The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: bsa-bill on 23.02. 2019 12:54

Title: polarity reverse
Post by: bsa-bill on 23.02. 2019 12:54
I'm struggling to get my old brain around this one, going to be one step at a time.
New battery on the Flash, Flash was negative earth I've put it back to positive in order to sell the bike.
So all was well battery connected correct way. dynamo flashed for positive earth, new dvr2 fitted (correct type +E), charging indicated on ammeter when engine revved, (battery was a cheapo, just needed to get the bike road worthy, better one in the pipeline for prospective buyer
to days ago I went to the bike and found the battery flat, I put it on charge but it didn't charge, tried a different charger - same thing, now modern chargers often will not charge a battery unless it has some charge in it so I put on an old charger and (confession) hospital appointments, bowls matches and shopping - yep left it on for more than a day, strangle all red lamps on the charger were still lit and the battery still flat, now here's the thing I changed  the meter to a digital one and noticed a very small Negative charge.
The battery has one cell full of chalk like substance, I flushed some of this out (about half of it) then put the battery on charger  the wrong way and it's charged upto 6.4 volt ( fell overnight to 5.5).
Questions for your consideration
1. I need to check dynamo polarity obliviously, easy enough to flash it again but I want to know what it is now, I have leds things in the shed so I can test poalrity, but is it safe to do so without harm to dvr2
2. could the old fast charger have reversed the battery polarity
3. how to reverse the battery back to normal.
4. anything else I should beware of that a 75 year old idiot might have inflicted on this innocent bit of machinery's electrical system
Title: Re: polarity reverse
Post by: duTch on 23.02. 2019 14:02

 Bill, I feel your grief having battery issues but for a bit different reason due to mechanical reg that needed adjusting- must've vibrated out *conf2*....can't help any with DVR2 though, but this is why I elected to go -ve earth so I don't (in theory) mix things up- oh yeah *eek*....though as I think you suggesting, I will say that the guy I go to for batteries said the other day that small lead-acid batteries shouldn't be on charge for more than about 45 min. (oh yeah *eek* again- guilty as charged *bash*)
Title: Re: polarity reverse
Post by: bsa-bill on 23.02. 2019 14:35
small lead-acid batteries shouldn't be on charge for more than about 45 min.

Cheers duTch, yes I'm thinking that might be a factor, yes to avoid confusing myself ( to easy) I had the Flash - earth, put it back to + in order to sell as original.
Battery voltage is down a bit but all right to fit wrong way around for the mo while I try to work out if the dynamo is putting out + or -
Glorious weather for Feb - Garden beckons also
Attached photo of battery, you can see the white stuff that's left I could probably flush this out but I'm thinking it's come from somewhere, probably from the plates in the cell so doubt I'll get full voltage out of it - it was cheap so to be expected I presume
Title: Re: polarity reverse
Post by: Swarfcut on 23.02. 2019 14:47
Bill, Bridging D and F on the dynamo and running to earth via a load such as a headlamp bulb and a test meter in series will establish dynamo polarity, and is the usual (backyard) way of testing dynamo output.  Check for current a drain with everything off, easy to measure current between the battery and earth. A drain here explains the flat battery, so time to go hunting. If all this checks out fine,  I would be inclined to discard that mucked up battery, it is the most likely cause of your problems.


Additional...Pic looks like some of the plates have started to disintegrate, so down to the recycling /scrappy for that one.
Title: Re: polarity reverse
Post by: Bsareg on 23.02. 2019 17:42
That white deposit looks like sulphate from the sulphuric acid. Normally takes some time (over weeks)  to precipitate from a discharged battery. Doesn't explain the voltage reversal though........
Title: Re: polarity reverse
Post by: bsa-bill on 23.02. 2019 18:31
Normally takes some time (over weeks)  to precipitate from a discharged battery.

Thanks - that's interesting, I've been more concerned with Carb issues for sometime so bike's been shed bound for some weeks, I had "sulphate" floating around in my head since discovering the chalky cell, school science battery acid " a weal solution of sulphuric acid" - thank you Mr Williams.
I thought if it was from the cell plates it would be fatal for the cell, I do  have some spare battery acid, so maybe if I flush the deposit out and refill the battery could be rescued
Title: Re: polarity reverse
Post by: BSA_54A10 on 24.02. 2019 12:14
Cheap batteries are cheap for a reason.
There are a lot more battery brands than battery factories.
Each & every batteruy off a production line eta a wide variety of tests.
The ones that fail get printed up in the colours of cheap battery brands.
Thus not uncommon to get a cheap battery that will not charge, or hold a charge.
Also not uncommon to get one with the wrong markings on the terminals because the wrong plate pack was put into a case.
In most factories now days the cells are assembled then attached to the top then the top attached to the lower half of the case.
Title: Re: polarity reverse
Post by: bsa-bill on 24.02. 2019 12:40
Also not uncommon to get one with the wrong markings on the terminals

