Fluke 45 battery pack - Page 1
Forum section: electronics / testgear

author: octal
author_rank: Contributor
timestamp: August 02, 2025, 03:46:59 pm
content: Hi,I got a used Fluke 45, it looks still in spec (I compared measurements to my recently calibrated HP3478A). I opened and cleaned it and I was suprised it had the internal battery pack. I kept it charging for more than 1 hour but when I disconnect it and power it on, there is absolutely nothing, I think the batteries cells are simply died.Do you know which batteries I can use? (looks like GEL batteries?) any reference would be very helpful.It is really worth repairing and replacing the battery pack?Also the VFD is a little bit dimmed, still very readable and all segments are ok, but not extra bright. Where do you find new (or at least good lookong) VFD modules? can I find them without the controller board (to lower the price)?Thx

author: David Hess
author_rank: Super Contributor
timestamp: August 02, 2025, 04:03:25 pm
content: I am sure the battery is made of Cyclon 2.5AH lead-acid cells:https://batteryspecialties.com/batteries/Enersys/0810-0004/0810-0004I think this is the whole battery:https://www.amazon.com/0819-0019-2-5AH-Enersys-Monobloc-Battery/dp/B0CNL3X5J9I would modify the charging circuit to use a pair of lithium ion batteries, but the service manual I have does not include a full schematic.

author: octal
author_rank: Contributor
timestamp: August 02, 2025, 04:39:06 pm
content: I found the service manual.The battery pack is specified as (chapter 7)SpecificationsSpecifications for the Battery Kit Option are as follows:• Type of Cell: Sealed Lead-Acid Rechargeable• Battery Voltage: 8 Volts Nominal• Charge Time: 16 Hours Typical w/meter offIn the schematics section (chapter 9) of the service manualhttps://www.rftesolutions.com/pdf/meters/multimeters/45--Service_Manual.pdfyou can find the schematic of the battery charger (I don't know if it's complete or not).I'm not really interrested in making the multimeter portable, I was just curious about if it were possible to restore it from original like parts. The battery pack you give link to in Amazon is the correct battery pack, unfortunately it can't be shipped to France.

author: bdunham7
author_rank: Super Contributor
timestamp: August 02, 2025, 07:34:07 pm
content: Quote from: octal on August 02, 2025, 03:46:59 pmIf you want it to work without mains power then the thing to do is somehow track down some lead-acid cells like the onesDavid Hesslinked to. There have been prior discussions about modifying these to use different battery types but I think that's a lot of work.https://www.eevblog.com/forum/testgear/battery-for-the-fluke-45/msg5887706/#msg5887706Those VFDs are long obsolete and I think it would be very unlikely that you'll come across any NOS units. You can still keep an eye out, but there are probably a few hundred others doing the same and many of them need one a lot worse than you do. Your VFD actually looks pretty good compared to others I've seen.

author: J-R
author_rank: Super Contributor
timestamp: August 02, 2025, 08:56:21 pm
content: You could also consider 6x NiCd cells in series. The voltage should be virtually identical and NiCd is compatible with float charging, as opposed to NiMH or Li-Ion.Tenergy is my go to for NiCd these days.

author: floobydust
author_rank: Super Contributor
timestamp: August 02, 2025, 09:03:10 pm
content: The Cyclon lead-acid cells are top quality... but have rape me pricing. Crazy expensive.You can get NiMh Panasonic Eneloops at 2.5Ah in AA size. Much smaller.I'm not sure- you could mod the charger to just shut off, make the Cycle/Float Rate Switch have a float charge rate of near zero.

author: bdunham7
author_rank: Super Contributor
timestamp: August 02, 2025, 09:35:33 pm
content: Quote from: floobydust on August 02, 2025, 09:03:10 pm$10/each is not too crazy IMO. It's not a volume product.I'd use 7 AA NiMH with a hand-selected 5W 9.45V zener in parallel and then make sure the charge rate was plenty low, <50mA @ 9.45V. And an extra fuse somewhere...

author: floobydust
author_rank: Super Contributor
timestamp: August 02, 2025, 11:34:38 pm
content: I've used the Cyclon batteries in a few products, they are rated -65°C to +80°C or -85°F to +176°F. Good in cold weather.But these are the cell prices I see lately. $43 each. Nowhere near $10 lol.

author: bdunham7
author_rank: Super Contributor
timestamp: August 02, 2025, 11:48:25 pm
content: Quote from: floobydust on August 02, 2025, 11:34:38 pmThere are several suppliers that have them for $10-15USD and show stock. $43/ea would be a bit much.. I doubt there's an active counterfeit market for these, I assume Digikey is justgougingoptimizing monetization of limited markets.

author: David Hess
author_rank: Super Contributor
timestamp: August 03, 2025, 01:02:59 am
content: Quote from: bdunham7 on August 02, 2025, 11:48:25 pmNo, they were always expensive; they are a premium performance lead-acid cell.Quote from: J-R on August 02, 2025, 08:56:21 pmNiCd cells are not common anymore, and the charger would need to be changed.I would first try a pair of lithium-ion cells float charged to 4.0 volts per cell, but the charger would need to be modified. I am actually doing this sort of thing now for a 12 volt lead-acid replacement, but of course with 3 lithium-ion cells in series. I need to come up with something better though.Quote from: bdunham7 on August 02, 2025, 09:35:33 pmInstead of a power zener diode, a cheaper low power zener diode can be used in a bipolar transistor shunt regulator, but I would not count on being able to select a zener diode of the proper voltage, and I am not sure how reliable NiMH cells would be like that.

