Measuring voltage sequentially across numerous individual batteries - Page 1
Forum section: electronics / general

author: jkrbasu
author_rank: Contributor
timestamp: July 17, 2017, 06:31:56 pm
content: Hi EEVbloggers!I'm a total newb and attempting to build a multi-channel battery cycler. I would love to be able to use a single DMM to take four-wire measurements of voltage on numerous batteries sequentially (to save me from having to buy multiple DMM's...I already have multiple power supplies..). Let's say I'd like to take measurements across 25 batteries once per second (so one measurement per 50 msec)I was thinking about using a matrix/switch like a Keithley 7001, but I'm worried that the relay switching time might be too slow. Does anyone know of a solid state switch that doesn't suffer from settling requirements of physical relays?Thanks very much!

author: branadic
author_rank: Super Contributor
timestamp: July 17, 2017, 06:44:57 pm
content: Well, you have to account for a few 100ms with relays. There is currently a Scanner/Multiplexer in the bay:http://www.ebay.de/itm/Prema-Scanner-2024-/232412002456?hash=item361cd5b898:g:lIYAAOSwH09ZLZmu-branadic-

author: nfmax
author_rank: Super Contributor
timestamp: July 17, 2017, 07:00:12 pm
content: An HPAK 34970A with a relay scanner card will do the job nicely. It does 47 readings/sec into internal memory at 5 1/2 digits, up to 600 at 4 1/2. The standard 34901A relay multiplexer will do 60 channels/sec, or the 34902A reed relay multiplexer will scan upto 250/sec. They are commonly found on eBay. The 34972A is rarer and more expensive, but has USB and network instead of GPIB and serial.

author: alm
author_rank: Super Contributor
timestamp: July 18, 2017, 12:15:02 am
content: Relay-based scanners are common because they are the most versatile due to the high insulation resistance in off state. Obviously switching/settling delay and relay wear is an issue. Some multimeters (e.g. Keithley 199/2000/2001, some Prema meters, HP 3457A) have an optional scanner/multiplexer plug-in. These are often limited to 8-10 channels. There are also stand-alone multiplexers for if you want more channels/flexibility.The HP/Agilent/Keysight 34970 was already mentioned. Keithley has the older 700 and the newer 7000 and 2700 series. HP had the 3488a. Make sure you get a matching scanner card.There are a couple of scanner cards using solid state relays. TheKeithley 7710is one such card for the 2700. There is currently oneon ebaywith 2700 for $540 or best offer, which is not a terrible deal in my opinion (no association to the seller, can not vouch for the seller or this unit). Note that the 2700, unlike a 7001, includes a multimeter.TheKeithley 7015(-C and -S only differ by the connector type) is another solid state scanner card, but for the 7001 mainframe. Did not see any affordable ones on eBay right now. Not sure what Agilent/Keysight solid state scanner modules might be affordable.

author: jkrbasu
author_rank: Contributor
timestamp: July 18, 2017, 06:44:19 am
content: Thanks for the recommendations! I really like the 2700- it's got the multiplexer, and the DMM is better than what I have now anyway!

author: tautech
author_rank: Super Contributor
timestamp: July 18, 2017, 07:30:47 am
content: Quote from: jkrbasu on July 17, 2017, 06:31:56 pm25 channels of measurements is a big ask but you could almost do it with 2 units of the SDM3000 bench meters with the SC1016 16 ch scanner card fitted.From the manual:The SIGLENT SC1016 is a multiplexer that provides multi-point measurement capabilities to the SDM3000 series of digital multimeters. The scanner features 12 multi-purpose + 4 current channels.The SC1016 is a factory fitted accessory.SDM3000 family product page:http://siglentamerica.com/pd.aspx?tid=37&T=1Thread:https://www.eevblog.com/forum/testgear/siglent-new-bench-dmm-sdm3055/

author: branadic
author_rank: Super Contributor
timestamp: July 18, 2017, 04:43:07 pm
content: Prema 5017 SC does have a scanner / multiplexer / switch on board, able to measure 80channels single-ended with common ground, 40 channels 2-wire and 20 channels 4-wire. It contains a 7.5 digit multimeter, although based on LM399. I'm happy to have one of those at work and another in my homelab.This gear would have been great if they would have include the better PRI 5610 E converter (linearity error typ. 0,05 ppm, max. 0,08 ppm) together with LTZ1000.-branadic-

author: macboy
author_rank: Super Contributor
timestamp: July 20, 2017, 03:10:08 pm
content: Quote from: nfmax on July 17, 2017, 07:00:12 pmI recently acquired a 34970A myself and it's a nice piece of kit. Two things to keep in mind: some early 34970A did not include the built in DMM and are therefore a scanner only (Option -001 has no DMM). You can add the DMM to it with the 34970-80010 retrofit kit, but that's probably a rare find on its own. Also, the 34901A is only 20 channels, so two cards would be required (unit supports three cards)I also have several Keithley 7001 units and 40ch cards for them. I certainly wouldn't mind parting with one. Or two

author: alm
author_rank: Super Contributor
timestamp: July 20, 2017, 03:27:28 pm
content: Quote from: macboy on July 20, 2017, 03:10:08 pmAre there any solid state relay cards available for the 34970? Switching all contacts every second for many hours translates to a lot of relay wear.

