Another battery question

That little 'dolls eye' only gives a rough indication of the state of charge of one cell, don't forget that you have another 5. It's only a little gimmick - it tells you battery state of charge (in one cell only, give ot take), and is not a battery failure diagnostic tool!

Julian.

Had exactly this on my Disco 1, turned out to be a corroded earth strap from engine to body, worth checking.

Andy

(snip)

I had one go with similar symptoms, except it was a solid fault. I reckon it was a high resistance in the battery. A new one fixed it...

But would that give a low voltage across the battery when trying to start?

It should be either 14.6V or 14.8V (depends on which book you read!) off-load, so I'd venture a cell has gone short, which produces a high resistance when cranking, hence the low cranking voltage (if I remember right, cranking voltage is allowed to go down to about 8.5V in the specs).

Richard

It would, V=IR and all that, which is why the battery gets blamed quite a lot for bad earths, as it appears to "go flat very quickly" as the voltage drops.

Richard

(snip)

But, if there is a bad connection, whether earth strap or something else, the volt drop is across the bad connection, not the battery itself.

I must read different books :-)

But the duff connection increases the resistance of the circuit, and as the battery is (to all intents and purposes) giving the all the I it can when cranking, I is constant, R increases, so V must fall.

Unless R gets so great that I goes through the handbrake cable instead, which is when W gets involved; with a fire extinguisher. Richard

On or around Mon, 12 Feb 2007 11:53:14 -0000, "Bob Hobden" enlightened us thusly:

check the earthing, like people said. If the battery's under the passenger seat it earths to the chassis underneath the seatbox. remember to check that one, too.

one other possibility: low electrolyte. If you get fully charged voltage and no power, it can be that. but not if it's an intermittent fault.

Obviously, if the electrolyte is half-way down the plates, you've effectively got a half-as-big battery. I once found on that somebody had dumped, with about 2" or less of electrolyte in the bottom. Topped it up, charged it and it ran me car for over a year.

The 12.6V (usually reagrdad as 13V) is the on-load voltage, unless everyone at Lucas Rists was wrong when I worked there! (designing and making ehicle harnesses, in cluding battery leads).

Richard

"Andy" wrote after>

Earth strap replaced and a new one added between engine and battery.

The I is limited by the resistance, so with a duff connection, I falls. If I remained constant even with high resistance, V would have to increase to maintain the V=IR relationship. A battery is much closer to a constant voltage supply than a constant current one. It depends a lot on exactly where the voltage measurement is made. I was assuming across the battery terminals, where an excessive drop in voltage when cranking indicates resistance within the battery. If, for example, the voltage drops from 12 to 6 when cranking, then the internal battery resistance is the same as that in the rest of the circuit.

"Austin Shackles" wrote after... "Bob Hobden" enlightened us thusly:

New battery earth strap bolted on and another added direct between engine and battery as suggested here a few weeks ago, symptoms remain the same. Yesterday I took the dash panel apart to see if there were any loose connections on the ignition switch but they were all tight and clean (look brand new). (As usual another problem reared it's head, the plastic strap holding the switches onto the steering column was broken allowing them to move about!! ) Electrolyte is fine. Battery is now on charge, when I started it charging this morning (11.30) it was taking just over 4 amps but it's now (18.30) down to well under 1 which is about right I think.

I'll do some checks on it tomorrow when it's fully charged, but I suspect it's knackered.

So what size battery, amperage, should I go for?

Could it be the starter btw? The replacement put on by the ins co's garage after it was stolen never sounded the same as the old original one. To discount the starter what symptoms should I look/check for?

The on-load voltage depends on the load, shirly :-) The 12.65V quoted above is off load, off charge, and surface charge removed. (and 12V nominal wet cell lead acid)

Whatever, it's not worth arguing about. The three batteries in the stores are apparently defective though, which is a pity.

Richard

No it wouldn't.

With a constant voltage the current flow drops proportionately to the increase in resistance so as less starter current is drawn because of an increase in resistance then the voltage taken across the battery will stay high (or higher in practice). If the battery voltage drops *so* low under starting conditions, the battery *may* be knackered or it may be just flat due to undercharging because of a high resistance affecting the charging circuit. Don't discount that the starter may also be drawing excessive current.

Check the connections with jump leads and get the starter and battery tested before spending the hard earned.

Martin

That voltage would be read under charging conditions, off load charge would be approximately 12.75 volts.

so I'd venture a cell has gone short, which produces a high

If a cell has 'gone short' as you say, that produces a low resistance and also a low voltage as the battery has less cells in the series. If a cell had a high resistance the voltage would also read low but with no cranking capacity.

It's easy to tell if the battery is knackered or not....Get it fully charged then nip into your local garage or kwickfit and ask them to "drop test" the battery...

Jon