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I'm an electrical engineer who could use some help understanding lead acid batteries.

I recently bought an old motorcycle and charged the battery on my trusty automotive style battery charger after it lost charge. After several hours, the water was boiling inside the battery. I'm fairly certain the battery is relatively new and the water level was correct the last time I checked. I didn't have my multimeter at the time so I didn't take any measurements, but the battery is 12V and charger was set to 10A.

  1. Is it normal for the water to be boiling?
  2. Is my charging current set too high?
  3. Can permanent damage result from boiling?
  4. Is this the reason they must be topped off with water from time to time?

Thanks!

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    \$\begingroup\$ How many amp-hours was the battery? Lead-acid rarely charges at even 1C (usually 0.2C), so unless you had a 200Ah motorcycle battery, you put it through a hell of a time. \$\endgroup\$
    – Bryan Boettcher
    May 19, 2017 at 20:52
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    \$\begingroup\$ That's not boiling. \$\endgroup\$
    – Harper - Reinstate Monica
    May 19, 2017 at 21:14
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    \$\begingroup\$ 10A! Yikes, that's probably almost ten times what it's designed for. \$\endgroup\$
    – David Schwartz
    May 19, 2017 at 22:50
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    \$\begingroup\$ Bubbles forming and occasionally rising to the top? Or actual boiling in the sense that the electrolyte itself had reached boiling temperature and was also boiling actively? The reason is that lead-acid batteries normally form bubbles on the plates during charging. And these get big enough and then rise. Some chargers will periodically reverse the charging voltage polarity for a moment in order to force the bubbles loose so as to keep them small, as the bubbles interfere with re-plating lead from solution back onto the plates, forming unwanted filaments of lead. \$\endgroup\$
    – jonk
    May 19, 2017 at 22:53
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    \$\begingroup\$ Add distilled water (not tap water) to cover the plates. Then charge slowly (1A ish) and hope for the best. My money is on the purchase of a new battery before you try to recapture your youth... \$\endgroup\$
    – StainlessSteelRat
    May 19, 2017 at 23:03

3 Answers 3

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  1. Yes it is normal for the water to boil if you are overcharging it, with a too high current. And it is bad.
  2. Yes your charging current is set too high. Also check that the final float voltage from your auto charger is correct to the final charging voltage of your battery.

From Yuasa batteries (pdf): yuasa techmanual

For the correct charge rate a rule of thumb is to divide the battery’s amp hour rating by 10. For example a 14 AH battery should be charged at 1.4 amps (14AH÷ 10 = 1.4 amps). See the section on “Choosing a Battery Charger” for
more details.

When charging amperage exceeds the level of the natural absorption rate, the battery may overheat, causing the electrolyte solution to bubble creating flammable hydrogen gas. Hydrogen gas, when combined with oxygen from the air, is highly explosive and can easily be ignited by a spark.

  1. Yes.
  2. You shouldn't boil them. But some evaporation will still occur. Even "sealed" batteries have a safety valve for venting if they are overcharged.
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    \$\begingroup\$ Thanks for the response. It was bubbling, perhaps it wasn't boiling I suppose. You would think I would have known better but I didn't check the charging current before hooking it up. I'll top off, set my charger to 1A, and see what happens. \$\endgroup\$
    – Fuffer
    May 22, 2017 at 17:09
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The electrolyte has a diffusion rate - the rate at which an electrical field can transit through the electrolyte (determined by the state and concentration of the acid). If you force the power by applying a higher voltage (Pressure) the excess energy is taken up in the water molecule and splits it - H and O2 The is gassing but not boiling. As above, lower the charge voltage under 14.2 and do not exceed an amperage of 30% of the C10 rate of the battery.

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  • \$\begingroup\$ This is a great answer, although I think you have meant "the rate at which electrical charge can transit through the electrolyte". The field can be assumed to be constant, for simplicity. \$\endgroup\$
    – Kuba hasn't forgotten Monica
    Aug 30, 2022 at 22:17
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U can begin charging on 'engine start. BUT...only for about an hour. Then back down to 'normal' (your charger likely has these two settings). It is normal for bubbles to rise even during normal charge (which exceeds the charge rate of your alternator). Hydrometer tells the truth, if you have one. my issue: charging a battery that has been in car that hasn't been run since 2019. The battery may well be toast.

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