Battery Capacity Tester Circuit

jateureka

jateureka

10 kW
Joined
Apr 6, 2010
Messages
824
Location
Brisbane, QLD Australia
Hi, I'm new to the forum and I need a circuit for a battery capacity tester that will allow me to determine the Ahr rating of some 36V nominal SLA and LiPo battery packs used in e-bikes. The battery packs are rated up to 14Ahr. I'm not sure that I can break the packs down to cell level for testing, but it would be good if the circuit could do both cell and pack voltages.
The circuit would basically consist of:
a. An active load, to allow a constant current discharge (adjustment from say 1A to 10A)
b. a low voltage disconnect, so the load is removed once the battery cut-off voltage is reached
c. a timer so that the Ahr rating can be determined

I have searched this forum also not much luck. This thread (no comments in almost a year) is a bit overkill for the 10Ahr and 14Ahr battery packs I need to test http://endless-sphere.com/forums/viewtopic.php?f=14&t=7446

I have found a few circuits on the web, but none that satisfy all the above requirements.
This one http://www.discovercircuits.com/H-Corner/battery-tester.htm for testing lithium button cells under 100mA current draw, but I really like the simplicity of his 'poor man's timer' flip flop circuit!

This circuit http://www.siliconchip.com.au/cms/A_107821/article.html is limited to a maximum battery voltage of 15V and 8A current.
My electronics theory is a little rusty, so can someone tell me what mods are required to the circuit to increase the allowed battery voltage to cater for a 36V nominal battery?
Would it be a case of stepping down the low voltage disconnect sensed voltage, with a voltage divider or zener diodes, to pin 13 of IC1d (ignoring the timer circuit for now)? :?:
 
i use an electric heater and a big soldering iron and light bulb for a load. about 3.6A so not too rapid at all.

i use a voltmeter and run the discharge through my 10A setting on another voltmeter to sample voltage and current over the discharge and integrate..
 
dnmun said:
i use an electric heater and a big soldering iron and light bulb for a load. about 3.6A so not too rapid at all.

i use a voltmeter and run the discharge through my 10A setting on another voltmeter to sample voltage and current over the discharge and integrate..
Click to expand...

Yep, it's what I do. This doesn't have the advantage of testing large currents and it's not *that* quick, but it gets the job done. I recently charged one of my a123 cells this way and by integrating the current over time, I calculated that it had charged it 2.2 Ah. Yep, that cell looks healthy.
 
Thanks for your replies.
Unfortunately I need something a little more accurate than that, which is why I'd like some help modifying this circuit to test 36V 14Ahr SLA and LiPo battery packs.
jateureka said:
This circuit http://www.siliconchip.com.au/cms/A_107821/article.html is limited to a maximum battery voltage of 15V and 8A current.
My electronics theory is a little rusty, so can someone tell me what mods are required to the circuit to increase the allowed battery voltage to cater for a 36V nominal battery? Would it be a case of stepping the sensed voltage down, with a voltage divider or zener diodes, to pin 13 of IC1d (ignoring the timer circuit for now)? :?:
Click to expand...
 
Probably the easiest way to test your battery capacity is to use a CycleAnalyst. After your done testing your battery off of your bike, you can then use it on your bike to monitor everything that's going on with your battery and motor.

Just my 2 cents.
Ambrose
 
If you really want to use that circuit, upgrade the current sense resistor (higher watts, lower resistance) and the mosfet. You may want to use a bank of mosfets if the current is great enough. You also may want to use a resistive load along with it as well if the current is great enough as it appears the shunt resistor is supposed to be the load which may place a limiting factor on your power testing capability.

It appears you'll need to drop the voltage to the components. A way to do this would be a resistive divider if the current draw was low enough, or you could use an amplified zener. Basically, bias a transistor using the appropriately rated zener (12v for example, or whatever voltage seems good for the chips) and using a current limiting resistor on the zener. You may want to use a transistor with a high HFE depending on the current draw of the components so, in other words, a darlington.

Another way would be to use an appropriately rated voltage regulator, however, it might be difficult to find a cheap regulator if the input voltage is too high.
 
After a little more research I found some more circuits that are just constant current dummy loads - no LVD or timer for Ahr
http://www.rcgroups.com/forums/showthread.php?t=1107072&page=15
http://electronicdesign.com/article/power/resistive-dummy-load-draws-constant-current-from-1.aspx Description for 1.2V to 50V and up to 10A
the circuit is here http://archive.electronicdesign.com/files/29/4056/figure_01.gif

I now have a Turnigy wattmeter and the LiPo battery has BMS/lvd and fuse, so I have started to make a dummy load with parts in my 'junk box'.

