Charging old alkaline batteries in the sun ?

[Jolly 7]

With the very warm sunny weather we are experiencing of late, has anyone tried to make a DIY solar charger for dead alkaline batteries ? I know a charge control circuit is needed to make a proper solar charger for rechargeable batteries, but can a suitable one be made for old alkalines too ? My makeshift charger is probably not very scientific as I have connected a 6V solar panel to two old AA batteries. I have fitted a Schottky diode to prevent current flow back to the panel, but could I make it better ?
I should add that the voltage of each of the dead batteries is around 0.8V each.

[Refugee]

I have seen a cheap calculator with a small solar panel along the edge of the display.
It also had a regular button cell inside.
I would guess that it would offer a very low charge rate.
Solar cells are basically a light dependent current source.

[Jolly 7]

My setup did not work as intended. After an hour or so of charging I tried to measure the voltage of the cells, but it was almost zero and one had leaked. Both are in the bin now.
My only success from today is getting an old tape motor to run as a solar fan !

[GMB]

I have played with this in the past. You need to use an asymmetric a/c waveform for this purpose, i.e. it charges on +ve half cycle and then partly discharges on the -ve one.

*BUT* My experience is that it seems to go well, but I found the batteries would often suddenly start rapidly leaking, maybe a week later - just as you thought it had all gone well and the battery was in use.

So I gave up on it.

[emeritus]

I used to do this to get more life out of zinc-carbon and Alkaline cells, but did it before the voltage had fallen below about 1V. Anything lower never worked. Never ever had any of the old Blue Ever-Ready U2 cells leak.

[G6Tanuki]

Rather than trying to charge them, just leave them in the sun for a few hours to accelerate the chemistry.

In times-past us schoolkids warmed-up our fading zinc-carbon cells and batteries so we could maybe get another couple of nights of Radio Luxembourg before having to dip into the pocket-money for a new "Flying Bomb" PP3.

[Jon_G4MDC]

I recall a row of PP9s lined up on top of a very hot rubber press in a research lab.
They went through the radio in strict rotation.

[emeritus]

Now I think of it, mum used to say how, during the war, they would put their No.8 batteries in the oven to revive them.

[Refugee]

Alkaline cells respond much better to a good bashing.
It shakes up the remains of the chemicals inside giving a little more life.
Anything else especially involving heat either electrically from charging or external heating just makes them leak.
Zinc carbon batteries used to go on a central heating radiator in winter.

[Maarten]

Some Philips portable sets revived their zinc carbon cells by running a small current through them when operating on mains.

['LIVEWIRE?']

I used to squeeze more life out of zinc carbon cells by heating them-this was many years ago before I knew a fraction of what I've learned and been taught about electronics. On a related topic, currently being advertised is a Charger which is claimed to be able to recharge ordinary alkaline batteries, and I also recall that PW once published a circuit for recharging 'single use' batteries, but I also seem to recall that it was said that under no circumstances should users attempt to recharge these (I never have) Can anyone shed more light on all the foregoing? Somewhere in my 'forgettory' the 'little grey cells' are telling me that the chemical process in zinc carbon & alkaline cells is strictly non-reversible!

[The Philpott]

You can stick them under your armpits if you're stuck half-way up a snowy mountain, but other than that I don't chance reviving primary cells.
Dave

[Mike. Watterson]

Alkaline cells can be somewhat recharged if not more than 1/3rd or 2/3rds discharged, I forget which.
Layer cells can have shelf life extended and a slight recharge with a very slight charge. Some Philips sets had a high value resistor from mains PSU to HT pack. They often had a DEAC (early NiCd) for LT Regulation and Battery operation. The D Cell holder for LT was only for temporary use as the DEAC (NiCd) would discharge it and cause it to leak.

Regular Zinc Carbon or Zinc Chloride (or flat Alkaline, that's < 1.05V, probably less than 1.25V) should never be charged.

Endpoint of NiCd or NiMH is about 1.1V. A stack can be damaged by reverse charging a cell at lower per cell voltages).
Endpoint of Alkaline and Layer cells is about 1V per cell. A end point of 0.9V is possible in motor or flash-lamp Zinc Carbon and Zinc Chloride. There is little advantage to Zinc Chloride except in higher current applications. For radio (Transistor or LT) there is no advantage in Zinc Chloride over Zinc Carbon. Alkaline is about x3 capacity. The x5 life is only high drain applications compared with old style Zinc Carbon.
An Alkaline PP3 is only advantageous in test meters, smoke alarms etc, applications with long life because most use 6 cells similar to AAAA, thus volumetric efficiency is poorer than Layer Cell PP3. Some Alkaline PP3 do use stacked rectangular cells thus a bit better, however PP3s don't usually leak and if used in a radio, the common Alkaline PP3 works out a lot more expensive.
Cylindrical Zinc Carbon and Zinc Chloride single cells have no use case at all. They should be banned.
NiMH have 1 month to 1 year self discharge depending on purity & Quality, the 2,700mA NiMH AA are usually far worse self discharge than 2000 or 2,200mA.
NiMH are a fire risk in gear not designed for them. Add a fuse otherwise a short can set the wiring insulation on fire.
More than 12V NiMH and over discharge 3V to 12V packs will have short life due to one cell getting reverse charged.
45V, 67V, 90V, 108, 120V etc packs best made from PP3 layer cells, Alkaline AAA or Alkaline AA cells according to space, but fused. A B101 pack can use six stacks of CR2032 if combined with fuses and diodes.
Even Primary Lithium cells are a fire/explosion risk if misused. The idea of Lithium 9V PP3 replacements seems dubious given price and chemistry. For non-critical use up to 1 year shelf life the Layer PP3 is fine. The Alkaline PP3 might have a 10 year shelf life and is the only choice for Smoke alarms or any low consumption device.

