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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!

user56reinstatemonica8
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  • have you looked into load tests? – race fever Jan 14 '16 at 16:42
  • Not yet, unless you count googling it after reading your comment! I must be looking in the wrong context because I'm reading things like "For accurate results with a load tester, the battery must first be recharged before it is tested" – user56reinstatemonica8 Jan 14 '16 at 16:49
  • Seems like I've read on Electronics SE that you can't charge a battery and check it's level at the same time. You have to momentarily stop charging, then check the voltage to see if it's moving. – JPhi1618 Jan 14 '16 at 16:51
  • @JPhi1618 so I should unhook the charger after a few hours, and if it's charging, the voltage of the battery will have increased? I guess the question then becomes "How do I calculate how long it should take for the battery's voltage to have increased by 1V" – user56reinstatemonica8 Jan 14 '16 at 16:56
  • I wonder if "you can't check the voltage while charging" would imply "If you try to check the voltage while charging and succeed, it's not charging, if it fails, it is charging"? – user56reinstatemonica8 Jan 14 '16 at 16:57
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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 '16 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. – Nate Eldredge Jan 14 '16 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. – Nate Eldredge Jan 14 '16 at 17:54
  • @NateEldredge my multimeter can measure amps so I don't see any reason why I couldn't. Can you write up a quick answer saying what order things should be in and what signs I should be looking for? – user56reinstatemonica8 Jan 14 '16 at 17:56
  • I spoke too soon - it can only measure AC amps, voltage is the only DC thing it can measure. – user56reinstatemonica8 Jan 14 '16 at 18:59
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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. – Pᴀᴜʟsᴛᴇʀ2 Jan 14 '16 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. – Nate Eldredge Jan 15 '16 at 01:46
  • @NateEldredge is right, you should connect the ammeter between the charger and the battery (in series) to measure the charging current. In the current measuring mode the meter has as close to zero impedance as it can manage. Which means if you connect it directly across the battery you are putting a dead short on it. That is going to result in a melted ammeter since a good battery will put over 100A through it, and many cant handle more than 20A. Also check the max charging current for the charger, and be sure it's not more than the meter can handle. – cdunn Jan 15 '16 at 14:02

1 Answers1

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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.

Nick C
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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 – user56reinstatemonica8 Apr 12 '16 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! – user56reinstatemonica8 Jul 19 '16 at 16:01