BK Precision 8500 DC Load behavior - Page 1
Forum section: electronics / testgear

author: valley001
author_rank: Frequent Contributor
timestamp: October 02, 2021, 04:12:47 am
content: Hi all,I bought a BK 8500 for evaluating lifepo4 batteries I am building for personal use (kids electric bikes and such). I had one of those power mosfet on a board deals from amazon, the power mosfet finally blew because I forgot to turn the pots on it down to zero before hooking up my battery...I was tired of the thing anyway.The BK is a huge step up for me, but I am curious about its behavior when doing the battery test. My arrangement is: 24 inch leads, 6 gauge copper stranded wire connected at both ends with a ring terminal crimped down good. I have the sense leads crimped into the ring terminal that connects to the battery. The ring terminals + and - connect directly to their corresponding battery terminals.If I hook it up to, for example, a 12v lifepo4 and drain it @ 12 amps, when the device is working the display reads 11.997 amps or so. The voltage also reads some 17mv low compared to my HP 3456a (I trust the 3456a).Is this a normal behavior out of this unit, or may it indicate a calibration issue? Could these errors be induced by the 1/2 inch or so of copper between where the sense lead is crimped in and the actual battery terminal? I would like to know that my capacity results are at least somewhat accurate.Thanks all.

author: HKJ
author_rank: Super Contributor
timestamp: October 02, 2021, 06:00:21 am
content: The load has terminals for voltage sense on the back, using them you can eliminate voltage drop in the wires.

author: threephase
author_rank: Regular Contributor
timestamp: October 02, 2021, 07:23:53 am
content: The nominal resistance of 6AWG wire is 0.395 Ohms/1000ft. So at 12A load you can work out that the voltage drop across 2ft of wire will be 9.48mV, multiply that by 2 for both the leads gives 18.96mV total voltage drop across your 6AWG leads.As HKJ advises, the BK8500 has sense terminals on the rear of the unit, so connecting your sense wires into those should produce a closer reading to the HP3456a.The BK8500 has a voltage accuracy of +/- (0.05% + 0.02%FS), which would be +/- 9.6mV at 12V. No idea what the voltage accuracy is for the HP.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 02, 2021, 09:10:23 am
content: Quote from: HKJ on October 02, 2021, 06:00:21 amQuote from: threephase on October 02, 2021, 07:23:53 amThe OP mentions the sense leads, so I expect he has them connected in the correct place.Quote from: valley001 on October 02, 2021, 04:12:47 amQuote from: threephase on October 02, 2021, 07:23:53 amThe accuracy of the HP3456A is nominally an order of magnitude and a half better than the load, and unless it's been abused I'd expect it to at least meet, and probably exceed its accuracy by a margin.You may have made a mistake here; as it's being used to discharge a battery in CC mode, the accuracy specs are +/-(0.2% + 0.15% FS) for the load regulation and measurement for the current, and +/-(0.02% + 0.02% FS) for the voltage measurement.An indicated current of 11.997A could therefore be any value between 11.859 and 12.135A. Without knowing what the actual voltage readings are it's difficult to be precise, but for a nominal 12V reading the uncertainty would give you something in the range of 11.994 to 12.006V. I'd expect an indicated 12V on the HP3456A to be spot on to millivolt accuracy, the uncertainty would be in the low 100's of microvolts.Of course, without knowing the calibration and usage history of either item there's a good deal of guesswork...One thing to check is the test leads for the meter, are they in good order, or old and stretched?I wouldn't expect much of a volts drop across the body of the crimps though obviously there will besome, but whether it's enough to make up the difference would very much depend on their quality. Are they tin plated copper? Or *coughs* aluminium..?

author: valley001
author_rank: Frequent Contributor
timestamp: October 02, 2021, 03:22:23 pm
content: Thank you for your reply. I am using kelvin clips on the 3456a. The crimped ring terminals are pure non plated copper, with the sense wire crimped in with the 6AWG stranded copper wire on the ring terminal. The other end of the sense leads, I tinned the stranded wire (22awg I think) so it would insert into the push connectors at the back of the 8500. Since we are talking millivolts there should be no issues with thermal offset correct?I have only paid attention to the 8500 voltmeter while the unit is operating under load, so I will:1- Observe the reading and compare to the 3456a while NOT under load, and2- Fabricate a set of crimped copper leads for the 3456a to rule out the cheap kelvin clips.Quote from: AVGresponding on October 02, 2021, 09:10:23 am

author: AVGresponding
author_rank: Super Contributor
timestamp: October 02, 2021, 05:49:40 pm
content: One thing you might consider in order to absolutely minimise any possible volts drop in the crimp lugs is to use separate ones for the sense wires, and connect these to the battery before the load crimp lugs. We're talking a few tens of ppm's here probably, but hey, if it gets you closer to the truth...Speaking of ppm's, adding an extra metal (by tinning the load end of the sense wires) always will give some level of effect; you'll just have to decide if it's a significant one. In this case, probably in the very low ppm's.

author: valley001
author_rank: Frequent Contributor
timestamp: October 02, 2021, 07:16:15 pm
content: I took a reading with the 3456a both at the battery terminals and at the input terminals of the BK 8500. The BK 8500 reads 3.34v at rest. With the 3456a I have:At the battery I have 3.35229VAt the 8500 input I have 3.35229VI think my 8500 is out of calibration by some 10-20mv or so. I don't suppose there is an easy way to change the calibration factors on this thing? I guess I discovered why it was surplus. How much error will this introduce into my battery capacity measurements?Quote from: AVGresponding on October 02, 2021, 05:49:40 pm

author: HKJ
author_rank: Super Contributor
timestamp: October 02, 2021, 07:29:46 pm
content: Quote from: valley001 on October 02, 2021, 07:16:15 pmBasically none.You can see a lot of battery discharge curves on my website, a few 10 tents of mV will not have any significant impact on them.The BK load can be calibrated electronically.

author: valley001
author_rank: Frequent Contributor
timestamp: October 02, 2021, 08:51:13 pm
content: OK.....so it turns out I inadvertently turnedoffthe sense option when fiddling with the menu.With sense turned on I have a somewhat more reasonable 9mv disagreement between the 8500 and the 3456a. I can live with that.