I have a YT7B-BS battery in my motorcycle. My motorcycle doesn't start because the battery voltage is only 12.05 Volts.

The battery has the following inscriptions:

  • Standard charge: 0.7 A x 5~10 hours
  • Quick charge: 3 A x 1 hours
  • 6.8 Ah (20 HR) / 110 A (CCA)
  • 12 V, 6.5 Ah (10 HR)

I tried to charge the battery with an external hard drive power supply rated as following:

  • Switching adapter
  • Model SYS1298-1812-W2E
  • Input: 100-240 V ~ 1.0 A MAX, 50-60 Hz
  • Output: +12 V continous 1.5 A
  • Output power: 18W max

I have plugged the positive terminal of the power supply to the positive terminal of the battery, and the negatives to the negatives. After a 2 hours, the battery voltage is exactly the same: 12.05 V. The power supply is a little warm but not too much.

I plugged the motorcycle battery to my car battery for a few minutes and the tension went up to 12.5V, allowing my motorcycle to start (battery alone, no cables), so the battery doesn't seem to be dead.

Why isn't the power supply charging the motorcycle battery?

  • 2
    \$\begingroup\$ If you were able to start your motorcycle why not just leave the motorcycle running and let it charge its own battery? \$\endgroup\$
    – Ross Ridge
    Apr 17, 2017 at 20:41
  • \$\begingroup\$ Because it would take at least an hour, waste gas, would pollute the garage etc. The alternator mainly powers the headlights etc... only a small amount of current flows to recharge the battery it idle speed (if any!). It would be very wasteful and polluting. \$\endgroup\$ Apr 19, 2017 at 14:41

3 Answers 3


A 12V battery needs a higher voltage to charge. You're using a 12V HDD power supply with outputs a constant voltage to charge a battery that already presents 12V across its terminals. No current will flow from the power supply to the battery. If it was discharged even more however, you will be able to charge it up to 12V (Whatever the equivalent State of Charge corresponding to that voltage is).

Try using a 14V supply, and preferably with a current limitation (@3Amp MAX) feature so that you won't damage it.

  • \$\begingroup\$ I will only be able to try that once I recover my motorcycle.. which has been stolen since I asked this question... \$\endgroup\$ Apr 19, 2017 at 14:39

Battery is near 50% SoC at 12V and >90% at 12.5V float after 1 hr and needs 14.2V charge

Most likely the CCA has degraded from aging, temp or abuse (V<<1.5V for extended periods. (Sulphation) The CA rating is the current shorted to 7.5V and is chiefly limited by plates ESR from aging which reduces the Ah capacity as well.

The CCA rating is about 30% lower at 0'C than at 20'C for the CA test. You state CCA is 110A new which means 110A at 7.5V at 0'C . If you cannot get 50A at 7.5V at 20'C , then the battery is < 40% start capacity of new and NG . I suspect it is even worse.

If you cannot do a V reading during starter and measure I or any other load test to drop V and raise I by 50A then I suggest you get a new battery.

First get a better charger for your battery with 14V.


Lead-acid battery chargers exist for a reason - because charging a battery isn't as simple as you might think (and, doubly, has the potential to be rather dangerous). Randomly connecting electrical things that aren't meant for each other when you have absolutely no clue what you are doing will usually end up with one of several outcomes : 1) It won't work but your stuff will be fine, 2) It won't work and your stuff will be broken, 3) It won't work and you'll end up causing yourself an electrocution or an explosion, 4) You get lucky and it works. In this case, you got lucky with outcome #1.

A real automotive battery charger will be labelled and sold as-such and will use a variety of voltage or current control methods while sensing the state of charge in the battery to adjust its delivery of power. Improperly charging a lead-acid battery, in the worst case, can lead to excessive generation of hydrogen gas and explosion so this isn't really something that should be toyed with.

The other answers have given good detail about why your battery didn't charge, but fundamentally this didn't work because entirely the wrong tool was used for the job. A commercial battery charger is not expensive and will generally be cheaper and do the job better (and more safely!) than a DIY hack. Designing and building a charger as a DIY project (that will actually work), while entirely feasible, is also most likely not going to be as cheap as simply buying a charger. It sounds like your objective here is to get your battery charged and not to embark on a didactic project to learn about electrical engineering so, do yourself a favour, and just go buy an appropriate charger.

  • 3
    \$\begingroup\$ Charging a lead acid battery is as easy as hooking it to a suitable voltage supply, it's not a Lithium battery. In your car it is hooked to the generator via a diode, it's not the definition of simple maybe, but that's pretty close. You can even parallel it with another battery to charge it (as op did)... \$\endgroup\$ Apr 17, 2017 at 21:07
  • \$\begingroup\$ @VladimirCravero the alternator output is also controlled by a voltage regulator (not sure what it actually is or how it's connected) and the excitation coil current. If the regulator stops working, it can start overcharging the battery, boiling and/or electrolyzing the electrolyte. \$\endgroup\$
    – Nick T
    Apr 17, 2017 at 22:48
  • 2
    \$\begingroup\$ @VladimirCravero Sure, but if OP has no idea why his 12V wall wart wasn't charging the battery then there's nothing to stop any other crazy idea like attaching a 24V supply or maybe even plugging it straight into the mains. The point is that, while it is in principle "simple" to charge a 12V lead battery, it can also be extremely hazardous if you don't understand what you are doing. Even "normal" charging voltages can eventually push a lead-acid battery into dangerous hydrogen production if the process is not regulated. It's a risky system to just slap whatever voltage onto and walk away. \$\endgroup\$
    – J...
    Apr 17, 2017 at 22:59
  • \$\begingroup\$ What @VladimirCravero said. Car battery chargers are (or at least historically were) trivial and had no charging control circuitry. Any DC voltage source 13-14 V is suitable for charging but might be slow unless it can provide a lot of current. \$\endgroup\$ Apr 18, 2017 at 2:23
  • \$\begingroup\$ I agree with the warning but this kind of battery is quite easy to charge and you can even overcharge them without big risks. Feels like a dumb question now :) Batteries sometime have a air valve to evacuate the hydrogen produced, not mine. \$\endgroup\$ Apr 19, 2017 at 14:44

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