I've been researching charging flat batteries and I think, if I've understood correctly:

  • Trickle charging isn't always possible from flat, because:

    • Some trickle chargers don't charge at all unless they detect a voltage close to 12v, and completely flat batteries will tend to be lower than this
    • Sometimes sulfur build up inside the flat battery prevents charging, and needs high voltages to beat
  • However, if it is possible, slow charging or trickle charging appears to be preferred over:

    • jump starting, which apparently can damage the alternator
    • regular charging, which I think can strain the battery if done from flat? I might have misunderstood this

How can I test if a particular charger is charging a particular battery?

  • Regarding minimum voltage, I can't find any manufacturer's info, but connecting the charger to a multi meter gives a reading of 19V. I'd imagine if it was the type to require 12V in the battery it would show 0V in the multimeter? (Not sure why it's 19V not 12V though [update] it's because its open circuit voltage is much higher than its voltage under load)
  • I have no idea where to start regarding checking if the sulfur issue might be preventing charging.
  • I tested the battery before connecting the trickle charger, and it gave a reading of 2 volts, which I believe is typical of a completely flat battery (we're talking "door and window switches don't respond, turning ignition makes no sound whatsoever" level of flat)
  • The charger itself gives no information at all.

Charger is a Nikkai Power 2.4W solar charger, so I'm expecting a very slow charge even if everything works. I'd like to know how to find out if it's doing nothing before having waited several days!

Update: one day later, I checked the battery voltage again and it had increased from around 2V to 3.8V. So, it is charging (and yes, it is extremely slow - this thing is more commonly used as a maintainer than a charger).

I also replaced my crappy multimeter than can't measure DC amps and can only measure DC volts to the nearest whole volt (why do they even make these?!?) with a better multimeter that does decimals and DC amps. The amp reading for the battery after a day's charging is (unless I'm reading it wrong) 40 mA. This seems insanely low - certainly explains why the car is unable to do anything, even central locking. When connected in series with the charger, the amp reading stayed pretty constant, when connected on its own with the charger disconnected, the amp reading was dropping quite fast (losing something like 0.4 mA a second). This also looks like additional confirmation that the charger is working. [update] Don't measure the amp reading of the battery without the charger like I did!

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    The problem is you can't separate out what the voltage from the charger is and the voltage of the battery. I think that unhooking every hour or two and charting the battery level would work, but I'm not sure at all. I do know that the charge level is NOT linear, so even if you know how long it took to get from 8v to 9v, you could draw no conclusions about how long it would take to get to 13v.
    – JPhi1618
    Jan 14, 2016 at 16:59
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    You could connect an ammeter in series with the charger and battery; this would at least tell you if the charger is doing anything. Whether it is actually charging the battery is a separate question. Jan 14, 2016 at 17:51
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    Note that based on a quick back-of-the-envelope computation, you will get a very slow charge indeed - it would probably take at least two weeks. Jan 14, 2016 at 17:54
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    @NateEldredge - Write your answer using your comments ... it's what I would do to test the battery, especially if I didn't have a charger which showed the condition. Jan 14, 2016 at 23:59
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    @Paulster2: I will if/when I get the time - I think I should really include a diagram. If someone else wants to do so before I do, they are most welcome. Jan 15, 2016 at 1:46

1 Answer 1


To summarize the various comments into an answer...

You can connect an ammeter (or a voltmeter in current mode) in series between the charger and the battery - this will tell you if a current is flowing through the circuit - and so whether the charger is at least attempting to charge the battery. Make sure that the meter you are using can take the maximum current that the charger can emit, and never connect the meter across the battery terminals in current mode - you'll probably destroy the meter...

This won't, however, tell you if the battery is actually holding a charge - for that you'll have to measure the voltage across the batter terminals after disconnecting the charger, and again a few hours later, to see if it has dropped.

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    I've also noticed that I can detect a small increase in voltage comparing the charger connected and disconnected, and that with the charger connected, and with a very sensitive digital multimeter set to show <20v accurate to nearest 0.01v, when the voltage was low to begin with, I can sometimes see the voltage (slowly!) increasing in real time Apr 12, 2016 at 10:50
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    I'd just like to add a clarification for people like me whose memory of series and parallel connections is not so good - "In series" here means that the multimeter is what completes the circuit. So to measure current, first you make the circuit incomplete (usually by removing the negative battery clip), then you connect the two disconnected points (negative terminal to negative clip) using the multimeter, so that the multimeter is part of the circuit. Don't connect positive post to negative ground (parallel connection) in current mode like you might when in voltage mode! Jul 19, 2016 at 16:01

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