PP9 battery - reversed polarity. - UK Vintage Radio Repair and Restoration Discussion Forum

brenellic2000 · 23rd May 2015, 10:52 am

I have had, over several years, the odd Ever Ready PP9 battery in my 'Hacker Sovereign II' go reverse polarity! As you'll know it has 2 x PP9 in series.

Worringly, I've had two reversals in almost as many months on the same radio -my other Sovereign II's batteries seem to last for years.

What is the cause?

- is there a fault in the radio? Seems unlikely as they are in series and the other is fine.

- or a manufacturing/internal fault in the battery - an individual cell failure, poor QA?

- is there a more reliable brand out there?

Barry

af024 · 23rd May 2015, 11:33 am

Does it read reverse polarity when disconnected from the set?

broadgage · 23rd May 2015, 1:17 pm

I suspect that one battery has a greater capacity than the other. When they are discharged in series, the lower capacity battery discharges first, and then as the good battery continues to provide current the bad one becomes reverse charged.

Batteries used in series should preferably be of the same brand, age, and type.

When using multiple AA or D cells in series I test each one on load before use, this sounds irksome but is very quick. I use a 1.5 volt torch bulb and a 1 ohm resistance in parallel and find that the brightness or otherwise of the lamp is very quickly observed.

For 9 volt batteries I would test with a 6 volt 3 watt bulb and no resistance. Such batteries are intended for relatively slow discharge, and a load test at over 0.5 amp is a severe test and a good guide to condition.
Do not worry about over running the bulb for such a brief test.

paulsherwin · 23rd May 2015, 1:54 pm

This was my thinking too.

brenellic2000 · 23rd May 2015, 3:21 pm

Thanks lads,

Yes, it does read reversed off set; and yes I do replace in pairs, but one (the first in the series) always seems to go flat quicker... so it does make sense that the good is trying to recharge the bad.

Do I take it then that this not a manufacturing fault in the battery, but luck of the draw?

TTFN

McMurdo · 23rd May 2015, 3:22 pm

It's a well known phenomena and why you shouldnt mix battery types as already mentioned. Some can leak badly when reverse charged...it happens with AA types as well which are very often series connected. I was confused when I first came across it! Maybe fit some reverse parallel diodes?

kalee20 · 23rd May 2015, 5:09 pm

Yes, the first cell to run out of capacity will be reverse charged by the rest.

Batteries in series though... Are they in a simple series connection, or is there a connection to the mid point? Some output stages needed the mid-point as a return point for the loudspeaker. If the balance of the O/P stage isn't quite right, there could well be a small, steady current draw from the mid-point, which would act to discharge one battery before the other. And then the other would reverse-charge it. Not good.

Anti-parallel diodes across each battery would be a good, cheap, protection method but sadly few, if any, manufacturers fitted them.

23rd May 2015, 10:52 am	#1
brenellic2000 Octode Join Date: Aug 2006 Location: Rye, East Sussex, UK. Posts: 1,647	PP9 battery - reversed polarity. I have had, over several years, the odd Ever Ready PP9 battery in my 'Hacker Sovereign II' go reverse polarity! As you'll know it has 2 x PP9 in series. Worringly, I've had two reversals in almost as many months on the same radio -my other Sovereign II's batteries seem to last for years. What is the cause? - is there a fault in the radio? Seems unlikely as they are in series and the other is fine. - or a manufacturing/internal fault in the battery - an individual cell failure, poor QA? - is there a more reliable brand out there? Barry

23rd May 2015, 11:33 am	#2
af024 Octode Join Date: Jan 2003 Location: Ashby-de-la-Zouch (it's not by the sea) Posts: 1,255	Re: PP9 battery - reversed polarity. Does it read reverse polarity when disconnected from the set? __________________ Now where on earth did I remove that from?

23rd May 2015, 1:17 pm	#3
broadgage Nonode Join Date: Jan 2010 Location: North Somerset, UK. Posts: 2,130	Re: PP9 battery - reversed polarity. I suspect that one battery has a greater capacity than the other. When they are discharged in series, the lower capacity battery discharges first, and then as the good battery continues to provide current the bad one becomes reverse charged. Batteries used in series should preferably be of the same brand, age, and type. When using multiple AA or D cells in series I test each one on load before use, this sounds irksome but is very quick. I use a 1.5 volt torch bulb and a 1 ohm resistance in parallel and find that the brightness or otherwise of the lamp is very quickly observed. For 9 volt batteries I would test with a 6 volt 3 watt bulb and no resistance. Such batteries are intended for relatively slow discharge, and a load test at over 0.5 amp is a severe test and a good guide to condition. Do not worry about over running the bulb for such a brief test.

23rd May 2015, 1:54 pm	#4
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: PP9 battery - reversed polarity. This was my thinking too.

23rd May 2015, 3:21 pm	#5
brenellic2000 Octode Join Date: Aug 2006 Location: Rye, East Sussex, UK. Posts: 1,647	Re: PP9 battery - reversed polarity. Thanks lads, Yes, it does read reversed off set; and yes I do replace in pairs, but one (the first in the series) always seems to go flat quicker... so it does make sense that the good is trying to recharge the bad. Do I take it then that this not a manufacturing fault in the battery, but luck of the draw? TTFN

23rd May 2015, 3:22 pm	#6
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,274	Re: PP9 battery - reversed polarity. It's a well known phenomena and why you shouldnt mix battery types as already mentioned. Some can leak badly when reverse charged...it happens with AA types as well which are very often series connected. I was confused when I first came across it! Maybe fit some reverse parallel diodes? __________________ Kevin

23rd May 2015, 5:09 pm	#7
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: PP9 battery - reversed polarity. Yes, the first cell to run out of capacity will be reverse charged by the rest. Batteries in series though... Are they in a simple series connection, or is there a connection to the mid point? Some output stages needed the mid-point as a return point for the loudspeaker. If the balance of the O/P stage isn't quite right, there could well be a small, steady current draw from the mid-point, which would act to discharge one battery before the other. And then the other would reverse-charge it. Not good. Anti-parallel diodes across each battery would be a good, cheap, protection method but sadly few, if any, manufacturers fitted them.