DIY Doctor

Battery Charger and Multimeter Reading

Postby greengrass » Fri Jan 18, 2019 12:56 am

Seeing as its electric though I'd try my luck can't find direct answers elsewhere
mm= multimeter
Test for a car charger.
Is following correct to find out if charger is pushing out a charge.
Charger plugged in not activated.
Leads on charger connected to a multimeter leads set at 20 range DC
Once connected charger switched on to rapid charge . Display on mm 19.43
Charger switched to trickle charge display on mm is 15 .35
Battery colour indication window shows red dot (recharge)
Multimeter across battery leads show 11.93 yet turn engine over half turn then nothing as though battery flat . Strange when meter gives 11.93 reading.
Battery is 1yr old.
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Postby Mr White » Fri Jan 18, 2019 7:36 pm

Batteries are not what they seem

With your meter across + & - on the battery, you are only testing for a potential difference between the two connections. So that is what the meter shows you in volts.
When you give the battery a "load" (something to do) it the requires current to operate that load, your battery cannot supply any current (Not enough to turn your vehicle starter) so the voltage drops as it tries to "work the load"

Your battery probably has a failed cell.

When testing any battery you should always give it a load, but since most batteries that are tested are for small things like a torch, a low battery voltage is accepted as an indication of condition, but this is not the case with lead acid batteries.
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Postby collectors » Sat Jan 19, 2019 12:29 pm

Fully charged automotive batteries should measure at 12.7 volts or above. When the engine is running, this measurement should be around 13.8 to 14.8 volts. Most chargers are a tiny bit higher on trickle charge. As mr white said, its quite likely a cell gone down. Is there any extra powerful thing in the car like amps?. as this wont help unless compensated for.
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Postby ericmark » Sun Jan 20, 2019 8:26 am

It will depend on make and type of charger, today both car own charging and external chargers have moved on, with changing a battery requiring the engine management needing to be told when new battery fitted.

A typical small charger will have stages, starts by pulsing a charge until a threshold voltage then highest charge dropping in stages as the voltage raises. Cheap Lidi charger has 3.8A, 3A, 0.8A and 0.1A and by end voltage will have hit around 14.4 volt which is well above what old chargers would have shown.

They also take a long time, it took 10 days to recharge my caravan battery. But they are designed to fit and forget, they will not over charge.

However quoting voltages does not really help any more, off charge no less than 12.4 volt, old regulated charger would have been 13.8 volt, but the Lidi charger seems to read 12.8 volt most of the time, what it does is as soon and volts drop below threshold it will increase charge rate, and volts will raise, but once at upper threshold it switches off again, so with casual glance your unlikely to see it during that short charge cycle, most of the time it's just above lower threshold.

The new Lidi charger actually has a volt meter built in, but the much more expensive Ctek does not and there are slight differences in all the small battery chargers so there is no one instruction fits all.

The cars have also changed, my wife's Jaguar XE will not allow you to even try starting if battery is flat, seems stupid, but guess it's to stop damaging starter motor! However you do realise there is a problem, if engine management detects low battery, the auto stop start is disabled.

It does not matter how big the battery charger is, or how big a battery is, a lead acid battery takes around minimum of 8 hours to fully recharge, as the battery once 80% charged will not accept a high charge rate, that last 20% takes longer than first 80% and if left flat it can take weeks to recover.

Batteries have sulphur on the plates when discharged, this hardens over time, so the longer a battery is left discharged, the longer it takes to recharge, this is why I like the connect and forget battery charger, as an experiment I connected a Lidi charger to an energy meter and watched how much power it took, the battery quickly went to 0.1A stage, although clearly not charged, it sat at this for 8 days, then jumped over an hour to 0.8A held at that for 24 hours, then dropped to 0.1A again at which point it was fully recharged.

I always knew it took time, but before never realised which a sulphated battery is charger how it suddenly reaches a point where it recharges, so if that battery was tested day 1 to 8 it would have shown US but by day 10 it showed A1.
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