I have an old (ca. 10 years old) 6V lead-acid battery and a mains charger for it and I know the combination worked years ago.

Now I tried to recharge the battery and the charger's LED turns on during charging, but when I use the battery with a LED bicycle light (for dynamo use) the bicycle light doesn't illuminate. Of course the light might be defective, so my question is:

How can I check whether the lead-acid battery and its charger are still working?

I do have a simple digital multimeter and after charging the battery over night it gives me a voltage of 4.5V at the battery (no other load attached,) but at the plugged-in charger I see strangely only 0.15V (no other load attached.)

I have no idea what's going on. Is all of this normal and the bicycle light is the problem?


The battery is a DiaMec SLA battery. Here a picture of it:

SLA Battery - Bike lighting stuff_IMG_0639

And the brand of the charger is PowerTech:

enter image description here

Update II / Resolution:

Finally, after reading all the great tips and comments, I did get a new battery (exactly same make and model) and the old charger works perfectly with it and the lamp works as well. So indeed, as people told me, it was the battery which was defective.

  • \$\begingroup\$ what is the battery voltage when the bicycle light is connected and turned on? \$\endgroup\$ – jsotola Apr 29 at 6:52
  • \$\begingroup\$ @jsotola You mean I attach the multimeter in parallel circuit with the light to the battery, right? (Sorry for the possibly silly question, but my times of fiddling with electronics are decades ago.) \$\endgroup\$ – halloleo Apr 29 at 7:06
  • 1
    \$\begingroup\$ It’s dead Jim. Buy a new one. \$\endgroup\$ – winny Apr 29 at 7:25
  • \$\begingroup\$ ... but not as we know it. \$\endgroup\$ – Andy aka Apr 29 at 10:01
  • \$\begingroup\$ @halloleo, put one probe on the negative terminal and the other probe on the positive teminal \$\endgroup\$ – jsotola Apr 29 at 14:14

With the battery connected to the charger, and charging up, I would expect to see something like 7V (give or take a fraction of a volt). Once the charger is disconnected, the battery would drop to about 6.5V when freshly charged.

If the battery is only giving 4.5V, then it is totally flat. If it has been left like that for years, it is unlikely to ever work again.

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  • \$\begingroup\$ Thanks @SimonB. And the charger? Is the charger broken when it gives only 0.15V without anything attached to it (apart from the multimeter)? \$\endgroup\$ – halloleo Apr 29 at 8:29
  • \$\begingroup\$ @halloleo That depends on whether it's a smart charger or a dumb one. I would expect a dumb charger to be putting out a voltage all the time. A smart one may detect that there's no battery connected, and shut itself down. \$\endgroup\$ – Simon B Apr 29 at 13:56
  • \$\begingroup\$ Not sure. I added a photo to the question. It says on the charger "Trickle Charging" - does this mean it is a smart charger? BTW, the charger never reaches the flashing trickle stage... \$\endgroup\$ – halloleo Apr 30 at 2:53
  • \$\begingroup\$ @halloleo It looks like it is. Most likely, it's never succeeding in charging the battery, so it never reached the trickle charge stage. \$\endgroup\$ – Simon B Apr 30 at 7:23
  • \$\begingroup\$ Thanks for the info! For the "resolution" see the last update at the bottom the question. \$\endgroup\$ – halloleo May 4 at 2:56

As you have mentioned in your very first line, the battery has been sitting in storage for a period of 10 years. I’m quite sure there has been a lot of sulfation of the plates going on during this period. This damages the plates of the battery quite significantly. Is your battery a Sealed Lead Acid battery or is it one of those flooded 6V motorcycle batteries?

I have tried restoring some fully dead more than 10-year-old lead-acid cells by topping it distilled water and adding three drops of battery acid to increase the conductivity (since your battery is charging you don’t need to add any acid). This helped to increase the charge holding capacity to a good extend. One thing to check is to shake mildly your battery if its VRLA and hear for something moving at the base of the battery when shaken. If you can hear something moving then the lead particles from the plates have disintegrated and are settling at the bottom of the cell and this causes shorts on some cell chambers. Now if your battery is like one of these conventional distilled water topped old automotive batteries then you can see it through the case. I think this might be the reason why you are getting only 4.5Volts.

A single lead-acid cell can output around 2.1-2.25V. A 6V battery is made of three 2.2V cells in series. So, one of your cells is likely shorted out or the battery acid in that cell had dried out. Try adding water to all the cells and then charging up. If you feel the heat building up in any of the three cell chambers (can be felt on the battery surface) then the battery is ready to go to recycling center. If you turn lucky and the battery manages to charge up finally to get fully charged to 6.45V. Then you may need to cycle the charge-discharge cycle a few times to build some capacity. But even then a significant part of the capacity will be lost. You may need to load test it to see the exact figure. Hope this helps.

As for your charger that you have stated above giving only 0.15V. Is it a smart charger? What brand and model is it? Try using a traditional charger that can give output no matter the voltage of the battery. Like a car/ bike charger with 6V setting at a slow charge rate of 2A or below that for the first 15 to 20 mins. Once it’s above 5.98 to 6.12V then you can continue charging the battery with your present charger and keep checking the voltage to see if it has increased.

Battery discharge-charge percentage chart

Charge indicator

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  • \$\begingroup\$ It is the SLA type. I don't think I want to open the battery... \$\endgroup\$ – halloleo Apr 29 at 23:07
  • \$\begingroup\$ Do you have access to a bench power supply or automotive charger with 6v setting at lowest current setting if available? Try charging on it for like 15mins to see if the voltage rises to 6V or above. All the older batteries that I’ve restored needed the water to be topped. If you are not comfortable then getting a new one is the only option. \$\endgroup\$ – The_Vintage_Collector Apr 30 at 1:31
  • \$\begingroup\$ No other power supplies available. Might get a new battery indeed. Hope the charger still works! \$\endgroup\$ – halloleo Apr 30 at 2:54
  • \$\begingroup\$ Looking at the picture that it has two modes means that it is definitely a smart charger. Taking a look at the specifications of the charger also confirms that. So, one thing to remember is that these smart chargers have a minimum voltage threshold, below which they will not start the charging process. So, assuming you are lucky maybe your battery could accept a charger but this charger would not allow you. The only way to bypass this is either an older non smart charger or connect another 6V battery in parallel and hookup the smart charger. Once you see the charging light flashing... \$\endgroup\$ – The_Vintage_Collector Apr 30 at 10:32
  • \$\begingroup\$ ..wait for like 20-30mins & measure with a multimeter to see the voltage is higher like 6.05V or above. At this point, you can unplug the good battery in parallel & see if the smart charger will charge you other battery without any issues. One thing is I don't have a lot of hope with the life of the battery or it's capacity. Since it's for your safety on a vehicle get a new battery from a trusted brand like Yuasa etc. If your battery holder can accept bigger batteries go with a 6V 7Ah or better 6V 12Ah capacity battery. Lasts longer on a single charge & less worry about deep discharge. \$\endgroup\$ – The_Vintage_Collector Apr 30 at 10:36

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