HP3478A DMM - battery replacement - Page 1
Forum section: electronics / testgear

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: March 09, 2017, 07:30:56 pm
content: Having recently acquired a calibrated HP3478A I have discovered that the calibration data is backed up in volatile RAM that is backed up by a lithium BR2/3 / CR17335 type battery. The battery is rated at 3V. I'm not quite sure how to date the meter, but the battery on the PCB looks original so it looks like careful surgery will be required.Having searched I find that the recommended replacement battery is usually of these:https://cellpacksolutions.co.uk/products/batteries/panasonic-br-23at-23-a-lithium-tagged-battery/https://cellpacksolutions.co.uk/products/batteries/panasonic-br-23a-e2sp-23-a-lithium-tagged-battery-pcb-version/The problem is that neither of these come in the same package, i.e. with the single PCB mounting pin on each end. I was wondering whether I could use this one instead?http://uk.rs-online.com/web/p/speciality-size-batteries/4364803/The voltage is slightly higher, but the IC runs at 5V anyway.Second question: someone suggested that a capacitor could be put across the battery? I can see how this might store charge and power a low current device during a replacement of removable batteries, but since this battery is not likely to be removed except once every 10 years or so when a new one is needed, I can't see any benefit for it here. In fact an aluminium electrolytic might become a problem if it deteriorates and shorts so my gut feeling is to avoid.

author: ap
author_rank: Frequent Contributor
timestamp: March 09, 2017, 07:53:57 pm
content: First of, Li batteries come in different voltages, mostly 3V and 3.6V, depending on the exact chemistry used. On your original one, 3V is printed, you should use also that voltage. So the battery you propose, which has 3.6V, should not be used. (It may work, but circuitry analysis would be required, I would not go that route though)Any such correct Li battery with leads, as long you can fit it in without unproper lead bending (appropriate workmanship) and as long as the capacity is in the same reagion, should be ok. Do not use a battery without leads and try to solder wires to it. Normally you find this type of batteries e.g. in the bay.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: March 09, 2017, 10:42:12 pm
content: Thanks for the advice. This one probably looks more like it and has exactly the same rating:http://uk.rs-online.com/web/p/speciality-size-batteries/4363513/The tagged Panasonic one is rated 3V but 1.2Ah, so a bit more than the existing HP and the Varta from RS which are both rated 0.95Ah. Does the higher energy density mean that it should last longer? If so, then I will probably go for that one. I think it should be possible to solder the tags to the existing posts if I snip the old battery off leaving plenty of length on the post, but I may need to shorten the battery tags. I was wary of soldering directly on to the battery and didn't favour that approach at all. The surface does not solder well and the last thing I want is for the solder joints to come apart. Also, who knows what effect applying direct heat to the terminals would have on the battery chemistry.Rather strangely, the ones I have seen on eBay have generally been much more expensive and this is one time that I do not want to risk fakes of poor quality far east products.

author: ap
author_rank: Frequent Contributor
timestamp: March 10, 2017, 03:21:55 am
content: This one looks much better. This is e.g. the type I use on similar gear for this type of applications (am more into higher resolution gear though, Datrons...).Often it is convenient to leave the original wires in and cut them close to the battery body, and connect the new battery to these. Make sure no heat is applied to the battery itself, dangerous to the chemistry inside.

author: xrunner
author_rank: Super Contributor
timestamp: March 10, 2017, 03:32:59 am
content: Quote from: WaveyDipole on March 09, 2017, 07:30:56 pmI just did a re-battering (is that a word) on a HP 3468B and used one of these -http://www.ebay.com/itm/Brand-New-Varta-CR-2-3-AA-3V-Lithium-Cylindrical-Batteries-1350mAh-CR14335-/302237090193?Is that something that will work for you?QuoteI simply applied 3V from a PS using two wires soldered to the traces at convenient places that run to the battery pins + and -. Just leave the 3V on until the new battery is soldered in.

author: mmagin
author_rank: Frequent Contributor
timestamp: March 10, 2017, 05:08:17 am
content: If you care about getting maximum life from it, I'd stick to the Carbon monofluoride cathode chemistry, e.g Panasonic BR.I think the two pin variant is BR-2/3AE5SPN and the three pin one is BR-2/3AE2SP. Digikey carries both here in the US (stupid lithium restrictions though).What I did when I replaced it with it powered on, but I took two precautions. First I soldered a 3 volt supply elsewhere via a battery holder with 2 AA batteries and a diode, in case power was lost. Then to make sure I didn't short the battery while soldering (negative terminal is at earth potential), I powered the 3478A with a cord that has the ground pin cut off (very naughty!) and verified with an ohmmeter that there wasn't a DC path from the chassis of the meter to the soldering iron tip.

