Multi-pack battery voltage monitor
in [Design]
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From: krw on 23 Jul 2010 18:37 On Sat, 24 Jul 2010 07:25:21 +1000, Grant <omg(a)grrr.id.au> wrote: >On Fri, 23 Jul 2010 06:38:49 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> wrote: > >>On Jul 22, 7:20 pm, Grant <o...(a)grrr.id.au> wrote: >>> On Thu, 22 Jul 2010 19:01:00 -0400, "tm" <no...(a)msc.com> wrote: >> >>> >...At least a 0.1 volt accuracy. That is non-trivial >>> >at 1000 volts above ground. >> >>> Non trivial? Easily enough done with opto couplers, there's 10kV and >>> up ones >> >>Opto couplers need circa 3V and 4 mA to drive 'em on and off. >>Is there a plan to make isolated power supplies for each of >>the seventy-two cells? Using opto couplers is not a real >>solution until you provide the support circuitry. You'd >>also want an ADC at each node, and put the digital output onto >>the optocoupler's drive amplifier. > >I see lots of power available there. Besides you can command the >things to be off most of the time, battery voltage is a fast moving >target ;) Two optos plus a small PIC per cell. Cheap, easy, not >much support components needed. Why two isolators?
From: Grant on 23 Jul 2010 19:06 On Sat, 24 Jul 2010 07:25:21 +1000, Grant <omg(a)grrr.id.au> wrote: >On Fri, 23 Jul 2010 06:38:49 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> wrote: > >>On Jul 22, 7:20 pm, Grant <o...(a)grrr.id.au> wrote: >>> On Thu, 22 Jul 2010 19:01:00 -0400, "tm" <no...(a)msc.com> wrote: >> >>> >...At least a 0.1 volt accuracy. That is non-trivial >>> >at 1000 volts above ground. >> >>> Non trivial? Easily enough done with opto couplers, there's 10kV and >>> up ones >> >>Opto couplers need circa 3V and 4 mA to drive 'em on and off. >>Is there a plan to make isolated power supplies for each of >>the seventy-two cells? Using opto couplers is not a real >>solution until you provide the support circuitry. You'd >>also want an ADC at each node, and put the digital output onto >>the optocoupler's drive amplifier. > >I see lots of power available there. Besides you can command the >things to be off most of the time, battery voltage is a fast moving Oops! ^^^^^--> not! >target ;) Two optos plus a small PIC per cell. Cheap, easy, not >much support components needed. Grant.
From: Gary Lynch on 23 Jul 2010 21:16 In article <mlmd469g97a9dc08udedrjurvbviveb061(a)4ax.com>, peter2(a)hipson.net says... >On Wed, 21 Jul 2010 07:15:53 -0400, Gary Lynch <bookworm(a)execpc.com> >wrote: > >> +------+ I am looking for an economical way to measure > Wow! What a turnout!! I apologize for my apparent disappearance. I had to change new readers before I could reply back. This also caused me to lose all posts between 8:00 am and 3:00 pm on July 22; so you wrote something in that window, I fear I can't see it. I don't have the answer to every question, but I'll give it my best shot. 1. I used the word 'pack' to refer to a 6-cell, lead-acid battery--the same type as you use to start your car. Open-circuit voltage about 12.6 V. If there's an unambiguous industry term for that, I wouldn't mind hearing it. A string of 72 would add up to 907 V. I have worked with busses close to 700 V, but this may be a record, especially if it triggers some kind of UL rule. 2. By 'offset' I presume you (PeterD) mean some kind of voltage regulator? Cost is a factor and I should explain the scenario. The appliance is a UPS. The batteries must be tested at regular intervals. The UL spec calls for measuring the voltage of the entire stack and warning the user when the average voltage per cell drops below a certain threshold. But real-world batteries don't deteriorate uniformly and we would like to measure them individually and sound an alarm when a single battery can not pull its own weight. This happens while the unit is driving its full rated load on the battery, so the voltage will be dropping as we go. The shortest test lasts 30 seconds, and fitting 72 tests into 10% of this window gives us 42 msec/conversion (assuming they are executed sequentially). If that proves a burden, I can also make the test longer. I don't want it to take so long that the last battery looks bad only because it was measured way down the discharge curve. 3. I don't have a spec for accuracy, but let's say we sound an alarm when a battery drops to 10 V. If I put 10 bit steps between 12 & 10; that's 0.2 V. That's crude but a starting point. I just realized I will have to account for the accuracy of my voltage regulator if it's in series with the A/D. Getting complicated. I would have to look at the curves for zener diodes. That's obviously cheaper than a regulator, but I lose more accuracy. 4. My boss wants to use a bank of resistive dividers enabled one at a time by relays into a single A/D, referenced to ground (similar to a recommendation posted here). I don't like this as the top battery will be subject to a large common-mode voltage, making that measurement much less accurate than the bottom battery's. 5. I don't have to control the current or voltage of the batteries, just want to monitor their health. 