Last night my radio failed again mid QSO.
OK on RX but not enough umph for TX.
It runs from 8 off Ni-MH AA cells (Vapex 2900 mAh).
They all seemed to charge OK in my Panasonic CC63 charger, and on failure, all show the same 1.2 volts.
So how can I find which cell or cells are failing?
Stuart - G1ZAR
As they are separate cells fitted into a holder, could it be a high resistance joint eg a bit of corrosion, loose rivet etc in the holder, rather than a faulty cell?
Otherwise, I guess measure the voltage across each cell under load - how to do that easily would depend on your junk box and metering facilities!
I put an appropriate resistor across the cell whilst checking the voltage. A 5 ohm resistor should draw 240 ma at 1.2 volts. If the voltage holds up, the cell is probably good. If not, it’s getting weak. You could use a lower value resistor to draw more current. Be sure to pick a resistor with enough wattage capability do you don’ t smoke it!
K6YK
If every cell measures @ 1.2v when you get a “failure” then they’re still at their nominal voltage. That sounds like the cells are perhaps okay? I would suspect a high-resistive connector or joint failure somewhere as already suggested.
Might it be cost effective to grab a set of 8x LADDA cells from IKEA and just test them to see if the problem goes away before digging into the battery connections etc? You’d have a new data point and a spare set of nice AA cells!
Rob - G5RMP
Recently I went through a set of eight 2500mAh cells from my 817ND. By the voltage they were all OK but in the rig they didn’t last long before it started cutting out, so I applied a more brutal test, I stuck a 10amp meter directly across them for a brief touch. Some of them showed six to eight amps, some of them barely made two amps, yet they all showed the same fully charged voltage. I replaced the lot, they didn’t owe me anything, I’d had them for over a decade!
You have to measure the voltage under load. As the cell gets old its internal resistance increases but it will still measure 1.2V as the meter will draw very little current.
There was a circuit in one of the magazines (RadCom TT, QST) in the 80s for a “battery boiler”. This was an electronic load consisting of 4 (maybe 8) 2N3055 and a simple control circuit. You clipped it on your battery along with a voltmeter and adjusted the “boil factor” knob to load the cell(s). You could set current up to about 25A on the one my old club had. The 2N3055 were on a big heatsink and it got hot. By setting a suitable “boil factor” you could load up your cells and see how they performed.
We found invaluable when a club member (now long retired ex-BT) obtained a significant number of Hawker/Gates SLA cells which were removed for disposal from phone exchanges during upgrades. I ended up with many 4V and 6V 100Ah Hawker cells about 15x15x25cm that weighed a lot. They ran the 10/25W 2m/70 contest station for so long we could often charge them at the start of the season and mid way through only and they never let us down. But that was because we chose the best cells by “boiling” them first.
So you need to make a smaller NiMH boiler and you can find which cells are pish and which are OK. However, apart from the fun of making a using a boiler, the cost of the parts probably exceed the cost of buying new quality cells. As I had some NiMHs that started failing in the cold, I simply went to IKEA and replaced them with the same IKEA LADDA 2450mAh AA cells. My new LADDAs knock spots off the well used old LADDAs.
Normally used to test 18650 and other lithium cells, there are purpose-built on-load testers like this one:
https://de.aliexpress.com/item/1005005690292845.html
I wonder if there is an equivalent for Ni-MH cells?
73 Ed.
UPDATE:
Actually, this simpler (and cheaper) one may be more suitable for NiMH:
Here’s a nice modern take on the old battery boiler… instead of many paralleled 2N3055s it uses a single MOSFET.
EDIT: the article is in English!
Thanks for the replies and suggestions.
I should have mentioned that I swapped the batteries for a new set and it was OK for the next 1.5 hours. So I could somewhat discount the bad joint theory. What I should have done was swap them over with the second radio and see if the fault stayed with the batteries in the new radio.
For the time being I will go the route of measuring voltage under load, but ultimately it will pay me to build an electronic load (battery boiler) as later this year I have press my ARDF TX’s into service. 6 transmitters with 10 cells in each. I’m not replacing all of those without testing.
73,
Stuart
I use the ‘battery test’ mode on my DMM, it applies a 100mA load for 1.5v
I’d always presumed this was sufficient for simple load testing of a small AA cell but perhaps not?
That chap Ohm said some interesting things… like P=VI
5W handy, 7.2V battery. 5 = 7.2 * I, I= 5/7.2 = 0.7A = 700mA
So 100mA is better than the uA load a DVM would place on the cell, it’s not really much in the case of a small transmitter.