I found a Razor electric dirt bike and figured I'd see if I could fix it.

Batteries were dead and the "control board" was dead too. It's an on and off throttle so really a relay is all that is needed, though I'll probably put in a motor controller to set max speed while I'm in there.

Now the batteries are SLA 7ah 12v batteries (two in series). I disconnected them and they were DEAD.

I figured I'd try to give them a charge and see what happened.

I got out my bench power supply, did some googling and set it to give 14.7v at a max of 2 amps.

I hooked it up and the (individual) battery was taking .3 amps at 14.7v.

I did some more googling to see that might be because of sulfation on the plates. Sure enough, as I let it sit, it started to increase in the amperage it was taking.

Now, I've put it on to charge for like 12 hours, several times over the last few days. I take it off the power supply when I leave the house for safety. I have now left it on for 24 hours and it's still taking 2 amps at 14.7v.

I'm just confused as to where this power is going.

Any ideas?

  • 1
    \$\begingroup\$ It's worse than that he's dead Jim, dead Jim, dead! \$\endgroup\$ – Bruce Abbott Feb 24 '18 at 18:21
  • \$\begingroup\$ it is going into heating what used to be a battery and is now a resistor. \$\endgroup\$ – jsotola Feb 24 '18 at 18:39
  • \$\begingroup\$ @BruceAbbott Ye cannot change the laws of physics! \$\endgroup\$ – bitsmack Feb 24 '18 at 19:01
  1. 14.7 V is very high for the float charge voltage of a typical "12 V" lead-acid battery. At that point you're doing damage. Something like 13.6 V is a much better value.
  2. Lead-acid batteries will take current at the float charge level indefinitely. 2 A is high for a 7 Ah battery, but then again so is your float charge voltage. Measuring the current while abusing the battery doesn't tell you anything useful.

Take the battery off the charger and see what you've got. It's open-circuit voltage will probably be around 12 V or a little more. However, the real test is how it performs under load.

A good test would be to connect a 12 Ω or so power resistor. 12 Ω should draw about 1 A, which is something that battery should be able to provide for a while. Monitor the voltage over time, but stop when it gets to 10 V. Letting it get lower will cause some damage in the form of loss of capacity.

Note that the resistor will dissipate about 12 W when things are working, so it should be a "15 W" or "20 W" resistor.

If a battery can sustain the 1 A load for maybe 3 hours before dipping to 10 V, then it still has some useful capacity left. If the voltage crashes within a few minutes, then this battery is gone. Dead. Kaput. Dispose of it properly.

  • \$\begingroup\$ I thought with a Lead Acid battery that the amperage being accepted would reduce. Once reduced then I would want to go into float charge. Are you saying that a healthy battery would just keep taking 2Amps? \$\endgroup\$ – Gilligan Feb 24 '18 at 20:30
  • \$\begingroup\$ @Gilligan Having produced half a million battery chargers with my name on it (the PCB of course, not the product), you’re not wrong. After ~0.8 times battery capacity divided by charging current, you should hit float and your current will drop. 14.7 V is a bit high for everyday charging but too low to revive half-dead batteries with too high internal impedance. You on the other hand seems to have found one with a dead cell. If you can see though the plastic or open the fill-plugs, you should be able to see bubbling on all but one cell during charging. \$\endgroup\$ – winny Feb 24 '18 at 20:39
  • \$\begingroup\$ I do not have a resistor of that wattage at the moment so I just hooked it up to the 24v motor (250W) which is I think a 1/2 ohm load if I remember right, and it ran pretty steadily for a good 30 seconds. I stopped running it and put the DMM on it and it was showing 13v. I'm anticipating having to change these batteries and will likely just go to some 18650 when I do. I was just looking to play around and see what I could get out of these batteries for the time being. I was perplexed at the continued draw after what seemed like enough time. \$\endgroup\$ – Gilligan Feb 24 '18 at 20:43
  • \$\begingroup\$ @winny these are SLA batteries so there are no ports or ways to see bubbling. :( \$\endgroup\$ – Gilligan Feb 24 '18 at 20:45
  • \$\begingroup\$ @Gilligan Unless black plastic, I usually shine a bright light from the backside to shine though or listen for bubbling. \$\endgroup\$ – winny Feb 24 '18 at 20:48

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