Thanks Trevor, Something I/we take for granted and I don't recall checking it, haven't got back to the bile yet due to very unseasonal weather here in UK allowing gardening to proceed.
The electrics on the bike seemed to be working, - showed a discharge when lights on changing to a charge when engine revved,  wondering about the wiring of the ammeter now (not sure it makes a difference ?), anyway new battery in process of being ordered might possible need a new dvr2 , first job though is to check dynamo polarity, I did flash it but must check
Title: Re: polarity reverse
Post by: RDfella on 24.02. 2019 13:18
Maybe it was a reverse charged battery? Not common, but did have an issue with a new battery for an MGB in the 70's. Couldn't figure what was up, but a quick test showed the larger, +ve terminal was in fact -ve, and the -ve terminal was +ve. The new battery had been supplied charged wrong way around, so I discharged it and charged it the right way. Seemed to be OK after that.
Title: Re: polarity reverse
Post by: bsa-bill on 25.02. 2019 12:16
Tested the dynamo (F+D) with bulb and meter - showed a negative output - good, not putting any trust in anything I also tried a led indicator from dynamo it worked with positive lead to frame.
So system is as it should be (DVR2 hopefully not harmed).
Thinking Trevor could well be correct, I had installed the battery taking polarity for granted, bikes only been run to check lights worked and working through carbie problems, so lights would work whichever way the battery was polarized , can't quite work out why it went flat, can now understand why it did not accept charge (DOH).
Can't get all the white deposit out of the cell but it has charged to 5.4 v and seems to be keeping it, new battery arriving Wed, will leave any further investigating of DVR2 until then.
Many thanks for help guys
Title: Re: polarity reverse
Post by: berger on 25.02. 2019 12:54
hellooooo I am going to the pub, I knackered a solid state rectifier DVR whatever because the battery I was using was of low amp hour 7A I think ,either that or I mistakenly wired it up wrong because I forgot which earth I had chosen, but I am swinging towards it was because of the battery. right enough of this shi* its not not helping anybody and i'm waffling again. anyway got a new DVR DRV whatever and fitted the biggest amp hour I could find to fit under the back of the seat 10 amp hour I think and everything is fine *smile*
Title: Re: polarity reverse
Post by: mikeb on 26.02. 2019 08:59
hi Bill
i'm not sure i'm following all of this but isn't your battery toast? white deposits, reverse charging and weird voltages would make me say goodbye to it whatever the cause. As Trevor says cheap may be for a reason. 

good the dynamo is functional. i'd be most worried about the dvr2. i've been a happy fan boy of those but talked to KiwiGF in the weekend who said he's had two of them die - one by a brief short and another i can't recall if he could figure out why. either way they are not bullet proof and I'd be wanting to disconnect it before testing the dynamo or anything other testing. they cost too much to risk. Also do you have a fuse installed for it - at least one is necessary (i have fuses all over the place). also i assume you reversed the ammeter connections when reversing polarity? else the readings will be inversed/confusing.

side note i hate digital meters for these occasions as polarity is less obvious. for this I'd always use an old analogue meter so I'm less prone to misread polarity when it matters - which it does here
Title: Re: polarity reverse
Post by: bsa-bill on 26.02. 2019 10:15
i'm not sure i'm following all of this but isn't your battery toast?

yes new battery due tommorow, I did get more of the white stuff out but not all of it, battery charged up to 6.7 but will fall a bit, will dispose of it, might have done as a spare but my memory and it's change of heart + dangerous, big felt pen maybe help but no it's due for recycle.

I'd be wanting to disconnect it before testing the dynamo

DVR2 yes I disconnected the F and D wire at the DVR2 to test the dynamo, Ammeter works correctly (tested when testing the dynamo) and worked correctly previous to the battery going flat, leads me to thinking the DVR2 might be OK and further thinking that the possibly the battery reversed polarity after I took it off the bike (did I connect the charger wrong way ????).

also i assume you reversed the ammeter connections when reversing polarity?
Yes, if you mean when I changed the bike from neg earth, and it was working correctly, never got my head around how ammeters work, managed the volts , amps, wattage correlations but why an ammeter needle swings - black arts.
Fuses yes one in the line from battery, in defence of digital stuff it was my digital meter that indicated the negative charge ( - sign before the reading, my analogue meter just does not move or kick the wrong way.

The battery concerned was a stop gap till I got a decent one, the original battery was fine but missing one rubber plug for central cell, I did not want to sell the bike with battery with a bit of cork in place of the rubber plug, wondering now how much as set of plugs would be IF you can get them.

However many thanks for your replies/concerns 
Title: Re: polarity reverse
Post by: mikeb on 27.02. 2019 04:40
all good - hope it works out Bill. let us all know.

FYI ammeters are just low voltage meters than measure the voltage across a low resistance shunt. as current thru the shunt increase so too does the voltage across it (V=IxR), and the meter measures that. but if those electrons are flowing the other way (charge vs discharge) then the voltage polarity is also reversed so the meter swings the other way. old school magic - amazing what can be made out of a coil of wire.
Title: Re: polarity reverse
Post by: bsa-bill on 27.02. 2019 09:08
Thanks for that Mike, I'll read it a few more times so it sinks in, started life as an electrician (apprentice, we moved so that fell through
) and have tinkered with cars and bikes ever since so I think I must have known that at some point, difficult at 75 to say whether I have never known a thing or did and just forgotten  *smile* *smile*