6. Whatever solution I implement will be under the control of a microprocessor. Must execute without human intervention. 7. I believe I can draw milliamps out of the batteries during the test. The challenge is the quiescent current, as many a customer will take delivery and stash it in a warehouse for 6 months before trying to install it, and the test circuit may not load it down while in storage. 8. I don't mind using opto-couplers, but 72 of them will add up fast. 9. PIC does make a uP with built-in A/D converter and UART for under $1. Isolation is still unresolved. We definitely cannot afford 72 isolated power supplies. Hope that gives you a better picture. ============================================================ Gary Lynch To send mail, change no$pam gary.lynch(a)no$pam.com in my domain name to ieee. ============================================================
From: Grant on 24 Jul 2010 02:16 On Fri, 23 Jul 2010 17:37:14 -0500, "krw(a)att.bizzzzzzzzzzzz" <krw(a)att.bizzzzzzzzzzzz> wrote: >On Sat, 24 Jul 2010 07:25:21 +1000, Grant <omg(a)grrr.id.au> wrote: > >>On Fri, 23 Jul 2010 06:38:49 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> wrote: >> >>>On Jul 22, 7:20 pm, Grant <o...(a)grrr.id.au> wrote: >>>> On Thu, 22 Jul 2010 19:01:00 -0400, "tm" <no...(a)msc.com> wrote: >>> >>>> >...At least a 0.1 volt accuracy. That is non-trivial >>>> >at 1000 volts above ground. >>> >>>> Non trivial? Easily enough done with opto couplers, there's 10kV and >>>> up ones >>> >>>Opto couplers need circa 3V and 4 mA to drive 'em on and off. >>>Is there a plan to make isolated power supplies for each of >>>the seventy-two cells? Using opto couplers is not a real >>>solution until you provide the support circuitry. You'd >>>also want an ADC at each node, and put the digital output onto >>>the optocoupler's drive amplifier. >> >>I see lots of power available there. Besides you can command the >>things to be off most of the time, battery voltage is a fast moving >>target ;) Two optos plus a small PIC per cell. Cheap, easy, not >>much support components needed. > >Why two isolators? For low power option, described upthread. Grant.
From: Paul Keinanen on 24 Jul 2010 02:20
On Fri, 23 Jul 2010 21:16:40 -0400, "Gary Lynch" <gary.lynch(a)ieee.org> wrote: >I apologize for my apparent disappearance. I had to change >new readers before I could reply back. This also caused me >to lose all posts between 8:00 am and 3:00 pm on July 22; so >you wrote something in that window, I fear I can't see it. Use groups.google.com and then select "Advanced Search" select some keywords and limit search to sci.electronics.design. > >4. My boss wants to use a bank of resistive dividers > enabled one at a time by relays into a single A/D, > referenced to ground (similar to a recommendation > posted here). I don't like this as the top battery > will be subject to a large common-mode voltage, > making that measurement much less accurate than the > bottom battery's. > >5. I don't have to control the current or voltage of the > batteries, just want to monitor their health. > >6. Whatever solution I implement will be under the control > of a microprocessor. Must execute without human > intervention. > >7. I believe I can draw milliamps out of the batteries > during the test. The challenge is the quiescent > current, as many a customer will take delivery and stash > it in a warehouse for 6 months before trying to install > it, and the test circuit may not load it down while in > storage. Use a separate optocopler to fire up the test. >8. I don't mind using opto-couplers, but 72 of them will > add up fast. > >9. PIC does make a uP with built-in A/D converter and UART > for under $1. Isolation is still unresolved. We > definitely cannot afford 72 isolated power supplies. Use a modular design, with say 9 batteries served by an isolated module. Depending on the physical battery layout, some other grouping might simplify cabling requirements. Anyway, the number of batteries is so low that dividing down the voltages from these 9 batteries can be done without extremely critical component tolerances. The processor in each module would require a 9 input multiplexor to select one of the voltage dividers at a time to the ADC. With eight isolated modules, the number of optoisolators is reduced, if the modules are addressed separately, a single series current loop can be used to read the measurements. A simple ON/OFF addressing would require eight separate wire pairs to the module command port. However, if the select signal contains an address, only a single loop would be required to command all modules. Normally, the command loop would be powered down, which would also put the modules to sleep. A common broadcast command would activate all modules to start measuring. Later on, each module would interrogated and each module would send the stored 9 measurements in a single message. Thus, only two pairs of wires would be needed from the main processor to the battery array, one for the command loop and the other for reading the results. A modular design is more flexible, allowing the same modules to be used with different number of batteries. |