[emeritus]

I only studied chemistry to GCE "O" level, but my understanding is that there are reversible and non-reversible chemical processes. The action in a lead-acid accumulator is reversible as long as discharge does not proceed beyond the point where insoluble lead peroxide is formed. Once significant amounts of insoluble lead peroxide are formed, it is very difficult to reverse the reaction to convert it back to spongy lead/lead oxide.

I suspect a similar situation exists in zinc-carbon batteries. As long as only a small amount of discharge has taken place, presumably when the depolariser hasn't had to do much, the reaction is reversible to some extent. Further down the discharge curve, the depolariser increasingly takes part in the chemical reactions, and so the reaction becomes progressively less reversible. By the time the voltage has reached 1V , forget it.

Regarding the Philips radio that trickle charged Leclanche cells, I used to do this with my Philips EL3585 tape recorder when I was a schoolboy and didn't know you were not supposed to. I originally fitted a 2.5mm jack socket to allow it to be run from an external mains unit (the nominal 12V DC auxillary supply from my Hornby Dublo train controller via a watty variable resistor to drop the odd 4V) and originally wired it conventionally so that plugging in the external supply disconnected the battery. I found that this resulted in hum, and thought that rewiring to leave the internal batteries connected might help to reduce it, as well as stabilising the voltage. This did work sufficiently well for playback, allowing me to only need to use it on batteries alone for recording, and I found that, after use with the external supply, the battery voltage was higher than it had been at the start (the EL3585 has a meter that shows battery voltage on playback and signal level on recording). I never had any Ever Ready Blue U2 cells leak, but when I tried it with a set of the more expensive later white "Sealed" cells that I had been obliged to buy due to non-nonavailability the cheaper blue ones, they did leak. I had stopped using the recorder by the time alkaline batteries became common.

[Mike. Watterson]

Alkaline chemistry is rather different to Zinc Carbon/Zinc Chloride. On Alkaline the case is +Ve and doesn't get eaten. On the Zinc Carbon/Zinc Chloride the + is a carbon rod and the case is zinc which is consumed. The depolariser in Zinc Carbon isn't the main barrier to charging, it's not really reversible. Early Victorian torches had carbon filaments and no depolariser, so the the light quickly died, hence "Flash light".

Layer cells while having carbon and being related to Zinc Batteries have a radically different construction and not exactly the same chemistry. That's why they have much higher internal resistance, longer shelf life and rarely leak. The Zinc cased cells inherently leak as they are consumed. The so called "leak proof" had a metal disk on the base and sealed sleeve instead of card outer. They may have then reduced the thickness of the Zinc case.

The Alkaline cell chemistry is reversible, but if too discharged the structure is too damaged to recover. They leak if the battery spring is too strong or construction is poor, not due to consumption like the Zinc, but the -V contact plate with the internal spike has a seal to the outer case with top cap has a seal that can fail, then the gel leaks out.

[Heatercathodeshort]

There was Fidelity radio that charged the PP9 through a charging circuit. It claimed to increase the life of a PP9 by around 4 times.

Nothing much was seen of the design. John.

[G6Tanuki]

There have been various circuits offered over the decades which claim to "recharge" primary-batteries.

Simple ones just trickle-charged the primary-battery with some DC, hopefully through a series-resistor!

Better ones used "dirty DC" - circuits typically being a half-wave rectifier often with a capacitor or a resistor paralleling the diode in order to deliver an 'electrolytic shake-up' to the primary-battery.

None of them really caught on, presumably because they didn't work!

[electronicskip]

Originally Posted by G6Tanuki

Rather than trying to charge them, just leave them in the sun for a few hours to accelerate the chemistry.

In times-past us schoolkids warmed-up our fading zinc-carbon cells and batteries so we could maybe get another couple of nights of Radio Luxembourg before having to dip into the pocket-money for a new "Flying Bomb" PP3.

I used to line all of mine up on a hot radiator to get a bit more out of them when I was a kid to my mothers horror! I'm sure she thought they were all going to blow up like bombs.

[Spec-chum]

Gosh I remember that trick when I was young 'un. My father use to try to squeeze more life out of his torch batteries doing that. One time he lined 4 Varta HP2's in row next to each other on top of the radiator. After about an hour there was a bit of bang and one of the HP2's was on floor. It hadn't exploded or leaked, nothing spectacular. But he didn't put that battery back on the radiator

[MrBungle]

My mother used to put them in the oven for 5 minutes!

I remember as a child longing for charged PP3 batteries. It gets bad now to the point that I can afford new ones and just shrug but I still treat them like a nugget of gold!