The basis for the load is an old pentium 100 computer. I'm using the case (cut down) to house the assembly, the ATX supply for the +12V to power the circuit and the CPU fan/heatsink assembly to cool the FETs - a couple of IRF1010N.

According to my calcs these FETs should be good for at least 9A without a heatsink at 40C ambient, so two in parallel on a heatsink should easily handle 20A.
 
Be careful...
The best each of those IRF1010N FETs can do, acting as an active load and without a heat sink is about 2W...maybe. At 36V, that's about 56mA! They will instantly burn out at the 320W (or so) power levels when handling 9A at 36V. With a decent CPU heat/fan assembly and two of those FETs, you'll get no more than about 125W, absolute max, before exceeding the 175C max rated temperature for those FETs. At 36V, that's about 3.5A or so. Since the pack voltage will drop under load, perhaps 5A.

Once you start using FETs as an active load, you have to think about the power each is dissipating, not just the current and voltage. The thermal resistance of the FET and the heat sink are critical to determining how hot the FETs will run for various power levels. Then you can use the pack voltage to determine how much current you can draw before exceeding this power level and burning out the FETs.

The rcgroups circuit is a good one (full disclosure...I had a bit of input towards its design) and a great place to start. If possible, I recommend selecting FETs that have a lower junction-sink thermal resistance. Your IRF1010N's are 1.35C/W and you can easily find 1.0C/W which will easily increase your power handling another 15W/FET using a decent heat sink. And for higher voltage packs, like your 36V ones, I highly recommend using a FET that's rated for use as a constant-current load. Check out Figure 8 in the IRF1010N data sheet, the Max. Safe Operating Area graph. It only shows the FET rated for up to 10mS at a time acting as a load. There's no DC plot line. This FET is susceptible to blowing out at much lower power levels, voltages and temperatures than the other specs might indicate. The rcgroups design uses a FET that is rated for use as a load and there are a few others out there.

For those whose heads are now hurting...sorry for the techno-babble. It's always a challenge to figure out where to stop when explaining something that might lead to lots of questions. :)
 
Thanks for the response CamLight

It seems I have based my calcs on operating as a switch instead of a load :oops: My electronics is very rusty!

The IRF1010N didn't cost me anything, same as the PC. I will keep a carefull eye on the case temps as I ramp up the current. Once I get a grasp of this cicruit I will either replace them or build another with better FETs.

Can someone tell me where in Australia I can get the IPP80N06SL FETs used in the RCGroups circuit?
 
jateureka said:
Can someone tell me where in Australia I can get the IPP80N06SL FETs used in the RCGroups circuit?
Click to expand...

Try this
http://www.mouser.com/Semiconductors/Transistors/Transistors-General-Purpose/MOSFET-Power/_/N-7h6e2Zscv7?Keyword=IPP80N06&FS=True
 
Thanks for the link. Those FETs are $1.98 AUD at Mouser.
Unfortunately postage from USA to Australia is a killer. Mouser want $39 AUD and Digikey $30 USD!!

I see that Active Components is an Australian reseller for Mouser, so will check pricing there.

Edit: Active components charge $15 AUD on p/h on orders under $120.
The IPP80N06Sxx ranges from $2.28 to $4.60 AUD depending on what xx is.
I got excited when I saw the FDA69N25 for $4.488 and then I realised the minimum buy was 205 pieces! :shock:
 
jateureka said:
Can someone tell me where in Australia I can get the IPP80N06SL FETs used in the RCGroups circuit?
Click to expand...
IIRC, the RCGroups load kit that "Nobert" is offering is about ready for release. He might have just the FETs for sale (unless you wanted an entire kit). Nobert is in the USA but if he's willing to ship to AU, the cost might be a lot less than ordering from other commercial sources.
 
jateureka said:
Thanks for the link. Those FETs are $1.98 AUD at Mouser.
Unfortunately postage from USA to Australia is a killer. Mouser want $39 AUD and Digikey $30 USD!!
Click to expand...
I'd volunteer to help, but last time I did that for someone, the USPS double-charged me (after a comedy of (their) errors online and then at the PO) and won't refund it. Since I can't afford that to happen again, I won't mail anything out that I can avoid (and I don't feel like giving them any business unless I absolutely have to).