author: BradC
author_rank: Super Contributor
timestamp: March 10, 2017, 05:24:35 am
content: I need to do this in the next year or so. I was just going to leave the meter powered on and use a gas soldering iron.

author: BU508A
author_rank: Super Contributor
timestamp: March 10, 2017, 06:24:38 am
content: Quote from: mmagin on March 10, 2017, 05:08:17 amYep. Very naughty. Better using something like this:http://www.ebay.com/itm/302242839382

author: mmagin
author_rank: Frequent Contributor
timestamp: March 10, 2017, 06:36:56 am
content: Quote from: BU508A on March 10, 2017, 06:24:38 amI'm not sure about that one, but most isolation transformers connect the ground pin on the input to the ground pin on the output, leaving it still ready for the grounded iron to short the battery.

author: BU508A
author_rank: Super Contributor
timestamp: March 10, 2017, 07:10:20 am
content: Quote from: mmagin on March 10, 2017, 06:36:56 amMost isolation transformers of this kind I know are excatly NOT doing this, because this would be stupid.I'm having a Grundig RT-5A (the blue one with black frontend) and the output socket does not have ground connectors at all.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: March 10, 2017, 10:44:13 am
content: Unfortunately the typical eBay UK price is somewhat higher :-http://www.ebay.co.uk/itm/BATTERY-LITHIUM-2-3AA-AXIAL-3V-VARTA-06237501301/361859303264So would I trust that these are genuine and reasonably fresh?http://www.ebay.co.uk/itm/Varta-CR-2-3AA-CR14335-BR14335-Tagged-Axial-leads-3v-Lithium-Battery-/112248629396?hash=item1a228a2894:g:m2EAAOSwJ7RYYMuZmmagin, thanks for the part codes for those battries. The part code BR-2/3AE5SPN seems to be the correct one with two pins, but unfortunately I am not able to find it available in the UK. DigiKey is quoting a 16 week lead time, and Mouser states 'shipping restricted'. The three pin one is available but will obviously be awkward to fit. I checked the PCB and there are no alternative mount points. I think I will stick with the Panasonic BR and go with the tagged version, soldering it to the existing posts.I appreciate the precautions regarding GND isolation and soldering connections to the temporary power supply. One slip of a a clip and the data is gone! My PSU has a floating non-ground referenced supply so I could use this for a backup supply. Alternatively I am considering two AA cells in a holder although sometimes the contacts on these can bit a little dodgy. Some have simply left the unit powered on and used a butane powered soldering iron. This is also an option I am considering.The layout of my unit seems different to the examples I have found online so far and to the service manual. The serial number information on the PCB indicates that it is a Revision C. I have been trying to identify the solder points for the backup battery/supply. I measured 3.03V across the battery terminals and 2.8V across the GND and VCC pins of the memory chip. I have measured DC voltages only as I'm a little nervous about probing for continuity in case I inadvertently short the supply to the IC! Anyway, I think I have identified suitable points.BTW, is the 'B9511' on the battery the date code? I.e. is is 1995, month 11?

author: SingedFingers
author_rank: Frequent Contributor
timestamp: March 10, 2017, 10:55:32 am
content: RS Sell them new here:http://uk.rs-online.com/web/p/speciality-size-batteries/4363513/6.66+VAT. Free next day delivery. No old stock risk.TBH I'd probably be naughty and stick kapton around the earth pin on the iron as well but only if it's not a fire stick and has an isolation transformer in a separate box.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: March 10, 2017, 12:18:24 pm
content: Well its funny how 7.45GBP can turn into 16.49GBP, which is what the Panasonic one will cost by the time I add VAT and postage costs, which makes the price not much difference to the trending eBay prices I am seeing. I have found the 1.35Ah version of the Varta at Rapid. They offer an economy postage option so this works out at 11.07GBP. I then discovered that the RS info is wrong. According to the datasheet, the CR2/3 is a 1350mAh battery, not 950mAh so the total price of 7.99GBP including VAT is very tempting.So now before I finally make a purchase lets talk chemistry. I am probably going into more detail than I really need to in practice, but for my own education the original battery appears to be Li Sulphur Dioxide and AFAIKT they are now unobtainium. The alternatives, Panasonic BR are Li Polycarbon Monofluoride, whereas the Varta are Li Manganese Dioxide with Organic Electrolyte. Both seem to have a low self discharge rate with long storage life of 10 years, but for maximum life it has been suggested that I should go with the Li Polycarbon Monofluoride. So the question is exactly what difference does the chemistry make in this case?I found the following references helpful in answering that question:http://www.marcompro.biz/samples/ces-plb.pdfhttps://en.wikipedia.org/wiki/Lithium_batteryAccording to these references, the Li Polycarbon Monofluoride battery has 'a longer shelf life' and 'provides more reliability' than Manganese Dioxide batteries. I am therefore inclined to go with the Panasonic BR and am willing to pay a bit extra to obtain the optimum longevity and peace of mind. But how much difference will it really make in practice? A few months? A year or two? 5 years? The HP battery seems to have lasted at least 20 years!

