Thread: Long Term Diagnostic Sessions - Battery Charger or Power Supply?

I ended up disconnecting the Bat +ve and hooking my power supply up directly to the lead. -ve was left connected to battery where I connected as well.

I'm still not quite sure if the power supply would overcharge the battery if left on long term so I chose to do it this way - but then again, if an alernator is driving at 13.8V wouldn't it overcharge a battery too?


Lukeyson

just curious...sorry to ask off topic, but what kind of interface are you using to scan for those Mode 22 Pids? that is GM mode 22 right..?

I'm using a Drewtech Mongoose ISO/CAN J2534 tool and the fantastic IPSECMsgMon utility which is part of the Cardix SDK.

Mode22 is not specific to GM. It's one of the standard Diagnostic modes as defined in ISO14229 (ReadDataByIdentifier), and also found in ISO15765-3 for Diagnostics on CAN and J2190. I'm doing this on an Australian Ford BA Falcon XR8.


Lukeyson

I bought a fairly inexpensive battery charger that I just clamp on to the car. I've never had a problem as long as I have everything turned off (fan, interior lights, headlights, radio).

You've got one hell of a power supply if can supply 23 amps @ 12 volts. Typical battery chargers adjust the current while charging the battery and offer protection against overvoltage and overcurrent. I still prefer my automotive charger but you're probably ok with the power supply since the battery is disconnected and the car's not running. Keep an eye on the current so you don't smoke your nice power supply.

Most (all?) diagnostics still work with the car running. Just have somebody with a laptop in the passenger seat and drive to a beach that's 2 hours away.

I've got a trickle Battery Charger here with all those features, but it doesn't supply enough current so tha battery still drains.

I still don't understand what it is that stops the alternator from overcharging the battery? Given that there's nothing other than the Alternator Regulator there, I can't see this doing Battery Recharge regulation since it needs to supply lots of grunt to the rest of the car. And there's certainly no smarts in the big fat wire that links the battery and the alternator that I can see...

I'm trialling connecting the power supply to the car without disconnecting the battery, and will monitor things closely for a little while. But to be honest, I'm not expecting any problems.


Luke

The smarts is in Kirchoff's Voltage Law.

the current going into the battery is a function of the difference between alternator output potential and battery voltage.
Eg = alternator output (rectified)
Vt = battery terminal voltage.

Eg>>Vt battery charges

but since as the battery charges Vt goes up, the more the battery charges the less current that flows. Unless the charging voltage knows to rise to a set differential voltage above Vt, the battery will only charge up to Eg and go no further. Current will not flow when they are equal(Vt=Eg), and the battery will not overcharge.

So to be clear, a 12V lead acid battery is not overcharged at 13.8V, right?


Lukeyson

No, in fact a battery cell's voltage varies with the state of charge discharge. 2.0V is the nominal voltage for a lead-acid battery cell (12V for 6 cells). A battery gets up as high as 2.3V per cell (13.8V) at a fully charged state, and discharged can get to voltages of 1.75V (10.5V) where any lower can cause irreversible damage.

Sometimes batteries are charged at higher voltages to rejuvenate batteries, but that is for short, controlled periods, to do things like drive all of the sulfates out of the porous plates. 14.4V is another limit to consider, that is a gassing limit for the 6 cell battery, charging above that limit will cause excessive Hydrogen and Oxygen production through the electrolysis of water.

So 13.8V is fully charged, 12.0V is nominal, >11V is discharged.