Recently purchased a Yuasa PowerSport (6N5.5-1D- 6V 5.5Ah) bike battery. Since Amazon does not sell it with the acid it has to be purchased separately from an auto store or self prepared. I went with the latter and prepared a 37% sulfuric acid solution using a 98% ACS H2SO4 reagent.

If I'm correct flooded Lead acid batteries use 36-38% sulfuric acid electrolyte.

I had prepared the acid solution yesterday. A little while ago I added the prepared acid to the battery and immediately upon adding the lead plates died/bubbled a bit and the battery is getting warm (not hot.) Is this normal or something is wrong with my battery?

The battery shows a voltage of 6.62V.

  • While charging the battery for the first time can I use a constant voltage charger?
  • Can the charging voltage be 7.2-7.5V like SLA or should it be lower for a flooded cell?
  • \$\begingroup\$ They charge the same. I don't know about the initial warmness though, I've not dealt with "building" a battery before. \$\endgroup\$
    – Aaron
    Commented Jun 30, 2020 at 15:47
  • \$\begingroup\$ The 6v battery should be fine if at 6.62v initially. If you see/hear bubbling while fully charged, that voltage is too high. \$\endgroup\$
    – rdtsc
    Commented Jun 30, 2020 at 16:54
  • \$\begingroup\$ @rdtsc I assume that the voltage is that high because Yuasa might have pre-charged it at the factory before drying the cell. The bubbling was when I had poured the acid solution into the battery. As, of now the bubbling has almost stopped except for occasionally tiny bubbles. The battery has also cooled off. I'm thinking maybe it was some kind of reaction between the acid and the plates? \$\endgroup\$ Commented Jun 30, 2020 at 18:33
  • 1
    \$\begingroup\$ A new battery with acid freshly added IS fully charged. You can't make the acid any stronger than it started out. \$\endgroup\$
    – Dave Tweed
    Commented Jun 30, 2020 at 19:58
  • \$\begingroup\$ @DaveTweed That's true. I guess initially the voltage might show high because the battery specific gravity was close to fully charged one. \$\endgroup\$ Commented Jul 3, 2020 at 20:43

1 Answer 1


Addressing your first question: “While charging the battery for the first time can I use a constant voltage charger?

The short answer is: Yes, but with specific voltage and current monitoring (and/or time).

Yuasa site recommends its batteries to be charged with their chargers. Searching for them, most are multistage charges, with at least bulk, absorption and floating modes, each one with different voltages and current values.

The following figure summarizes such characteristics for 6V and 12V chargers - helping a broader range of readers too.

Please see the information within the highlighted rectangles. Yuasa’s Chargers for 6V & 12V Lead Acid batteries

About your second question: “Can the charging voltage be 7.2-7.5V like SLA or should it be lower for a flooded cell?

As could be seen, the absorption mode would be the “main” charging mode, constant voltage mode set at 7.2V for 6V batteries (or 14.4V for 12V models).

Additional information for an implicit question about how to charge, or when the absorption charging mode should be terminated?

There are 3 stages or modes applicable for normal charging:

  1. Bulk mode: Charging current is limited up to a “safe“ value, while the battery voltage increases. It is a constant current (CC) mode. When current starts to reduce, the battery is charged at aprox. 80% of rated capacity.
  2. Absorption mode: When the battery voltage reaches the “absorption charging voltage”, it enters the absorption mode, operating in constant voltage mode. Depending of literature sources bulk mode shifts to the next mode when the charging current reduces to about 10% to 20% of bulk current value or 3% to 5% of AH. At this point, battery would be fully charged (100%).
  3. Floating mode: The constant voltage mode value is lowered to the “floating charging mode”. It can be maintained for extended periods (days) without overcharging the battery, but for that, the precise value should be fine tuned for ambient/battery temperature.

The recommended current for bulk charging (first mode) is 10% of rated AH. Some articles cite up to 20% as maximum to be considered still within “slow charging” class. In this case (6V x 5.5AH), the maximum current for your battery should be 1A, while 0.6A would be better.


Yuasa’s smart chargers

Recommended charging for lead acid batteries - Battery University

Chargetek’s charging and equalization - additional information


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