Car battery "miracle"

I squeeze extra years out of batteries by charging them every few months at a current-limited higher voltage to break down sulfation.

They say up to about 20V at a controlled low current. I haven't recovered useful life from a battery that needed more than 16~17V to begin accepting current, or found a published value for the current limit so I arbitrarily chose 0.5A which doesn't overheat my home-made LM317 adjustable power supply. Commercial trickle chargers don't go high enough and would dry out and ruin batteries if they did and were left on too long.

Sealed VRLA batteries don't withstand overcharging very well, look up the maker's specs. Flooded batteries with filler caps are more tolerant since you can monitor the specific gravity and top them up if low.

Analog ammeters are cheap and good enough to set the charging level:

I prefer a digital meter with 10mV resolution to check the voltage. A digital multimeter is fine since you can disconnect it and turn it off, unlike the current meter which has to stay in the circuit. I haven't fried an HF multimeter by turning it off while connected to voltage or current, but I think that's a bad habit.

I bought one of these because it looked safer to leave unattended overnight with the hood closed:

It charges OK, 1.5A initially falling to 0.7A as the battery reaches

14.5V and then ~0.1A at 13.4V, but slowly drains the battery when unplugged. Like several of the reviewers I put a connector on the battery side, 45A Andersons that I use for all 12V wiring.

-jsw

I generally don't condemn a battery untill it's been on a charger for an entire week

Or been failed by a Midtronics type tester.A Midtronics has been known to say a bad battery is good, but I've never seen it call a good battery bad. The one "false good" I am painfully aware of was one with an intermittent open intercel connector that went "BANG" 2 days after the tester said that although it wouldn't crank the battery 20 minutes earlier, it was fine!!!.

Do you know how it tests? I downloaded the PBT-300 manual which gives nothing away.

When I was the Lithium battery tech at Segway the impedance test was to preload the battery with a small current, then pulse a large one with an electronic load and record the currents and voltage dip with a storage scope. The impedance was the slope of the line connecting the two points, R = E / I, and didn't pass through zero. The readings changed quickly as the electrolyte was locally depleted, so I took the values at a specified time after the pulse start. The test required a very expensive lab-quality DC current scope probe.

Here's a 1 milliOhm current-measuring shunt for experimenters:

The problem with a shunt is that it ties your oscilloscope to battery voltage, which is at worst a hazard and at best a voltage measurement offset issue.

-jsw

They are called "transcondictance" testers and they work even on a 80% depleted battery - I suspect it is a similar method.

It looks to me as if AC conductance is good for a mechanic's quick check but no better than the old resistive load tester or measuring the discharge V-I curve for home use, since we can let a test run for hours and then wait while the battery recharges.

See comment #7

What that doesn't show is that the battery will jump back to the same fully discharged voltage after each load is removed. The lower endpoint voltages at higher currents include the large IR drop through the depleted electrolyte.

-jsw

[ ... ]

Yep.

Put the shunt in series with the ground side, and measure voltage referenced to ground. (Then measure voltage across the battery with two voltage probes in differential mode, if a 75mV error is too much.) Set scope to A+B with B channel inverted.

Voltage hazard handled by putting the shunt in the ground side, offset by the differential input setup. (Unless the shunt burns out, at which point the ground side of the battery is now floating. :-)

Enjoy, DoN.

That's a good explanation of the setup except that it requires a 3 (or

4) channel scope, since the point of connecting the shunt was to display the current.

Using a battery ground braid to connect the battery to the shunt removes most of the voltage offset from IR drop in the cable so one voltage probe should be good enough. The scope can be grounded to the battery side of the shunt and the other side considered part of the load. Its voltage will be positive during discharge, negative if the charging current passes through it.

Do NOT let the probe ground touch the (+) battery terminal.

I'd say "and park it on a hill", but think of all the ways you'd have to qualify that statement!

IF it's old enough to not have a computer, and IF it has a manual transmission THEN park it on a hill.

Mine has a computer and a manual transmission and I park it on a slight slope every night. Half the time I don't use the starter when backing out of the driveway.