author: ap
author_rank: Frequent Contributor
timestamp: March 10, 2017, 03:25:25 pm
content: Quote from: mmagin on March 10, 2017, 05:08:17 amThats actually illegal. An isolation transformer must not have a secondary earth connection (otherwise what sense woul isolation have) to the socket.

author: mmagin
author_rank: Frequent Contributor
timestamp: March 10, 2017, 05:54:16 pm
content: Quote from: BU508A on March 10, 2017, 07:10:20 amQuote from: ap on March 10, 2017, 03:25:25 pmWell, apparently in Germany, isolation transformers actually make senseNot so elsewhere. (See other threads about the variations.)Anyway, my point was basically just "make sure that there is no DC path from the soldering iron tip to the 3478A chassis".

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: March 19, 2017, 12:50:23 pm
content: Well I performed the 're-battering' surgery today and it was successful! I did find that using the gas soldering iron is a bit tricky and one has to watch which way the gas exit hole is facing so as not to burn anything adjacent. I now have a nice scorch mark on the edge of the workbench!The new Panasonic BR battery had a voltage of 3.41v without load when I checked it before soldering it in. After soldering in a couple of wires for the backup supply and connecting them up to my PSU which has a floating supply, I carefully snipped out the old battery leaving as much of the pins in the PCB as possible. Since I couldn't source a battery with the same pin configuration, the replacement battery had tags and these were rather long, so I bent the ends back over outwards and made sure that I soldered the lower end of the tags only. The new battery fitted between the pins of the old battery and soldered easily soldered to them. I checked that there was a voltage present and removed the PSU wires.I did notice that after I had removed the PSU wires, the voltage was down to 2.96v. This did rise to 3.02v after 5 min or so when things had cooled down a bit. Is this normal?In any case, the meter came up with the usual 'SELF TEST OK' message when it was powered up so I assume that all is OK and hopefully I am good to go for the next 10 years?

author: SingedFingers
author_rank: Frequent Contributor
timestamp: March 19, 2017, 01:28:57 pm
content: Glad it worked. Interested in the outcome as I have my eye on one of these.Out of interest, what happens if the calibration data is lost entirely?

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: March 20, 2017, 08:07:25 am
content: From what I've read, you get 'Error 1' on the display which indicates that that calibration data has been lost or corrupted. If failed, the battery needs to be replaced and the meter then needs to be calibrated, which I understand can be quite expensive.The one I acquired came calibrated, but the battery seemed to be the original which would have been installed at the time of manufacture. If so, then it has probably lasted some 20 years or more! As I understand it, they do die rather suddenly without warning, and as a result the calibration data is immediately lost. Although it was an extra cost, its would still be much less than the cost of a re-calibration so I thought it prudent to replace it at earliest opportunity.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: March 20, 2017, 09:23:10 am
content: Makes perfect sense to me.I'm going to skip this one and stick to 4.5 digit meters I think as I can calibrate them at home. Plus they are "good enough" for what I need to do which at most is roughly trimming offsets on opamps.

author: lmester
author_rank: Regular Contributor
timestamp: March 20, 2017, 02:50:24 pm
content: Quote from: WaveyDipole on March 20, 2017, 08:07:25 amI bought a HP3478A a year ago. After verifying that it worked, the first thing that I did was to replace the battery. I paid about $150 for the meter. A proper calibration could cost more than that. You certainly don't want to lose the calibration. Also, I was able to get an exact replacement battery from Digi-key electronics.Finally, for anyone interested in buying one, the LCD display has no backlight. If this will be your only bench meter, you might want to look for something with a better display. I use mine mostly for data logging so the display is not a big problem for me.