I have a 12 V 6 Ah lead acid battery, and have been instructed to charge it using a configurable power source.

They told me I should use a float charge, setting the voltage at a fixed level according to the battery datasheet, and end the charge when the charging current is lower than about 5% C (5% of 6A in this case).

Doing this, I have barely gotten about 80% of the rated capacity. Now, doing some research on my own I've found that what I should really do is a cycle charge, which as I understand it is a constant current charge until voltage reaches a higher level than with the float charge (also specified in the datasheet).

Long story short is, what is the proper way I should charge this kinds of batteries? Should I do constant voltage then constant current, just constant current, whatever? What I really wanna do is understand how to evaluate a given battery.

  • 1
    \$\begingroup\$ Who is "they"? Have you checked out Battery University yet? I have found this book very useful. It predates rechargeable Lithium batteries, but I don't think that lead-acids have changed since it was written. \$\endgroup\$
    – TimWescott
    Commented Nov 12, 2018 at 15:47

1 Answer 1


The bulk of the battery charge would be a constant current (CC) mode to the battery, this only gives you about 70-80% of the battery capacity.

If you want full battery capacity utilization you would then switch to a constant voltage (CV) mode. If you wish to maintain the battery in a charged state you would then switch over to a lower CV float charge mode.

The primary difference between the topping charge and float charge is the latter has a lower voltage, otherwise overcharge will permanently (if done long enough) reduce the battery capacity.

Lead acid battery charging voltage values are temperature sensitive, which can complicate things.

If you have a severely discharged battery, you will need to trickle charge it until it reaches about 75% of the normal battery voltage. In most cases such a battery will have permanently reduced capacity.

Reviewing some integrated battery charge controllers such as Texas Instruments bq24450 or Consonance CN3717 is a good way to get introduced to the pitfalls as well. Tim's recommendation will also give you good info.

  • \$\begingroup\$ I thought a topping charge applied to a battery in storage as opposed to one in use and being charged. Whereas a float or hysteresis charge applies to an in use battery being charged. The use of float vs. hysteresis being a bit controversial especially in regards to stationary SLA. Did you read the sources recommended by @TimWescott or what? BTW the bq24450 is obsolete. \$\endgroup\$ Commented Dec 9, 2018 at 4:46

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