Bit off topic electronis help

I have a Fagor DRO on my ToolMaster Mill. Works great, had it for years. NiCad battery back-up so if Power is lost memory is alive. Often I would leave it on at end of day and if there was power interruption nothing was lost. I actually don't remember if table was moved when power was off it it would detect movement but not important for this discussion. Batteries went bad so I purchased a AAA battery holder, removed the batteries that were soldered in and leaking. Wired in battery holder, inserted batteries and works OK. I was wondering if I were to install a large capacitor it could serve as a Battery. I'm thinking I would have to limit the charge current so if the Cap. is totally discharged I will not burn-up the charging circuit. I'm thinking a resistor large enough to limit current to the same as charge current for 2 NiCad AAA batteries though I forget if they are in series or parallel. The resister will be paralleled with a diode so only charge current is limited. Reason I want to do this is that I'm doing some electrical changes. Power for the light and DRO is supplied via extension cord. My RPC is wired so that no power is applied to the 3 phase line until activated. I purchased a 240/120vac transformer with fused input and output and will mount it in the electrical control section of the ToolmMster. I have a few TVS &Transient Voltage Suppressor) to mount to the transformer output to eliminate any voltage spikes. Therefore when 3 phase is shut down my DRO will not be powered.
Anyone know what the charge current is for AAA NiCad battery?

Can be anything.

Needs to be the load plus charge.

You need to determine if the battery just maintains the memory and nothing else then micro amps of load.

If you can measure this then easy to do.

A "super cap" could maybe fill the need.

We had some old scada stuff that had a 9 volt transistor style ni-cad that was charged by internal power then had a buffer resistor to drop down to needed.

We replaced with 6 volt gel cell and it holds for weeks.

In your case you need to see what is in the circuit

It may be as simple as getting a larger battery. Capacity not voltage

It is designed for a battery so it should already have current limit.

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Been a while since I looked at the circuit but it's not easy to determine what it's doing. I'm not sure but I think there was a fuse in the battery circuit but it too is soldered in. When the battery went bad I connected a small Cap. in the circuit, it lasted a few days but after that it would forget setting like inch/metric and direction of count. worked OK until I got the battery and holder. So that every time I shut it down or lost power the X axis was backwards and it was in metric. If I used bigger batteries charge current will increase, I guess I can wire in a resister/diode assembly too.

Batteries and capacitors are different....

Capacitor: Voltage drops as charge is taken out of the capacitor, falling steadily from "fully charged" to empty.

Battery: Voltage stays relatively steady as charge is taken from the battery, until it is nearly discharged, at which point it begins to drop fairly rapidly.

The characteristic of a battery is generally what electronics want. Only if the capacitor is ridiculously too large, so that the current drawn does not change the voltage much over the time the capacitor needs to supply current, does the capacitor behave similarly to the battery. (if the electronics can work over a wide range of voltage, a capacitor may be OK)

Batteries stop working as well after many "cycles" of charge and discharge. They lose capacity, and hold less charge. If the circuit worked OK when new, then a replacement battery may not need to be any bigger than what it was originally. It should work fine until the new battery "wears out", many years later.

I don't understand why folks like the OP think they know more than the engineer who designed the end device in the first place. It is easy to understand wanting to improve something, but in so many cases like this one, no improvement is required. The original design works very well as is. Just fix it.

It is very simple!! I have fixed many problems and made things better. I am an Engineer both EE and ME! For example I have an older CNC Bridgeport Mill that kept blowing transistors used to drive the stepper motors. I simply replaced them with transistors with higher current capability and the problem stopped! Also while an Engineer for various companies I had to go back and re-engineer circuits to find a way to lower cost! The problem is that cost reduces sales! Working for Fedders Corp. if you think you will get more than 9,000BTU from a 10,000BTU air conditioner your dreaming! People compare cost to BTU output, in addition we checked all the other brands and they were the same except Carrier at that time (1968) they were 100%+. Same with data plate amp. draw, that was fudged too! During UL witness test we tested input power ratings (BTU rating was not any concern by UL) We had meters that were calibrated by a well known company so that they would confirm data plate ratings! In addition UL would double check by going to a distributor and selecting a unit, a secret mark was placed on the unit to insure they received the selected item, then had it shipped to UL labs! But that unit did not go direct, it came to us where we did what was necessary to get the unit to match data plate info! I have been involved with other similar situations and have been told of the same by other Engineers! Every attempt I have made to improve something I purchased was successful!
Yes JST I know the Cap will lower voltage as charge is used, but also battery charge voltage is higher than battery voltage so Cap will last quite a while if large enough. Very large Cap will take a while to charge due to R/C time constant but power loss is short compared to supply availability.

What's wrong with battery of original design type?

Worked when new, we assume, should still. Redesigns are not always helpful, and take time you may not need to take.

As I mentioned I did that. Removed the soldered in NiCad Batteries. Purchased AAA 2 battery holder and wired it to the terminals from where I removed the leaky batteries. It's working, when if first happened I used a small electrolytic cap. 50mfd if my memory is correct. That worked until battery holder and batteries came from ebay. However original batteries kept memory alive for only a few days. If I supply power for the DRO from transformer mounted in ToolMaster base power can be off for longer time. Yes I can add larger batteries but charge current will increase, I assume I can add resister so batteries charge at slower rate yet diode in parallel will apply full voltage to the unit.
I like the circuit I have that adds or removes All power from RPC 3 phase line. To get around the transformer connected to the 240VAC single phase of the 3 phase RPC created line I would need 6 wire connection. That way power will be supplied to DRO all the time.

JST said:

What's wrong with battery of original design type?

Worked when new, we assume, should still. Redesigns are not always helpful, and take time you may not need to take.

