikes.ca. at only $40 for a known quantity it seemed like a better way to go.

6.8 Ohm 250 Watt load resistor in extruded aluminum shell for discharge testing battery packs. Will drain a 36V battery at 5 amps, and a 48V battery at 7 amps. Connect multiple loads in parallel for higher discharge currents.

https://www.ebikes.ca/shop/electric-bicycle-parts/test-equipment/load-resistor-6-8.html

 

[pilotg2]

Ah ha, thank you for taking the time to reply in simple, understandable terms - it makes more sense now. I now know what is missing on this little adventure - keeping the load at a constant current for accuracy. I cannot test the same pack as it wears out accurately without a constant static load. Watching this video helped too - https://www.youtube.com/watch?v=cxkVxi9P0EA

What i wanted to achieve with some accuracy was working out the remaining capacity of a pack. Sort of a benchmark I could test every 3 to 6 months for interest or if i pickup a second hand pack, etc. I wasn't aiming as high as my LiitoKala lii-500S charger results, where it fully charges a cell then discharges at a constant 500mA and then fully charges again and I note down the mAh of each cell.

As the budget for this quick side project is "free only" I may just leave it as it is.

However, with expectations lowered, my element contraption is more like real world testing, in that the Ah result on the meter is equivalent to if I had tested riding my bike where the load would change? Have i got that right. Like the pack i tested in my first post is at 91% with a varying discharge rate. I get how that % is somewhat meaningless but it gives me an ideal that under those conditions the pack is still good and i know what to expect from it.

The elements are all connected in parallel (I think) as my thinking was if i had a smaller pack I can disconnect an element or two.

This is all part of my learning as I'm leading up to building a spot welder and making my own battery pack or repairing one that may have a row of dead cells, etc.

 

[pilotg2]

The load is from oven, stove and oil heating elements. I am useless and describing things so I have attached pics -

Ognoring your questions etc,....
... but that is one hell of an impressive piece of modern art !
Maybe a dash of bright red/orange paint on the elements....and you have a real showpiece !

I thought so too lol. Strange my wife's reaction was not so warming when she discovered it on the outdoor dining table.

 

[john61ct]

Using a coulomb counting meter to report Ah in and out with varying current rates is not as accurate as a precisely timed CC discharge test

some are better than others

might be "good enough" for your purpose

but none are free.

 

[pilotg2]

Hi john61ct

On consideration I'm going to continue this side project but it looks like the DIY route has headed beyond my capabilities. I'll still use my contraption to drain the odd battery for storage or something.

I was going through your links earlier and was interested in this https://usa-m.banggood.com/150W-Constan ... 55707.html

I'm an Aliexpress man and saw this one https://a.aliexpress.com/_siej7L

Or this

https://a.aliexpress.com/_s8ZatT

I'd greatly appreciate it if you could take a look and et me know which would hopefully be more reliable? Also using one of the above I presume with constant load set I can achieve accurate capacity results?

 

[john61ct]

no way to know without trying really

maybe a new thread in the right forum with a specific title

with pics & links asking if anyone's bought one?

If you buy one do please let us know