Replacement battery for Yashica Electro 35 series

Some time ago, I bought a Yashica Electro 35 GT. It came without a battery and, knowing the original mercury oxide battery was no longer available, I searched for a recommended replacement. The most commonly recommended replacement seemed to be a PX28A (4LR44) alkaline along with an adaptor to make it fit. I bought an adaptor and battery but had problems. Below are described the problems I had along with what I discovered and some recommendations. If you want to get straight to the important bit, skip to the bottom of the page where you'll see that I don't recommend the PX28A but recommend a couple of alternatives.

My problem was that the battery check lamp wouldn't come on when the battery check switch was pressed. I assumed a fault was with the camera but, after extensive investigation, decided the only possible reason for it was the battery itself. The battery was replaced with another of the same type but the symptoms were the same. But by replacing the battery with a lithium equivalent, the battery light started working and the under/over lamps were also a lot brighter. Given that the nominal battery voltage was the same for both the alkalines and the lithium, it started to look as if the voltage of the alkalines was reducing, on-load, to a value below the triggering voltage of the battery check circuit. I confirmed this by making temporary connections directly to the battery terminals and measuring the battery voltage when operating the camera. The battery volts dropped to approx. 3.7V when the battery check switch was pressed. The service manual states that the battery check lamp will turn on with a battery voltage of 3.9V, so it's not surprising that mine wasn't working.

Before going further, I'm sure there's people out there, using the PX28A, who will say they are not having problems. I can believe it. The 3.7V that I measured is so close to the specified working voltage that tolerances between components and slight differences between batteries, could be enough to have the battery check lamp work in some cases but not in others. I have seen reports from some people who say their battery check lamp worked for a short while, but no longer does, but as everything else seems to work, they've ignored it. This could be attributable to the reduction in battery voltage through normal use which has caused the on-load voltage to drop below the threshold of the circuit. I've seen reports from others that their battery check lamp has never worked and they usually assume a fault with the camera, but as the camera appears to work otherwise, they too ignore it. These instances may also be attributable to the wrong type of battery being used.

Having decided the alkaline battery wasn't up to the job, despite being recommended by many people, I looked for specifications of the battery to compare with the requirements of the camera. The important specification, in this case, is the internal resistance or the maximum current that can be drawn. Manufacturers will normally give a typical current rating though, in reality, it will be possible to draw a much higher current than specified, it's just that, the more you draw, the lower the battery output voltage will fall and the greater the power dissipated inside the battery (and hence, the hotter the battery gets). I was unable to find a value for the internal resistance but typical current ratings range from 1mA downwards. The GP battery data sheet shows a life span calculated on a 6Kohm load (1mA at 6V) but most other manufacturers quote currents of around 0.2mA, the lowest I've found being 0.12mA.

Compare this battery specification to the needs of the camera. The service manual says the current drawn when any of the lamps are on is 60-70mA. That is, at least, 60 to 70 times more than the battery is specified for (depending on which manufacturer's data sheet you read). I was now understanding the problem.

So the answer is simply that the PX28A alkaline battery is unsuitable and an alternative needed to be found. As I said, the camera appeared to work fine with Lithium cells but I thought it still worth looking at the specifications for the alternatives. The alternatives I looked at were 2CR1/3N lithium, A32PX alkaline and 4SR44 silver oxide. All these give a nominal 6V output.

I searched for data sheets first and found them for the 2CR1/3N, and the 4SR44 but not for the A32PX. The data sheet for the lithium suggests it's capable of delivering the required current and looks like a suitable replacement in that respect. The data sheet for the 4SR44 is less forthcoming but  suggests the battery is capable of driving a 200 ohm load, at least for a short period. Problem is, the camera presents a load of approx. 100 ohms.

As the data sheets weren't able to give me a definitive answer as to which, if any, is a suitable replacement, I did an on-load test for each using the battery check lamp of the camera and using direct connections to the battery in each case to monitor it's output. The results (including the PX28A for comparison) were thus:


From the results, the A32PX is clearly the best option in terms of on-load volt drop and I was surprised the lithium wasn't better. The PX28A is obviously struggling and given that it also has the lowest capacity of any of the batteries, it's difficult to understand why people use them.

It's a fact, though, that many people do use the PX28A in their Electro 35s (with or without the battery check lamp working) so I decided to see if there are any other effects of using them. The most obvious one I could see was that the over/under lamps are much dimmer though not to the point of being unusable. The other area of possible effect is exposure. The Electro 35 series are quite tolerant of large battery variations. Some have suggested the voltage is regulated but that's not so. The tolerance comes from the design. The circuit charges a capacitor via the LDR (light dependant resistor). The rate of charge is dependant on the battery voltage of course but because the threshold at which the voltage on the capacitor stops the exposure is also dependant on the battery voltage, the two change similarly (essentially a bridge circuit) and the result is a constant exposure. At least, that's the theory but I decided to check. I replaced the LDR with a fixed value resistor to ensure the exposure didn't vary due to any change in light conditions. I then connected the camera to a variable power supply and set up my shutter speed tester to measure the shutter speed. The resistor value I chose to replace the LDR with gave me an exposure of approximately 1/60th second at f/5.6. I checked the shutter speed with the power supply at 6V and at 4V and there was no measurable change. I chose these two voltages as being the usual limits of the battery voltage (as the battery check circuit operates at 3.9V that's, presumably, the minimum voltage Yashica intended the camera to operate at). I also checked the exposure at 3.5V as well as it's quite likely the PX28A will get that low if used. Again, there was no measurable change in the exposure. Pretty good I think. I also repeated the test with the LDR replacement resistor removed and the LDR left disconnected. There is a resistor normally connected across the LDR so, in this configuration, the capacitor was being charged only through this resistor and the exposure obtained would represent the maximum exposure possible with the camera. The results were not so good but not bad. With a 6V supply, the exposure (with f/1.7) was 18 seconds. With 4V it was 14 seconds which is approximately half a stop under exposure. With 3.5V it was 12.5 seconds. Given that it's very unlikely that exposures this great will be used in normal use, I don't see this as a problem.

Conclusions

Whichever battery is used, the camera appears to work successfully in that it will take correctly exposed photographs. The only real problem is that the PX28A may not activate the battery check lamp (or, if it does, it may not for very long). There is then the problem of knowing what the state of the battery is and of not knowing when it may fail. Given that it has the lowest capacity, knowing when it's going to fail is probably quite important. Of the other batteries, the A32PX seems the best option, especially as it doesn't require an adaptor of any sort to use it. There has been suggestions that it's no longer made and the ones currently available are old stock. In fact the Small Battery Company in the UK, who I bought mine from, assures me they are still in production and will continue to be available. In case of any difficulty obtaining the A32PX though, the other alternative is clearly the lithium, though the silver oxide isn't far behind.

Addendum

The A32PX battery is actually 4 off PC640A batteries combined together so another alternative is to buy PC640A batteries instead.

Supply of the A32PX and PC640A batteries appears to be variable. At the time of writing this (December 2019), the Small Battery Company can supply both types but this isn't guaranteed so buy them when you can. The shelf life of the batteries is years.