author: macboy
author_rank: Super Contributor
timestamp: July 20, 2017, 04:55:37 pm
content: Quote from: alm on July 20, 2017, 03:27:28 pmthe relays are rated 100 million cycles at no load (i.e. measuring voltage). That's 38 months of continuous operation at one cycle per second. Considering the amount of time the charger will be used, they will likely never wear out.

author: nfmax
author_rank: Super Contributor
timestamp: July 20, 2017, 06:02:51 pm
content: Regarding 34970As. If you buy a second hand relay card, you can read the cycle count for each relay. A tip: the higher - number channelsalwayshave lower cycle counts - everybody starts using channels from 1! The option 001 no DVM is still available from Keysight if you want it.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 20, 2017, 06:09:17 pm
content: What about using ordinary CMOS switches (like 4051, 4066, etc.) and a "sample-and-hold" capacitor. You could select the capacitance value for optimal speed vs. "sag". Surely the cells have a low enough source impedance that you could charge up (or down) the capacitor in a few milliseconds and then switch it over to an ADC. Sounds like an ideal Arduino (et.al.) project.

author: alm
author_rank: Super Contributor
timestamp: July 20, 2017, 06:39:46 pm
content: Quote from: Richard Crowley on July 20, 2017, 06:09:17 pmMight work if the battery voltage is within range. Depends on if you want a project or a tool.Quote from: macboy on July 20, 2017, 04:55:37 pmSo no solid state relay cards for the smaller Agilent multiplexers?

author: edpalmer42
author_rank: Super Contributor
timestamp: July 20, 2017, 06:46:15 pm
content: I didn't see any mention of what type of batteries you're monitoring or what resolution you need, but a BMS (Battery Monitoring System) for a Lithium battery pack does exactly what you're talking about. It monitors the voltage of each cell in the pack to make sure that the voltage doesn't go too high on charge or too low on discharge. You might want to check a few data sheets and see if you can find one that fits your needs.Ed

author: jkrbasu
author_rank: Contributor
timestamp: July 21, 2017, 06:33:56 am
content: I didn't see any solid state relay cards for the 34970A, however, there is a reed relay card (34902A), which is faster than the 34901A EMR card (250 ch/s vs 60 ch/s).They're pretty cheap on eBay, and I was just able to acquire the 34972A DMM plus switch unit from a friend, so I think I'll go with this and the reed relay card...any thoughts? What's the lifetime of reed relays vs EMR relays like?Thanks!

author: jkrbasu
author_rank: Contributor
timestamp: July 21, 2017, 06:35:47 am
content: Also I imagine the reed relay cards are less clicky than the EMR cards! The noise might drive me kind of crazy!

author: jkrbasu
author_rank: Contributor
timestamp: July 21, 2017, 06:38:18 am
content: Hi Ed, I'm building a battery cycler, not so much a battery management system. The goal is to cycle something like 25 batteries, each with their own dedicated power supply, but using only one DMM + switch to monitor their voltage. I'll be taking measurements at least once a second on each battery, because it is important to keep a close eye on voltage as it indicates the endpoint of charge/discharge. I am testing rechargeable Lithium ion coin cells

author: daqq
author_rank: Super Contributor
timestamp: July 21, 2017, 06:50:58 am
content: If you don't mind making your own system, you may want something like this:http://cds.linear.com/docs/en/datasheet/68021fa.pdfAlso, how much precision do you need? It's nice to have a single 6.5 digit DMM, but for the same price you probably could get a few lesser DMMs. With less muxing (not one of 25 per cycle, but, say, 6 of 25 per cycle) the speed requirements on the relays would drop significantly.

author: e61_phil
author_rank: Frequent Contributor
timestamp: July 21, 2017, 08:22:27 am
content: If price is an argument: You get the HP 3488 switch controller for almost nothing (<10$ sometimes). The modules are also very cheap. I only found a speed specification for the newer 20Channel model. That says 43Ch/s.

author: nfmax
author_rank: Super Contributor
timestamp: July 21, 2017, 10:12:55 am
content: From the module specifications, the rated life of all relay types is 100,000,000 cycles at no load, 100,000 cycles at rated load. Rated load is 300V/1A/50W for the armature relays, 300V/50mA/2W for the reed relays. The service manual has a procedure for measuring and verifying the relay contact resistance (which is the most significant indicator of wear).

author: jkrbasu
author_rank: Contributor
timestamp: July 21, 2017, 05:33:10 pm
content: Quote from: e61_phil on July 21, 2017, 08:22:27 amWow that's pretty nice- I always wonder why people buy the expensive new stuff when eBay is flooded with cheap alternatives like this!Do you happen to know of a solid state or reed relay switch that is similarly old but good and similarly priced?

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: August 02, 2017, 07:47:32 pm
content: Quote from: jkrbasu on July 17, 2017, 06:31:56 pmVoltage is always atwo-wire measurement, afaik.

author: alm
author_rank: Super Contributor
timestamp: August 02, 2017, 08:24:15 pm
content: I assumed that four-wires meant two for charging and two for measuring the voltage. That is the only way it would make sense.

author: jkrbasu
author_rank: Contributor
timestamp: September 19, 2017, 01:15:52 am
content: Correct- sorry for the confusion!