But someone else might be able to do that for you--have you pay them to order the stuff here in USA by cheapest shipping, then USPS flat-rate it to you, which should cost about $13USD for the small flat rate box, about the size of a large paperback or small hardback book. Should still work out significantly cheaper than directly from Mouser/Digikey, I'd think.
 
The data sheet for IPP80N06S3L-08 on the Mouser web site link above does not show a DC load line in the SOA graph.

ALso IRF540PBF ($1.88 AUD Farnell) does not show a DC load line in the SOA graph either but IRF540A ($2.93) does.

It looks like you need to look at the data sheet for each part as they are all different and it tends to be the cheaper parts that do not have the DC load line.
 
Amberwolf,

Active Components is an Australian reseller for Mouser and they only charge $15 AUD p/h, or if your order is over $120 then it's free.

I have also found Farnell sell Fairchild parts and they have free p/h on web orders but their base price is high - $11.25 AUD for FDA59N25 compared to $6.82 at Active Components.
 
jateureka said:
The data sheet for IPP80N06S3L-08 on the Mouser web site link above does not show a DC load line in the SOA graph.

ALso IRF540PBF ($1.88 AUD Farnell) does not show a DC load line in the SOA graph either but IRF540A ($2.93) does.

It looks like you need to look at the data sheet for each part as they are all different and it tends to be the cheaper parts that do not have the DC load line.
Click to expand...
Good catch! Looks like Nobert at RCGroups substituted FETs after he and I discussed his design and I did the thermal calculations for the FDP047N08 FET he was originally going to use.

I had forgotten what FET we had originally spec'd and never realized he made the change. :oops: :oops: :oops:
 
I just put in an order with Farnell for some FDA79N15 at $3.48 AUD plus GST and some other parts including W57 resettable thermal circuit breakers 20A 50V for just over $5 with free p/h for web orders.
.
 
I am currently using a Computerized Battery Analyzer (CBA) from West Mountain Radio
http://www.westmountainradio.com/cba.php

I borrowed one from my friend and now I am seriously considering buying one of my own. The device connects by USB to your computer and plots the discharge curve. The setup is completely customisable. You can set a LVC and adjust the load. You can overlay the results on top of the previous discharge curves. Export spreadsheets and images.
It is a bit pricey, but even the base model has many, many features.
 
As part of pulling together a semi-automated battery tester (SABT) I am testing components which are UL listed for a small commercial environment to expand upon those who already own a West Mountain Computerized Battery Analyzer (CBA). A question:

?Has anyone coupled a West Mountain CBA, West Mountain CBA Controller, and a multi-chemistry charger such as a Grin Technologies Satiator for a semi-automated battery tester?

This SABT solution currently combines:
  • a Grin Technologies Satiator (or other Charger with a TTL output or well defined end of charge cycle current)
  • West Mountain Radio CBA (Computerized Battery Analyzer)
  • West Mountain CBA Charge Controller and Temperature Probe
  • West Mountain Extended Software
  • Fireproof Battery Bag or Fireproof Safe

This solution is NOT the lowest cost set of alternatives out there. The selection of components is part of a solution for SABT which can pass muster with a fire marshall or an insurance adjuster!

Thank you for any feedback!

Best, Mark

PS - yes we are aware the Grin Technologies Grinstation provides the same function. Many already own the West Mountain Radio CBA some own the CBA amplifier for larger battery pack testing. So we are looking to expand the capabilities of the existing CBA base.
 
We have found a few quirks in setup of the West Mountain CBA IV, CBA Charge Controller, and Grin Tech Satiator as a Semi Automated Battery Analysis (SABA)

  • the default configurations of CBA Charge Controller have a 55v limit, and that limit IS enforced when encountered!
    • BUT there is a mod that can be performed by West Mountain Radio to raise the limit to 64v
    • AND our CBA IV has worked for years with peaks up to 60v at 2A or less
  • the Grin Tech Satiator must have the default profile match the battery under test and then power reset before starting any test cycles
  • the profiles for CBA software testing can be saved IF you purchase the extended CBA software license
  • CBA software defines a "cycle" as each test, so a "charge-discharge" run for 3 cycles performs a charge, a discharge and another charge test

We will post some drawings and other details as part of prep for our next Light Electric Vehicle Technicians Training:
http://levassociation.com/leva-certified-technician-training

Best, Mark
 
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