author: J-R
author_rank: Super Contributor
timestamp: August 03, 2025, 03:36:49 am
content: NiCd cells are still plenty common. As stated, Tenergy has a wide selection available:https://www.ebay.com/itm/296447671968I used their D cells with tabs to restore a Fluke 8600A to original operation.NiMH and Li-Ion should not be float charged as it damages the cells. They require charge complete detection. NiCd tolerates this much better (as well as heat and overcharging) and is similar to lead acid which essentially requires float charging. The information about the differences between these chemistries is well known.So I don't think there would be any need to modify the charge circuitry with 6 NiCd in series, since that will result in a full charge voltage of between 8.4V and 8.7V which is within range of the 8.56V 100% SOC of the original lead acid cells.With all that said, I think I'd go this route:https://www.ebay.com/itm/388511506554

author: bdunham7
author_rank: Super Contributor
timestamp: August 03, 2025, 04:59:49 am
content: Quote from: J-R on August 03, 2025, 03:36:49 amYou can, in fact, float both. I mean float, not float charge since you simply select a voltage below full charge and hold it there where there is no charge current beyond self-discharge. This is much different than float charging SLA or NiCd where you use the battery as the regulator by forcing it to continuously absorb charge current. If you select 1.35/cell for NiMH or 3.85V/cell for Li-Ion, you'll have reduced capacity but it will work.

author: David Hess
author_rank: Super Contributor
timestamp: August 03, 2025, 01:37:28 pm
content: Quote from: bdunham7 on August 03, 2025, 04:59:49 amThat is what I have been doing, sacrificing some capacity and float charging to 4.0 volts instead of 4.2 volts.I wonder if the admonishment not to float charge lithium-ion cells at any voltage is about safety rather than endurance. When I wrote earlier about changing what I am doing, I meant adding safety features to my charger.

author: J-R
author_rank: Super Contributor
timestamp: August 03, 2025, 11:02:14 pm
content: The research I've seen points to an unfortunate conclusion that technically speaking any amount of charge in Li-Ion causes degradation. So the longest life is storing the battery at nearly 0% SOC (not 0V), and the shortest life is storing the battery at 100% SOC. Obviously the battery has a job to do, so it's been determined that a happy medium is cycling Li-Ion in the middle of its range, such as perhaps 30% to 70%. The reason they didn't recommend going any lower is because there is a risk of fully discharging a cell when in a multi-cell configuration. So avoid going too low.Also, I have seen various laptops offer an option to extend the battery life by stopping the charging at 80% SOC.I do not recall and could not find any research that discussed floating the voltage of the cell at some reduced voltage. Is there some degradation caused by this alone, or is it always just the fact that the cell is storing a charge, per the other research?Even perhaps decades ago I recall discussions on the Candle Power forums where heavy users of Li-Ion reported a much longer cell life by halting the charge at perhaps 80% or 90% SOC instead of 100%. So it's been a thing for a very long time. However, battery chemistries have also changed and we even do have higher voltage Li-Ion cells available on the market that top out at 4.30V or 4.35V, so stopping at 4.2V would be a similar concept.So a valid conclusion could be that given the low cost of Li-Ion and the continued developments, it is probably not really an issue to just go with whatever solution is safe/easy/inexpensive to implement and so what if the cells last 8 years instead of 10.

author: David Hess
author_rank: Super Contributor
timestamp: August 04, 2025, 04:47:46 am
content: At high voltage the electrolyte decomposes according to some power function of the voltage, so raising the voltage by 0.2 volts might cause the electrolyte to decompose 10 times faster. (1) When the charger disconnects at low current, the voltage across the cell remains and self discharge occurs, but that voltage is still breaking down the electrolyte.So if I have a cell which after charging to 4.2 volts, sits at 4 volts after an extended time, then float charging to 4 volts (or less) should not cause significant additional wear. All I am doing is making up for the self discharge.At low voltage a different process happens which also damages the cell, so maximum lifetime is somewhere in the middle.(1) Somewhere I have a chart which shows this, but I could not find it online.

author: BradC
author_rank: Super Contributor
timestamp: August 04, 2025, 08:04:57 am
content: Quote from: floobydust on August 02, 2025, 09:03:10 pmI bought a pair of 2.5Ah cells in 2017 for $20/aud each. They are now $48.95 each (~$30 USD at todays exchange rate).https://au.rs-online.com/web/p/lead-acid-batteries/0591461They went into a 35kg workshop scale that has a very poorly implemented charging system. They've been in use now for 8 years and I still get days out of them, so even at those prices I'd buy them again. The original 4V SLA it came with lasted about the warranty period (just over a year).Edit : Just spotted this for those in the Northern Hemisphere :https://batteryspecialties.com/batteries/Enersys/0810-0004/0810-0004