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Simple answer:- NiCd batteries are a thing of the past unless the DoD has need of them.

JST said:

Batteries stop working as well after many "cycles" of charge and discharge. They lose capacity, and hold less charge. If the circuit worked OK when new, then a replacement battery may not need to be any bigger than what it was originally. It should work fine until the new battery "wears out", many years later.

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My feelings exactly. And if you are concerned about tearing up the PCB from multiple battery replacements over the years the best way is to insert solid wire leads and solder the battery to those away from the PCB. Back when PCs largely used soldered coin cells that's how I handled it when replacing batteries for friends.

Mark Rand said:

Simple answer:- NiCd batteries are a thing of the past unless the DoD has need of them.

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For new designs yes, but for repairing equipment that originally incorporated them into the design why
re-engineer the damn thing to repair it?

Froneck said:

I am an Engineer both EE and ME!

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As one EE to another, why are you asking electrical questions on a machinist web site? Google is your friend.

CarlBoyd

It is repaired! I can simply replace batteries now that were soldered in. But as I have mentioned the only power available to the DRO is via extension cord from near-by wall outlet. I want to change that but if I supply 120VAC power not extracted from machine 3 phase power created by RPC I will require 6 wire connector or wire direct. I will not change complete disconnect from machine power when 3 phase RPC is off, I do not want any single phase power on 3 phase line while RPC is not running! Locking contactor will shut off all power requiring RPC restart if power is lost!
Why ask electrical questions here?? This is an electrical topic, there is also Gunsmith topic! Plus I have found some of the smartest electrical minds here in the electrical topic section of Practical Machinist!! Like it or not the days of all mechanical machinery is gone. Many here have the older machines that we have added DRO and converted single phase to run 3 phase motors! Some added VFD. I have one that I intend to use on a 24" Hendey lathe that was probably equipped with a DC motor because all the selections on the "Speed" chart are in speed range not single speeds. I also build Target Pistols, there is no American made brand that might be called Target Pistol capable of accuracy required for Competition target use. I currently make what is considered the Best trigger Shoe for Target Pistols and currently working on a .22 conversion for 1911 type .45 Pistol.

Mark Rand said:

Simple answer:- NiCd batteries are a thing of the past unless the DoD has need of them.

Click to expand...

Not hardly....

Those lovely chinese consumer (and pro) lithium cells just love to explode when paired with cheapo chinese made chargers. Which is basically what you buy with most battery tools regardless of brand or nominal market target.

It's a known thing. Folks have resorted to making fireproof battery safes to contain the blaze "when" it happens.

I won't have them in the place outside of a computer, or phone, which gets a lot of testing and qualification. (yeah, Samsung, but....)

Got me! They are a thing of the past from right-pondian suppliers. Though I see that I could get them from Mouser or Digikey.

Froneck said:

Anyone know what the charge current is for AAA NiCad battery?

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Look at the label or printing on the battery. The charging current should be 10%.
So if the label says 600mAh then use 60mA. That's what I do. Forget the fast charge methods.
You should start your charge when the cells are at zero otherwise memory conditions occur.

Thanks rons; I guess I'm going to have to measure available charge current in that the circuitry exists. Batteries in the unit were unmarked and soldered in. I'll look at the open circuit (batteries removed) with an o'scope to see what the charge voltage looks like. Discharge the batteries I'm using and check charge current. That should give me an idea as what I can do. I might just have to find a way to get 120VAC single phase to the machine.
I'm thinking the batteries are always charging and that the DRO on/off switch simply shuts down the display. What I'm currently doing is running conduit on the ceiling (14') and supplying 3 phase power via SO cable attached to the box with strain relief. Keeps SO cable off the floor and out of the chips!

I should have mentioned that me rechargeable NiCad AAA's are printed as 300mAh. My AA's ar 600 mAh.
The most desirable way is to slowly discharge them through as resistor and then start the charge.

I use a LM317K and a 10 ohm resistor to charge two AAA batteries in series.
A very simple circuit. Imagine that R2 is you battery.
My power supply has a current/voltage switch so I just dial it to 30-40mA.

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rons said:

Look at the label or printing on the battery. The charging current should be 10%.
So if the label says 600mAh then use 60mA. That's what I do. Forget the fast charge methods.
You should start your charge when the cells are at zero otherwise memory conditions occur.

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Almost correct. The rule is "not to exceed 10%". It can be less without issue.

steve-l said:

Almost correct. The rule is "not to exceed 10%". It can be less without issue.

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Except when quick charging ... repent ...

I have no qualms about answering old threads. The whole point behind such a site is what updates may apply any time afterward, No?
There is clearly serious electronic wizardry on tap here. I didn't have any such assistance some years ago, 2001 or so, DRO'ing my ATW Pacemaker. Wasn't so much about the unit per se', but getting applicable voltage. Didn't see worth of pulling 110v conduit 25' from a wall in rented space. Sat down with a McMaster-Carr until one page said "here you go, sport".
Disclaimer, IIRC, YMMV.
Bought a single 240v-110v transformer, made a little bracket, jumped 1 incoming 3ph conductor to said transformer, inside lathe control box, and wired a hospital grade outlet applied outside same control box. This particular reader isn't a central display signaled by scales, it's a electronic version of a Trav-A-Dial, the display right atop it, trammed in just like a Trav-A-Dial, carriage only. It has an internal battery, wall wart, wire lead and small jack. Made in Netherlands/ Sweden or so, plugged in til I use it, goes for hours. Most convenient? No. Does it beat flex cables getting crushed or wound up in chips? Sure! Will a travel indicator address the cross-slide readings? Well, it works for a few people, yes.